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Forest Ecology and Management 302 (2013) 107–121

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Forest Ecology and Management

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Review

The harvested side of edges: Effect of retained forests on there-establishment of biodiversity in adjacent harvested areas

0378-1127/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.foreco.2013.03.024

Abbreviations: VR, variable retention; ECM, ectomycorrhizal fungi.⇑ Corresponding author. Tel.: +61 3 6235 8306; fax: +61 3 6226 2698.

E-mail addresses: [email protected] (S.C. Baker), [email protected](T.A. Spies), [email protected] (T.J. Wardlaw), [email protected](J. Balmer), [email protected] (J.F. Franklin), [email protected] (G.J.Jordan).

Susan C. Baker a,⇑, Thomas A. Spies b, Timothy J. Wardlaw c, Jayne Balmer d, Jerry F. Franklin e,Gregory J. Jordan a

a University of Tasmania, School of Plant Science, Private Bag 55, Hobart, Tasmania 7001, Australiab US Department of Agriculture Forest Service, PNW Research Station, 3200 Jefferson Way, Corvalis, OR 97331, USAc Forestry Tasmania, Division of Research and Development, GPO Box 207, Hobart, Tasmania 7001, Australiad University of Tasmania, School of Geography and Environmental Studies, Private Bag 78, Hobart, Tasmania 7001, Australiae School of Environmental and Forest Science, College of the Environment, University of Washington, Seattle, WA 98195, USA

a r t i c l e i n f o

Article history:Received 14 January 2013Received in revised form 18 March 2013Accepted 19 March 2013Available online 27 April 2013

Keywords:Forest influenceEdge effectsVariable retentionClearcuttingDispersalRe-colonisationNatural disturbance

a b s t r a c t

Most silvicultural methods have been developed with the principal aim of ensuring adequate regenera-tion of commercial tree species after harvesting. Much less effort has been directed towards developingmethods that benefit the re-establishment of all forest biodiversity. The concept of ‘forest influence’relates the probability of species re-establishment to the distance from mature forest. This idea is centralto contemporary retention forestry practices as well as connectivity theory in natural landscape manage-ment. Some species from all major forest biodiversity groups respond to forest influence following har-vesting, however, the temporal and spatial scales of forest influence are mostly poorly known. This paperreviews global knowledge of mechanisms and scales at which forest influence operates, and shows thatthese are highly variable. Important general factors and mechanisms that underlie the ability of organ-isms to re-establish include qualities of retained elements, dispersal capacity, suitability of habitat con-ditions, and interspecific interactions, all of which may vary with distance from intact mature forest.Forest influence may enable species to persist in harvested areas through buffering of microclimate,and/or assist re-colonisation via proximity to source populations or essential habitat elements. Althoughforesters have often applied a ‘‘rule of thumb’’ that the extent of forest influence is within one tree heightof mature forest, existing scientific literature provides little evidence of a universal relationship betweencanopy height of retained forest and re-establishment success. One-tree-height-from-retention guide-lines can help plan harvest layouts, but only as long as plans allow for variation in re-establishment suc-cess among species and groups. The evidence from this review is that variability in harvest layouts willpositively benefit biodiversity conservation in managed forest landscapes.

� 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1082. Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093. Retention silviculture and forest influence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 109

3.1. Overview of retention silviculture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1093.2. Silvicultural management of forest influence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110

4. General factors and mechanisms leading to forest influence on biodiversity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
4.1. Qualities of retained elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1104.2. Dispersal limitation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
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108 S.C. Baker et al. / Forest Ecology and Management 302 (2013) 107–121

4.3. Microclimatic gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1114.4. Distance to critical habitat elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1124.5. Interspecific interactions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1124.6. Temporal factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

5. Impacts on different groups of organisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
5.1. Vascular plants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1125.2. Bryophytes and lichens . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.3. Vertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113
5.3.1. Mammals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.3.2. Birds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1135.3.3. Amphibians and reptiles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

5.4. Invertebrates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1145.5. Fungi and microbes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114

6. Synthesis. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114
6.1. Mechanisms and scales. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1146.2. Comparison with edge effects into unlogged forest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1166.3. Harvesting patterns and forest influence. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 116
7. Knowledge gaps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.1. Recommendations for sampling designs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1177.2. Recommendations for future research . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117
7.2.1. Mechanisms of forest influence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1177.2.2. Spatial and temporal scales . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1177.2.3. Harvest design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117

8. Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118

1. Introduction

The direct and indirect effects of nearby mature forest on dis-turbed areas are known as ‘forest influence’ (Keenan and Kimmins,1993). These edge effects are diverse and complex, but someimportant aspects relate to dispersal of individuals and seed, aswell as microclimatic gradients such as shading. Proximity toundisturbed mature forest may assist forest organisms to re-estab-lish in areas disturbed by timber harvesting or natural disturbance.The magnitude and distance of forest influence varies betweenspecies and environmental characteristics, such as slope, aspect,latitude and microclimate (Keenan and Kimmins, 1993). Forestinfluence may be positive or negative, depending on the factor orspecies involved (Bradshaw, 1992).

In forests managed for wood production, foresters have tradi-tionally developed harvesting techniques designed to ensureprompt and adequate regeneration of commercial tree species.However, the degree to which harvesting practices enable otherorganisms to be sustained in the post-harvest forest is much lesswell understood. Understanding the mechanisms, spatial scalesand ecological impacts of forest influence on biodiversity (the topicof this review) can assist forest ecologists and managers in design-ing ecologically sustainable forest harvesting practices, predictingrecovery from natural disturbances, and mitigating the adverse ef-fects of forest fragmentation.

Historically, forest conservation efforts have focussed more onreserving areas of unharvested forest, and much less on the re-establishment of biodiversity in harvested areas. However, it isbecoming increasingly clear that harvested areas are importantfor maintenance of regional biodiversity, and that characteristicsof the surrounding landscape affect the population sustainabilityof many species (Lindenmayer and Franklin, 2002). Adequate con-servation in managed forests requires both reservation and appro-priate management practices on those areas available forharvesting (Lindenmayer and Franklin, 2002).

Forest influence is also relevant to the management of forestssubject to natural disturbance. In temperate and boreal forests,large-scale natural disturbances like wildfire, insect outbreaks

and windthrow kill stands of mature trees, although enough usu-ally survive to enable re-establishment of a new tree cohort and as-sist re-establishment by other biodiversity (Turner et al., 1998;Lindenmayer and Franklin, 2002; Dale et al., 2005).

Both timber harvesting and natural disturbance can result in lo-cal elimination of some species that are adapted to conditions inmature forest (Lindenmayer and Franklin, 2002; Whelan et al.,2002; Baker, 2006). Re-colonisation of the regenerating forest,and the subsequent establishment of viable populations by thesespecies, depends on dispersal either from nearby intact mature for-est or from surviving individuals within the harvested area. Theeffectiveness of this dispersal depends on species’ life history traitsand the availability of suitable habitat. Dispersal limitation canconstrain colonisation by species from all taxonomic groups (Bull-ock et al., 2002). Indirect effects of proximity to forest edges on bio-diversity also affect habitat suitability. For example, shading by thenearby mature forest may allow some species to persist and/or re-establish in otherwise hostile harvested areas by providing sitesthat are buffered from extreme environmental conditions.

Although studies of edge effects on biodiversity in managed for-ests are common, these studies have almost always been focusedon gradients from the harvest unit into the unlogged forest and ig-nored the forest influence gradients into harvested areas (Harperet al., 2005). This orientation is related to the interest in impactsof logging on the conservation of biological diversity in adjacentareas. While there is a large literature on recruitment of seedlingsinto gaps (including micrometeorological gradients), the focus ofgap studies is usually from a silvicultural rather than biodiversityperspective (Gray and Spies, 1996; Coates, 2002). However, con-temporary forest management is based on the premise that it isimportant to integrate wood production with conservation objec-tives within the harvest site. Under these new approaches, ecolog-ical research and practical application have usually focused onmaintaining refugia of mature-forest species and structures withinharvested sites without explicit consideration of the role thisretention plays in re-establishment of biodiversity in harvestedareas (Baker, 2011). Nevertheless, ‘rules of thumb’ are sometimesapplied to facilitate forest influence in retention forestry systems

S.C. Baker et al. / Forest Ecology and Management 302 (2013) 107–121 109

(Mitchell and Beese, 2002; Baker and Read, 2011). Such efforts areconstrained by inadequate understanding of the ecologicalprocesses.

Our synthesis of forest influence builds on Bradshaw’s (1992)re-view of forest influence which focused on silvicultural aspects suchas regeneration establishment, stand development, felling damageand burning along with aesthetics. Bradshaw (1992) provided littlediscussion of how forest influence can impact biodiversity, exceptwhere animals impacted on production outcomes (e.g. seedlingbrowsers). The impact of forest influence on biodiversity is there-fore the focus of our review. The aim of this paper is to reviewthe current literature to assess the different components that makeup forest influence, their relationship to biological diversity andtheir role in conserving biological diversity in forests managedfor wood production. An understanding of how and why forestinfluence affects re-establishment by biodiversity can assist ecolo-gists and managers develop practices that encourage re-establish-ment of mature-forest species in disturbed areas. We summarisethe limited information on the spatial extent of forest influenceand describe common factors and mechanisms underlying forestinfluence, and their relevance to different elements of biodiversity.Until there is better empirical data on the spatial and temporalscales of forest influence, general knowledge of factors and mech-anisms that underlie biodiversity responses should assist manag-ers to predict the efficacy of different silvicultural options forbiodiversity conservation.

2. Methods

We have reviewed a wide range of ecological information onthe processes and scales of forest influence on major groups of for-est biodiversity, primarily focusing on research that directly relatesto forest harvesting. The paper first outlines retention forestrymethods and their value for increasing forest influence in har-vested areas. We then review and synthesise what is known aboutthe underlying factors and mechanisms of forest influence, andtheir effects on particular biodiversity groups. Finally, we highlightkey knowledge gaps that require further research effort.

We focus on harvesting of native forests in temperate and bor-eal regions where one management aim is to conserve biodiversityat the landscape scale, regardless of whether sites are regeneratednaturally or artificially, such as by planting or seed sowing. The lit-erature was largely derived from studies of retention forestry alongwith a variety of silvicultural systems that would traditionally havebeen classified as even-aged, including clearcutting, gap cutting,seed tree, and shelterwood. However, our review is relevant more

Fig. 1. The concept of forest influence illustrated with schematics of a harvested coupewith either the aggregated retention form of variable retention (VR) (left) or traditional clare designated for long-term retention. These, and the adjoining strip of reserved foresources for seeds, spores and animals from old forest to more recently harvested areas. Acheight of long-term retention (putative zone of forest influence). Under this definition,considered in forest-influence calculations. The example shows that VR sites with uncut aof the area in aggregates results in 51% putative forest influence, compared to only 6% i

broadly, including to reduced-impact logging in the tropics (e.g.Ruslandi et al., 2012), since, as Bradshaw (1992) points out, distinc-tions between even-aged versus uneven-aged management areartificial, especially when one takes a global overview.

We aimed to achieve a broad review of the topic, and can onlyconsider each part briefly. Only the most relevant papers were in-cluded, omitting much detail. The papers were selected on the ba-sis of personal knowledge and database searches. Search termsincluded, but were not limited to: variable retention, green-treeretention, forestry, clearcutting, harvesting, dispersal, re-colonisa-tion, re-establishment, biodiversity. Search terms identifying theindividual biodiversity groups were also used. We also approachedregional experts in retention forestry to obtain additionalreferences.

3. Retention silviculture and forest influence

The proportion of harvested area influenced by the edge is animportant characteristic of any silvicultural system (Bradshaw,1992), although historically the focus was usually on maximisingtree establishment, survival and growth. However, retention for-estry approaches also have biodiversity conservation as a keyobjective, and thus maintaining forest influence over a large pro-portion of the harvested unit is often a primary consideration. Toillustrate the concept, Fig. 1 contrasts the hypothetical zones of for-est influence in a clearcut and a retention forestry site.

3.1. Overview of retention silviculture

Retention forestry was developed in western North Americabased on insights into species recovery following the volcaniceruption of Mt St. Helens in 1980. Refuges enabled some organismsto survive within the blast area and distance to these source pop-ulations influenced the pattern and rate of re-establishment (Daleet al., 2005). Retention silvicultural systems are flexible approachesto forest harvesting, in which small patches of forest (with aggre-gated or group retention) and/or scattered individual structuralelements such as trees, snags and logs (with dispersed retention)are retained, for the long-term, across a harvested area (Franklinet al., 1997; Lindenmayer et al., 2012). Many silvicultural prescrip-tions actually retain both forest patches and individual structures(mixed retention). Diverse forms of retention silviculture, com-monly named variable retention (VR) and green tree retention,are now used to balance economic, social and conservation objec-tives in many parts of the world (Gustafsson et al., 2012). AlthoughFranklin et al. (1997) did not use the term, the underlying

Coupe boundary

Protected

Area with influence

Aggregates

situated next to a strip of forest protected within a reserve. The site was harvestedearcutting (right). The variable retention site contains unharvested aggregates whichst, are expected to provide buffered microclimatic conditions and re-colonisationcording to some definitions of VR, more than 50% of a site should be within one-tree-areas surrounding the site that are not designated for long term-retention, are notggregate have greater areas within one-tree-height of retained forest – retaining 23%n the clearcut site.


principles of ‘forest influence’ (sensu Keenan and Kimmins, 1993;Beese et al., 2003) are implicit in their primary objectives for reten-tion forestry. These principles are lifeboating of biota, structuralenrichment of post-harvest stands, and enhanced connectivity.Retention (‘‘lifeboating’’) of species and structures (e.g. logs, snags,cavity-bearing trees) from the pre-harvest stand may allow somemature forest species to persist through the early stages of standdevelopment (Beese and Bryant, 1999; Baker et al., 2009; Stephenset al., 2012), and then provide a source for re-establishment ofspecies into the harvested area (Hill and Read, 1984; Luomaet al., 2006). Structural retention moderates habitat conditions,making the harvested matrix a less hostile environment for somespecies, e.g. through shading of harvested areas by retained trees(Heithecker and Halpern, 2006, 2007). Reduction in the distancebetween unlogged habitats or important structural features isbeneficial, both for species moving among areas of suitable matureforest, and for species re-establishment into harvested areas(Chan-McLeod and Moy, 2007; Söderström, 2009). Thus, regener-ated forest stands enriched with retained biological and structuralfeatures may enable re-establishment of some species earlier inthe rotation (Fisher and Bradbury, 2006). However, the extent towhich it is possible for a retention area to help will depend onthe current successional stage and pathway for the particularpre-harvest ecosystem, which will be impacted by previous naturaldisturbance and harvest history. Restoration of mature forest con-ditions may be required in some second or third growth forests(Bauhus et al., 2009).

3.2. Silvicultural management of forest influence

The concept of forest influence is explicitly factored into rulesfor designing VR site layouts in Tasmania and coastal BritishColumbia. In these regions, a VR site is one in which the majority(>50%) of the cutblock (British Columbia) or harvested area (Tas-mania) is within one canopy tree-height of long-term retention,with the implication that this is the extent of forest influence(Fig. 1; Mitchell and Beese, 2002; Baker and Read, 2011). Underthis definition, only long-term retention counts as providing for-est influence. This recognises that re-establishment may be de-layed until conditions become suitable in the harvested area,and areas available for harvesting will not provide this ongoingservice. Of course, in reality, adjacent areas may in fact contrib-ute to forest influence to a certain extent. This definition arosein part as a method to distinguish VR from clearcutting, basedon Keenan and Kimmons’ (1993) definition of a clearcut as hav-ing approximately the area of a circle of four tree heights indiameter. However, this definition of forest influence is notwidely applied outside of British Columbia and Tasmania,although in some regions there are guidelines for maximum dis-tances between retention trees or aggregates (Baker, 2011).Encouraging foresters to explicitly manage for forest influenceis hindered by the generally poor understanding of biodiversityresponses.

Although mature forest influence is often considered to dimin-ish significantly at distances greater than one tree length from theedge (Mitchell and Beese, 2002), the actual temporal and spatialscales of forest influence are still poorly known, although somestudies indicate that canopy height is not directly scaled to dis-tance of forest influence. For example, studies of a variety of eco-system variables at the Sicamous Creek Trial in mainland BritishColumbia (Huggard and Vyse, 2002) recorded very narrow zonesof forest influence approximately 3–6 years after harvesting. Thoseauthors concluded that using a ‘one or two tree heights from edge’rule for forest influence is not appropriate for most biodiversity inthat ecosystem, and advocated reducing the distance between re-tained patches.

4. General factors and mechanisms leading to forest influenceon biodiversity

Proximity to source populations can be especially important forre-colonisation by species that have been locally eliminated byharvesting or other disturbance. However, the biotic and abioticenvironments also affect the probability of re-establishing in dis-turbed areas. Many environmental factors vary systematically withdistance from edge and contribute to forest influence (see Sec-tion 4.3). There are also many factors contributing to forest influ-ence through the process of re-colonisation. Fig. 2 illustratessome of the factors and processes involved with forest influence.

Forest influence will only be significant for a proportion of spe-cies. Some species will survive on the disturbed areas in the firstplace, e.g. survival of some vascular plant and bryophyte speciesvia a soil-stored diaspore bank or vegetative regeneration (Halpernet al., 2005; Stark et al., 2006; Caners et al., 2009) or of inverte-brates inhabiting legacy coarse woody debris (Hjältén et al.,2010). For other species, long-distance dispersal abilities removethe necessity for a local propagule source, provided habitat condi-tions are suitable for re-colonisation. The relative importance ofthese processes varies among forest types and successional stages,for example, plants from stable habitats generally have seeds withlower persistence in the soil (Thompson et al., 1998).

Major factors likely to influence re-establishment success in-clude the functionality of retained elements as source populations(see Section 4.1.) and traits of organisms including dispersal ability(see Section 4.2), habitat specificity (Halpern et al., 2012), toleranceto altered microclimatic conditions (Martinez Pastur et al., 2013),relative density (Solarik et al., 2010) and reproductive output andsuccess, competition and survival (Siipilehto, 2006). Time sincedisturbance will also be an important factor affecting the scaleand degree of re-establishment (e.g. Tabor et al., 2007). All of thesefactors may interact, complicating prediction of individual speciesresponses. It is also possible that processes and time scales for for-est influence may differ relative to the underlying growth rates ofdifferent forest types and the quality of retained forest elements.The following sections describe some of the main mechanisms thatunderlie biodiversity responses to forest influence. More details fordifferent biodiversity groups are given in Section 5.

4.1. Qualities of retained elements

If retained elements are to facilitate colonisation of harvestsites, it follows that they must have influencing characteristics.The seral stage of the forest will affect the species compositionand reproductive maturity of the species within it. Thus, retainedsecond growth may, at least until it ages, be a poorer populationsource for re-establishment than current oldgrowth. To operateas population sources, retained elements must be sufficiently largeto sustain mature forest species for enough time to enable re-col-onisation to take place (e.g. see Section 6.3). Depending on the spe-cies concerned, size requirements may relate to aggregate size tominimise edge and area effects (Halpern et al., 2012), windthrow(Steventon, 2011), and regeneration burn impacts (Scott et al.,2012). Likewise, patch size will determine habitat suitability inrelation to the home ranges of some animals (e.g. Stephens et al.,in press). The size and age of individual habitat elements may alsobe important; e.g. older, larger trees may provide more seed or cav-ities for vertebrates (Palik and Pregitzer, 1994; Koch et al., 2008)and large logs may contain appropriate rotten wood types for cer-tain saproxylic invertebrates and fungi (e.g. Yee et al., 2006). Thetotal amounts of mature forest retained in the landscape may alsoaffect the capacity of retained patches to sustain viable source pop-ulations, and thereby affect forest influence. Wardlaw et al. (2012)

Fig. 2. A conceptual diagram illustrating forest influence.


found that mature forest sustained viable source populations ofbirds and vascular plants in landscapes that varied widely in theamounts of mature forest retained. However, populations of somespecies of flighted beetles in retained mature forest decline in land-scapes with little remaining mature forest.

4.2. Dispersal limitation

Logging removes some species from harvested areas; either di-rectly, through tree harvesting, or indirectly, if species either die ordisperse elsewhere in response to unsuitable habitat conditions.Re-colonisation of harvested areas by such species therefore de-pends on dispersal from source populations outside the loggedarea. Hence, the extent of forest influence is likely to be relatedto dispersal distances for species with relatively poor dispersalcapacities. This will relate to the dispersal modes employed by dif-ferent kinds of organism. Most dispersal of plants and fungi is viaspecialised life history stages (seed, spores or vegetative propa-gules). Re-colonisation is a function of the distance these propa-gules travel from the parent (Matlack, 1994a; Tabor et al., 2007)and the time to reproductive maturity (Matlack, 1994a). A highproportion of insects have a flying adult life-history stage to enabledispersal, although many other animals lack life history phasesspecialised for dispersal, and instead depend on adult or juvenilesflying or moving across the ground. Dispersal strategies varygreatly within taxonomic groups, and many species may be dis-persed by multiple mechanisms (e.g. Higgins et al., 2003). Evenfor groups of species with generally similar dispersal mechanisms,there is much variation in relative dispersal ability, e.g. differing

aerodynamic properties of seeds of wind dispersed plants (Mul-ler-Landau, 2010).

4.3. Microclimatic gradients

Microclimatic conditions in disturbed areas can be strongly re-lated to edge proximity, often resulting in better habitat for ma-ture-forest species near the disturbance edge (e.g. Dynesius et al.,2008). In particular, areas near edges may be shadier, moister, lesswindy, with lower vapour pressure deficit, lower air and soil tem-peratures, less subject to temperature extremes, and experiencedelayed snow melt (e.g. Davies-Colley et al., 2000; Huggard andVyse, 2002; Redding et al., 2003; Huggard et al., 2005; Heitheckerand Halpern, 2007). Although the extent of microclimatic bufferingdiffered among variables and also among studies, most gradients inthese studies appeared to be strongest within approximately 10–20 m of edges. However, as shown for soil temperature and mois-ture (Redding et al., 2003), gradients within the edge transitionzone are likely to be minor in comparison to the strong contrast be-tween unlogged and cleared conditions, especially in the earlyyears after disturbance. Gradients in microclimate can affect pro-cesses such as evapotranspiration, moisture availability to plants,nitrogen mineralisation and CO2 efflux, and therefore ecosystemfunction (Redding et al., 2003, 2004).

Heithecker and Halpern (2007) found steeper gradients intransmitted light along transects from aggregates into harvestedareas at a site with taller trees, indicating a relationship with can-opy height, although the 20–30 m zone of reduced solar radiationwas less than the height of dominant/co-dominant trees, and was


narrower still for effects on temperature. In some cases the scale ofshading effects also varies significantly with the orientation ofedges to solar radiation (Huggard et al., 2005; Heithecker andHalpern, 2007; Prevost and Raymond, 2012), which can result indifferential biodiversity responses and tree re-establishment onshaded and exposed edges (e.g. LePage et al., 2000; Bolibok andSzeligowski, 2011). The slope of the site can also modify the shad-ing effect of border trees on the radiation regimes (Prevost andRaymond, 2012). Differences in microclimate between loggedand unlogged forest are likely to dissipate rapidly as the regenera-tion ages, in particular when the canopy closes.

Sensitivity of biodiversity to edge proximity may vary amongsites along temperature, moisture or productivity gradients, forexample edge proximity may be less important on cooler, moistersites for species that respond strongly to microclimatic conditions(Chan-McLeod and Moy, 2007; Rosenvald and Lõhmus, 2008).

4.4. Distance to critical habitat elements

Proximity to specialised habitats will also determine the capac-ity of some species to occupy certain sites. For example, many birdsand mammals depend on trees with nest cavities (e.g. Gibbons andLindenmayer, 1997; Cooke and Hannon, 2011), and invertebratespecies with saproxylic larval phases require snags or logs (Hjälténet al., 2010; Grove and Forster, 2011). Similarly, the presence ofcertain ectomycorrhizal fungi in harvested areas is likely to be di-rectly associated with the roots of retention trees, thus limiting thedistance into harvested areas where they might be able to inocu-late seedlings (Luoma et al., 2006). The unlogged edges will alsoprovide inputs of certain habitat elements and nutrients into har-vested areas, for example coarse woody debris, bark and leaf litter.

4.5. Interspecific interactions

Species occurrences may be positively or negatively correlatedwith the occurrence of other species, so that forest influence onone species can create forest influence on another. This may beespecially relevant to species with strong interactions, e.g. mycor-rhizal fungal association with tree roots (see Section 5.5), specialistherbivores, or plants and fungi relying on other species for dis-persal. In some cases the forest influence effect could have negativeconsequences, e.g. reduced growth of regeneration or increasedriskof browsing, predation, pests and disease (Bauhus et al., 2009).Competition within and between species for resources may alsolead to changes in habitat occupancy in relation to distance fromedge (e.g. Dzwonko, 1993; Coates, 2000; McCoy et al., 2004). Pre-dation risk may be higher further from the cover of mature forest,e.g. for some herbivorous mammals (Kremsater and Bunnell, 1999;While and McArthur, 2005). Patterning of herbivores can then leadto indirect effects on vegetation communities, through, for exam-ple, greater browsing of seedlings of preferred food plants nearerto edges.

4.6. Temporal factors

The temporal component of forest influence is poorly under-stood. It nevertheless seems reasonable to say that time since dis-turbance will be an important factor in re-establishment ofharvested areas, particularly for dispersal-limited species and spe-cies requiring habitat conditions (e.g. microclimate) that developover a delayed period. However, the likely rates of progressionfor forest organisms that benefit from forest influence are largelyunknown. For species with limited mobility, spread into harvestedareas may be gradual or intermittent, and in some cases this mayfurther depend on time to reproductive maturity (Matlack,1994a; Brunet et al., 2000; Tabor et al., 2007). Species succession

could additionally cause changes to inter-species interactions suchas competition or predation.

Unlike the edge influences of logged areas on adjacent forest,which tend to diminish with time since edge creation (Matlack,1993; Harper and Macdonald, 2002), the forest influence distanceinto harvested areas may increase with time, as species progres-sively disperse and establish further into harvested areas (Brunetet al., 2000). This rate of progression may depend on species’ dis-persal abilities, and in some cases, e.g. plants, to their age of repro-ductive maturity. At a certain point in time, initially limited speciesmay become fully established in harvested areas such that forestinfluence gradients are no longer apparent. The successional stageof the adjacent retained forest will also determine the species thatare available to re-colonise nearby harvested areas.

5. Impacts on different groups of organisms

The scales and mechanisms of forest influence are highly vari-able both within and between different biodiversity groups. Theamount of previous research effort also varies considerably be-tween and within these groups.

5.1. Vascular plants

A number of studies have shown significant relationships be-tween vascular plant recruitment and distance from edge (e.g.Matlack, 1994b; Asselin et al., 2001; Huggard and Vyse, 2002; Ta-bor et al., 2007). Seed dispersal ability, and factors determiningestablishment (gradients of microclimate, competition and herbiv-ory pressure) may all lead to forest influence on plant speciescomposition.

Plant dispersal patterns, and therefore dispersal limitation, varydepending on plant functional traits such as dispersal method(Dzwonko, 1993; Matlack, 1994a; Clark et al., 1998). Matlack(1994a) suggested a generalised ranking of migration rates of for-est herbs and shrubs depending on dispersal mode:ingested > adhesive�wind P ants P none. Seed dispersal bywind of several tree species showed an exponential decline withdistance from source, and was related to tree height, with deposi-tion at a distance equivalent to five forest heights of around 3%(Greene and Johnson, 1996) to 14% (Tabor et al., 2007) of edgedeposition. Proximity to edge may be more important for wind-dispersed species with relatively large seeds, since these tend tobe less well dispersed (Greene and Johnson, 1993). For animal-dis-persed species, flying fauna and large herbivores like deer are likelyto disperse seeds over longer distances than small ground-dwellingmammals (Matlack, 1994a; White et al., 2004), while ant dispersalmay occur over relatively short distances (Dzwonko, 1993; Mat-lack, 1994a). Proximity to perching sites in mature forest or dis-persed retention trees will also affect animal dispersal of plantseeds.

There is clear evidence that distance-related changes in physicalor biotic environment contribute to forest influence for plants. Insome cases (e.g. LePage et al., 2000) these gradients can vary be-tween exposed and shaded edge aspects. Light intensity was re-lated to edge responses of certain plant species (Dzwonko, 1993)and moisture stress is a factor for Korean red pine establishment(Lee et al., 2004). Shade-tolerant plants, such as many late-seralspecies, may be more common nearer to edges than further into re-cently harvested sites (Battaglia et al., 2004; Huggard et al., 2005).Vegetation gradients in relation to distance from an old field bor-der were found to co-vary with gradients in canopy cover, soilmoisture, soil reaction and soil nitrogen (Brunet et al., 2000).LePage et al. (2000) attributed harsh microclimatic conditions to


explain reduced tree seedling dispersion into clearcuts relative togroup cuts, since substrate availability was similar.

There are a number of examples illustrating how inter-specificinteractions can lead to gradients in plant occupancy with distancefrom edge. Reduced below-ground competition and reduced shad-ing can result in increased growth rates further from edges,although this will vary depending on species’ shade-tolerance(Dzwonko, 1993; Coates, 2000; York et al., 2003; McCoy et al.,2004). Predation on seeds or seedlings can affect survival alongedge gradients (e.g. Holl and Lulow, 1997; Lee et al., 2004). Forexample, numbers of the Australian marsupial herbivore, the pade-melon, in recently established plantations decreased with distancefrom unlogged forest, resulting in increased survival of eucalyptseedlings further from the edge (Bulinski and McArthur, 2000;While and McArthur, 2005).

5.2. Bryophytes and lichens

Some studies have demonstrated relationships between bryo-phytes and lichens and distance into harvested areas (e.g. Peckand McCune, 1997; Dettki et al., 2000; Baker, 2010) while othershave not (e.g. Hylander, 2009; Rudolphi and Gustafsson, 2011).For both these groups, long-distance dispersal up to the scale ofmany kilometres (e.g. Lattman et al., 2009) is thought to occur pri-marily via spores, whereas asexual propagules or fragments con-tribute mostly to local dispersal (e.g. Coxson and Stevenson,2007; Lobel and Rydin, 2009) and in a few cases to long-distancedispersal (Pohjamo et al., 2006). Peck and McCune (1997) notedthat species producing large fragments are likely to disperse short-er distances than species producing smaller fragments.

The shading received by harvested areas from retained treesmay be particularly important for some bryophytes and lichens be-cause of their sensitivity to microclimatic conditions, in particularmoisture (e.g. Dovciak et al., 2006; Dynesius et al., 2008; Rudolphiand Gustafsson, 2011). Lõhmus et al. (2006) found that desicca-tion-resistant lichens were more robust to harvesting impacts thanbryophytes. Lõhmus and Lõhmus (2010) found that microclimate-caused declines in epiphytic bryophyte populations stabilised be-tween 2–3 years and 5–6 yearspost-harvest, while lichen numbersincreased during the same period.

Many bryophyte and lichen species are most commonly foundon specific substrates such as bark, dead trees, tree fern trunksand logs (Roberts et al., 2005; Turner and Pharo, 2005; Rudolphiand Gustafsson, 2011), which in some cases might be more com-mon as a result of inputs from nearby retained forest.

5.3. Vertebrates

Proximity to mature forest impacts some vertebrate species inharvested areas (e.g. Steventon et al., 1998; Huggard and Vyse,2002). Unlike plants, terrestrial vertebrates lack life history phasesspecialised for dispersal, and instead depend on adult or juvenilesflying or moving across the ground. Most terrestrial vertebrate spe-cies are relatively well-dispersed, so that forest influence on theseorganisms may be more related to microclimate, distance to criti-cal habitat features, food distribution or interspecific interactions,especially predator avoidance.

5.3.1. MammalsThe preference of pademelons for edges discussed in Section 5.1

applies more widely among macro-herbivores. Thus, deer and elkbrowse more heavily near forest edges, e.g. within approximately200 m from edges (Marcot and Meretsky, 1983) and use the for-ested side for cover, which is thought to relate to predation riskand forage abundance (Kremsater and Bunnell, 1999). Linear ele-ments such as the edges between logged and unlogged forest can

create foraging habitat for bats, with elevated activity near theseedges (Grindal and Brigham, 1999; Law and Law, 2011; Webalaet al., 2011). Proximity of nearby forest may also impact speciescrossing between areas of mature forest (Marcot and Meretsky,1983; Selonen and Hanski, 2003). Thus, retained trees in clearcutsallowed movement of flying squirrels, while open gaps > 100 m ex-ceed their gliding ability (Selonen and Hanski, 2003). Proximity tohollow-bearing trees is also expected to be important for manyAustralian marsupials (Gibbons and Lindenmayer, 1997). In somecases the benefits of structural retention may be delayed. Forexample, hollow-bearing trees retained within harvested siteswere rarely used for dens or nests by brush-tailed possums 8–10 years following harvest, but were used at 17 years followingharvest (Cawthen and Munks, 2011), presumably increasing thelikelihood that the possums would forage in the surrounding har-vested area.

5.3.2. BirdsForest influence effects have occasionally been documented in

studies of birds, with species either preferring (Steventon et al.,1998) or avoiding (Schlossberg and King, 2008) harvested forestnear edges. In some cases, edge preference by birds may relate toelevated abundance of invertebrate prey (Helle and Muona,1985). Interspecific interactions may also lead to preferences forconditions further from intact mature forest. Predation or nest-par-asitism may be increased closer to bird perch sites in mature trees(Kremsater and Bunnell, 1999). Certain birds have been found tonest preferentially on retention trees in logged sites rather thanin unlogged forest, possibly due to lower risks of nest predation(Rosenvald and Lõhmus, 2008).

Edge responses can also relate to habitat conditions such as theamount of herbaceous growth (Preston, 2006), the distance thatbirds need to fly to key habitat structures like nesting or perchingsites in mature trees (Schieck and Song, 2006; Rosenvald and Loh-mus, 2007; Atwell et al., 2008) and the amount of suitable habitatin the wider landscape (Betts et al., 2006; Wardlaw et al., 2012).For example, Betts et al. (2006) suggested aural cues may beimportant attractors for conspecifics. This may result in aggrega-tions of higher bird density in landscapes with more retained ma-ture forest. Preston (2006) considered that group retentionadvantaged forest birds by providing greater connectivity, withforest patches in greater proximity facilitating movement aroundthe landscape. For example, a study of gap-crossing decisions offorest birds across cleared agricultural areas found that birds rarelyventured >25 m from forest edges, and would instead take longerroutes under forest cover (Bélisle and Desrochers, 2002).

5.3.3. Amphibians and reptilesAmphibian abundances have been found to decline with dis-

tance into clearcuts (deMaynadier and Hunter, 1999). This is likelyto relate to the narrow habitat tolerances of these organisms,which often prefer shaded, moist conditions (deMaynadier andHunter, 1998; Maguire et al., 2004; Chan-McLeod and Moy,2007). Specific habitat elements, e.g. coarse woody debris, plantcover or leaf litter conditions, can also be important (deMaynadierand Hunter, 1999; Maguire et al., 2004). In some cases, these hab-itats vary in abundance with distance from edge, e.g. as a conse-quence of retention trees falling into harvested areas. Manypond-breeding amphibians inhabit upland terrestrial sites in thenon-breeding season. Forest microclimate can be particularlyimportant for juvenile dispersal from aquatic habitats, especiallysince amphibians generally have limited mobility compared toother vertebrates (deMaynadier and Hunter, 1999; Patrick et al.,2006; Chan-McLeod and Moy, 2007). The distance between aggre-gates was shown to affect movement patterns of dispersing red-legged frogs in British Columbia; frogs had to be within 5–20 m


of aggregates for increased probability of movement in that direc-tion (Chan-McLeod and Moy, 2007).

Less is known about the responses of reptiles to silviculturalpractices than amphibians, and no studies have directly assessedgradients of edge response in harvested areas. Based on studiesof logging impacts, it appears likely that reptiles may respond toedge gradients in relation to microclimate such as the amount ofsunlight reaching the forest floor (Goldingay et al., 1996; Limaet al., 2001; Alexander et al., 2002). This may be important for rep-tiles, especially in cool climates where basking is used to increasebody temperatures (Alexander et al., 2002). Inputs of habitat ele-ments from edges, including leaf litter habitat and logs as baskingplatforms, could also lead to forest influence on reptiles (Brownand Nelson, 1993; Alexander et al., 2002; Todd and Andrews,2008).

5.4. Invertebrates

Forest influence has been documented in a number of inverte-brate orders: beetles, spiders, gastropods and both ant andnon-ant hymenopterans. Overall, research has been stronglybiased towards the ground habitat, which is readily sampled bypitfall traps, and towards spiders and especially beetles (but seeHelle and Muona, 1985; Siira-Pietikäinen and Haimi, 2009). Forestinfluence responses may be related to dispersal ability (Jonsellet al., 1999; Koivula, 2002; Jonsson and Nordlander, 2006), micro-climatic gradients and canopy openness (Koivula, 2002; Lemieuxand Lindgren, 2004), or availability of food or specialised habitats(Nordlander et al., 2003; Jacobs et al., 2007; Hyvärinen et al.,2009; Siira-Pietikäinen and Haimi, 2009). Serving as source habi-tats, edges can contribute to re-colonisation by ground beetlesafter disturbance (Molnar et al., 2001; Koivula, 2002). Differentinvertebrate orders and functional groups vary in their responseto harvesting practices and forest influence (Helle and Muona,1985; Siira-Pietikäinen et al., 2003; Jacobs et al., 2007; Hyvärinenet al., 2009), and specialist species and those of higher trophic lev-els may be especially sensitive (Jonsson and Nordlander, 2006).

In most studies showing forest influence in invertebrates, spe-cies affiliated with uncut forest became less common with distanceinto cut areas (Tim Work, personal communication; Helle andMuona, 1985; Pearsall, 2003; Klimaszewski et al., 2008). However,in one study (Pearce et al., 2005) species that were more commonnear edges were usually associated with open rather than matureforest habitats, suggesting that in this case the edge response re-lated more to differences in habitat conditions than re-colonisationfrom the nearby mature forest.

5.5. Fungi and microbes

There is a growing body of knowledge of forestry impacts onectomycorrhizal (ECM) fungi and some symbiotic nitrogen fixingbacteria, but little research into free-living fungi, viruses, bacteria,protozoans and blue-green algae. The soil microbial community issensitive to changes to microclimate such as temperature and soilmoisture availability (Marshall, 2000; Grayston and Rennenberg,

Table 1Hypothesised most important mechanisms relating to re-establishment of harvested area

Limiting factor Taxonomic group

Vascular plants Bryophytes and lichens Mam

Dispersal � �Microclimate � �Habitat elements � � �Interspecific interactions � �

2006), and could therefore be expected to respond to microclimaticedge gradients. Since many soil organisms are sensitive to temper-ature and moisture extremes, retention forestry practices that re-sult in microclimatic buffering are expected to benefit thesegroups. For example, Grayston and Rennenberg (2006) found thatthinning had different effects on microbial biomass, activity andpopulation structure depending on site aspect, and attributedthese differences to variation in soil moisture availability. Somegroups have poorer dispersal and colonisation abilities, includingnitrogen fixing species and plant pathogen species with partialdependence on suitable hosts (Bissett et al., 2010), and thus mightalso be sensitive to distance from edge for dispersal. Redding et al.(2004) found soil nitrate content and net nitrification (but notammonification) increased markedly beyond 2–6 m into harvestedforest from clearcut edges, while Hope et al. (2003) did not docu-ment an edge effect on litter decomposition. These results suggestforest influence distances may be small for microbial activity.

Forest influence from edges or individual trees has been rela-tively well quantified for ECM fungi. Because of their symbioticrelationship with tree roots, close proximity of host seedlings tothe roots of retained trees appears to be the main mechanismunderlying relationships with distance into harvested areas (e.g.Cline et al., 2005; Luoma et al., 2006). Narrow zones of forest influ-ence of generally less than 10 m have been documented in severalstudies of mycorrhizal root colonisation and/or diversity (Parsonset al., 1994; Durall et al., 1999; Hagerman et al., 1999a,b; Outer-bridge and Trofymow, 2004, 2009; Luoma et al., 2006; Joneset al., 2008). Although a small sub-set of species have effectivelong-distance colonisation via spores, local dispersal, either byspores or mycelial spread among host roots, is more common (Peayet al., 2011). Outerbridge and Trofymow (2004) found that old-growth trees provided greater inoculum potential than retainedsecond growth trees. In a related study, the same authors foundthat increased levels of green tree retention increased overallECM fungi richness and root colonisation over longer distances(Outerbridge and Trofymow, 2009).

6. Synthesis

6.1. Mechanisms and scales

Although several mechanisms underlie forest influence, it isimportant to re-emphasise that many individual species will notbe subject to these factors because they are well adapted to re-establishing in harvested areas. However, such species are rarelyof conservation interest because they tend to be favoured byanthropogenic processes. Several important processes and mecha-nisms relate to the biodiversity responses to retained forest influ-ence. These vary from species to species, but based on theforegoing literature review, some general hypotheses can be posedabout which mechanisms are particularly important for varioustaxonomic groups (Table 1). Although dispersal limitation maybe the primary limiting factor for some types of biodiversity (suchas some plants and flightless invertebrates), for others,microclimatic gradients (e.g. for lichens, bryophytes, reptiles and

s in relation to distance from edge for major groups of forest biodiversity.

mals Birds Amphibians and reptiles Invertebrates ECM fungi

� ��

� � � ��

1 10 100 1000

VASCULAR PLANTS Balsam fir regeneration, Asselin et al. 2001

Cool temperate rainforest trees, Tabor et al. 2007Vascular plants, B. Beese pers. comm. 2012

Hemlock seedlings (S facing), LePage et al. 2000Hemlock seedlings (N facing), LePage et al. 2000

Spruce seedlings, LePage et al. 2000Douglas-fir height, York et al. 2003

Giant sequoia height, York et al. 2003Ponderosa pine height, York et al. 2003

CRYPTOGAMS Lichen litterfall, Peck & McCune 1997

Moss cover & richness, B. Beese pers.comm. 2012Bryophytes, ground & log substrates, Baker 2010

VERTEBRATES Deer and elk, Marcot & Metetsky 1987

Syberian flying squirrel, Selonen & Hanski 2003Avifauna, Wardell-Johnson pers. comm. in Bradshaw 1992

Red-legged frogs, Chan-McLeod & Moy 2007Amphibians, deMaynadier and Hunter 1998

INVERTEBRATES Forest interior spiders, Larrivée et al. 2008

Ground-active invertebrates, T. Work, pers. comm. 2010Carabid beetles, Pearsall 2003

Pine weevil feeding, Norlander et al. 2003ECM FUNGI

ECM fungi from aggregates, Jones et al. 2008ECM fungi at aggregates, Outerbridge & Trofymow 2004

ECM fungi from dispersed trees, Luoma et al. 2006ECM fungi from cutblock edges, Durall et al. 1999

ECM fungi from gap edges, Parsons et al. 1994

Approximate distance into harvested area (m)

Taxonomic group

Fig. 3. Estimates of the extent of forest influence into harvested areas for different types of biodiversity. Distance into harvested area is displayed on a log scale, and themidpoint is displayed when a range was given. Differences in study designs, time since harvest, silvicultural systems and gap sizes and approaches to analysis andinterpretation mean that these comparisons are illustrative only. (See above-mentioned references for further information.)


amphibians), the proximity to, and amount of, key habitat ele-ments (e.g. for saproxylic invertebrates), or interactions with otherspecies (e.g. for mycorrhizal fungi) may be of similar or greaterimportance. Habitat elements appear to be important for at leastsome species from all the taxonomic groups we assessed, confirm-ing the value of structural retention as a valuable forest manage-ment tool to encourage re-establishment of biodiversity.

Estimates of the distance that forest influence extends are sum-marised in Fig. 3. Because of differences in design, sampling effortand analysis approach, these measures cannot be directly con-trasted (Harper and Macdonald, 2011), but do serve to provide apicture of the relative distances for the various biodiversity groups.The documented scales vary from less than 10 m to approximately200 m, and there was variability within, as well as between, majorbiodiversity groups (Fig. 3). More mobile biodiversity groups (e.g.mammals and birds) show generally greater forest influence dis-tances than others, such as ECM fungi, for which distance to rootsof residual trees appears to be a plausible mechanism. Except forvertebrates, estimates of forest influence did not exceed 100 m inextent, although this is possibly biased by the scales (spatial andtemporal) of assessment.

Most studies were carried out during the first few years afterharvesting, with the notable exception of the retrospective studyof bryophytes by Baker (2010) in a 48 year-old clearcut. The largesize of this clearcut also enabled sampling much further from theedge (to approximately 500 m) than the majority of studies, makingdetection of long edge gradients possible if present. However, thesize of cutting units usually prohibits assessment over such longdistances. It is therefore important to keep in mind that estimatesof depth of forest influence will be within the context of the maxi-mum distance from edge sampled, and may be limited by the size ofthe cut area which may prevent detection of longer gradients.

Research is required to determine how forest influencechanges with time. However, we hypothesise that the strengthand distance of forest influence will increase over time, at leastinitially, for species where the primary mechanism for influenceis dispersal limitation (Matlack, 1994b). By contrast, for speciesfor which microclimate limits establishment, forest influenceeffects may be most pronounced in the short-term, but at somestage the entire regenerating forest will reach suitablemicroclimatic conditions for sensitive species to establish. Forexample, we would expect that for species limited by microcli-mate, gradients in forest influence will diminish after canopyclosure.

We observed in our review that canopy height of edge vegeta-tion was rarely reported, precluding analysis of the relationship be-tween this measure and forest influence distance. Height of canopytrees along with crown size and width may play a role in determin-ing the extent of the zone of forest influence for some factors;either directly, e.g. through dispersal of tree seed, or indirectlythrough microclimatic amelioration. For example, it is possible thatcanopy height sets an approximate upper limit for forest influenceon vascular plants, for which seed dispersal is likely to be a primarylimitation.

Based on this review, we suggest some hypotheses aboutcondi-tions under which forest influence may be maximised, in the hopethat further research will test these ideas, along with the relation-ship between forest influence and height and structure of retainedforest:

� forest influence should be of greater relative importance forlate-successional species, particularly those species with poorerdispersal abilities and requirements for shaded microclimaticconditions, than for early-successional species;


� seral-stage, species composition and condition of the retainedmature forest elements are likely to affect their functionalityas a source population, with intact late-seral areas providingforest influence gradients for a higher proportion of species;� forest influence should be of greater relative importance for

habitat specialists than for generalists;� mature forest influence should be of greater depth and magni-

tude where local site conditions in the harvest area favour re-establishment from mature forest, such as shaded aspects andpositions down slope and downwind of mature edges.

6.2. Comparison with edge effects into unlogged forest

The dominant processes and mechanisms underlying forestinfluence are related to, but different from, those that underlieedge effects into unharvested forest (see Harper et al., 2005). Edgeeffects in unlogged forest relate to removal of the buffering influ-ences of intact vegetation, whereas forest influence on harvestedareas relates to additional influence from the retained forest. Thus,while edge effects in unlogged forest often result in increasedevapotranspiration, decomposition and growth, and reduced can-opy cover and tree density near edges (Harper et al., 2005), forestinfluence may result in the opposite effects on the same variablesfor harvested areas near edges. Other factors, such as composi-tional diversity, amount of coarse woody debris, and rates of dis-persal or nutrient cycling, are also likely to show contrastinggradients either side of edges. These distinctions could then resultin differences in the relative distances, magnitudes and time-spansof edge responses by biodiversity either side of boundaries, andcould affect different suites of species. Nevertheless, certain attri-butes of forest may render them particularly susceptible to bothedge effects into unlogged forest, and forest influence on the har-vested side. In particular, the magnitude and distance of edge ef-fects are considered to be emphasised in forest systems in whichthe mature forests naturally experience rare disturbance, containfew pioneer species and have much taller and more closed cano-pies than the logged forests (Harper et al., 2005). We hypothesisethat the same is true of forest influence. Research studies that as-sess gradients both sides of edges would be particularly usefulfor making direct comparisons of scales and mechanisms of edgeresponses either side of boundaries.

Fig. 4. A variable retention site with 23% retention in aggregates illustrates the impacaggregates have higher levels of calculated forest influence (i.e. areas within one-tree-heigsusceptible to windthrow and regeneration burn impact. This may impact their functioecological processes.

6.3. Harvesting patterns and forest influence

Forest influence will impact biodiversity responses to a certainextent under all silvicultural systems (Bradshaw, 1992), but is par-ticularly relevant to the objectives of retention forestry ap-proaches. The choice of aggregated versus dispersed retentionpattern, and distances between retained elements may have strongimplications for re-establishment success. In particular, sinceaggregates maintain a more intact assemblage of the species pres-ent within sites, they might be expected to better facilitate re-col-onisation compared to dispersed retention which typically onlyretains trees and snags. However, for species where microclimaticamelioration is more important, the greater degree of shading fromdispersed retention may be beneficial. Retained tree densities im-pact the degree of microclimatic forest influence with dispersedretention. For example, a study of dispersal of canopy lichens founda positive relationship between cyanolichen litter biomass and thenumber of retained trees (Peck and McCune, 1997). Since trees areusually not retained within harvested areas of aggregated reten-tion sites, establishment of some bryophytes and lichens occupy-ing habitats developing on mature trees will be delayed untiltrees reach sufficient age to provide that habitat, which may notoccur within the time-period of a silvicultural rotation. For speciesthat prefer these substrates, dispersed retention may facilitateestablishment in harvested areas (Dovciak et al., 2006; Lõhmuset al., 2006; Caners et al., 2010). Dispersed retention is also ex-pected to be beneficial for mycorrhizae associated with retainedtrees.

Because of their isolation and small size, dispersed trees andsnags and small aggregates are particularly susceptible to edgeand area effects (e.g. Aubry et al., 2009; Baker et al., 2009; Lefortand Grove, 2009). These legacies are also more susceptible towindthrow (Scott and Mitchell, 2005; Jonsson et al., 2007) andregeneration burn impacts (Scott et al., 2012). These factors maythus compromise their ability to retain species, and in turn limittheir function as a source of re-introduction.

For a given retention level with aggregated retention, there willinevitably be a trade-off between the potential loss of habitat qual-ity if using more small or narrow aggregates versus the decreasedratio of influence to aggregate area resulting from using fewer lar-ger, less edge-effected, aggregates (Fig. 4). Simulation modelling byChan-McLeod and Moy (2007) shows that doubling the retention

t of aggregate size on the amount of putative forest influence. Sites with smallerht of long-term retention). However, smaller aggregates are more edge-affected and

nality for both lifeboating and for providing actual forest influence on species and


level or halving patch size for the same retention level (i.e. havingmore but smaller aggregates) had similar effects on decreasing theinter-patch distances (i.e. increasing forest influence). By contrast,patch shape (circular compared to rectangular) had relatively littleeffect, apart from slightly greater inter-patch distances with circu-lar aggregates. However, this modelling did not consider the im-pact of patch size on functionality of retention.

7. Knowledge gaps

As much as anything, our review highlights what is not knownabout biodiversity responses to forest influence, and although wespeculate about likely responses, a number of questions remainunanswered.

Research is needed to better understand scales and the mecha-nisms underlying the forest influence response and test thehypotheses posed in Section 6. Although more work is requiredfor all taxonomic groups, some groups are particularly poorly stud-ied, e.g. soil microbial community (other than ECM fungi), reptiles,and most invertebrate orders. Sampling a number of biodiversitygroups along transects within the same silvicultural experiment,and applying the same analysis approach across taxa, would enabledirect comparison of scales of influence. The designs of most re-search experiments do not allow us to easily tease out these under-lying mechanisms, and, as a result, ascribing cause is at best basedon correlation with habitat conditions. Although measuring rela-tionships with habitat variables is useful for understanding biodi-versity responses, this approach could be complemented withmanipulative research experiments specifically designed to allowdetermination of the relative importance of different mechanisms.

7.1. Recommendations for sampling designs

Different studies inevitably use different methodological ap-proaches to address specific study questions, and forest influencestudies can locate plots randomly or along transects, either uni-formly or with concentrated sampling effort nearer the edge. Care-ful thought to the sampling design is merited in every case, since adesign that was perfect for one study, may not be ideal for a similarresearch project. The advice on sampling effort provided by Harperand Macdonald (2011) is likely to be equally relevant to studies offorest influence. However, future meta-analysis and/or synthesis ofthe magnitude and scale of forest influence would benefit fromreporting the following:

� the height of the forest either side of the boundary, the timesince harvest, and the approximate expected age of canopy clo-sure, as well as an estimate of the successional stage of themature forest;� how estimates of the extent of forest influence were derived,

the error associated with these estimates and the magnitudeof changes. If practical, consider using the existing randomiza-tion test approaches for estimating the extent of forest influence– see Harper and Macdonald (2011) for an approach for singlespecies data, and Millar et al. (2005) for NCAP analysis of com-munity data; and� the proportion of species that appear to be sensitive to forest

influence, and reflect on whether they share any common attri-butes that contrast with species that are not sensitive to edgeproximity.

7.2. Recommendations for future research

Considerable knowledge gaps about forest influence remain,especially related to mechanisms, spatial and temporal scales,and harvest design. We briefly characterise those gaps below:

7.2.1. Mechanisms of forest influenceA better understanding of the relative importance of factors and

mechanisms that underlie forest influence can assist managerspredict responses for new situations. For example, it is unclear ifthe disturbance regime of the forest type affects species responsesto forest influence. In landscapes with less frequent disturbances,the species pool contains a larger portion of slowly dispersing spe-cies or species with preference for closed canopy habitats. On theother hand, forest influence may not be that important in distur-bance-prone landscapes where most species are well adapted tore-colonisation of disturbed areas and presence of undisturbed for-ests is not a strong control on post-disturbance colonisation andgrowth.

Differences in life history characteristics may also explain vari-ation in species responses to forest influence. For example, wewould hypothesise that forest influence would have more of an ef-fect on late-successional species needing to re-establish in har-vested areas compared to early-seral species that might bedisadvantaged by shaded conditions near edge. Combinations ofspecies life history attributes may also render some more sensitiveto forest influence. For example, species that are dispersal-limitedand sensitive to high solar radiation or temperature extremescould be more sensitive to forest influence than those that do notshare this combination of characteristics.

Micro-habitat needs of species may also explain differences inresponses of species to forest influence. Species that require coarsewoody debris or particular types of litter may only be present inharvest units near forests where those habitat elements are pres-ent. It may be possible to test for the effects of those elementsusing manipulative experiments that either maintain existing mi-cro-habitats in logging units or add them to the units where theyhave been eliminated by the disturbance.

7.2.2. Spatial and temporal scalesAs with many ecological processes, forest influence effects are

likely to change with spatial and temporal scale, and research onthis topic as well as long-term studies have been lacking (Rosen-vald and Lõhmus, 2008; Bauhus et al., 2009). The effects of harvestunit size and size of remnant patches on forest influence have notbeen studied. We also lack knowledge of how contrast between thestructure of the disturbed patch and the adjacent forest patch af-fect forest influence. The temporal aspects of forest influence areparticularly important since the recovery time of microclimateand biota within disturbances will control the rate and pattern ofharvest activities in landscapes where maintaining biodiversity isan objective. The successional dynamics of post-disturbance com-munities may be controlled by the sequence of species invasions ofthe disturbed patches. In other words, the first forest species to col-onise the sites may control the rate of succession and the compo-sition of subsequent colonisers (Read and Hill, 1988). It is quitelikely that some species will recover rapidly across the harvest unitwhile other may move quite slowly and perhaps never recover be-fore the next harvest event. It will be important to know the rela-tive rates among species, to identify the species with the mostvulnerable populations to the cumulative effects of harvestingacross landscapes over time. Some studies suggest that some ofthese slow species may be lichens and bryophytes (e.g. Kantvilasand Jarman, 2006).

7.2.3. Harvest designMany harvest unit design questions require additional research.

One important one is the use of rules-of-thumb such as the ‘one-tree-height from long-term retention’ rule that is sometimes usedto characterise edge effects and design management systems. Thevalidity of this rule will vary and management designs may need totake into account other rules that fit better for particular taxa or


processes. The relative merits of retaining aggregates compared todispersed trees in units are not fully understood and are likely tobe dependent on the details of the aggregates and the patterns ofthe individual trees. Some simple modelling of the rate and patternof spread of organisms with different life history characteristicsunder different harvest designs could provide some insights toguide managers. Such research could also identify thresholds forretention densities and aggregate sizes that optimise the trade-off between lifeboating and influence functionality. Another areaof research interest would be comparing forest influence from har-vest residuals such as aggregates to natural fire residuals. There issome research indicating these residuals have different character-istics and habitat value (Gandhi et al., 2004; Dragotescu and Knee-shaw, 2012), so it is reasonable to expect that they might not becomparable in how they provide forest influence.

8. Conclusions

Forest influence is an important emerging field of applied forestecology. We argue that managing for forest influence is as impor-tant to conservation outcomes of forest management as is mini-mising edge effects into unlogged forest. The limited amount ofresearch into forest influence has shown that proximity to retainedforest does impact a proportion of forest biodiversity. Other thanfor vertebrates, in most cases influence was estimated to extend<100 m, but in some cases much shorter still, e.g. generally<10 m for ECM fungi. Therefore harvested areas of retention for-estry sites designed with increased forest influence as a manage-ment objective should develop biological communities richer inmature-forest affiliated species sooner than clearcuts. The distanceand underlying mechanisms of forest influence vary broadlyamong taxonomic groups, as well as between individual specieswithin taxonomic groups, and even potentially among individualswithin species. Further research is needed to better understandthese variables so that forest managers have better informationto help with site planning. However, since different species maybe favoured at different distances from edge, and no single sizeof cut area will be ideal for all types of biodiversity (Bradshaw,1992), variation in harvest layouts and gap dimensions is recom-mended. The rule of thumb that forest influence extends one treeheight into harvested areas is an easily applied management toolthat ensures variable retention cutblocks have greater forest influ-ence than clearcuts. However, it should not be assumed that onetree height is directly scaled to actual depth of forest influencefor biodiversity.

The paucity of research on edge gradients into harvested areasto-date, combined with long time-frames of ecological responsesto forest harvesting, mean that forest managers are by necessityhaving to implement harvesting practices in the absence of de-tailed knowledge of likely biodiversity outcomes. Priorities for fu-ture research include more studies of the spatial and temporalscales and identifying factors that underlie biodiversity responses.The vast majority of studies on forest influence and retention for-estry more broadly (Rosenvald and Lõhmus, 2008) are short-term,thus long-term outcomes are poorly understood. Ongoing monitor-ing of silvicultural trials is therefore of great importance.

Acknowledgements

We wish to thank numerous forest ecologists from around theworld for email discussions and suggestions for relevant literature.In particular Bill Beese played a critical role in bringing our atten-tion to the ecological importance of forest influence, and how itcould be factored into harvest planning. We also acknowledge staffat Forestry Tasmania involved with policy development to

encourage forest influence at harvest. Journal referees includingKaren Harper provided detailed and thoughtful comments whichconsiderably improved the manuscript. Funding for this projectwas provided by ARC linkage grant LP100100050 with supportfrom Forestry Tasmania and the Forests and Forest IndustriesCouncil. A World Forest Institute fellowship supported SCB in Port-land during part of the project. Robyn Scott produced Figs. 1 and 4.

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