woody colonization on unmanaged urban and ex-industrial sites

Woody colonization on unmanagedurban and ex-industrial sitesS. J. HODGE AND R. HARMER

Forestry Authority, Research Station, Alice Holt Lodge, Wrecclesham, Farnham, Surrey, GU10 4LH,England

Summary

Forty-six unmanaged urban and ex-industrial sites in the county of Avon, South Staffordshireand the West Midlands were visited in 1992/93 during a survey which aimed to relate site factorswith natural colonization by trees and shrubs. The factors observed included: type and texture ofsubstrate; the predominant type of vegetation; aspect; the presence of parents in the near vicinityof the site and the numbers and species of colonizing woody plants. The extent of woody colo-nization was very variable and despite an average stocking density of 1500 stems ha"1, only 19per cent of the land surveyed was colonized to an acceptable woodland standard. Woodland pro-duced by natural colonization was species poor with an average of three per site; the most abun-dant species were ash, birch, goat willow and hawthorn, which made up 85 per cent of thecolonization recorded. Although there was variation between species, the presence of the parentwas usually the most important factor associated with colonization; when potential parent treeswere visible from the site there was, on average, a 58 per cent probability of finding colonizationof the corresponding species. In general, colonizing plants were positively associated with sub-soil, small stony substrates such as gravel and ballast, and a weak grass sward. Conversely colo-nization was negatively associated with loam soils, sand, unfractured rock/concrete substrates,and sites with grazed sward or no vegetation.

Introduction u s e d f o d a y ( M o r t i m e r ; 1 7 0 g . Schlich, 1904;

During this century there has been an increasing James, 1981; Luke, 1984; Harmer and Kerr,recognition of the benefits of expanding wood- 1995). Alternatively, woodlands can be createdland cover in the UK. The principal means of by natural colonization; the process by whichachieving this has been by planting and success- plants establish from seeds dispersed naturallyful methods of establishing transplanted trees from local sources. The natural colonization ofhave been developed for most tree species on a abandoned land by woody plants is a wellwide variety of site types. Other methods which known process which has been studied fre-rely on plants derived from seeds germinating in quently on old fields in North America (Raup,situ have rarely been used in Great Britain. Cre- 1940; Pickett, 1982; Gill and Marks, 1991; Mys-ation of woodlands by direct sowing has a long ter, 1993). Detailed observations are less corn-history (Fernow, 1913) and early forestry texts mon in Britain but long-term studies at thedescribe suitable techniques which can still be Broadbalk and Geescroft wilderness areas atO Iroctute of Ounered ForcitOT, 19% Forestry, Vol. 69, No. 3, 1996

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246 FORESTRY

Rothamsted (Lawes, 1895; Brenchley andAdam, 1915; Witts, 1965), the formation ofbeech woodland on the Chilterns (Watt, 1934),the development of woodland on lowlandheaths (Summerhayes and Williams, 1926), andthe range of examples cited by Peterken (1993),all catalogue the process of natural colonizationon abandoned farmland in Britain. Other stud-ies have described the sequence of colonizationon abandoned industrial land (e.g. Hall, 1957;Leisman, 1957; Bradshaw and Chadwick, 1980;Roberts et al., 1981).

The last decade has seen an increased empha-sis on the creation of woodlands composed ofpredominantly native species, principally tomeet amenity, landscape and conservationobjectives (Anon, 1993; Anon, 1994b; Rodwelland Paterson, 1994). While this can be carriedout by planting, natural colonization is oftenthe preferred method of woodland creation as itis an essentially natural process. It has someadvantages over afforestation by planting: thetrees cost nothing; the trees will be patchily dis-tributed across the site and thus the woodlandsmay be more natural looking with greater struc-tural diversity; the species will be matched tothe site and be of the local genetic stock. How-ever, natural colonization is unpredictable andwoodlands may take several decades to develop(e.g. Brenchley and Adam, 1915; Hall, 1957;Witts, 1965; Maycock and Guzikowa, 1984).Reasons for this unpredictability are complexand related to the diverse processes involved incolonization. These have been discussed byMyster (1993) and include site conditions at thetime of abandonment, season and year of aban-donment in relation to tree seed availability,tree seed dispersal, and survival and growth ofseedlings. Dispersal of seed is related to theproximity of seed sources, quality of seed yearand mobility of the seed produced. Growth andsurvival of seedlings will depend on the avail-ability of resources such as water and nutrients,competition with other vegetation and brows-ing.

The inception of initiatives in the UK such asthe Black Country Urban Forestry Unit and the12 Community Forests, and the provision of tar-geted grant incentives (Anon, 1994a) has pro-vided a stimulus for woodland creation in andaround towns, and it is in these areas that the

desire to harness natural colonization is oftenstrongest. This is due to a number of factorsincluding:

• recognition of the importance of natural col-onization in the creation of existing wood-lands. For example a recent survey within theWest Midlands Borough of Sandwell hasshown that 35 per cent of the woodland pre-sent arose by natural colonization (Anon,1991);

• the amount of unmanaged land resulting fromstructural changes in heavy industry, mineralextraction and urban land use;

• observations of rapid colonization of somederelict sites;

• the desire to minimize the costs of woodlandcreation;

• the generally low importance of timber pro-duction in urban areas.

However, at present, it is not possible to pre-dict with any confidence the probability ofachieving woodland creation on unmanagedurban sites, the time it is likely to take, or thespecies composition of the resulting woodland.The following paper describes the results of asurvey of natural colonization on unmanagedurban land in the UK county of Avon, the For-est of Mercia and the Black Country. The sur-vey was carried out to investigate the speed,extent and species composition of colonizationon urban and industrial sites, how it was relatedto a variety of site factors, and to develop prac-tical guidelines which help to identify siteswhere natural colonization is likely to be a suc-cessful method of woodland establishment.

Methods

Site selection

The survey was carried out during 1992 and1993 in three areas within designated urban orcommunity forest initiatives: the County ofAvon; the Forest of Mercia (South StaffordshireDistrict and Walsall Metropolitan Borough);the Black Country (in the Metropolitan Bor-oughs of Sandwell and Dudley in the West Mid-lands). Overall 46 sites were surveyed; 17 in thecounty of Avon, 14 in the Forest of Mercia and


WOODY COLONIZATION ON UNMANAGED URBAN AND EX-INDUSTRIAL SITES 247

Table 1: Characteristics of the three study areas

Main soil types Degree of soildisturbance and

removal

Rainfall(mm a"1)

Avon Calcareous or neutral clays and loamsMercia Acidic sands with some slightly

acidic coarse loamsBlack Country Coarse sands and fine silty loams

ModerateHigh

Extreme

840670

670

15 in the Black Country. Regional rainfall andsoil characteristics are given in Table 1.

Sites within the study area, that had beenunmanaged for between 10 and 50 years andthat were between 1 and 7 ha in area, werelocated using local authority records, referenceto maps, and observation on the ground. Siteswhere abandonment could not be dated or per-mission for access was denied, were discounted.A random sample of suitable sites was surveyed.The presence of woody colonization was notused as a criterion for site selection.

Site assessment

The following information was collected foreach site as a whole:

• Time since abandonment.• Previous land use.• Species of trees large enough to produce seed

within about 50 m of the site.

Detailed assessments were made in 4 X 4 mquadrats placed at 40-m intervals along severaltransects parallel to the boundary of the site.The transects were 15 m apart up to a maxi-mum of 60 m from the boundary. The numberof quadrats studied at each site depended on itssize and varied between 7 and 55 covering 2 percent of the area. The following data wererecorded for each quadrat:

1 Vegetationa. Trees and shrubs: cover of each species (10

per cent intervals); number of trees (maxi-mum of 50 for each species); estimatedmean height for each species (in heightclasses: <1.0m; 1.0-2.0 m; 2.1-3.0 m;3.1-4.0 m;>4.0m).

b. Non-woody plants: the predominant typeof non-woody vegetation present in thefollowing categories was noted: no vege-tation; weak, open sward; continuous,grazed sward; dense matted ungrazedsward; bramble; other herbaceous(includes mosses).

2 Bare ground: percentage of quadrat with novegetation in 10 per cent intervals.

3 Texture of substrate: The following cate-gories were recognized: clay; silt; loam; sand;soil-less, large particle size (e.g. rubble,brick); soil-less, small particle size (e.g. bal-last, clinker, gravel); soil-less, unfractured(e.g. rock, concrete in large blocks/areas ofunbroken material).

4 Type of substrate: The following categorieswere recognized: colliery spoil; pulverizedfuel ash (PFA); rubble; top soil; sub-soil;small-stony (e.g. gravel, ballast, sand); solid(e.g. rock, concrete); various infill.

5 Penetration: Surface vegetation was removedand penetration was crudely estimated bymeasuring the depth to which a spade, pro-pelled vertically downwards from a height ofabout 50 cm, penetrated the substrate at thefour corners of each quadrat. Three cate-gories of penetration were recognized: low(not penetrable to 5 cm), medium (penetrable5-10 cm), high (penetrable to 10 cm or more).

6 Aspect: north, east, south, west or flat (lessthan 25° slope).

7 Damage: Evidence of vandalism, bark strip-ping, browsing and fire damage to any treesor shrubs was noted.

The common names of species are usedthroughout and botanical names are given inAppendix 1.


248 FORESTRY

Data presentation and interpretation

A major characteristic of the data-set was thehigh degree of variability between sites. Conse-quently, some of the key descriptive and data-fields are presented on an individual site basis inTables 2, 3 and 4.

In order to maximize the practical usefulnessof the survey results, the extent of colonizationis expressed in two ways:

1 stocking density at the site level (stem ha"1);2 proportion of each site stocked to an 'accept-

able woodland standard' (defined as a 4.0 X4.0 m quadrat containing at least four trees ofany size or two trees >1 m height).

The relationships between woody coloniza-tion and site factors was investigated in twoways:

1 General site characteristics associated withnatural colonization were investigated bylooking at the relationships between individ-ual factors and either the presence/absence ofwoody colonization or the number of plantscounted regardless of species.

2 In order to identify which were the mostimportant site features influencing coloniza-tion by the five most common species, all lev-els of all factors were included in a singleanalysis for each species.

Most analyses were by stepwise log-linearmodelling using a binomial model for presenceor absence of seedlings and a Poisson model forseedling numbers (Crawley, 1993).

Results

The 46 sites surveyed covered an area of 98.3 haand more than 2500 colonizing woody plantswere found in the 1115 quadrats that wereassessed. Previous land use fell into four broadcategories: agriculture, industry, transport, andlandfill. The distribution of these variedbetween areas; a majority of sites in the Countyof Avon were associated with roads or railways,whereas ex-industrial sites were the most com-mon type in the Forest of Mercia and the BlackCountry. All sites supported some vegetation,the type and vigour of which varied between

and within sites, but over all sites grass swardwas the most common vegetation type (Tables2, 3 and 4).

Fewer than 250 of the plants were damaged;of those, 60 per cent were browsed and a major-ity of the remainder subjected to debarking orbreakage. This information was not used in theanalyses.

Few quadrats had large areas of bare groundand approximately 75 per cent of the quadratsassessed were well vegetated with less than 10per cent bare ground. Most of the 110 quadratswith more than 80 per cent bare ground were onsubstrates composed of rock and concrete.

Most of the quadrats fell on areas of flat land;of the 300 with distinct slopes approximately 30per cent had northerly and 30 per cent southerlyaspects, the remainder were more or less equallydivided between east and west. Aspect had someoverall influence on both the presence of colo-nization and the number of plants present (bothP^0.05) but the effect varied with species: forexample, ash showed a significant positive asso-ciation between north facing quadrats and thepresence of colonizing plants (P^O.001) whereasfor oak there was a negative associationbetween presence and flat sites (P^O.001).

The survey recorded 25 species of woody col-onization, although ash, birch, goat willow andhawthorn made up 85 per cent. Species mix var-ied dramatically between the three study areas(Figure la, b, c) with ash and hawthorn domi-nating in Avon, birch and ash in Mercia, andgoat willow and birch in the Black Country.The number of species recorded also variedbetween study areas with 17 in Avon, 13 in theForest of Mercia and 11 in the Black Country.Over all sites, the mean number of colonizingspecies per site, on sites where some coloniza-tion had occurred, was 3.2 (Avon 3.7; Mercia3.6; Black Country 2.4).

Where woody colonization did occur ittended to be sparse, four sites had no coloniza-tion and more than half of the sites had fewerthan 500 stems ha"1. The maximum recordedstocking density was 11000 stems ha"1 for site30 (Table 3). The mean stocking density over all46 sites was approximately 1500 stems ha"1.However, on average only 19 per cent of landsurveyed was stocked to an acceptable wood-land standard and 14 sites had no areas with


Table 2: Characteristics of abandoned sites and natural colonization in Avon

Sitenumber

12

34

5

67

89

10

111213

1415

1617

Time sinceabandonment

(years)

1834

1726

26

1217

151417

192627

2626

2620

Principalsubstrate

type

RockColliery spoil

Top soilSmall-stony

Top soil

Top soilTop soil

Top soilTop soilSubsoil

Top soilTop soilTop soil/variousTop soilSubsoil

Top soilTop soil/various

Principalvegetation

type

Herbaceous

Dense swardBramble

Mixed

MixedDense sward

MixedMixedDense sward/bramble

Dense sward/brambleDense sward/weak swardWeak sward

HerbaceousDense sward/weak sward

Dense swardWeak sward/herbaceous

Bareground

(%)

9032

045

19

33

10

19

36

35

09

016

Stemsha"1

2731 142

274 091

4 450

542 908

2230

1311

14459

125

894 509

1812146

Establishmentto woodland

standard(% of site)

434

036

32

457

70

24

000

054

207

Species mix (%)

Ash 92, oak 4, sycamore 4Elder 47, oak 19, ash 11, goatwillow 11, elm 10, hawthorn 2

Hawthorn 100Ash 88, hawthorn 6, oak 3, field maple 1,rowan 1, goat willow 1Ash 53, blackthorn 35, oak 3, elder 3,dogwood 2, hawthorn 2, holly 2Elder 100Hawthorn 86, ash 10, dogwood 1, oak 1,goat willow 1Hawthorn 100—Ash 37, dogwood 16, hazel 15,hawthorn 14, elm 4, oak 5, sycamore 5,

blackthorn 1, holly 1, lime 1, goat willow 1Hawthorn 34, ash 33, field maple 33Hawthorn 50, oak 50Blackthorn 50, hawthorn 50

Elder 100Ash 58, blackthorn 18, hawthorn 10,wayfaring tree 6, cherry 3, dogwood 3,apple 1, sycamore 1Blackthorn 97, holly 3Goat willow 43, hawthorn 43, elder 14

OOanor-OZN

—|

5zozczZ

omDcCD

z£*

DmX

Zaon

i

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ron

In Tables 2, 3 and 4 the term 'mixed' for principal vegetation type indicates no predominant type.


o

Table 3: Characteristics of abandoned sites and natural colonization in the Forest of Mercia

Sitenumber

18

192021

22

23

2425

2627

28

29

30

31


(years)

11

104211

10

11

3522

1315

23

13

41

33

Principalsubstrate

type

Various/small-stonyTop soilTop soilRubble

Top soil

Top soil

Top soilSubsoil

Top soilTop soil

Top soil/variousSolid/subsoil

Various/small-stonySubsoil

Principalvegetation

type

Weak sward/denseswardDense swardDense swardWeak sward/denseswardDense sward

Dense sward/weakswardDense swardWeak sward

Dense swardDense sward/weakswardDense sward/weakswardWeak sward/densesward

Weak sward

Mixed

Bareground

(%)

19

00

17

0

2

014

45

3

19

62

3'

Stemsha-1

4 840

2 452500391

429

9 025

991094

3171

815

1971

11027

1667


standard(% of site)

47

800

11

48

011

00

22

40

79

58

Species mix (%)

Birch 58, goat willow 38, oak 2,Scots pine 1, hawthorn 1Hawthorn 98, oak 2Oak 100Oak 80, birch 7, goat willow 7, Scotspine 6Goat willow 63, oak 25, hawthorn 8,ash 4Ash 94, hawthorn 2, oak 1, goat willow 1,sycamore 1, whitcbeam 1Elder 100Birch 84, goat willow 10, oak 4, Scotspine 2Goat willow 100Hawthorn 50, pear 50

Birch 43, oak 17, goat willow 17,hawthorn 10, elder 10, dog rose 3Hawthorn 36, goat willow 35, birch 13,oak 11, hazel 2, elder 1, rowan 1,sycamore 1Birch 90, oak 7, goat willow 3

Birch 78, goat willow 16, sycamore 6

-nOFfl

•HJO•<


Table 4: Characteristics of abandoned sites and natural colonization in the Black Country

Sitenumber

32

3334

35

36

37

38

39

40

41

42

43

44

45

46


(years)

18

1318

19

24

24

33

10

28

24

13

13

23

13

17

Principalsubstrate

type

Top soil/variousConcreteTop soil/variousSmall-stony/rubbleVarious/topsoilTop soil/rubbleTop soil/variousRubble/small-stonyRubble/variousTop soil/rubbleVarious/small-stonyVarious/small-stonySmall-stony/top soilRubble/ash

Rubble/various

Principalvegetation

type

Weak sward

Dense sward/grazedswardWeak sward

Dense sward/bramble

Dense sward/grazedswardDense sward

Weak sward/denseswardWeak sward/denseswardDense sward/weakswardDense sward/weakswardGrazed sward

Dense sward

Dense sward/weakswardDense sward

Bareground

(%)

20

9239

30

3

6

0

17

5

6

8

20

1

34

1

Stemsha-'

312

294208

6771

76

0

454

1969

393

568

2000

0

164

3125

0


standard(% of site)

11

611

75

3

0

5

40

11

18

27

0

3

22

0

Species mix (%)

Goat willow 64, birch 29, hawthorn 7

Goat willow 75, birch 25Birch 67, goat willow 33

Birch 57, goat willow 41, hawthorn 2

Elder 50, ash 25, hawthorn 25

—

Cherry 94, birch 6

Goat willow 87, birch 13

Goat willow 50, hawthorn 32, birch 14,privet 4Birch 70, goat willow 25, hawthorn 5

Goat willow 49, birch 46, elder 3, ash 1,pear 1—

Broom 50, hawthorn 20, elder 10,hazel 10, robinia 10Goat willow 97, birch 3

—

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252 FORESTRY

(a)

Oak 3%Dogwood 3%

Elder 4%Blackthorn 14%

Hawthorn 10%

Goat Willow 17%

Other 3%

Cherry 3%

Hawthorn 3%

Figure I. Species mix of woody colonization: (a) overthe Avon sites; (b) over the Forest of Mercia sites;and (c) over the Black Country sites.

establishment to an acceptable woodland stan-dard (Tables 2, 3 and 4).

Twenty-four species of potential parent treewere recorded, the most common being goafwillow and hawthorn which were present on 32sites (Table 5). Species such as beech, fieldmaple and rowan which were found at fewerthan four sites are not included in the table. Thespecies of woody colonization present wereclosely related to the potential parents speciesvisible from the site. Where a parent was visibleat a site, over all species there was a 0.58 prob-ability of finding corresponding seedlings. Incontrast, a much lower proportion of sites wascolonized by any species if the parent was notvisible (Table 5). Sites with potential parenttrees of birch, blackthorn, hawthorn, oak andgoat willow visible were most commonlyaccompanied by corresponding colonization(blackthorn colonization appeared to be mainlyby suckering). Alder and poplar were neverrecorded as seedlings despite the presence ofpotential parent trees. The probability of find-ing sycamore colonization within sight of apotential parent tree was only 27 per cent sug-gesting (along with its general scarcity withinthe survey) a relatively poor ability to colonizeopen ground in these areas of the country.

As most colonization sources were on orbeyond the perimeter of the sites, an increasedamount of colonization might be expectedaround the edge of abandoned sites. However,the Jonckheere-Terpstra test showed thatstocking density, proportion of area colonizedto an acceptable woodland standard, and meanheight of colonization did not change signifi-cantly with distance from the perimeter (Table6). There were relatively few quadrats morethan 45 m from the perimeter and hence datafor such distances are less reliable.

Although the oldest site surveyed had beenabandoned for 42 years, the majority of siteshad been abandoned for between 10 and 20years (Table 7). There were few sites that hadbeen abandoned for more than 31 years anddata for the >36 years category is strongly influ-enced by site 30 which was particularly heavilycolonized (Table 3). The presence and numberof plants of some species were associated withtime since abandonment (Table 10) but the Jon-ckheere-Terpstra test showed that there were



Table 5: Potential parent species present at more than three sites and the percentage of sites with colonizingseedlings

Species

AlderAshBirchBlackthornElderHawthornOakPoplarSycamoreGoat willow

Sites

Number

818244

1632216

1132

with potential parents

% with seedlings

05571755669710

2769

Sites without

Number

38282242301425403514

potential parents

% with seedlings

07057

21809

14

no significant relationships between time andeither number of species present or the totalnumber of woody plants growing on a site. Theamount and size of colonization present 10years after abandonment was similar to that onsites abandoned one and two decades earlier.Similarly, over all species, the mean height ofcolonizing trees was not significantly related totime since abandonment (Table 7).

There were significant relationships betweenmost site factors (Table 8). The penetrability ofthe substrates was associated with both its typeand texture, high penetrability being associatedwith top and sub-soils of loam, clay and silt.Soil-less substrates were less penetrable. Densesward was associated with loamy top soils ofmedium/high penetrability, whereas weaksward was associated with less penetrable soil-less substrates. Other types of vegetation weregenerally not associated with substrates of anyparticular type, texture or penetrability. Thisinter-relationship between the factors makes

interpretation of the data difficult but analysessuggest that overall the four most importantfactors determining the presence of woodyseedlings regardless of species are, vegetationtype > type of substrate > texture of substrate >aspect. The same four factors were most impor-tant in determining the numbers of plants ofany species but the positions of type and textureof substrate were reversed.

The type of substrate present usually variedwithin a site. Although one was very heteroge-neous with six types recorded, most had two orthree. Top soil was the most commonly occur-ring substrate being found on 35 sites and in 505quadrats. Overall, PFA was least common,being found in only 10 quadrats (Table 9). Theproportion of quadrats colonized with one ormore seedlings of any species varied between nocolonization for PFA and 51 per cent forquadrats with sub-soil (Table 9). Greatestseedling densities were found on small-stonysubstrates and there were significant positive

Table 6: The nature and extent of woody colonization at increasing distance from site perimeter

Distance from perimeter

15 m 30 m 45 m+

Number of quadratsStocking density (stems ha"1)Extent of colonization to an acceptable woodlandstandard (%)Mean height of colonization (m)

7261489

18

1.2

3091541

22

1.5

801617

16

0.8


254 FORESTRY

Table 7: The relationship between time since abandonment and woody colonization

Time since abandonment (years)

Number of sitesMean number of species (ha~')Stocking density (stems ha"1)Extent of colonization to an

acceptable woodland standard (%)Mean height of colonization (m)

10-15

161.4

1845

201.3

16-20

101.5

1071

191.3

21-25

62.2447

91.5

26-30

S2.6

2160

220.8

31-35

42.1793

223.2

>36

22.0

6641

460.9

associations between the presence of coloniza-tion and sub-soil or small-stony substrates.

Substrate texture also varied with 70 per centof sites having two or more substrates. Loamwas the most common texture occurring in 642quadrats on 41 sites; soil-less large was the leastcommon being found in nine quadrats on foursites (Table 9). The proportion of quadrats col-onized varied between 19 per cent for sand and67 per cent for soil-less large, with greatest den-sities of colonization occurring in quadrats withsoil-less-small texture (Table 9). The substrate'stexture had some influence on the presence ofcolonization and there were significant negativerelationships with loam, sand and soil-lessunfractured substrates (Table 2).

More than 50 per cent of quadrats had sub-strates which could be penetrated to a depth ofover 10 cm and colonization was recorded in asignificantly greater percentage of quadrats withhighly penetrable substrates (37 per cent) thanthose with low or medium penetrability (both30 per cent, P^O.01 by Kruskal-Wallis test).However, there was no difference in theseedling densities on quadrats with the lowestand highest penetrabilities.

Although two sites had only dense sward,

vegetation was usually heterogeneous with anaverage of three types on each site. On threesites all six categories were found. Dense swardand grazed sward were the most and least com-mon types respectively (Table 9). Weak swardwas the type of vegetation most commonly col-onized, with grazed sward and no vegetationtypes the least. Similarly, seedling densities weregreatest in quadrats with weak sward (Table 9).

There were 25 species of woody plant foundcolonizing the sites surveyed but many of theseconsisted of isolated plants at a few sites. Ingeneral, a species did not colonize a site unlessits parent was nearby and this was included asa factor when data for the five most commonspecies were analysed. In these analyses, amodel was developed which related coloniza-tion to all variables and the individual levels ofeach factor, the first six components fitted foreach species are given in Table 10. For allexcept goat willow (Salix caprea L.) the pres-ence of the parent was the most important char-acter determining the presence of colonizingplants. The numbers of colonizing plants werealso related to the presence of parents and, withthe exception of hawthorn {Crataegus mono-gyna Jacq.), to a weak sward. In general, the

Table 8: Relationship

VegetationNature of substrateTexture of substrate

between site factors

Aspect

•»

Nature

»*»

Texture

••»•••

Penetration

»»•*•*#•*

Values are levels of significance for association between factors by Fisher's exact test except those including penetrationwhich used Kruskal-wallis test.



Table 9: Relationships between type and texture of substrate, and vegetation type with seedlings of any colo-nizing species

Factor

Nature of substrateColliery spoilPFARubbleTop soilSub-soilVarious infillSmall-stonySolid

Texture of substrateClaySiltLoamSandSoil-less, unfracturedSoil-less, smallSoil-less, large

Vegetation typeDense swardGrazed swardWeak swardBrambleHerbaceousNo vegetation

Assocationwith

colonization

+ , » "

+ > **

**#*»

~J • S-S-

»

+ , ***

**

No. ofsites

34

18351326146

209

411611104

401537191817

_r _-

No. ofquadrats

2810

1105051361577394

16573

6428489549

52853

2736780

112

Presence ofseedlings*

430

342851324830

41523219155167

311749313316

Density ofseedlings5

3.902.01.92.93.05.81.3

2.15.62.11.20.96.91.8

1.61.24.90.73.21.5

5%, 1% and 0.1% respectively.* % of quadrats with 1 or more seedlings of any species.s Mean number of seedlings in each quadrat.

presence and abundance of colonization presentappeared to be adversely affected on sites withboth the worst and the best substrates.

Discussion

While sampling was extensive, the patchy dis-tribution and low number of plants overallresulted in many quadrats with no colonizationand insufficient data for independent analysis ofdata for most species. Furthermore the interre-lationship between the type of substrate, its tex-ture and the type of vegetation present had tobe borne in mind during data analysis and inter-pretation. Use of a stepwise model, where fac-

tors are added sequentially in declining order ofimportance, may give a false impression if fac-tors are closely related, as the addition of onefactor accounts for some of the variation whichmay be explained by another related factor. Inaddition, many site factors which can influencethe process of plant establishment but requirefrequent site visits or specialist equipment werenot measured (e.g. monitoring of microclimateand soil chemical analysis). These limitationswere considered at the survey design stage andaccepted in order to allow an extensive surveyto be undertaken. Accordingly, interpretation ofresults has been largely descriptive but designedto offer advice to practitioners wishing to usenatural colonization.


Table 10: Characters associated with colonization by the most common species found

Ash

Presence of colonization Parent +North (A) +Rubble (N) -Infill (N) -Loam (T) -Solid (N) -

Number of plants Parent +Topsoil (N) +Loam (T) -Weak sward (V) +Small-stony (N) +Sub-soil (N) +

Birch

Parent +Topsoil (N) -Weak sward (V) +Time +Small-stony (N) +Colliery spoil (N) -

Weak sward (V) +Parent +Soil-less small (T) +Topsoil (N) -East (A) -Small-stony (N) +

Hawthorn

Parent +Time -Soil-less impervious (T) -Sand (T) -North (A) +Infill* (N) -

Penetration +Parent +Time -Loam (T) +Sand (T) -Herbaceous (V) +

Oak

Parent +Flat (A) -Silt (T) +Time +Soil-less impervious (T) -Topsoil (N) -

Parent +Flat (A) -Time +Weak sward (V) +West (A) -Topsoil (N) -

Goat willow

Weak sward (V) +Parent +Topsoil (N) -Sand (T) -Small-stony (N) +Time -

Weak sward (V) +Time -Topsoil (N) -Parent +Loam (T) +Soil-less small (T) +

•nOfapnon

Hfa•<

+/- = positively/negatively associatedCharacters: A = aspect; N = type of substrate; T = texture of substrate; V = type of vegetation; Time = Time since abandonmentAll significant at PS0.001 except *, which is P5O.O5



In the absence of management, the occur-rence of woody colonization depends on thechance coincidence of suitable soil conditions,ground vegetation characteristics, disturbancefactors and colonization sources. The detaileddata analysis for individual species (Table 10)showed that the presence of parent trees was themost important factor influencing the presenceof colonizing plants. There was a close rela-tionship between the identity of species coloniz-ing and those present as parents around thesites. Failure of tree seed to disperse on to aban-doned land is an important factor limiting col-onization and is related not only to theproximity of parents but also their fecundityand dispersal mechanism, all of which should beconsidered before assessing site suitability forwoodland establishment by natural colonization(Robinson and Handel, 1993; Gill and Marks,1991; Myster, 1993). Goat willow was the onlymajor species where the presence of parent treeswas not the most important factor. This may bedue to the nature of goat willow seed which issmall and light with a tuft of long hairs thatfavour long-distance dispersal.

The species composition of colonization var-ied considerably between the three study areas,the number of species found was greatest inAvon and least in the Black Country. Althoughresults showed that this was probably related tothe species of parent trees present, it may alsoreflect their method of dispersal (Bramble andAshley, 1955; Wagner et al., 1978; Gibson,1982); and the quality of the sites available forcolonization (Gibson et al., 1985). Seed disper-sal in the parent species observed can be cate-gorized into three broad types: light winddispersed (such as birch and goat willow); heavywind dispersed (such as ash and sycamore); andanimal dispersed (such as hawthorn and oak).Classification of colonization by dispersal mech-anism showed that: in Avon 2 per cent ofseedlings were light wind dispersed, 41 per centheavy wind dispersed, and 56 per cent animaldispersed; in Mercia 56 per cent light wind dis-persed, 27 per cent heavy wind dispersed, and16 per cent animal dispersed; in the Black Coun-try 91 per cent light wind dispersed, 2 per centheavy wind dispersed, and 7 per cent animaldispersed. There appears to be a marked reduc-tion in the extent of bird and animal dispersed

and heavy wind dispersed seed, and an increasein the extent of light wind dispersed seed, withincreasing urbanization. Reasons for this mayinclude a decrease in the activity of seed dis-persing animals or reflect a difference in thetypes of substrate present in each area.

Many of the sites in Avon on which colo-nization had occurred had a mix of woodyspecies characteristic of those found in theNational Vegetation Classification native wood-land type W8: ash—field maple—dog's mercury(Rodwell, 1991). This woodland type is appro-priate to the predominantly base rich, relativelyfertile soils common in this area, but is unlikelyto occur on impoverished sites such as those inthe Black Country which would tend to supportless diverse woodland types. Woodland derivedfrom natural colonization tended to be speciespoor (on average three species per site) anddominated by ash, birch, goat willow andhawthorn. Irrespective of the merits of differentspecies and mixtures, the species mix resultingfrom natural colonization tends not to befavoured by those planting woodland. Forexample, a survey of nine sites in the BlackCountry showed that an average of 16 specieswere planted per site (Hodge, unpublished).Willow made up 6 per cent of the trees plantedcompared with 58 per cent in naturally colo-nized areas, and birch 9 per cent compared with33 per cent. The relative simplicity of naturallycolonized woodlands must either be accepted orexpensive respacing and supplementary plantingundertaken to increase richness.

The types of site available for colonizationvaried from fertile agricultural land throughsites reclaimed with a covering of natural soil ofvarying composition to extremely disturbedsites with little or no soil. These substrates dif-fer substantially, not only in their ability to pro-vide suitable sites for germination and sufficientresources for establishment of woody species,but also in their influence on the growth ofcompetitive vegetation. All these factors areimportant in the process of colonization (Mys-ter, 1993). Fertile soils provide the best condi-tions for growth of woody seedlings, but theyalso promote the growth of ground vegetationwhich inhibits seedling growth either directly bycompetition for resources (Davies, 1987) orindirectly by providing cover for seed eating or


258 FORESTRY

browsing animals such as mice and voles (Mys-ter and McCarthy, 1989; De Steven, 1991; Gilland Marks, 1991).

Sites of moderate fertility may be more read-ily colonized if they have a physical structurethat traps seed and offers germination sites withgood contact with the mineral substrate for suc-cessful radicle penetration. The survey foundthat good colonization often occurred on small-stony soil-less substrates of ballast, clinker andgravel. These substrates are similar in structureto the coal pit heaps on which Brierley (1956)recorded successful vegetation establishment. Incontrast, sites with unfractured rocks and con-crete failed to colonize except in cracks andcrevices which represent the only suitable placesfor germination.

Although the type of vegetation that developsis largely a product of substrate characteristics,it is easy to observe and provides a good indi-cator of the likelihood of natural colonization.Weak sward was the vegetation type in whichcolonization was most often found whereas itwas less common in areas of dense and grazedsward. This observation is typical of woodlandsites being restocked by natural regenerationwhich will occur readily in sparse grass swardon thin infertile soils but is less likely in therank vegetation which develops on fertile soils.The nature of ground vegetation will also influ-ence the likelihood of successful germinationand growth of different species. Theoreticallysmall seeded species are least likely to competeadequately with dense vegetation and the surveydata did show that the presence of a weak grasssward was more closely associated with theestablishment of small seeded birch (Betulaspp.) and goat willow than oak. (Quercus spp.),ash (Fraxinus excelsior L.) or hawthorn.

Although the data cannot be interpreted as aliteral time series, the evidence from this surveysuggests that the environmental conditionsrequired for colonization are most likely tooccur soon after abandonment. Detailed obser-vations relating natural colonization with timehave been rare in Britain but Watt (1924) sug-gested that scrub on the South Downs was onlyinvaded by trees after about 50 years and Hall(1957) found that there were generally fewwoody species at low abundance on coal spoilheaps less than 20 years old. Other types of

spoil and agricultural land can also be slow tocolonize (Brenchley and Adam, 1915; Witts,1965; Bradshaw and Chadwick, 1980; Robertset al., 1981; Maycock and Guzikowa, 1984).The many studies of old-field succession inNorth America have shown that woody speciesrichness increases with time (Myster, 1993).Similar results have been reported for miningsites (Hall, 1957; Johnson et al., 1982). How-ever, in this study there was no significant rela-tionship between time since abandonment andnumber of colonizing species on the site. Rea-sons for this may include the short time periodbetween abandonment and surveying whichwas generally less than 25 years, the lack of par-ent tree species such as oak and hawthorn withseeds capable of colonizing vegetated sites andthe lack of long distance dispersal of this typeof seed which is dependent on the activity ofbirds and mammals (Gibson et al., 1985).

The results of this survey emphasize theunpredictable nature of natural colonization.Where the coincidence of favourable environ-mental variables occurs, woody colonizationcan be relatively rapid and widespread, such asat site 21 of which 75 per cent was colonized toan adequate stocking density (with an averagetree height of about 2 m) by 19 years after aban-donment. However, this site was the exceptionrather than the rule and 23 of the 46 sites hadless than 10 per cent of their area colonized toan adequate woodland standard. Woody colo-nization was characteristically patchy, reflectingsite variability which is likely to be high on manmodified urban sites (Craul, 1992). In additionparent trees can be widely scattered in built upareas, which also promotes a patchy pattern ofcolonization. Where it did occur, woody colo-nization was often very dense. For example, onsite 34 the 2452 stems in the 1.0 ha site wereconcentrated into just 8 per cent of the area,within which the stocking density was equiva-lent to 30 650 trees ha"1. While the patchy dis-tribution of woody colonization leads towoodlands of natural appearance, there may bea need for costly operations to improve thevalue of the site in meeting management objec-tives. Heavily colonized patches may needrespacing to create an open woodland structuresuited to amenity uses and to allow light pene-tration to the field layer. Poorly colonized areas



may need to be planted to increase the amountof woodland on the site.

Although natural colonization can sometimesbe an effective way of producing woodland it isunpredictable. If an abandoned site has alreadybeen colonized, and the distribution and speciesmix of trees accord with the overall manage-ment plan then the natural colonization shouldbe accepted. However, the characteristics ofsites with no existing trees should be carefullyappraised before the decision to rely on naturalcolonization woodland establishment is made.

Acknowledgements

Ian Collier and Nick Smith undertook the substantialtask of site selection and investigation of historicalrecords. They, along with Nigel Rylance, GarethHopkins, Tom Jackson, Peter Flynn, Andrew Gra-ham, Anthony Reeves and Stephen Harper, carriedout the site surveys. Roger Boswell undertook allanalysis and organization of the data set. Sandwell,Dudley and Walsall Metropolitan Borough Councils,the Black County Development Corporation, theBlack Country Urban Forestry Unit, the Forest ofMercia team, Avon County Council and the AvonCommunity Forest team provided invaluable back-ground information.

Conclusions

Analysis of the survey data from 46 unmanagedurban and ex-industrial sites indicated that:

1 Although sites surveyed had been abandonedfor between 10 and 42 years, only 19 per centof this land had been colonized to an accept-able woodland standard, despite stockingbeing, on average, 1514 stems/ha.

2 Most colonization occurred shortly afterabandonment.

3 The nature and extent of woody colonizationwas highly variable and generally patchy.

4 Woody colonization tended to be speciespoor and dominated by ash, birch, goat wil-low and hawthorn. It was less diverse, andcomposed of a different balance of species,than most urban woodland plantings.

5 The seed bearing trees close to the abandonedsites gave a good indication of the likelyspecies composition of woody colonization(but not of the rate at which colonization islikely to occur).

6 Over all species natural colonization wasmost likely on sites with sub-soil or small-stony substrates supporting a weak sward. Incontrast, fertile sites, with loamy topsoils anddense sward, and the most severe sites such asimpervious rock and very free draining infer-tile sands did not favour colonization bywoody species.

7 Unless it is already present in an acceptableform, natural colonization may needrespacing and infill planting may be requiredin order to create woodlands that will meetmulti-purpose objectives.

Appendix 1. Botanical names of speciesencountered in the survey as colonization(*) and as potential parent trees (#).Nomenclature follows Stace (1991)

Alnus spp.Malus spp.Fraxinus excelsiorFagus sylvaticaBetula spp.Prunus spinosaCytisus scopariusPrunus aviumCornus sanguineaSambucus nigraUlmus spp.Acer campstreSalix capreaCrataegus monogynaCorylus avellanaIlex aquifoliumAesculus hippocas-

tanumChamaecyparis

lawsonianaTilia spp.Acer platanoidesQuercus spp.Pyrus hybridsPopulus spp.Ligustrum spp.Kobinia pseudoacaciaRosa spp.Sorbus aucuparia

*# Alder# Apple*# Ash# Beech*# Birch*# Blackthorn*# Broom*# Cherry# Dogwood*# Elder•# Elm*# Field maple*# Goat willow*# Hawthorn•# Hazel*# Holly# Horse chestnut

# Lawson's cypress

# Lime# Norway maple*# Oak# Pear# Poplar# Privet*# Robinia

Rose*# Rowan


260 FORESTRY

Scots pineSycamoreWayfaring treeWhitebeam

Pinus sylvestrisAcer pseudoplatanusViburnum lantanaSorbus aria

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Received 1 March 1995


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