WORKSHOP REPORT 5

The impact of N enrichment on heathland and the implications for vegetation management

Report of a Workshop held at the

Witley Centre and Thursley

Common, Godalming, Surrey

27 May 2004

Prepared by Helen Adamson and Sarah Gardner of ADAS Consulting Ltd

July 2004

Contents

BACKGROUND...................................................................................................................................... 3

NITROGEN DEPOSITION AND HEATHLANDS- TOO MUCH OF A GOOD THING? - DR. SALLY POWER (IMPERIAL COLLEGE)..........................................................................................3

NITROGEN AS A POLLUTANT.................................................................................................................. 3PLANT AND ECOSYSTEM RESPONSES TO NITROGEN..................................................................................4THE HEATHLAND PROBLEM.................................................................................................................... 4NITROGEN AND MANAGEMENT EXPERIMENTS IN A SURREY HEATHLAND.................................................5

Phase 1............................................................................................................................................. 5Phase 2............................................................................................................................................. 5Conclusions from experimental work................................................................................................7

PRELIMINARY CONCLUSIONS.................................................................................................................. 7A FOOTNOTE ON PHOSPHORUS............................................................................................................... 8QUESTIONS ARISING.............................................................................................................................. 8

EFFECTS OF NITROGEN AND GRAZING ON PLANT AND INSECT COMMUNITIES OF UPLAND HEATH – DR. SUE HARTLEY (UNIVERSITY OF SUSSEX)...........................................9

INTRODUCTION AND AIMS...................................................................................................................... 9EXPERIMENTAL DESIGN AND METHODS..................................................................................................9RESULTS AND DISCUSSION................................................................................................................... 10

Heather.......................................................................................................................................... 10Other plant species......................................................................................................................... 11Insects............................................................................................................................................ 12How soil type/site affects vegetation communities...........................................................................13Effect of soil type on nutrient uptake of different plant species........................................................15

FURTHER EXPERIMENTS....................................................................................................................... 15CONCLUSIONS..................................................................................................................................... 17

Below-ground................................................................................................................................. 17Above-ground................................................................................................................................. 17

SUMMARY:.......................................................................................................................................... 18Mechanisms of heather-grass competition.......................................................................................18Drivers of community composition..................................................................................................18Two final points to go away with.....................................................................................................18

QUESTIONS ARISING............................................................................................................................. 18

FIELD TRIP......................................................................................................................................... 19

CONTROLLING OVER-MATURE GORSE...................................................................................................19CONTROLLED BURNING AND CUTTING OF HEATHER..............................................................................19CONTROL OF BRACKEN........................................................................................................................ 20

ACKNOWLEDGEMENTS.................................................................................................................. 20

AUTHOR CONTACTS........................................................................................................................ 20

REFERENCES & FURTHER INFORMATION................................................................................21

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BackgroundThis workshop was held at the National Trust’s Witley Centre, near Godalming in Surrey. The main theme was the impact of N enrichment on heathland and its implications for vegetation management. The workshop was prepared for local land managers and project officers and included formal talks, informal discussions and a field visit to Thursley Common to look at experimental work presented by Dr. Sally Power (Imperial College) and to see some examples of gorse, heather and bracken management.

Nitrogen deposition and heathlands- too much of a good thing? - Dr. Sally Power (Imperial College)

Nitrogen as a pollutant

There are many natural sources of nitrogen including:

Biological nitrogen fixation from free-living and symbiotic organisms. Lightening and fire which cause the oxidisation of atmospheric nitrogen gas into a

form available to plants Excretion products from animals which add to the amount of ammonia in the

atmosphere.Anthropogenic activities in recent decades have resulted in the amount of nitrogen available to plants and micro-organisms more than doubling. These include:

An increase in agricultural fertiliser production and use, which generates reduced nitrogen (ammonia).

Fossil fuel combustion particularly for transport but also for power generation increase the amount of oxidised nitrogen (usually referred to as NOx emissions).

To a lesser extent, the increased use of legumes in agriculture, these generally produce reduced nitrogen.

Of these anthropogenic sources it has been shown that nitrogen from fossil fuels has been gradually increasing since the 1950s, though it is thought that this will be kept under control by legislation and new technology. The nitrogen produced by legumes and rice has increased slowly and steadily but it is thought that this too will be controlled. Nitrogen from fertiliser has increased dramatically particularly in the last 20-30 years and it is thought that it will continue to increase because fertiliser use is linked to food production, which is linked to global population. An overall increase in the amount of the reduced form of nitrogen is therefore predicted.

In the UK, the deposition of nitrogen (nitrogen that falls onto the surface of the vegetation and is available to the ecosystem) in the oxidised form is quite low (1.6 m tonnes in the year 2000 (Dalton & Brand-Hardy 2003)). In the reduced form levels are higher, often at 14 kg ha-1 yr-1 and reaching nearly 30 kg ha-1 yr-1. The total nitrogen received by an ecosystem, ranges from 2 kgha-1yr-1 to over 50 kg ha-1 yr-1; Surrey receives approximately 20 kg ha-1 yr-1. The rate of uptake of nitrogen by the vegetation depends on

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its characteristics. Rough, tall vegetation receives a higher load than shorter, smooth vegetation. Forest for instance, receives around 25 kg ha-1 yr-1as an average across the UK, whilst grass receives 15 kg ha-1 yr-1. Heathland is more akin to grassland in structure and receives an average of around 16 kg ha-1 yr-1.

Plant and ecosystem responses to nitrogen

Most natural and semi-natural ecosystems are adapted to very low nitrogen inputs. Plant competition and nutrient cycling are geared towards a very conservative nutrient conservation strategy. Enhanced nitrogen levels can therefore be detrimental to these ecosystems as the plants are poor competitors in a high nutrient environment.

The effects of nitrogen deposition can be divided into two categories; those associated with acidification and those with eutrophication (nutrient loading). Acidification occurs particularly when reduced nitrogen is deposited, because plants take up the positively charged ammonium ions and to counteract the positive charge, hydrogen ions are excreted thus lowering the pH of the soil. Microbial activity such as nitrification also results in the production of hydrogen ions.

In heathlands, the effects of eutrophication are more biologically relevant than those of acidification. Because nitrogen is usually limiting, elevated levels can result in increased plant growth and associated changes in chemistry e.g. higher nitrogen concentration. Quality and production of litter increase which in turn affects nutrient cycling. There is often a reduction of mycorrhizal fungi associated with the plants, along with an increase of microbial activity and consequently of nutrient cycling.

All of these effects combine to cause a disturb natural nitrogen cycling within the heathland ecosystem.

The heathland problem

In the early 1980s in the Netherlands, nitrogen deposition levels reached more than 80 kg ha-1 yr-1. This stimulated an increase in the frequency of heather beetle attacks, which, in turn, led a breakdown in the heather (Calluna vulgaris) canopy. The resultant gaps created sites for grasses, particularly wavy hair grass (Deschampsia flexuosa), to invade. Most of the heathland in the Netherlands is now lost and the evidence suggests that nitrogen is the main causative factor (Brunsting & Heil, 1985).

The Netherlands experience generated concern in the UK and in the late 1980s a survey of UK lowland heathland revealed that these had been declining for a number of years, with Surrey exhibiting the steepest decline. The decline is attributed to changes in management, land use and increased urbanisation, but it is believed that elevated nitrogen levels have been a contributing factor.

This concern stimulated work at Thursley Common to investigate the effects of current rates of nitrogen deposition on heathland. This work contrasted with previous work that looked at high rates rather than natural rates of nitrogen deposition.

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Nitrogen and management experiments in a Surrey Heathland

Phase 1Nitrogen addition experiments at Thursley Common began in 1989. The rates of nitrogen added experimentally were very low at 0, 7.7 or 15.4 kg ha-1 yr-1 which represented 0, 50% and 100% of the current deposition at the site. The total of added nitrogen and deposited nitrogen, therefore, did not exceed 30 kg ha-1 yr-1, which is well within the UK range. Parameters measured included biomass, changes in plant and soil chemistry and phenology (e.g. bud burst and growth).

After seven years, the results showed that, on adding nitrogen, the total above-ground vegetation biomass increased substantially suggesting that the system was nitrogen limited. Litter production and hence, nutrient cycling also increased. Soil pH decreased as expected and flowering increased. More subtle effects included earlier bud burst, making the plants more susceptible to frost damage. There was no increase in below-ground biomass, and thus the root:shoot ratio was reduced. This shift in ratio will increase the susceptibility of plants to drought, since water loss is increased but no corresponding increase in uptake is achieved. An increase in foliar nitrogen concentration was also recorded, which could increase the likelihood of an outbreak of insect herbivores such as heather beetle (Lochmaea suturalis).

No significant invasion of grasses was recorded at Thursley, unlike the Netherlands, but there was a large build up of nitrogen in the system, leaving it potentially primed for a catastrophic event. It was felt that this nitrogen should be removed by management techniques in order to prevent catastrophe, and this was the basis for the next phase of the experiment.

Phase 2The aim of this phase was to assess the way in which management might affect the system’s ongoing response to nitrogen inputs. Two experiments were devised.

Experiment 1: Four management practices were carried out in 1998 on an area of heathland independent from that of phase 1. Nitrogen was added at 0 or 30 kg ha-1 yr-1

in that and in subsequent years. Experiment 2: This used the Phase 1 plots and looked at vegetation recovery

following cessation of nitrogen addition in 1996 and the introduction of four management practices in 1998.

The management treatments applied to the plots were based on information derived from a survey of management of lowland heaths, and on their practical suitability for the site. The four treatments were:

1) Low intensity mow in which all the above-ground material was removed by a forage harvester. This represents a removal of 18% of the nutrients.

2) Low temperature burn carried out in February, which removed about 30% of the nutrients.

3) High intensity mow is the same as the low intensity mow except that litter was also removed, representing approximately 30% of the nutrients. This treatment was probably more intense than the low temperature burn.

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4) Simulated high temperature (accidental burn). A low temperature burn was undertaken then all the remaining plant material, litter and humus was taken to a laboratory and burned. The resulting ash was returned to the site a few days later. This procedure removed most of the nutrients and in this way, is similar to a turf cut.

These management treatments therefore represent a gradient of intensity with (1) being the least intense and (4) the most.

Experiment 1:How does management affect heathland response to ongoing ni trogen addit ion Shoot growth: Nitrogen addition increased shoot growth in the short term (1999), the

size of the response was inversely related to the intensity of management. By 2002 there was no difference between the management treatments for shoot length.

Percent vegetation cover: Total vegetation cover remained fairly low (up to 50%) until 2000 but increased in subsequent years, particularly on the nitrogen addition plots. On both the control and nitrogen plots, vegetation cover was lowest and bare ground cover highest on the most intensively managed plots.

Litter decomposition (nutrient cycling): High intensity mowing led to a higher rate of nutrient cycling than low intensity mowing. Nitrogen increased the rate of nutrient cycling, but cycling on the high mowing plus nitrogen treatment was less than on the low mowing without nitrogen treatment. This suggests that high intensity management may have counteracted the effects of nitrogen addition.

Soil microbial community: Nutrient cycling in the soil is driven by the microbial community. Nitrogen addition resulted in an increase in microbial biomass but no difference in microbial biomass was observed between the high temperature burn and low intensity mow treatments. The fungi:bacteria ratio increased with the addition of nitrogen leading to an increase in the mineralisation of organic nitrogen to mineral nitrates. The change in the fungi:bacteria ratio and increase in available N is likely to favour grasses such as wavy hair grass or purple moor grass (Molinia caerulea). Under low nutrient conditions heather has the competitive advantage, but under high nitrogen conditions the converse is true and grass can become dominant.

Seedling invasion: In 1999, more grass seedlings were recorded on the high intensity burn with added nitrogen treatment than for any of the other treatments. This treatment was associated with a lower heather cover and greater proportion of bare ground, compared with the other treatment plots. The seedlings did not persist, however, possibly due to the dryness of the site. Different results might have been obtained on the wetter areas of Thursley Common.

Experiment 2: Can appropriate management accelerate recovery? Soil nitrogen content: Two years after the start of the experiment, soil nitrogen was

lower on the more intensely managed plots (i.e. treatments 3 and 4 above), suggesting that management had reduced soil nutrients.

Shoot growth: Before the introduction of management, nitrogen addition had increased heather shoot growth by 46%. Three years after the introduction of management, shoot growth was 25% higher on plots that had received nitrogen. Shoot growth was still 20% higher on these plots in 2003, even though they had not

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received nitrogen for six years. Management treatment had no significant effect on heather shoot growth.

Phenology: In 2003 and 2004, bud burst on plots that had received nitrogen in the Phase 1 experiment was still earlier than on the ‘no nitrogen’ control plots. This illustrates the persistence of the nitrogen effect and results in the plants remaining potentially vulnerable to spring frosts.

Percent vegetation cover: The difference in percent vegetation cover between the control and the high nitrogen plots was calculated for each management treatment for the period 1998-2003. The difference in heather cover between control and low intensity mown plots was greater than the difference observed between controls and high intensity burned plots. This suggests that high intensity management is effective in reducing the legacy of previous nitrogen addition undertaken during Phase 1 of the project. .

Lichen community: Percent cover of lichens had been reduced by the nitrogen additions in phase 1 of the work. Following cessation of nitrogen, the effects persisted for 4-5 years but by 2001 the community had recovered. The species present were Cladonia finbriata, C. digitata and Parmelia perlata.

Soil microbial community: The effects of nitrogen on soil microbial communities persisted six years after cessation of the nitrogen addition treatment. The effects were most marked on the high intensity burn where fungal communities on the ex-nitrogen plots remained low and were fewer in number in 2002 than those on the no-nitrogen control plots.

Conclusions from experimental workNitrogen inputs caused changes in heather growth, chemistry, flowering and phenology and accumulation of nitrogen in the soil. Inputs also affected lichen and microbial communities and disturbed nitrogen cycling. More intensive habitat management reduced the soil nitrogen store and to some extent (absolute) plant responses to nitrogen. It also interacted with N addition to influence seedling invasion and lichen re-colonisation. Ecosystem recovery was very slow.

A critical load is the maximum deposition of nitrogen that can be tolerated (indefinitely) by an ecosystem without damage taking place. The critical load for lowland dry heath is 10-20 kg ha-1 yr-1. Many areas in the UK are receiving N deposition loads that are expected to have negative effects on ecosystems, and on much of the UK heathland, except perhaps in Scotland and Northern Ireland, critical loads are exceeded. Mitigating action such as habitat management may therefore be very important.

Preliminary conclusions At current N deposition levels, ecosystem function is affected. If nutrient loading persists, changes in heathland plant community composition can

be expected. Management can be used to counteract the effects, but careful consideration needs to

be given to the type, intensity and frequency of management.

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A footnote on phosphorus

Work is now being carried out on the response of phosphorus limited heathland to the addition of nitrogen and phosphorus. Unlike nitrogen limited heathland, the addition of nitrogen to phosphorus limited heath does not result in an increase in growth. However, the concentration of nitrogen in the foliage does increase, which increases the susceptibility of the plant to herbivore attack. Although little is known yet about phosphorus limited systems, there are indications that they may also be under threat from nitrogen deposition.

Questions arising1) What loss of nitrogen is there from the system? Almost nothing as gas or leachate. A

small amount is lost from rabbit and deer grazing but most of the nitrogen inputs are accumulated.

2) How does gorse interact in the system given that it is a legume and fixes nitrogen? An increase in gorse is bringing more nitrogen into the system, but we are interested in how things are changing so if gorse is a consistent feature in the community it shouldn’t be a problem in terms of nutrient loading. It should be managed in the usual way. Legumes tend not to be favoured by high nitrogen therefore nitrogen is unlikely to cause gorse to increase.

3) How does soil moisture affect the process? Soil moisture affects microbial nutrient cycling. If the soil is wet there will be more denitrification thus more gaseous losses of nitrogen. If it is too dry there will be no microbial activity. Little work has been done on wet heath but it is thought that these have a slightly higher tolerance as indicated by the critical load (10-25 kg ha-1 yr-1). Conversely they support more lower plants which tend to be more sensitive to pollution of any kind.

4) Question about phosphorus limitation in Netherlands and UK. In the Netherlands it was apparent from the growth response to nitrogen deposition that heathlands were clearly nitrogen rather than phosphorus limited. In the UK, burning returns phosphorus to the system therefore phosphorus will not be limited. If turf is stripped then the community will have less nitrogen and phosphorus therefore responses to nitrogen will be less. Nitrogen deposition is pushing systems towards being phosphorus limited but even in a phosphorus limited situation, there will still be adverse responses to added nitrogen in terms of plant chemistry if not of growth.

Effects of Nitrogen and Grazing on Plant and Insect Communities of Upland Heath – Dr. Sue Hartley (University of Sussex)

Introduction and aims

Upland heath is in decline; it is being lost to grassland and to forestry plantations. This loss to grassland is attributed to various factors including heavy grazing and nutrient deposition. The aims of this experiment were to determine:

The effects of grazing and nutrients on vegetation structure and composition? How changes in the plant community might affect the invertebrate community?

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Experimental design and methods

There were 4 experimental sites in the Cairngorms - two in Glen Clunie (Sites 1 and 2) and two in Glen Shee (Sites 3 and 4). At each site there were two sets of fenced plots (indicated by the black line in Figure 1) and two sets of unfenced plots, giving a total of four blocks per site. Each block was divided into four 5 x 3 m experimental plots to which the fertiliser treatments were applied. Nitrogen (75 kg ha-1 yr-1), phosphorus (12.5 kg ha-1 yr-1) and potassium (25 kg ha-1 yr-1) were applied in the combinations shown in Figure 1 below, such that four fenced and unfenced plots at each site received N and four did not. The nitrogen addition treatment represents half of what is being deposited already in this area. Further details of the experimental design are given in (Hartley 1997).

N P K C NP NK PK NPK

N P K C NP NK PK NPK

Figure 1: Layout of plots at Glen Clunie and Glen Shee

Vegetation measurements were carried out on three randomly selected 1m2 sub-plots on each of the 64 experimental plots.

Parameters measured included:

Heather cover Heather canopy structure Total plant biomass Plant species richness Abundance (%cover of plant species) Abundance and species richness of Hemiptera Soil nutrient availability

Results and discussion

For the purposes of this workshop, the results from the nitrogen and no-nitrogen control treatments on the fenced and unfenced plots will be summarised.

HeatherOn the unfenced (grazed) plots, heather cover decreased, but the rate of decrease was higher on the nitrogen than on the control plots (Figure 2). On the fenced plots, heather increased under both treatments at the expense of grass, suggesting that at these rates of nitrogen addition and over these time scales, nitrogen is not harmful to heather. The effect of nitrogen was influenced by grazing and there was a significant interaction between the two factors.

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Figure 2: Heather and grass cover changes on fenced and unfenced plots with and without nitrogen addition

On both the fenced and unfenced plots, more shoots were browsed on the nitrogen than on the control plots (Figure 3). (Some grazing by voles, grouse and hares was detected on the fenced plots.) Sheep selectively graze on heather with a higher nutrient content, which explains why heather cover decreased particularly on the unfenced nitrogen plots (Figure 2). Heather height was lower overall on the unfenced plots than on the fenced ones (Figure 3). On the fenced plots heather was taller on the nitrogen treatments than on the controls, but there was no difference in heather height between the nitrogen and control treatments for the unfenced plots. It is thought that although nitrogen increases the growth of heather, this extra growth was consumed by herbivores, which tend to prefer nutrient rich heather.

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Figure 3: Percent heather shoots browsed and heather height on fenced and unfenced plots, with and without nitrogen.

Other plant speciesFigure 2 shows that when heather was exposed to both grazing and nutrient addition, its cover was reduced. Plant species richness was highest on the grazed and nitrogen treatments and lowest on the ungrazed (fenced) nitrogen plots, where heather cover was high. (Figure 3). It appears that other species were able to establish when heather cover was reduced and that there was an interaction between the effects of nitrogen and grazing on species richness.

A key to heather resisting grass invasion is canopy occupancy. Preventing grazing allows heather to close its canopy and exclude competing species. Temporal changes in canopy cover measurements revealed that canopy closure differed between sites. Canopy closure rates at sites 1 and 2 (Glen Clunie) were higher than at sites 3 and 4 (Glen Shee) for the unfenced plots. This was because stocking rate was higher at site 3, as confirmed by measurements of shoots browsed in 1995. At site 3, over 60% of shoots were browsed compared with sites 1 and 2 where the figure was less than 50%.

Figure 4: Species richness in control and nitrogen plots for fenced and unfenced

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Multivariate analysis techniques were used to examine the influence of the grazing and nutrient addition treatments in determining the composition of the plant community. This analysis suggested that both treatment factors were important. The separation of plant species in Figure 5 suggests that the X-axis is associated with a grazing gradient. Thus, heather (Calluna vulgaris), which in this experiment was abundant on fenced, low grazing treatments, is positioned at one end of the X-axis in Figure 5, whilst mat-grass (Nardus stricta), which was abundant on the unfenced, high grazing treatments, is positioned at the other end. Similarly on the Y-axis, wavy hair grass (Dechampsia flexuosa), which was abundant on the nitrogen treated plots, was well separated from mosses such as Hypnum cupressiforme, which was associated with the low nutrient control plots. This suggests that the Y-axis in Figure 5 is associated with a nutrient gradient.

With respect to the experimental treatments, the fenced (ungrazed) control plots tended to be located towards the lower grazing end of the X-axis, and the unfenced control plots tended to be located towards the low nutrient end of the Y-axis (Figure 5).

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Calluna vulgaris V. myrtillus

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Hypnum cupressif ormePleurozium schreberi

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Figure 5: Multivariate analysis on vegetation species

InsectsAssessment of the experimental treatment effects on the insect community focused on the Hemiptera (leafhoppers & spittlebugs), which include a large range of plant-sucking insects that feed directly on the plant. A total of 29 species of Hemiptera were identified, with sites at Glen Shee (sites 3 and 4) being more species rich than those at Glen Clunie (sites 1 and 2) (Hartley et al. 2003). Hemipteran species richness also tended to be greater on the nitrogen plots. Hemipteran species richness was positively related to plant species richness, particularly the richness of grasses and herbs, which has been shown above to be related to the cover of heather. Canopy closure rates were higher at Glen Clunie than at Glen Shee, and it is likely that this was one of the factors influencing the greater hemipteran species richness observed at sites 3 and 4 (Glen Shee) compared to sites 1 and 2 (Glen Clunie).

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A multivariate analysis of the insect species caught was used to determine the main environmental drivers that affect the hemipteran community (Figure 6).

Figure 6: Multivariate analysis to show the main environmental drivers for insect communities

This suggested that the two main drivers of insect community composition were the volume (an indicator of canopy occupancy) of Nardus (VOL NARD) and the organic content of the soil (ORG CON), with the proportion of young heather shoots (PPN CUR) also having a small influence. The composition of the hemipteran community would, therefore, appear to be most strongly influenced by the grazing levels present at the site and by soil type.

Within this experiment the soil type differs between the two experimental areas. At Glen Clunie (sites 1 and 2) the soil is wet and peaty, whilst at Glen Shee (sites 3 and 4) the soil is more of a brown earth with a lower organic content.

How soil type/site affects vegetation communitiesTo investigate the effect of site/soil differences on plant community composition, a further multivariate analysis was carried out on the vegetation but this time with the sites identified (Figure 7). Grazing and nutrient levels were again the two main gradients explaining the separation of the treatment plots. In this analysis, however, nutrient level emerged as the primary gradient (the X-axis) and grazing as the secondary gradient (the Y-axis). The Glen Shee plots (sites 3 and 4) were all located towards the high nutrient end of the graph, whilst the Glen Clunie plots (sites 1 and 2) were located towards the low nutrient side (Figure 7).

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Figure 7: Multivariate analysis on vegetation species for each site (Glen Clunie – circle; Glen Shee – square)

Soil characteristics were then added to the analysis (Figure 8). On the resulting graph, organic soil matter formed the primary (X) axis with nitrogen forming the secondary (Y) axis. Glen Shee plots were found at the mineral soil end of the graph and Glen Clunie at the organic soil end. The effect of nitrogen addition was greater at Glen Shee with the nitrogen plots being closer to the top of the graph, whilst on Glen Clunie, the nitrogen and control plots were less clearly separated.

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From this analysis, it is appears that the soil characteristics of the different sites interact with the nitrogen addition treatment to influence the composition of the plant community present on each plot. On wet peaty soils (sites 1 and 2), decomposition rates are slow and nutrients tend to remain locked up so that there is little mineralisation and little nitrogen available to plants. On drier brown earth soils (sites 3 and 4), there is more nutrient turnover and nitrogen is more readily available. The effect of nutrient addition may therefore be significantly greater on sites 3 and 4 than on sites 1 and 2.

Effect of soil type on nutrient uptake of different plant species

Figure 9: How soil type affects nutrient uptake

Analysis of nitrogen uptake by heather and mat grass occurring on the different treatment plots indicated the following. When nitrogen was added to heather, the increase in growth was more marked on wet, peaty soils (Glen Clunie plots - sites 1 and 2) than on mineral soils (Glen Shee – sites 3 & 4). When nitrogen was added to mat-grass, the increase of growth was more marked on the dry mineral soils (Figure 9). These results suggest that when nitrogen is deposited on heathland, the vulnerability of the dwarf shrub heath to invasion by grasses will be dependent on soil type.

Further experiments

Additional work was done to investigate the role that soil factors play in heather - mat-grass competition.

Heather and mat-grass were planted together in pots of homogenised mineral soil with or without (control) the addition of nutrients. In this homogenous soil, the addition of nutrients resulted in a significant increase in the above-ground biomass of mat-grass but had little effect on the biomass of heather (Figure 10). This occurred because the addition of nutrients favoured the development of roots by the mat-grass plants (Figure 10). Mat-grass roots occupied most of the soil volume in the pots and so were able to take up most of the nutrients.

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Figure 10: The response of mat-grass and heather above and below ground in homogeneous soil

A further experiment was carried out using organic soil. In organic soils, nutrients are not evenly distributed throughout the soil but are limited to the top organic layer. In this situation, the ability of the mat-grass to develop a penetrating root system that can extend throughout the pot is not useful since the nutrients are confined primarily to the top organic layer. Moreover, in this soil type, nitrogen is generally present in its organic form, which is not usually accessible to plants. Heather can, however, form an association with ericoid mycorrhizal fungi, which colonise the heather roots. These fungi can utilise nitrogen and phosphorus from a wide range of organic substrates (Genney et al. 2000) and thus enable heather to utilise the organic nitrogen also.

In a second pot experiment a layer of mixed sand and humus was placed on top of a layer of sand. A mix of heather plants (with and without mycorrhizae) and mat-grass were sown. Half the plots had nitrogen added. Mat-grass leaf production was measured for mat-grass competing against heather with or without ericoid mycorrhizae. The results indicate that the presence of the mycorrhizae gave heather a competitive advantage over mat-grass in this organic soil (Figure 11).

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Figure 11: Mat grass leaf production in competition with heather with and without ericoid mycorrhizae

The experiment also investigated mat-grass growing on its own in pots. By measuring root length of the grass, it was also shown that when mat-grass was in competition with heather, it had less root in the peat layer than when growing alone. The presence of ericoid mycorrhizae caused the difference.

Conclusions

Below-ground Mat-grass is a superior competitor in mineral soils due to greater root depth and

higher nutrient uptake But heather is a superior competitor in organic soils because of its ability to

concentrate root growth in the organic layer and to access organic N via mycorrhizae. Heather can exclude mat-grass roots from this layer especially if its roots are

colonised by ericoid mycorrhizae. Below-ground interactions are most marked in the soil layers of greatest nutrient

availability Soil type and structure is the key to heather’s competitive abilityAbove-ground Changes in vegetation in response to grazing and nutrients are mediated by changes

in heather dominance. A tall dense canopy is the key to heather resisting invasion by grasses

Grazing and nutrient inputs interact. Nutrients cause a much bigger decrease in heather cover in the presence of grazing

Responses to nutrient treatments are highly site dependant, reflecting spatial variation in herbivore abundance and soil type

Insect diversity reflects plant diversity but is determined by site not treatment because plant diversity is very dependent on soil type

Summary:

Mechanisms of heather-grass competitionInvasion of heather by mat-grass is promoted by:

Above ground factors particularly light availability: gaps in the canopy created by grazing increase the availability of light

Below ground factors particularly increased access to nutrients: soil structure, organic content and the presence of ericoid mycorrhizae all influence the availability of nutrients to heather.

Drivers of community compositionThe richness (number of species) of the plant-feeding insect community is influenced by the diversity of the plants. The latter is influenced by management and by environmental factors on the site. These factors include grazing levels and nutrient availability, the latter in turn being influenced by soil processes such as nutrient cycling which in turn is

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influenced by soil type and structure. By altering the composition of the plant community, these factors also act indirectly on the diversity and composition of the insect community

Two final points to go away with Site base factors are important whether managing for vegetation or insects. Below-ground processes drive what is seen above-ground. Don’t forget about the

soil.

Questions arising1) What was the difference in stocking rates on the sites? Glen Clunie was grazed

predominantly by deer with a low sheep stocking rate in the summer. Glen Shee was grazed by sheep all year round at a fairly high rate. Supplementary feed was provided close to the experimental site.

2) Can you extrapolate the information about mat-grass to other grasses? The principal driver for other non-palatable grasses will be grazing as it is for mat-grass. You could consider the nutrient requirements, palatability and growth rates of each species (with assistance from (Grime et al. 1988) possibly) and predict which grasses are likely to dominate under which conditions.

3) Ericoid mycorrhizae are important in conferring a competitive advantage to heather, how does that interact with N, given that high N reduces the infection rate? Nitrogen addition didn’t reduce heather infection as much as the presence of mycorrhizae reduced the mat-grass roots. Heather was still dominant. The big question is what happens in the field, and what infection rates are like in the field; they are usually pretty high. It would be good to try an experiment with uninfected heather in the field, but this is difficult. Uninfected mat-grass was planted into infected heather and mat-grass stands of varying canopy. The mat-grass remained uninfected and performed badly when surrounded by heather.

4) What do you think will happen to these interactions if the climate changes as it is predicted to do? Temperature is likely to increase, which will cause an increase in nutrient cycling and nutrient availability. The interaction between the increase in temperature and the change in rainfall will be interesting. Drought will cause problems for heather, which roots only in the top 2cm where the nutrients occur, grasses which root deeper will be more resistant.If there is less spring frost, this will react in a positive way with the N deposition effects on phenology. In the southern lowlands, moisture is more of a constraint than temperature; therefore an increase in temperature might have a negative effect on nutrient cycling. This will depend on the spatial pattern of the increased overall rainfall.

Field trip

Controlling over-mature gorse

One of the management issues at Thursley Common is the management of stands of dense, inaccessible mature gorse (Ulex europaeus). No ground flora can persist under

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these stands, the ground is covered by gorse detritus, and control by mechanical means is not possible. Burning is the common practice to manage this situation. In the case of the burned gorse stand observed on the visit, an area on the edge of the stand adjacent to a footpath had been cleared mechanically a year or two prior to burning. This provided a flat exposed area to which the fire brigade had access. Immediately before burning in the winter of 1999/2000, the surrounding area was wetted with a ‘fire fogging system’. This machine operates with a low volume under high pressure to provide a fine mist of water. It is portable and can be transported easily by landrover. The purpose of wetting the surrounding area is to reduce the heat and rate of spread of the fire. A flame gun is required to start the fire. After the gorse was burnt the kindling1 was removed using saws, and a supplementary burn carried out on the ground litter layer.

The area has now successfully regenerated with common heather and some bell heather (Erica cinerea) also present. Rabbits have kept any gorse regeneration in check and although they have suppressed the heather to an extent, they have not prevented it from becoming dominant. Silver studded blue butterfly (Plebejus argus) is now established in the area. This is particularly notable as this species is thought to be sedentary with weak dispersal ability, though the nearest population is a considerable distance away.

The local residents, who are not generally familiar with lowland heath management, have been kept informed of the procedures and the objectives at Thursley and this has helped to win over their support.

Controlled burning and cutting of heather

Some areas of heather on Thursley are cut using a forage harvester. This is undertaken in a random fashion across the site. The randomness helps to maintain a greater spatial diversity of heather ages and structures across the site than if this were undertaken in pre-selected blocks. Most of the regeneration is from seed.

On other areas, mechanical cutting of heather is not an option because of the steepness of the site or because it has been used for military manoeuvres. Many of these areas are north facing and have only a small herpetological population. Burning is used to manage these steep north-facing areas, since the risk to the herpetological population is small. Burning is undertaken on a small scale with burn size being a maximum of approximately 75m x 50m and more commonly 40m x 40m. Before burning, the boundaries of the proposed burn area are cut with a reciprocating blade mower and the cuttings thrown into the middle. The ‘fire fogging system’ can also be used on the boundaries to contain the fire.

Control of bracken

Two main methods, cutting or spraying, are used to control bracken at Thursley.

Cutting is carried out using a forage harvester, 3 times a year on a 6-week cycle, with the arisings being removed from site. The bracken is cut to ground level though care is taken to retain hummocks and other structural features on the ground. This method achieves a 1 Traditionally these blackened sticks were used for fuel in kindling ovens

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90% bracken kill for 3-4 years and then the process has to be repeated. The advantages of this method are that other non-target species are not affected and that once the machinery has been purchased, there is no additional capital expenditure. The disadvantage is that the first cut stimulates the dominant growth nodes on the rhizomes so, if subsequent cuts are not carried out, growth will increase.

Spraying is carried out using the fern specific herbicide, Asulox, which can kill non-target ferns. The opportunity to spray is limited to 10-20 days at the end of July. If spray is followed by drought then it will be unsuccessful and spraying can be repeated the following year. If it is unsuccessful again then it must not be used again for a few years as the effectiveness will fall off. Heavy rain does not appear to adversely affect the herbicide.

AcknowledgementsWe would like to thank Sally Power and Sue Hartley for their comprehensive presentations and Simon Nobes for his informative tour of Thursley Common. Thanks too to Sharon Zyta and the staff at the National Trusts Witley Centre and for making us so welcome and for holding the fort whilst the organisers arrived. We apologise for the unavoidable delay at the beginning of the workshop and for the consequent loss of discussion time. Thanks also to Defra who funded the workshop under contract BD1234.

Author contactsFor further copies of this report, contact Helen Adamson at ADAS Redesdale, Rochester, Otterburn, Newcastle-upon-Tyne, NE19 1SB, helen.adamson@adas.co.uk or Dr. Sarah Gardner at ADAS Preston, 15 Eastway Business Village, Preston, Lancs, PR2 4WT, sarah.gardner@adas.co.uk.

References & further informationBrunsting, A.M.H. & Heil, G.W. (1985). The role of nutrients in the interactions between a herbivorous beetle and some competing plant species in heathlands. Oikos, 44, 23-26

Dalton, H. & Brand-Hardy, R. (2003). Nitrogen: the essential public enemy. Journal of Applied Ecology 40, 771-781.

Genney, D.R., Alexander, I.J. & Hartley, S.E. (2000). Exclusion of grass roots from soil organic layers by Calluna: the role of ericoid mycorrhizae. Journal of Experimental Botany, 51, 1117-1125.

Genney, D.R., Alexander, I.J. & Hartley, S.E. (2002). Soil organic matter distribution and below-ground competition between Calluna vulgaris and Nardus stricta. Functional Ecology, 16, 664-670.

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Grime, J.P., Hodgson, J.G. & Hunt, R. (1988). Comparative plant ecology. Unwin Hymen Ltd., London.

Hartley, S.E. (1997). The effects of grazing and nutrient inputs on grass-heather competition. Botanical Journal of Scotland, 49, 315-324.

Hartley, S.E. & Amos, L. (1999). Competitive interactions between Nardus stricta (L) and Calluna vulgaris (L) Hull: the effect of fertiliser and defoliation on above- and below-ground performance. Journal of Ecology 87, 330-340.

Hartley, S.E., Gardner, S.M. & Mitchell, R.J. (2003). Indirect effects of grazing and nutrient addition on the hemipteran community of heather moorlands. Journal of Applied Ecology 40, 793-803.

Power, S.A., Ashmore, M.R. & Cousins, D.A. (1998) Impacts and fate of experimentally enhanced nitrogen deposition on a British lowland heath. Environmental Pollution 102, 27-34.

Power, S.A., Ashmore, M.R., Terry, A.C., Caporn, S.J.M., Pilkington, M.G., Wilson, D.B., Barker, C.G., Carroll, J.A., Cresswell, N., Green, E.R. & Heil, G.W. (2004) Linking field experiments to long-term simulation of impacts of nitrogen deposition on heathlands and moorlands. Water, Air and Soil Pollution 40, in press

Power, S.A., Barker, C.G., Allchin, E.A., Ashmore, M.R. & Bell, J.N.B. (2001). Habitat management: a tool to modify ecosystem impacts of nitrogen deposition? Scientific World 1, 714-721.

Terry, A.C., Ashmore, M.R., Power, S.A., Allchin, E.A. & Heil, G.W. (2004). Modelling the impacts of atmospheric nitrogen deposition and Calluna-dominated ecosystems in the UK. Journal of Applied Ecology 41, 897-909.

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