Alison J. Hester

THE PERILS OF HAVING TASTY NEIGHBORS: GRAZING IMPACTS OF LARGE HERBIVORES AT VEGETATION BOUNDARIES

The boundaries between vegetation patches are focal points for dynamic interactions between plant communities, particularly in grazed ecosystems where vegetation types may differ in their acceptability to herbivores. Here we show that key vegetation resources attract herbivores, and the surrounding vegetation receives a higher impact than if it is associated with patches of less preferred vegetation (an example of apparent competition). We studied the influence of proximity to preferred grass patches on utilization of the less preferred dwarf shrub, heather (Calluna vulgaris) by red deer (Cervus elaphus) and sheep (Ovis aries) at a range of spatial scales in the Cairngorm Mountains of Scotland, UK. There was a sharp decline in heather utilization with increasing distance from the edges of grass patches. The proportion of grass in the local landscape (within 1 km) had a significant positive effect on heather utilization both at the grass–heather boundary and beyond 5 m from the grass patch. There was also a significant effect of dominant grass species on the utilization of heather within 50 cm of the grass-patch edge, with utilization around Agrostis/Festuca patches (most preferred) being greater than around Nardus-dominated patches, and lowest around patches of Molinia. The greatest contribution to variation in heather utilization was at the smallest scale, and variance components decreased as spatial scale increased, making it impossible to predict local heather utilization (i.e., at the scale of individual plants and of individual bites by foraging ungulates) from large-scale parameters alone, such as herbivore density. These findings emphasize that vegetation–herbivore interactions are localized within the landscape, and that it is these hot spots which are the key fulcrum for vegetation dynamics.

Experimental evidence for herbivore limitation of the treeline

The treeline ecotone divides forest from open alpine or arctic vegetation states. Treelines are generally perceived to be temperature limited. The role of herbivores in limiting the treeline is more controversial, as experimental evidence from relevant large scales is lacking. Here we quantify the impact of different experimentally controlled herbivore densities on the recruitment and survival of birch Betula pubescens tortuosa along an altitudinal gradient in the mountains of southern Norway. After eight years of summer grazing in large-scale enclosures at densities of 0, 25, and 80 sheep/km2, birch recruited within the whole altitudinal range of ungrazed enclosures, but recruitment was rarer in enclosures with low-density sheep and was largely limited to within the treeline in enclosures with high-density sheep. In contrast, the distribution of saplings (birch older than the experiment) did not differ between grazing treatments, suggesting that grazing sheep primarily limit the establishment of new tree recruits rather than decrease the survival of existing individuals. This study provides direct experimental evidence that herbivores can limit the treeline below its potential at the landscape scale and even at low herbivore densities in this climatic zone. Land use changes should thus be considered in addition to climatic changes as potential drivers of ecotone shifts.

Effects of season and intensity of sheep grazing on tree regeneration in a British upland woodland

Abstract This paper reports some effects of experimental alteration of season (summer, winter) and intensity of sheep grazing on tree regeneration, growth and browsing damage in an upland broadleaved woodland in Cumbria, UK. During the first 3 years of grazing treatments, seedling recruitment was primarily Betula pubescens, Sorbus aucuparia and Fraxinus excelsior . After 7 years of grazing treatments, seedlings and saplings of many species were numerous, with fewer seedlings in winter-grazed plots but a higher proportion of these were browsed. More seedlings had reached sapling stage in winter-grazed plots. Sapling numbers increased with decreasing grazing intensity, with lower proportions browsed at low grazing intensities. Within the first 3 years grazing treatments, recruitment of Betula pubescens seedlings was greater in grazed than ungrazed plots, and greater in winter-grazed than in summer-grazed plots. Browsing damage to these seedlings increased with grazing intensity in winter, and height increments were greatest in ungrazed plots. After 7 years, B. pubescens seedlings and saplings showed no significant treatment effects. After 3 years, recruitment of Sorbus aucuparia seedlings was greater in grazed than ungrazed plots, and greater in summer-grazed than winter-grazed plots. Fewer seedlings were browsed in low grazing intensity plots, and seedling height increments were greatest in ungrazed plots. After 7 years, more S. aucuparia seedlings were browsed in winter than summer, but saplings showed no treatment effects. After 3 years, recruitment of F. excelsior was greatest in low grazing intensity plots and smallest in medium grazing intensity plots. More seedlings were browsed at higher grazing intensities, but in summer-grazed plots height increments tended to be greatest in the most heavily grazed plots. After 7 years, F. excelsior seedlings were more numerous in summer-grazed plots but were not differentially browsed; sapling numbers were not affected by grazing treatments but browsing damage was very high in all except summer low grazing plots. The implications of the results are discussed in relation to woodland grazing management issues.

Is vegetation composition or soil chemistry the best predictor of the soil microbial community

With the species composition and/or functioning of many ecosystems currently changing due to anthropogenic drivers it is important to understand and, ideally, predict how changes in one part of the ecosystem will affect another. Here we assess if vegetation composition or soil chemistry best predicts the soil microbial community. The above and below-ground communities and soil chemical properties along a successional gradient from dwarf shrubland (moorland) to deciduous woodland (Betula dominated) were studied. The vegetation and soil chemistry were recorded and the soil microbial community (SMC) assessed using Phospholipid Fatty Acid Extraction (PLFA) and Multiplex Terminal Restriction Fragment Length Polymorphism (M-TRFLP). Vegetation composition and soil chemistry were used to predict the SMC using Co-Correspondence analysis and Canonical Correspondence Analysis and the predictive power of the two analyses compared. The vegetation composition predicted the soil microbial community at least as well as the soil chemical data. Removing rare plant species from the data set did not improve the predictive power of the vegetation data. The predictive power of the soil chemistry improved when only selected soil variables were used, but which soil variables gave the best prediction varied between the different soil microbial communities being studied (PLFA or bacterial/fungal/archaeal TRFLP). Vegetation composition may represent a more stable ‘summary’ of the effects of multiple drivers over time and may thus be a better predictor of the soil microbial community than one-off measurements of soil properties.

How important is plot relocation accuracy when interpreting re-visitation studies of vegetation change?

Background: Re-visitation studies are often based on phytosociological survey data where the precise location of the original plots is unknown. Attempts to evaluate the error associated with relocation uncertainty are rare, yet this is important in interpreting the results with any degree of confidence. Aims: Using a 50-year re-visitation study of upland vegetation in the Scottish Highlands, we aim to assess the potential for, and implications of, uncertainty in relocating plots. Methods: At nine sites, three to five replicate plots were surveyed within a stand of vegetation relocated to the nearest 100 m using the original plot location data. Results: The compositional difference (measured by the Bray–Curtis distance) between the original plot and new replicate plots was greater than that among the replicate plots, both for the combined data and individual vegetation types. Temporal species turnover was greatest in the lower cover-abundance categories (< 5%). Conclusions: We demonstrate that if temporal change in vegetation can be shown to be greater than local spatial heterogeneity today, patterns of change at the local scale detected by re-visitation data can be interpreted with some confidence if other sources of error are minimised. Recommendations for best practice in re-visitation studies are made.

Landscape-scale vegetation change in the Cairngorms, Scotland, 1946-1988: implications for land management

Abstract Land cover change data were derived from interpretation of aerial photographs taken in 1946 and 1988 for 1000 km2 area of the Cairngorms area, Scotland. These data are compared with qualitative predictions based on the successional models of Miles (1985 J. Soil. Sci., 36). The impacts of man are shown to have been of the greatest magnitude, with conifer planting showing the greatest net increase in area between 1946 and 1988. Successional changes in seminatural vegetation did occur but none totalled more than 5 km2. Areas of semi-natural woodland were already scarce by 1946, but the extensive areas of heather also showed few successional transitions to other communities, such as grassland, scrub or woodland. The findings are discussed in relation to changes in land use and herbivore densities over this time period. The theoretical successional framework of Miles (1985), with the important addition of the more direct effects of man, is shown to be a valuable tool for the examination of vegetation change data and the implications for management in an area such as the Cairngorms.

Environmental myopia: a diagnosis and a remedy.

Long-term ecological observation affords a picture of the past that uniquely informs our understanding of present and future ecological communities and processes. Without a long-term perspective, our vision is prone to environmental myopia. Long-term experiments (LTEs) in particular can reveal the mechanisms that underlie change in communities and ecosystem functioning in a way that cannot be understood by long-term monitoring alone. Despite the urgent need to know more about how climate change will affect ecosystems and their functioning, the continued existence of LTEs is extremely precarious and we believe that dedicated funds are needed to support them. A new non-profit organization called the Ecological Continuity Trust seeks to provide a solution to this problem by establishing an endowment that will be specifically earmarked to sustain LTEs as a scientific tool for the benefit of future generations.

A ROLE FOR LARGE HERBIVORES (DEER AND DOMESTIC STOCK) IN NATURE CONSERVATION MANAGEMENT IN BRITISH SEMI-NATURAL WOODS

Summary Large herbivores are a part of most natural woodland systems, but in Britain many of the species present in the wildwood some 7,000 years ago have been lost, while native woodland cover has...

Introducing spatial grazing impacts into the prediction of moorland vegetation dynamics

Grazing by large herbivores is a major determinant of vegetation dynamics in many semi-natural ecosystems, including the replacement of heather moorland by rough grassland in the British uplands. Herbivore foraging is influenced by vegetation patterns and, in turn, their grazing drives vegetation dynamics. Although vegetation impacts are local, spatially heterogeneous local impacts can have different consequences as would the same impacts distributed uniformly. We constructed a simulation model of the spatial effects of grazing by sheep on the vegetation dynamics of heather moorland, a vegetation community of international conservation importance in the UK. The model comprised three submodels to predict (1) annual average heather utilisation, (2) spatial variation in heather utilisation (higher near the edge of grass patches) and (3) competition between heather and grass. Here we compare the predicted heather utilisation and vegetation dynamics of the spatial model, relative to those of a non-spatial model. The spatial model resulted in a reduced loss of heather cover for a given sheep stocking rate. The model demonstrates how spatial interactions between large herbivores and their forage drive vegetation dynamics, leading to changes in community structure and composition. Indeed, omitting spatial effects in grazing models may lead to inaccurate predictions. We have shown that ecosystem modelling, based around an iterative dialogue between developers and experienced researchers, has the potential to make a substantial contribution towards the conservation and management of vulnerable landscapes. Combining modelling with experimental studies will facilitate progress towards understanding long-term vegetation/herbivore dynamics.

Relationship between fragmentation, degradation and native and exotic species richness in an Andean temperate forest of Chile

Impactos humanos tales como la fragmentacion y degradacion de bosques pueden tener fuertes efectos en las comunidades de especies vegetales nativas y exoticas. Ademas, perturbaciones antropicas ocurren principalmente en menores altitudes produciendo mayores grados de fragmentacion y degradacion que en mayores altitudes. La invasion de plantas exoticas deberia ser mayor en bosques mas fragmentados o degradados y, por lo tanto, en menores altitudes dentro de un tipo de bosque o piso altitudinal. En cambio, la riqueza de especies nativas deberia ser negativamente afectada por la fragmentacion y degradacion, encontrandose mayor riqueza en mayores altitudes dentro de un tipo de bosque determinado. En este trabajo evaluamos estas hipotesis en un bosque templado andino de la Region de la Araucania, Chile. Registramos la composicion de plantas vasculares en doce fragmentos de diferente tamano, razon perimetro/area, altitud y degradacion antropica (cortas, incendios, fecas de ganado). En base a estas variables construimos un indice de fragmentacion y uno de degradacion para estos fragmentos. Se analizaron las relaciones entre estas variables a traves de correlaciones de Pearson. Nuestros resultados sugieren que la fragmentacion y degradacion estan positivamente relacionadas y que ambos tipos de perturbacion ocurren en altitudes mas bajas del tipo de bosque estudiado. Ademas, la fragmentacion y degradacion estan afectando en diferente forma a la riqueza de especies nativas y exoticas. La invasion se incremento como consecuencia tanto de fragmentacion como de degradacion, y como consecuencia del patron de distribucion altitudinal de estas perturbaciones, la invasion aparentemente ocurre principalmente en zonas bajas. En cambio, la riqueza de especies nativas fue negativamente afectada solo por la fragmentacion, y no se relaciono con la degradacion interna de los bosques ni con la altitud.