Sarah J. Woodin

Can the foliar nitrogen concentration of upland vegetation be used for predicting atmospheric nitrogen deposition? Evidence from field surveys

The deposition of atmospheric nitrogen can be enhanced at high altitude sites as a consequence of cloud droplet deposition and orographic enhancement of wet deposition on hills. The degree to which the increased deposition of nitrogen influences foliar nitrogen concentration in a range of upland plant species was studied in a series of field surveys in northern Britain. A range of upland plant species sampled along altitudinal transects at sites of known atmospheric nitrogen deposition showed marked increases in foliar nitrogen concentration with increasing nitrogen deposition and altitude (and hence with decreasing temperature). For Nardus stricta L., Deschampsia flexuosa (L.) Trin., Calluna vulgaris (L.) Hull, Erica cinerea L. and Hylocomium splendens (Hedw.) Br. Eur. on an unpolluted hill, foliar nitrogen increased by 0.07, 0.12, 0.15, 0.08 and 0.04% dry weight respectively for each 1 kg ha(-1) year(-1) increase in nitrogen deposition. Most species showed an approximately linear relationship between foliar nitrogen concentration and altitude but no trend with altitude for foliar phosphorus concentration. This provided evidence that the tissue nutrient status of upland plants reflects nutrient availability rather than the direct effects of climate on growth. However, differences in the relationship between foliar nitrogen concentration and atmospheric nitrogen deposition for N. stricta sampled on hills in contrasting pollution climates show that the possibility of temperature-mediated growth effects on foliar nitrogen concentration should not be ignored. Thus, there is potential to use upland plant species as biomonitors of nitrogen deposition, but the response of different species to nitrogen addition, in combination with climatic effects on growth, must be well characterised.

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.

HABITAT TYPE DETERMINES HERBIVORY CONTROLS OVER CO2 FLUXES IN A WARMER ARCTIC

High-latitude ecosystems store large amounts of carbon (C); however, the C storage of these ecosystems is under threat from both climate warming and increased levels of herbivory. In this study we examined the combined role of herbivores and climate warming as drivers of CO2 fluxes in two typical high-latitude habitats (mesic heath and wet meadow). We hypothesized that both herbivory and climate warming would reduce the C sink strength of Arctic tundra through their combined effects on plant biomass and gross ecosystem photosynthesis and on decomposition rates and the abiotic environment. To test this hypothesis we employed experimental warming (via International Tundra Experiment [ITEX] chambers) and grazing (via captive Barnacle Geese) in a three-year factorial field experiment. Ecosystem CO2 fluxes (net ecosystem exchange of CO2, ecosystem respiration, and gross ecosystem photosynthesis) were measured in all treatments at varying intensity over the three growing seasons to capture the impact of the treatments on a range of temporal scales (diurnal, seasonal, and interannual). Grazing and warming treatments had markedly different effects on CO2 fluxes in the two tundra habitats. Grazing caused a strong reduction in CO2 assimilation in the wet meadow, while warming reduced CO2 efflux from the mesic heath. Treatment effects on net ecosystem exchange largely derived from the modification of gross ecosystem photosynthesis rather than ecosystem respiration. In this study we have demonstrated that on the habitat scale, grazing by geese is a strong driver of net ecosystem exchange of CO2, with the potential to reduce the CO2 sink strength of Arctic ecosystems. Our results highlight that the large reduction in plant biomass due to goose grazing in the Arctic noted in several studies can alter the C balance of wet tundra ecosystems. We conclude that herbivory will modulate direct climate warming responses of Arctic tundra with implications for the ecosystem C balance; however, the magnitude and direction of the response will be habitat-specific.

Herbivore impacts to the moss layer determine tundra ecosystem response to grazing and warming

Herbivory and climate are key environmental drivers, shaping ecosystems at high latitudes. Here, we focus on how these two drivers act in concert, influencing the high arctic tundra. We aim to investigate mechanisms through which herbivory by geese influences vegetation and soil processes in tundra ecosystems under ambient and warmed conditions. To achieve this, two grazing treatments, clipping plus faecal additions and moss removal, were implemented in conjunction with passive warming. Our key finding was that, in many cases, the tundra ecosystem response was determined by treatment impacts on the moss layer. Moss removal reduced the remaining moss layer depth by 30% and increased peak grass biomass by 27%. These impacts were probably due to observed higher soil temperatures and decomposition rates associated with moss removal. The positive impact of moss removal on grass biomass was even greater with warming, further supporting this conclusion. In contrast, moss removal reduced dwarf shrub biomass possibly resulting from increased exposure to desiccating winds. An intact moss layer buffered the soil to increased air temperature and as a result there was no response of vascular plant productivity to warming over the course of this study. In fact, moss removal impacts on soil temperature were nearly double those of warming, suggesting that the moss layer is a key component in controlling soil conditions. The moss layer also absorbed nutrients from faeces, promoting moss growth. We conclude that both herbivory and warming influence this high arctic ecosystem but that herbivory is the stronger driver of the two. Disturbance to the moss layer resulted in a shift towards a more grass-dominated system with less abundant mosses and shrubs, a trend that was further enhanced by warming. Thus herbivore impacts to the moss layer are key to understanding arctic ecosystem response to grazing and warming.

Is there a cost of parasites to caribou

SUMMARY Macroparasites potentially play a significant but often ignored role in the ecology and dynamics of wild ruminant populations. In the Arctic, parasites may impact on host populations by exacerbating the effects of seasonal and limited forage availability on the condition, fecundity and survival of individuals. We studied the effects of abomasal nematode parasites and warble flies, Hypoderma tarandi, on condition and pregnancy of caribou Rangifer tarandus in the Dolphin-Union herd, Nunavut, Canada. By the end of winter, female caribou over 2 years old showed a significant decrease in body weight with increasing nematode burden, and a decrease in back fat depth with increasing warble abundance. These effects were exaggerated in the non-pregnant fraction of the population. High warble larvae burdens were also associated with significantly reduced probability of being pregnant. Our research demonstrates a negative relationship between parasites and caribou condition that may have consequences for their fitness. Additionally, we discuss the possibility that muskox Ovibos moschatus share some parasite species with the caribou and could lead to elevated burdens in the sympatric host. Parasites may have been a contributory factor in a previous winter range-shift of the caribou herd and this may reflect a form of apparent competition between the two ungulate species.

Impacts of sulphur and nitrogen deposition on sites and species of nature conservation importance in Great Britain

A significant proportion of important nature conservation sites in Britain are subject to rates of deposition of sulphur and/or nitrogen which may perturb their ecology. This paper reviews studies of some of the effects of such pollutants on woodlands, upland and lowland heaths, and freshwaters. The weight of evidence shows that acidic deposition is causing damage on many sites which receive statutory protection. Many affected communities occur in areas where critical loads for H+ and S are exceeded, and would remain so even after implementation of current targets for reducing pollution. Little can be done on site to prevent such damage; conservation interests will best be protected by consideration of ecological data in the formulation of strategies for major reductions in emissions.

Differential allocation of carbon in mosses and grasses governs ecosystem sequestration: a 13C tracer study in the high Arctic.

*This study investigates the influence of vegetation composition on carbon (C) sequestration in a moss-dominated ecosystem in the Arctic. *A (13)C labelling study in an arctic wet meadow was used to trace assimilate into C pools of differing recalcitrance within grasses and mosses and to determine the retention of C by these plant groups. *Moss retained 70% of assimilated (13)C over the month following labelling, which represented half the growing season. By contrast, the vascular plants, comprising mostly grasses, retained only 40%. The mechanism underlying this was that moss allocated 80% of the (13)C to recalcitrant C pools, a much higher proportion than in grasses (56%). *This method enabled elucidation of a plant trait that will influence decomposition and hence persistence of assimilated C in the ecosystem. We predict that moss-dominated vegetation will retain sequestered C more strongly than a grass-dominated community. Given the strong environmental drivers that are causing a shift from moss to grass dominance, this is likely to result in a reduction in future ecosystem C sink strength.

Impacts of Grazing and Climate Warming on C Pools and Decomposition Rates in Arctic Environments

Arctic ecosystems are important habitats for Arctic breeding geese which in their turn have a profound impact on the vegetation in these areas. The herbivore pressure is currently on the rise in the region and may interact with climate change to drive changes in both productivity and decomposition rates. In this study, we aim to determine the combined impact of warming and geese on C pools and decomposition in two high Arctic habitats, mesic heath and wet meadow. We employed field-experimental warming and grazing treatments in a fully factorial design and quantified their impact on ecosystems C pools and turnover. High grazing levels reduced vascular biomass and litter C pools at both sites, whereas warming reduced moss biomass at the mesic site only. Grazing increased decomposition rates and reduced the amount of labile C in the soil at the mesic site, whereas warming substantially reduced C concentrations by approximately 25%. Intermediate level of grazing had an opposite effect and resulted in increased soil C storage at the mesic site. In contrast, no effects of the treatment on belowground C cycling were found at the wet site. In conclusion, the two levels of grazing and warming impacted on different aspects of the C cycle investigated in this study. The high grazing and warming treatment promoted reduced carbon storage/decomposition both above- and belowground with the strongest effects seen at the mesic site.

Root traits predict decomposition across a landscape‐scale grazing experiment

Root litter is the dominant soil carbon and nutrient input in many ecosystems, yet few studies have considered how root decomposition is regulated at the landscape scale and how this is mediated by land-use management practices. Large herbivores can potentially influence below-ground decomposition through changes in soil microclimate (temperature and moisture) and changes in plant species composition (root traits). To investigate such herbivore-induced changes, we quantified annual root decomposition of upland grassland species in situ across a landscape-scale livestock grazing experiment, in a common-garden experiment and in laboratory microcosms evaluating the influence of key root traits on decomposition. Livestock grazing increased soil temperatures, but this did not affect root decomposition. Grazing had no effect on soil moisture, but wetter soils retarded root decomposition. Species-specific decomposition rates were similar across all grazing treatments, and species differences were maintained in the common-garden experiment, suggesting an overriding importance of litter type. Supporting this, in microcosms, roots with lower specific root area (m2 g−1) or those with higher phosphorus concentrations decomposed faster. Our results suggest that large herbivores alter below-ground carbon and nitrogen dynamics more through their effects on plant species composition and associated root traits than through effects on the soil microclimate.

A field assessment of the success of moorland restoration in the rehabilitation of whole plant assemblages

ABSTRACT Question: How successful is restoration that is focused on a single dominant plant at enabling the reassembly of the whole vegetation assemblage and what factors affect the relative success of such restoration? Location: Moorlands in England and Scotland, UK. Methods: Vegetation composition was sampled in grass-dominated (degraded), restored and long established Calluna vulgaris-dominated (target) areas within eight moorland restoration sites. Additional soil and biogeographic data were collected. Data were analysed by Canonical Correspondence Analysis, which allowed the impact of moorland management to be examined. Results: All sites showed good restoration success when the dominant managed species (Calluna vulgaris, Molinia caerulea and Nardus stricta) were considered. Restoration success of the remainder of the plant assemblage, disregarding these dominant species, was lower with restored samples at some sites differing little from their respective degraded samples. Moors restored solely by grazing exclusion were more similar to their respective targets than were those restored by more intensive mechanical means. The most important factors in explaining vegetation assemblages were management status (i.e. whether samples represented degraded, restored or target parts of the moor) and latitude. Conclusions: The project demonstrates that, where possible, restoration should be attempted by grazing exclusion alone. Furthermore the importance of applying local restoration targets and of monitoring the whole plant assemblages when assessing the success of moorland restoration is highlighted. Nomenclature: Species names follow Stace (1991) for higher plants and Smith (2004) for bryophytes. Abbreviations: CCA = Canonical Correspondence Analysis, LOI = Loss on Ignition