Changes in species richness and composition in European acidic grasslands over the past 70 years: the contribution of cumulative atmospheric nitrogen deposition

Our study investigates the negative impact of nitrogen (N) deposition on species richness in acidic grasslands, based on a temporal comparison of vegetation data spanning a period of almost 70 years. We compiled a large data base of plots assigned to the Violion caninae grassland type, composed of managed, but unfertilized semi-natural grasslands on nutrient-poor, acidic soils. In total 1114 plots, mainly from Great Britain, the Netherlands and Germany, were compiled, dating back to 1939. Environmental site information included geographical and soil (mean Ellenberg values) variables as well as estimates of cumulative N and sulphur (S) deposition since 1939. Statistical analyses were carried out separately for the data subsets from the three regions. In all regions, the vegetation differentiation was mainly related to soil acidity and nutrient availability, as well as to the year of sampling and the cumulative amounts of N and S deposition. Plot-species richness of vascular plants and bryophytes (analysed for Great Britain only) decreased with time and analyses suggest these are affected by various factors, notably soil pH, but also latitude and cumulative N deposition. The latter explained more of the variation in species number than the year of sampling and cumulative S deposition, which supports the interpretation that the decline in species richness is mainly caused by increasing N availability and less by altered management and soil acidification. For Great Britain and Germany, cumulative N deposition showed a strong negative relationship with several biodiversity measures, especially the proportion of dicots, whereas it was positively related to the proportion of grass species. In general, our results give temporal evidence for the negative effect of N deposition on species richness in semi-natural vegetation.     

Growth of ectomycorrhizal mycelia and composition of soil microbial communities in oak forest soils along a nitrogen deposition gradient

Deciduous forests may respond differently from coniferous forests to the anthropogenic deposition of nitrogen (N). Since fungi, especially ectomycorrhizal (EM) fungi, are known to be negatively affected by N deposition, the effects of N deposition on the soil microbial community, total fungal biomass and mycelial growth of EM fungi were studied in oak-dominated deciduous forests along a nitrogen deposition gradient in southern Sweden. In-growth mesh bags were used to estimate the production of mycelia by EM fungi in 19 oak stands in the N deposition gradient, and the results were compared with nitrate leaching data obtained previously. Soil samples from 154 oak forest sites were analysed regarding the content of phospholipid fatty acids (PLFAs). Thirty PLFAs associated with microbes were analysed and the PLFA 18:2ω6,9 was used as an indicator to estimate the total fungal biomass. Higher N deposition (20 kg N ha⁻¹ y⁻¹ compared with 10 kg N ha⁻¹ y⁻¹) tended to reduce EM mycelial growth. The total soil fungal biomass was not affected by N deposition or soil pH, while the PLFA 16:1ω5, a biomarker for arbuscular mycorrhizal (AM) fungi, was negatively affected by N deposition, but also positively correlated to soil pH. Other PLFAs positively affected by soil pH were, e.g., i14:0, a15:0, 16:1ω9, a17:0 and 18:1ω7, while some were negatively affected by pH, such as i15:0, 16:1ω7t, 10Me17:0 and cy19:0. In addition, N deposition had an effect on the PLFAs 16:1ω7c and 16:1ω9 (negatively) and cy19:0 (positively). The production of EM mycelia is probably more sensitive to N deposition than total fungal biomass according to the fungal biomarker PLFA 18:2ω6,9. Low amounts of EM mycelia covaried with increased nitrate leaching, suggesting that EM mycelia possibly play an important role in forest soil N retention at increased N input.     

Impacts of local and regional factors on vegetation of boreal semi-natural grasslands

Using data from 46 sites in southern Finland and ordination methods, we examined plant-environment relationships in boreal mesic semi-natural grasslands at two spatial scales (grain sizes), using plots of 0.25 ha and 1 × 1 m. We applied the variation partitioning approach to determine the pure fractions of environmental variable groups and their joint effects on plant species compositional variation in the studied grasslands. The variables related to land-use intensity and high nutrient level (especially phosphorus) had a major role in explaining the species composition at both scales, although soil heterogeneity and habitat characteristics also accounted for a notable amount of the species compositional variation at the 0.25 ha grain size. At the 1 × 1 m grain size, the majority of the species compositional variation was related to the “pure” spatial differences between the studied grasslands (i.e. the site identity (dummy 0/1) variable), whereas the impacts of within-site variation of local environmental factors were considerably smaller. High nutrient levels and variables related to low land-use intensity, e.g. litter accumulation, were also significantly correlated with floristic variation at the 1 × 1 m grain size. Rare and declining grassland species are associated with low-nutrient grassland sites and patches. The main recommendation for the management planning of boreal semi-natural grasslands is that the first restoration attempts should be targeted to areas where nutrient levels, particularly that of phosphorus, are relatively low. Soil properties and plant species composition can provide useful guidelines for defining the correct management procedures for different sites.     

Direct and indirect effects of nitrogen deposition on species composition change in calcareous grasslands

Atmospheric nitrogen (N) deposition has been identified as a major threat to biodiversity, but field surveys of its effects have rarely focussed on sites which are actively managed to maintain characteristic species. We analysed permanent quadrat data from 106 plots in nature reserves on calcareous grassland sites in the United Kingdom collected during a survey between 1990 and 1993 and compared the data with the results from resurvey of 48 of these plots between 2006 and 2009. N deposition showed no significant spatial association with species richness, species diversity, or the frequency of species adapted to low nutrient conditions in the 1990–1993 dataset. However, temporal analysis showed that N deposition was significantly associated with changes in Shannon diversity and evenness. In plots with high rates of N deposition, there was a decrease in species diversity and evenness, a decline in the frequency of characteristic calcareous grassland species, and a lower number of rare and scarce species. As all sites had active management to maintain a high diversity and characteristic species, our results imply that even focussed management on nature conservation objectives cannot prevent adverse effects of high rates of N deposition. Structural equation modelling was used to compare different causal mechanisms to explain the observed effects. For change in Shannon diversity, direct effects of N deposition were the dominant mechanism and there was an independent effect of change in grazing intensity. In contrast, for change in herb species number, indirect effects on soil acidity, linked to both N and S deposition, were more important than direct effects of N deposition.     

The early effects of afforestation on biodiversity of grasslands in Ireland

The target rate of afforestation in Ireland over the next 30 years is 20,000 ha per year, which would result in an increase of the forest cover from the current 10% to 17%. In order to promote sustainable forest management practices, it is essential to know the composition and conservation value of habitats where afforestation is planned and the effects of subsequent planting upon biodiversity. The objectives of this study were to investigate changes in vegetation composition and diversity of grasslands 5 years after afforestation with Sitka spruce (Picea sitchensis) and determine the primary ecological and management factors responsible for these changes. Species cover, environmental and management data were collected from 16 afforested and unplanted improved and wet grassland site pairs in Ireland. Our results indicate that 5 years after tree planting, there were significant changes in richness, composition, and abundance of species. Competitive and vigorous grasses were more abundant in planted than in unplanted sites, as were generalist species found in both open and wooded habitats, while small-stature shade-sensitive species were less abundant. Vascular plant species richness and Shannon’s diversity index were higher in unplanted wet grassland, than in the planted sites. Bryophyte species richness was higher in planted improved grassland than in unplanted sites. The differences were primarily the result of the exclusion of grazing, ground preparation, changes in nutrient management and drainage for afforestation. Drainage ditches provided a temporary habitat for less competitive species, but the overall effect of drainage was to reduce the diversity of species dependent on wet conditions. Variance partitioning showed differences in the relative influences of environmental and management variables on biodiversity in the two habitats, probably due to the greater pre-afforestation grazing pressure and fertilisation levels in improved grasslands. The differences in biodiversity between planted and unplanted grasslands indicate that afforestation represents a threat to semi-natural habitats where distinctive and highly localised plant communities could potentially occur.     

Diversity patterns in grasslands along a landscape gradient in northwestern France

Abstract. Several measures of biodiversity were calculated (species richness SR, species diversity H', species evenness J', mean similarity, mosaic diversity and factorial diversity) in vascular plant communities along a landscape gradient in the Seine valley, Normandy, France. For these communities, we also recorded environmental and management data. Species and environmental data were analysed simultaneously by Canonical Correspondence Analysis (CCA) in order to study their relationships. CCA identifies one main landscape gradient linked to a set of highly linked ecological factors. Three community types were identified along this gradient: calcicolous communities on chalk slopes, mesophilous communities on colluvium and hydrophilous communities on alluviums. The measures of biodiversity between these groups and their variations along the landscape gradient indicate similar patterns for H', J' and SR. Between‐community biodiversity measures allow consideration of the distribution of species among communities in the landscape. Factorial diversity accounts for the organisation of the communities with reference to the basic mechanisms of species coexistence. Affinity analysis (similarity and mosaic diversity) measures the compositional pattern diversity, which is the function of the variation in species richness. We discuss the indicative versus the predictive value of these measures of biodiversity as regards ecological factors and processes and their application for conservation purposes.     

Plant diversity patterns in semi-natural grasslands along an elevational gradient in southern Norway

Leaf demography and growth of six common, co-occurring woody plant species that varied in stature (tree vs. shrub) and leaf texture (sclerophyllous, coriaceous, malacophyllous) were examined in a subtropical savanna parkland in southern Texas, USA. We tested the hypotheses that, (a) leaves of plants with evergreen canopies would have longer life spans than those of deciduous species; (b) supplementation of soil moisture would decrease leaf life span in both evergreen and deciduous species; (c) species responses to increased soil moisture availability would be inversely related to leaf longevity; and (d) deciduous growth forms would exhibit a greater growth response to increased soil moisture availability than their evergreen counterparts.A variety of seasonal leaf habits (evergreen, winter-deciduous and summer-deciduous canopies) and leaf life spans (median = 66 to 283 days) were represented by the targeted species, but there was no clear relationship between seasonal leaf habit and leaf longevity. Among species with evergreen canopies, median leaf longevity ranged from short (Zanthoxylum fagara = 116 days; Condalia hookeri = 158 days) to long (Berberis trifoliolata = 283 days) but did not exceed 1 yr. In fact, leaf longevity in evergreen shrubs was often comparable to, or shorter than, that of species with deciduous canopies (Ziziphus obtusifolia = 66 days; Diospyros texana = 119 days; Prosopis glandulosa = 207 days). Augmentation of surface soil moisture had no detectable effect on median leaf life span in any species and there was no clear relationship between leaf longevity and species growth responses to irrigation. Contrary to expectations, species with evergreen canopies responded to irrigation by producing more leaf biomass, longer shoots and more leaf cohorts/year than did deciduous species.Species differences in the annual cycle of leaf initiation, leaf longevity and canopy development, combined with contrasts in root distributions and a highly variable climate, may allow for spatial and temporal partitioning of resources and hence, woody species coexistence and diversity in this system. However, the lack of expected relationships between leaf longevity, leaf habit and plant responses to resource enhancement suggests that structure-function relationships and functional groupings developed in strongly seasonal environments cannot be applied with confidence to these subtropical savannas and thorn woodlands.     

Changes in nitrogen resorption traits of six temperate grassland species along a multi-level N addition gradient

Nitrogen (N) resorption from senescing leaves is an important mechanism of N conservation for terrestrial plant species, but changes in N-resorption traits over wide-range and multi-level N addition gradients have not been well characterized. Here, a 3-year N addition experiment was conducted to determine the effects of N addition on N resorption of six temperate grassland species belonging to three different life-forms: Stipa krylovii Roshev. (grass), Cleistogenes squarrosa (T.) Keng (grass), Artemisia frigida Willd. (semishrub), Melissitus ruthenica C.W.Wang (semishrub and N-fixer), Potentilla acaulis L. (forb) and Allium bidentatum Fisch.ex Prokh. (forb). Generally, N concentrations in green leaves increased asymptotically for all species. N concentrations in senescent leaves for most species (5/6) also increased asymptotically, except that the N concentration in senescent leaves of A. bidentatum was independent of N addition. N-resorption efficiency decreased with increasing N addition level only for S. krylovii and A. frigida, while no clear responses were found for other species. These results suggest that long-term N fertilization increased N uptake and decreased N-resorption proficiency, but the effects on N-resorption efficiency were species-specific for different temperate grassland species in northern China. These inter-specific differences in N resorption may influence the positive feedback between species dominance and N availability and thus soil N cycling in the grassland ecosystem in this region.     

Epiphytic lichen indication of nitrogen deposition and climate in the northern rocky mountains,USA

Lichen bioindication can provide economical and spatially extensive monitoring of climate and pollution impacts on ecological communities. We used non-metric multidimensional scaling of lichen community composition and generalized additive models to analyze regional climate and pollution gradients in the northern Rocky Mountains, U.S. Temperature extremes, relative humidity, and N-deposition were strongly related to lichen community composition. Eutrophic species (genera Physcia, Xanthomendoza, and Xanthoria) were associated with high N deposition, low precipitation, and temperature extremes. Estimated N deposition in our study ranged from <0.5 to 4.26 kg N ha⁻¹ year⁻¹ with degradation to lichen communities observed at 4.0 kg N ha⁻¹ year⁻¹, the indicated critical load. The resulting model can track changes in climate and N pollution related to lichen communities over time, identify probable sensitive or impacted habitats, and provide key information for natural resource management and conservation. The approach is broadly applicable to temperate ecosystems worldwide.     

Local soil characteristics determine the microbial communities under forest understorey plants along a latitudinal gradient

The soil microbial community is essential for maintaining ecosystem functioning and is intimately linked with the plant community. Yet, little is known on how soil microbial communities in the root zone vary at continental scales within plant species. Here we assess the effects of soil chemistry, large-scale environmental conditions (i.e. temperature, precipitation and nitrogen deposition) and forest land-use history on the soil microbial communities (measured by phospholipid fatty acids) in the root zone of four plant species (Geum urbanum, Milium effusum, Poa nemoralis and Stachys sylvatica) in forests along a 1700 km latitudinal gradient in Europe.Soil microbial communities differed significantly among plant species, and soil chemistry was the main determinant of the microbial community composition within each plant species. Influential soil chemical variables for microbial communities were plant species-specific; soil acidity, however, was often an important factor. Large-scale environmental conditions, together with soil chemistry, only explained the microbial community composition in M. effusum and P. nemoralis. Forest land-use history did not affect the soil microbial community composition.Our results underpin the dominant role of soil chemistry in shaping microbial community composition variation within plant species at the continental scale, and provide insights into the composition and functionality of soil microbial communities in forest ecosystems.     

Impacts and indicators of nitrogen deposition in moorlands: Results from a national pollution gradient study

Damage to terrestrial ecosystems from long-term atmospheric nitrogen pollution is a key conservation challenge in many industrialized countries. An important research and management priority is the identification of bioindicators to allow pollution exposure and ecological impacts to be determined at an individual site. We evaluate the impacts of nitrogen (N) deposition and identify methods with bioindication potential across a national-scale pollutant deposition gradient for British heather moorlands. Nitrogen deposition is associated with distinct changes in plant community structure, including reduced bryophyte and vascular plant species richness, and changes in the frequency of many species. Notable results include positive correlation with nitrogen for the invasive bryophyte Campylopus introflexus and negative correlation for the pollution-sensitive Hylocomium splendens and Pleurozium schreberi. Higher nitrogen deposition is associated with increased plant foliar N in a dwarf shrub and a bryophyte, increased extractable litter N, and reduced activity of the enzyme phenol oxidase. Although gradient study results cannot prove causation it is clear that Nitrogen deposition exerts a widespread impact on the ecology and biogeochemistry of heather moorlands. Bioindicators can be used to evaluate exposure and impacts, a promising approach could combine plant species richness and litter nitrogen analyses.     

Relationship between plant species diversity and soil microbial functional diversity along a longitudinal gradient in temperate grasslands of Hulunbeir,Inner Mongolia,China

Numerous experiments have been established to examine the effect of plant diversity on the soil microbial community. However, the relationship between plant diversity and microbial functional diversity along broad spatial gradients at a large scale is still unexplored. In this paper, we examined the relationship of plant species diversity with soil microbial biomass C, microbial catabolic activity, catabolic diversity and catabolic richness along a longitudinal gradient in temperate grasslands of Hulunbeir, Inner Mongolia, China. Preliminary detrended correspondence analysis (DCA) indicated that plant composition showed a significant separation along the axis 1, and axis 1 explained the main portion of variability in the data set. Moreover, DCA-axis 1 was significantly correlated with soil microbial biomass C (r = 0.735, P = 0.001), microbial catabolic activity (average well color development; r = 0.775, P < 0.001) and microbial functional diversity (catabolic diversity: r = 0.791, P < 0.001 and catabolic richness: r = 0.812, P < 0.001), which suggested thatsome relationship existed between plant composition and the soil microbial community along the spatial gradient at a large scale. Soil microbial biomass C, microbial catabolic activity, catabolic diversity and catabolic richness showed a significant, linear increase with greater plant species richness. However, many responses that we observed could be explained by greater aboveground plant biomass associated with higher levels of plant diversity, which suggested that plant diversity impacted the soil microbial community mainly through increases in plant production.     

Plant species responses along a grazing disturbance gradient in Australian grassland

Abstract. Floristic data from paired roadside-paddock analyses from grassland in central Queensland, Australia, were ordinated. The mean direction of the vectors between these pairs was almost perfectly aligned with the indirect gradient represented by the first axis of Non-metric Multi-dimensional Scaling. It confirms anecdotal evidence of a trend from infrequently grazed roadsides to constantly grazed paddocks. The increasing abundance of annual herbs and grasses along this putative gradient is consistent with documented trends from elsewhere in the world. The response patterns of individual species along the disturbance gradient is consistent with ecological theory predicting unimodal peaks in abundance along physical environmental gradients. The ancestral perennial dominants of the grasslands, Dichanthium sericeum and D. queenslandicum, exhibited a declining response to grazing disturbance. Even the generally unpalatable perennial grass Aristida leptopoda declined considerably in the upper segments of the grazing disturbance gradient. A suite of herbaceous trailing legumes had peaks in their abundance near the middle of the grazing disturbance gradient, trends that can be readily explained by the combination of their palatability and intolerance to competition from tall perennial grasses. Several species including the noxious exotic herb Parthenium hysterophorus showed increasing abundance along the grazing disturbance gradient. The methodology may have application as a rapid method of assessing disturbance impacts elsewhere, and is most suited where a management differential between paired plots can be reliably generalized and where the physical environment is relatively monotonous.     

大气氮沉降影响草地植物物种多样性机制研究综述

大气氮沉降对草地生态系统结构和功能的影响已成为全球变化生物学研究重点。大气氮沉降导致草地群落物种多样性降低已成为全球普遍现象,但其生物学机制还不清楚,因此有必要系统梳理大气氮沉降对全球不同草地生态系统的研究结果,以便在氮沉降背景下为我国草地生态系统的研究和管理制定科学决策。系统综述了氮沉降降低草地群落物种多样性的可能机制,主要包括资源竞争排斥、群落更新限制、土壤酸化及其离子毒害、养分失衡、氮素本身的毒害、次生胁迫。氮沉降导致草地物种多样性降低是多种机制综合作用的结果,每种机制在不同时空具有不同的相对贡献。同时,与欧洲酸性土壤草地和美国高草草原相比,我国草地土壤类型和植被属性具有明显差异。因此,应根据我国草地生态系统的特征、不同植物功能利用养分策略,从土壤养分变化、根系养分吸收转运、叶片生理过程等方面的整合研究思路,探讨氮沉降影响我国草地群落物种多样性的生物学机制,为我国草地生态系统的科学管理提供理论依据。     

降水量变化对藏北高寒草地养分限制的影响

生态系统净初级生产养分限制的模式是现代生态学关注的重要问题。养分的可利用性是草原生态系统生产力动态变化的关键决定因素, 但土壤养分可利用性与整个生态系统中养分限制之间的关系尚不清楚。该研究通过在藏北降水梯度上4种类型高寒草地(从东到西依次是高寒草甸、高寒草甸草原、高寒草原和高寒荒漠草原)设置氮磷养分添加试验, 系统研究氮磷养分添加对不同类型高寒草地的影响, 并探讨降水梯度上高寒草地的氮磷限制模式。结果表明: (1)氮磷添加对不同高寒草地的影响存在差异: 氮添加显著提高了高寒草甸和高寒草甸草原地上生产力, 而对高寒草原和高寒荒漠草原无影响; 单独磷添加对4种高寒草地均无显著影响, 而氮磷添加对4种高寒草地地上生产力均有促进作用。(2)通过计算氮磷共同限制指数发现: 随着降水量减少, 高寒草地氮限制指数从1.18逐渐降低到0.52-0.64, 养分限制模式从氮限制过渡到氮磷共同限制; 磷限制指数在高寒草甸草原和高寒草原为负值, 说明单独磷添加对高寒草甸的生产力有负向作用, 高寒草甸主要受氮限制; 高寒草甸草原介于氮限制与氮磷限制之间, 受到氮磷共同限制, 单独磷添加有负向作用; 高寒荒漠草原受到氮磷共同限制。研究表明, 高寒草地氮磷限制模式存在环境梯度上的递变规律, 随着降水量减少, 高寒草地养分限制模式从氮限制逐渐过渡到氮磷共同限制。由此推断, 未来气候变化条件下氮沉降增加对不同类型高寒草地的影响可能存在差异。同时, 利用养分添加恢复不同类型退化高寒草地时也应将氮磷限制模式的差异考虑进去。     

不同降水梯度下草地生态系统地表能量交换

通过对不同降水梯度下的蒙古中部针茅草原(KBU)、内蒙古羊草草原(NM)、海北高寒灌丛草甸(HB)和当雄高寒草甸草原(DX)4个草地生态系统的能量通量连续4-5 a的测定,分析了影响青藏高原和蒙古高原草地生态系统生长季中地表能量交换的主要因素。研究表明:相对于KBU、NM和DX,HB高寒灌丛草甸NDVI(0.58)和土壤含水量(28.3%)最大,因而地表短波反射率(αk)最低(0.12),从而获得了最大的净辐射(Rn)。KBU、NM和DX 3个草地生态系统生长季中αk随着植被的生长而降低,在生长季末期,随着植被的凋落而增加;HB的αk季节变化趋势与其它生态系统相反。从蒙古高原(KBU和NM)到青藏高原(HB和DX),随着降水量的增加,波文比(β)逐渐减小(2.25-0.53),即生态系统与大气的能量交换从显热(H)占主导转变为潜热(LE)占主导。植被状况对草地生态系统与大气之间能量交换的季节动态有重要的调控作用,在NDVI较低的时候,4个生态系统H/Rn都大于LE/Rn,LE/Rn随着NDVI的增加而增加,而H/Rn呈现出与LE/Rn相反的季节变化趋势。     

Determining the important environmental variables controlling plant species community composition in mesotrophic grasslands in Great Britain

Question: What is the relative importance of local site effects and selected important environmental variables in determining plant species composition? How do species respond to these environmental variables? Location: Ten mesotrophic grassland sites of high conservation value in southern England. Methods: Species cover was assessed in between 10 and 25 randomly selected 1‐m² quadrats at each site. At each quadrat degree of waterlogging (W), soil Olsen extractable phosphorus (P) and soil pH were measured. Variation partitioning was used to separate site and soil effects, and HOF (Huisman, Olff & Fresco) modelling was used to produce response curves for the major species on soil gradients, based on coenoclines derived from partial canonical correspondence analysis (pCCA). Results: Variation partitioning identified Site as the most important environmental variable (34.6%). Only 18.7% was accounted for by the three soil variables together; W (degree of waterlogging), P and pH accounted for 11.1%, 5.7% and 4.3%, respectively in raw form with 2.4% shared. However, when Site and the other soil variables were removed the variation explained reduced to 2.3% for W, 1.1% for P and 1.0% for pH. The species responses to each of these soil environmental factors could be separated into four types on each gradient. Most species were abundant at low W, low soil P and intermediate pH. Conclusions: Site‐based factors were more important than the three soil variables, which were assumed to be directly or indirectly associated with productivity. This implies that each site has unique properties that are more important than the soil variables. The three soil factors were, however, significant and the groups of the most common species, based on significant response curves, can be used as a first approximation of indicators of environmental conditions in British mesotrophic grasslands for conservation. However, W accounted for most variation, and the current reliance on soil available P and soil pH for assessing conservation/restoration potential should be viewed with caution.     

Invertebrate herbivory along a gradient of plant species diversity in extensively managed grasslands

Increasing plant diversity has long been hypothesized to negatively affect levels of invertebrate herbivory due to a lower number of specialist insect herbivores in more diverse sites, but studies of natural systems have been rare. We used a planned comparison to study herbivory in a set of 19 semi-natural montane grasslands managed as hay meadows. Herbivory was measured in transects through the plant communities, and in individuals of Plantago lanceolata and Trifolium pratense that were transplanted into each meadow. In addition, plant community biomass and arthropod abundances were determined in the grasslands. Before the first mowing in June, mean herbivory levels correlated negatively with plant species richness, as predicted by theory, but they were also significantly affected by plant community biomass and plant community composition. After mowing, herbivory levels were only significantly related to plant community composition. Damage levels in the transplants were lower than herbivory levels in the established plant communities. Most insect herbivores were generalists and not specialists. The number of insect herbivores and spiders were positively correlated and tended to increase with increasing plant species richness. Herbivory levels were correlated negatively with spider abundances. We conclude that while the predicted negative relationship between plant species richness and insect herbivory can be found in grasslands, the underlying mechanism involves generalist rather than specialist herbivores. Our data also suggest a role of natural enemies in generalist herbivore activities.Electronic Supplementary Material Supplementary material is available to authorised users in the online version of this article at .     

Changes in vegetation and soil characteristics in coastal sand dunes along a gradient of atmospheric nitrogen deposition

A field survey was conducted to detect signals of atmospheric nitrogen (N) in 11 dune systems along a nitrogen deposition gradient in the United Kingdom. In the mobile and semi-fixed dunes, above-ground biomass was positively related to N inputs. This increase was largely due to increased height and cover of Ammophila arenaria. In the long term, this increased biomass may lead to increased organic matter accumulation and consequently accelerated soil development. In the fixed dunes, above ground biomass also showed a positive relationship with N inputs as did soil C : N ratio while soil available N was negatively related to N inputs. Plant species richness was negatively related to N inputs. In the dune slacks, while soil and bulk vegetation parameters showed no relationship with N inputs, cover of Carex arenaria and Hypochaeris radicata increased. Site mean Ellenberg N numbers showed no relationship with N deposition either within habitats or across the whole dataset. Neither abundance-weighting nor inclusion of the Siebel numbers for bryophytes improved the relationship. The survey reveals that the relationships of soil and vegetation with atmospheric N deposition vary between sand dune habitats but, despite this variability, clear correlations with N inputs exist. While this survey cannot establish causality, on the basis of the relationships observed we suggest a critical load range of 10 - 20 kg N ha(-1) yr(-1) for coastal sand dunes in the UK.     

Changes in soil carbon and nitrogen cycling along a 72-year wildfire chronosequence in Michigan jack pine forests

We investigated the changes in soil processes following wildfire in Michigan jack pine (Pinus banksiana) forests using a chronosequence of 11 wildfire-regenerated stands spanning 72 years. The objective of this study was to characterize patterns of soil nutrients, soil respiration and N mineralization with stand development, as well as to determine the mechanisms driving those patterns. We measured in situ N mineralization and soil respiration monthly during the 2002 growing season and used multiple regression analysis to determine the important factors controlling these processes. Growing-season soil respiration rates ranged from a low of 156 g C/m² in the 7-year-old stand to a high of 254 g C/m² in the 22-year-old stand, but exhibited no clear pattern with stand age. In general, soil respiration rates peaked during the months of July and August when soil temperatures were highest. We used a modified gamma function to model a temporal trend in total N mineralization (total N mineralization = 1.853−0.276 × age × e ^{−0.814 × age}; R ² = 0.381; P = 0.002). Total N mineralization decreased from 2.8 g N/m² in the 1-year-old stand to a minimum value of 0.5 g N/m² in the 14-year-old stand, and then increased to about 1.5 g N/m² in mature stands. Changes in total N mineralization were driven by a transient spike in N turnover in the mineral soil immediately after wildfire, followed by a gradual accrual of a slow-cycling pool of N in surface organic horizons as stands matured. Thus, in Michigan jack pine forests, the accumulation of surface organic matter appears to regulate N availability following stand-replacing wildfire.