Effects of grazing on patch structure in a semi‐arid two‐phase vegetation mosaic

Abstract. Question: What are the grazing effects in the spatial organization and the internal structure of high and low cover patches from a two‐phase vegetation mosaic? Location: Patagonian steppe, Argentina. Methods: We mapped vegetation under three different grazing conditions: ungrazed, lightly grazed and heavily grazed. We analysed the spatial patterns of the dominant life forms. Also, in each patch type, we determined density, species composition, richness, diversity, size structure and dead biomass of grasses under different grazing conditions. Results: The vegetation was spatially organized in a two‐phase mosaic. High cover patches resulted from the association of grasses and shrubs and low cover patches were represented by scattered tussock grasses on bare ground. This spatial organization was not affected by grazing, but heavy grazing changed the grass species involved in high cover patches and reduced the density and cover of grasses in both patch types. Species richness and diversity in high cover patches decreased under grazing conditions, whereas in low cover patches it remained unchanged. Also, the decrease of palatable grasses was steeper in high cover patches than in low cover patches under grazing conditions. Conclusions: We suggest that although grazing promotes or inhibits particular species, it does not modify the mosaic structure of Patagonian steppe. The fact that the mosaic remained unchanged after 100 years of grazing suggests that grazing does not compromize population processes involved in maintaining patch structure, including seed dispersal, establishment or biotic interactions among life forms.     

Size traits and site conditions determine changes in seed bank structure caused by grazing exclusion in semiarid annual plant communities

1. Contrasting patterns of change in the seed bank of natural grasslands are frequently found in response to grazing by domestic herbivores. Here, we studied the hypotheses that a) patterns of change in seed bank density and composition in response to grazing depend on spatial variation in resource availability and productivity, and b) that variation among species in patterns of seed bank response to grazing is linked to differences in species size traits (i.e. size of plant, dispersal unit and seed). 2. Effects of sheep grazing exclusion on the seed bank were followed during five years in a semiarid Mediterranean annual plant community in Israel. Seed bank density and composition were measured in autumn, before the rainy season, inside and outside fenced exclosures in four neighboring topographic sites differing in vegetation characteristics, soil resources and primary productivity: Wadi (dry stream terraces, high productive site), Hilltop, South‐ and North‐facing slopes (less productive sites). 3. Topographic sites differed in seed density (range ca 2500–18000 seed m^?2) and in seed bank response to grazing exclusion. Fencing increased seed density by 78, 51 and 18% in the Wadi, South‐ and North‐facing slopes, respectively, but had no effect in the Hilltop. At the species level, grazing exclusion interacted with site conditions in determining species seed bank density, with larger or opposite changes in the high productive Wadi compared to the other less productive sites. 4. Changes in seed bank structure after grazing exclusion were strongly related to species size traits. Grazing exclusion favored species with large size traits in all sites, while seed density of tiny species decreased strongly in the high productive Wadi. Species with medium and small size traits showed lesser or no responses. 5. The size of plants, dispersal units and seeds were strongly correlated to each other, thus confounding the evaluation of the relative importance of each trait in the response of species to grazing and site conditions. We propose that the relative importance of plant size vs seed size in the response to grazing changes with productivity level.     

Regional productivity mediates the effects of grazing disturbance on plant cover and patch‐size distribution in arid and semi‐arid communities

Patch‐size distribution and plant cover are strongly associated to arid ecosystem functioning and may be a warning signal for the onset of desertification under changes in disturbance regimes. However, the interaction between regional productivity level and human‐induced disturbance regime as drivers for vegetation structure and dynamics remain poorly studied. We studied grazing disturbance effects on plant cover and patchiness in three plant communities located along a regional productivity gradient in Patagonia (Argentina): a semi‐desert (low‐productivity community), a shrub‐grass steppe (intermediate‐productivity community) and a grass steppe (high‐productivity community). We sampled paddocks with different sheep grazing pressure (continuous disturbance gradients) in all three communities. In each paddock, the presence or absence of perennial vegetation was recorded every 10 cm along a 50 m transect. Grazing effects on vegetation structure depended on the community and its association to the regional productivity. Grazing decreased total plant cover while increasing both the frequency of small patches and the inter‐patch distance in all communities. However, the size of these effects was the greatest in the high‐productivity community. Dominant species responses to grazing explained vegetation patch‐ and inter‐patch‐size distribution patterns. As productivity decreases, dominant species showed a higher degree of grazing resistance, probably because traits of species adapted to high aridity allow them to resist herbivore disturbance. In conclusion, our findings suggest that regional productivity mediates grazing disturbance impacts on vegetation mosaic. The changes within the same range of grazing pressure have higher effects on communities found in environments with higher productivity, markedly promoting their desertification. Understanding the complex interactions between environmental aridity and human‐induced disturbances is a key aspect for maintaining patchiness structure and dynamics, which has important implications for drylands management.     

Grazing effects on intraspecific trait variability vary with changing precipitation patterns in Mongolian rangelands

Functional traits are proxies for plant physiology and performance, which do not only differ between species but also within species. In this work, we hypothesized that (a) with increasing precipitation, the percentage of focal species which significantly respond to changes in grazing intensity increases, while under dry conditions, climate‐induced stress is so high that plant species hardly respond to any changes in grazing intensity and that (b) the magnitude with which species change their trait values in response to grazing, reflected by coefficients of variation (CVs), increases with increasing precipitation. Chosen plant traits were canopy height, plant width, specific leaf area (SLA), chlorophyll fluorescence, performance index, stomatal pore area index (SPI), and individual aboveground biomass of 15 species along a precipitation gradient with different grazing intensities in Mongolian rangelands. We used linear models for each trait to assess whether the percentage of species that respond to grazing changes along the precipitation gradient. To test the second hypothesis, we assessed the magnitude of intraspecific trait variability (ITV) response to grazing, per species, trait, and precipitation level by calculating CVs across the different grazing intensities. ITV was most prominent for SLA and SPI under highest precipitation, confirming our first hypothesis. Accordingly, CVs of canopy height, SPI, and SLA increased with increasing precipitation, partly confirming our second hypothesis. CVs of the species over all traits increased with increasing precipitation only for three species. This study shows that it remains challenging to predict how plant performance will shift under changing environmental conditions based on their traits alone. In this context, the implications for the use of community‐weighted mean trait values are discussed, as not only species abundances change in response to changing environmental conditions, but also values of traits considerably change. Including this aspect in further studies will improve our understanding of processes acting within and among communities.     

Effects of grazing intensity in grasslands of the Espinal of central Chile

Question: How is grazing intensity associated with species and morpho‐functional traits (MFTs) composition, productivity and richness of annual dominated grasslands? Have native and exotic species similar associations to this gradient? Location: Anthropogenic grassland in the Espinal vegetation in the sub‐humid area of the mediterranean type climate region of Chile (35°58’ S, 72°17’ W). Methods: Data were obtained from a long‐term (eight years) experiment with six stocking rates (1 to 3.5 sheep/ha). Detrended Correspondence Analysis (DCA) and regression analysis were used to determinate the relationship between grazing intensity and biomass, richness, abundance and traits of the species. Results: The first DCA axis was related to grazing intensity and explained most of the floristic variation (69.3%); the abundance of some non‐native species, e.g. Vulpia megalura were highly correlated with this axis. In the DCA for MFTs the first axis explained 87% of the variance and was also related to grazing intensity; the abundance of small size plants and shallow roots increased with grazing intensity. The relative abundance of grasses and composites, but not of legumes, changed with stocking rate: as grazing intensity increased composites became the predominant species to the detriment of grasses. The above‐ground biomass measured in exclusion cages declined with increasing grazing pressure. The richness of exotic species was greater compared to native ones at low stocking rates, but they converge to similar values at higher stocking rates. However, the relative abundance of exotic species was greater than 75% in all stocking rates. Conclusions: Grazing intensification has large effects in the structure of grassland in central Chile. With grazing intensities greater than 1 sheep/ha species characteristics change; evolving in a few years (6–8) towards a similar community regardless of the stocking rate. The overgrazed community has more native than exotic species richness, possibly due to greater defence traits against herbivory of this group of species.     

Floristic patterns and plant traits of Mediterranean communities in fragmented habitats

Aim To contrast floristic spatial patterns and the importance of habitat fragmentation in two plant communities (grassland and scrubland) in the context of ecological succession. We ask whether plant assemblages are affected by habitat fragmentation and, if so, at what spatial scale? Does the relative importance of the niche differentiation and dispersal‐limitation mechanisms change throughout secondary succession? Is the dispersal‐limitation mechanism related to plant functional traits? Location A Mediterranean region, the massif of Albera (Spain). Methods Using a SPOT satellite image to describe the landscape, we tested the effect of habitat fragmentation on species composition, determining the spatial scale of the assemblage response. We then assessed the relative importance of dispersal‐related factors (habitat fragmentation and geographical distance) and environmental constraints (climate‐related variables) influencing species similarity. We tested the association between dispersal‐related factors and plant traits (dispersal mode and life form). Results In both community types, plant composition was partially affected by the surrounding vegetation. In scrublands, animal‐dispersed and woody plants were abundant in landscapes dominated by closed forests, whereas wind‐dispersed annual herbs were poorly represented in those landscapes. Scrubby assemblages were more dependent on geographical distance, habitat fragmentation and climate conditions (temperature, rainfall and solar radiation); grasslands were described only by habitat fragmentation and rainfall. Plant traits did not explain variation in spatial structuring of assemblages. Main conclusions Plant establishment in early Mediterranean communities may be driven primarily by migration from neighbouring established communities, whereas the importance of habitat specialization and community drift increases over time. Plant life forms and dispersal modes did not explain the spatial variation of species distribution, but species richness within the community with differing plant traits was affected by habitat patchiness.     

Variations in resistance to canopy disturbances and their interactions with the spatial structure of major species in a cool‐temperate forest

Question: How does typhoon‐related disturbance (more specifically, disturbance in the understorey due to tree‐fall and branch‐fall) affect different species mortality rates in a vertically well‐structured forest community? Location: Cool‐temperate, old‐growth forest in the Daisen Forest Reserve, Japan. Methods: We investigated the canopy dynamics and mortality rate trends of trees ≥5 cm diameter at breast height in a 4‐ha study plot, and analysed the effects of tree diameter and spatial structure on the mortality risks for major tree species in the understorey. Results: Significant differences were found in the mortality rates and proportions of injured dead stems between census periods, which were more pronounced in the understorey than in the canopy. Acer micranthum, which showed increased mortality during typhoon disturbance periods, had a clumped distribution. In contrast, Acer japonicum and Viburnum furcatum, which showed similar mortality rates between census periods, had a loosely clumped spatial distribution and a negative association with canopy trees, respectively. In the understorey stems of Acanthopanax sciadophylloides and Fagus crenata, whose spatial distribution patterns depended on canopy gaps, significant increases in mortality rates were observed only during severe typhoon‐related disturbance periods. Conclusions: The sensitivity of trees to typhoon‐related canopy disturbance is more pronounced in the lower layers of vertically structured forest communities. Differences in mortality patterns generated through the combined effects of spatial variation in disturbance regime and species‐specific spatial distribution patterns (spatial aggregation, association with canopy trees, and canopy gap dependency) contribute to the co‐existence of understorey species in forest communities that are subject to typhoon‐related disturbance.     

Grazing as a factor structuring grasslands in the Pyrenees

Questions: What are the relative roles of abiotic and grazing management factors on plant community distribution in landscapes? How are livestock type and stocking rate related to changes in vegetation structure and composition? Location: Sub‐alpine grasslands in the central and eastern Pyrenees. Methods: Multivariate analysis and variance partitioning methods were used to evaluate the relative roles of environmental factors in structuring vegetation composition and diversity patterns in three surveys on differently managed grasslands. Results: Vegetation composition within a region was affected by environmental factors hierarchically, changing first according to abiotic factors and then to grazing management. At landscape scales, abiotic factors explained two‐fold more variation in vegetation composition than grazing factors. Within landscape units, cattle grazing increased vegetation heterogeneity at landscape and patch scales, while sheep grazing favoured the presence of a specific set of species with high conservation value. Species composition was highly responsive to management variables compared to diversity components. Conclusions: The combination of sheep and cattle grazing at various stocking rates is an effective tool to preserve the diversity of plant species and communities within a region with a long tradition of livestock management, through the scaling up of effects by local processes occurring in patches at smaller scales.     

A community‐level test of the leaf‐height‐seed ecology strategy scheme in relation to grazing conditions

Question: Is the assumption of trait independence implied in Westoby's (1998) leaf‐height‐seed (LHS) ecology strategy scheme upheld in a Mediterranean grazing system dominated by annuals? Is the LHS approach applicable at the community level? Location: Northern Israel. Methods: LHS traits (specific leaf area [SLA], plant height and seed mass), and additional leaf traits (leaf dry matter content [LDMC], leaf area, and leaf content of nitrogen [LNC], carbon [LCC], and phosphorus [LPC]), were analyzed at the species and community levels. Treatments included manipulations of grazing intensity (moderate and heavy) and protection from grazing. We focused on species comprising 80% of biomass over all treatments, assuming that these species drive trait relationships and ecosystem processes. Results: At the species level, SLA and seed mass were negatively correlated, and plant height was positively correlated to LCC. SLA, seed mass, and LPC increased with protection from grazing. At the community level, redundancy analysis revealed one principal gradient of variation: SLA, correlated to grazing, versus seed mass and plant height, associated with protection from grazing. We divided community functional parameters into two groups according to grazing response: (1) plant height, seed mass, LDMC, and LCC, associated with protection from grazing, and (2) SLA, associated with grazing. Conclusions: The assumption of independence between LHS traits was not upheld at the species level in this Mediterranean grazing system. At the community level, the LHS approach captured most of the variation associated with protection from grazing, reflecting changes in dominance within the plant community.     

Groupings of life‐history traits are associated with distribution of forest plant species in a fragmented landscape

Questions: 1. Do relationships among forest plant traits correspond to dispersability‐persistence trade‐offs or other inter‐trait correlations found in the literature? 2. Do species groups delineated by trait similarity, differ in occurrence in ancient vs. new forests or isolated vs more continuous forest patches? 3. Are these patterns consistent for different forest types? Location: Central Belgium, near Leuven. Methods: We investigate the distributions of a large set of plant traits and combinations among all forest species occurring in patches with varying forest continuity and isolation. Through calculation of Gower's similarity index and subsequent clustering,‘emergent’ species groups are delineated. Then, the relative occurrence of these different groups in forest patches of different age and size, sustaining different forest types (alluvial vs. Quercion), and having different isolation status is compared through multivariate GLM analysis. Results: Correlations among several life history traits point towards trade‐offs of dispersability and fecundity vs. longevity. We distinguished three species groups: 1= mainly shrubs or climbers with fleshy or wind dispersed fruits and high dispersal potential; 2 = dominated by small, mainly vegetatively reproducing herbs; 3 = with spring flowering herbs with large seeds and mainly unassisted dispersal. Relative occurrence of these groups was significantly affected by forest age, area, isolation and forest type. Separate analyses for alluvial and Quercion forests indicated that the relative importance of these factors may differ, depending on forest type and species group. Both forest continuity and isolation are important in restricting the relative occurrence of forest species in alluvial forests, whatever their group membership. In Quercion forests forest patch area was the primary determinant of relative occurrence of species groups. Conclusions: It is very important to preserve the actual forest area including the spatial setting and the dispersal infrastructure within the landscape. Next, forest connectivity may be restored, but it is inherently a long process.     

Nested patterns of community assembly in the colonisation of artificial canopy habitats by oribatid mites

An observed species–area relationship (SAR) in assemblages of oribatid mites inhabiting natural canopy habitats (suspended soils) led to an experimental investigation of how patch size, height in canopy and moisture influence the species richness, abundance and community composition of arboreal oribatid mites. Colonisation by oribatid mites on 90 artificial canopy habitats (ACHs) of three sizes placed at each of three heights on the trunks of ten western redcedar trees was recorded over a 1‐year period. Fifty‐nine oribatid mite species colonised the ACHs, and richness increased with the moisture content and size of the habitat patch. Oribatid mite species richness and abundance, and ACH moisture content decreased with increasing ACH height in the canopy. Patterns in the species richness and community composition of ACHs were non‐random and demonstrated a significant nested pattern. Correlations of patch size, canopy height and moisture content with community nestedness suggest that species‐specific environmental tolerances combined with the differential dispersal abilities of species contributed to the non‐random patterns of composition in these habitats. In line with the prediction that niche‐selection filters out species from the regional pool that cannot tolerate environmental harshness, moisture‐stressed ACHs in the high canopy had lower community variability than ACHs in the lower canopy. Colonising source pools to ACHs were almost exclusively naturally‐occurring canopy sources, but low levels of colonisation from the forest floor were apparent at low heights within the ACH system. We conclude that stochastic dispersal dynamics within the canopy are crucial to understanding oribatid mite community structure in suspended soils, but that the relative importance of stochastic dispersal assembly may be dependent on a strong deterministic element to the environmental tolerances of individual species which drives non‐random patterns of community assembly.     

Spatial heterogeneity and plant species richness at different spatial scales under rabbit grazing

Herbivores influence spatial heterogeneity in soil resources and vegetation in ecosystems. Despite increasing recognition that spatial heterogeneity can drive species richness at different spatial scales, few studies have quantified the effect of grazing on spatial heterogeneity and species richness simultaneously. Here we document both these variables in a rabbit-grazed grassland. We measured mean values and spatial patterns of grazing intensity, rabbit droppings, plant height, plant biomass, soil water content, ammonia and nitrate in sites grazed by rabbits and in matched, ungrazed exclosures in a grassland in southern England. Plant species richness was recorded at spatial scales ranging between 0.0001 and 150 m(2). Grazing reduced plant height and plant biomass but increased levels of ammonia and nitrate in the soil. Spatial statistics revealed that rabbit-grazed sites consisted of a mixture of heavily grazed patches with low vegetation and nutrient-rich soils (lawns) surrounded by patches of high vegetation with nutrient-poor soils (tussocks). The mean patch size (range) in the grazed controls was 2.1 +/- 0.3 m for vegetation height, 3.8 +/- 1.8 m for soil water content and 2.8 +/- 0.9 m for ammonia. This is in line with the patch sizes of grazing (2.4 +/- 0.5 m) and dropping deposition (3.7 +/- 0.6 m) by rabbits. In contrast, patchiness in the ungrazed exclosures had a larger patch size and was not present for all variables. Rabbit grazing increased plant species richness at all spatial scales. Species richness was negatively correlated with plant height, but positively correlated to the coefficient of variation of plant height at all plot sizes. Species richness in large plots (<25 m(2)) was also correlated to patch size. This study indicates that the abundance of strong competitors and the nutrient availability in the soil, as well as the heterogeneity and spatial pattern of these factors may influence species richness, but the importance of these factors can differ across spatial scales.     

Grazing effects on the species‐area relationship: Variation along a climatic gradient in NE Spain

Questions: Does grazing have the same effect on plant species richness at different spatial scales? Does the effect of spatial scale vary under different climatic conditions and vegetation types? Does the slope of the species‐area curve change with grazing intensity similarly under different climatic conditions and vegetation types? Location: Pastures along a climatic gradient in northeastern Spain. Methods: In zones under different regimes of sheep grazing (high‐, low‐pressure, abandonment), plant species richness was measured in different plot sizes (from 0.01 to 100 m2) and the slope of the species‐area curves was calculated. The study was replicated in five different locations along a climatic gradient from lowland semi‐arid rangelands to upland moist grasslands. Results: Species richness tended to increase with grazing intensity at all spatial scales in the moist upland locations. On the contrary, in the most arid locations, richness tended to decrease, or remain unchanged, with grazing due to increased bare soil. Grazing differentially affected the slope (z) of the species‐area curve (power function S=c A^z) in different climatic conditions: z tended to increase with grazing in arid areas and decrease in moist‐upland ones. ß‐diversity followed similar pattern as z. Conclusions: Results confirm that the impact of grazing on plant species richness are spatial‐scale dependent. However, the effects on the species‐area relationship vary under different climatic conditions. This offers a novel insight on the patterns behind the different effects of grazing on diversity in moist vs. arid conditions reported in the literature. It is argued that the effect of spatial scale varies because of the different interaction between grazing and the intrinsic spatial structure of the vegetation. Variations in species‐area curves with grazing along moisture gradients suggest also a different balance of spatial components of diversity (i.e. a‐ and ß‐diversity).     

Differences in species richness and life-history traits between grazed and abandoned grasslands in southern Sweden

Disturbance has profound effects on plant community composition. This paper deals with the influence of grazing on species richness and proportions of life‐history attributes of grassland vegetation at six spatial scales (0.001–1000 m²) in two provinces of southern Sweden. The study comprised 33 dry grassland sites, including 22 grazed and 11 abandoned localities, and 28 sites of coastal brackish meadows, divided into five management types (from “heavily grazed” to “abandoned since long time”).
In general grazed sites were species‐richer than abandoned sites, especially at small plot sizes. However, there was a steeper increase in species number towards larger plot sizes in the abandoned sites. Heavy grazing in the coastal meadows resulted in a comparatively low number of species, corroborating the intermediate disturbance hypothesis.
The analysis of life‐history traits indicated the importance of taxonomic group, canopy structure, height, regenerative strategy and, in particular, life form. Leaf anatomy and seed dispersal seemed to be less important. The responses to grazing as regards species traits differed somewhat between grassland types. Grazed sites generally had high proportions of legumes, therophytes, species with basal position of leaves and with regeneration by means of a persistent seed bank. Abandonment of grazing favoured monocots, geophytes, species with vegetative regeneration and (partly) leafy canopy structure. Some differences between grazed and abandoned sites were confined to either the smallest or largest plot sizes, indicating different responses of matrix and interstitial species. Various positive associations (attribute syndromes) or negative associations between individual traits were identified. There was, for example, a positive link between the attributes “geophytes” and “ability of vegetative regeneration”. The recognition of such links is important to avoid misinterpreting certain attributes as functional adaptations to grazing while they are only positively correlated to other attributes of larger significance.     

Patterns of plant species turnover along grazing gradients

Questions: How are plant species distributed along grazing gradients? What is the shape of species richness patterns? How can we test for the existence of potential discontinuities in species turnover pattern? Location: Semi‐deserts in the eastern Caucasus, Azerbaijan, Gobustan district. Methods: We studied the distribution of vascular plant species along transects 900‐m long, perpendicular to five farms, and estimated grazing intensity as current livestock units per distance. We modelled species response curves with Huismann–Olff–Fresco (HOF) models and calculated species turnover by accumulating the first derivatives of all response curves. To test for potential discontinuities in changes of vegetation composition along the grazing gradient, we introduce a new null model based on the individualistic continuum concept that uses permutations of the observed pattern of species responses. Results: Most species show a sigmoidal negative response to grazing intensity, while a few species respond with a unimodal pattern. The monotonic decrease in species richness with increasing grazing intensity marks a process of overgrazing that leads to the complete extirpation of plant species. Although the species turnover pattern shows a clear peak, it does not deviate significantly from the null model of individualistic continuous changes. Conclusions: Our approach offers a method for differentiating between transition zones and continuous shifts in species composition along ecological gradients. It also provides a valuable tool for rangeland management to test state‐and‐transition concepts and gives deeper insights into ecological processes affected by grazing.     

Convergent elevation trends in canopy chemical traits of tropical forests

The functional biogeography of tropical forests is expressed in foliar chemicals that are key physiologically based predictors of plant adaptation to changing environmental conditions including climate. However, understanding the degree to which environmental filters sort the canopy chemical characteristics of forest canopies remains a challenge. Here, we report on the elevation and soil‐type dependence of forest canopy chemistry among 75 compositionally and environmentally distinct forests in nine regions, with a total of 7819 individual trees representing 3246 species collected, identified and assayed for foliar traits. We assessed whether there are consistent relationships between canopy chemical traits and both elevation and soil type, and evaluated the general role of phylogeny in mediating patterns of canopy traits within and across communities. Chemical trait variation and partitioning suggested a general model based on four interconnected findings. First, geographic variation at the soil‐Order level, expressing broad changes in fertility, underpins major shifts in foliar phosphorus (P) and calcium (Ca). Second, elevation‐dependent shifts in average community leaf dry mass per area (LMA), chlorophyll, and carbon allocation (including nonstructural carbohydrates) are most strongly correlated with changes in foliar Ca. Third, chemical diversity within communities is driven by differences between species rather than by plasticity within species. Finally, elevation‐ and soil‐dependent changes in N, LMA and leaf carbon allocation are mediated by canopy compositional turnover, whereas foliar P and Ca are driven more by changes in site conditions than by phylogeny. Our findings have broad implications for understanding the global ecology of humid tropical forests, and their functional responses to changing climate.     

放牧对内蒙古锡林河流域草地群落植物茎叶生物量资源分配的影响

以内蒙古锡林河流域沿水分梯度分布的灰脉苔草(Carex appendiculata)、贝加尔针茅(Stipa baicalensis)、羊草(Leymus chinensis)、大针茅(Stipa grandis)、小叶锦鸡儿(Caragana microphylla)和冷蒿(Artemisia frigida) 6个草地群落为对象, 研究了围封禁牧与放牧样地中144个共有植物种的高度、丛幅面积、茎、叶和株(丛)生物量、茎叶比等性状。结果表明: 1)在个体水平上, 放牧样地中植物的生殖枝高度、营养枝高度、丛幅面积、单株(丛)生物量、茎、叶生物量和茎叶比均显著低于围封禁牧样地, 植物在放牧干扰下表现出明显的小型化现象; 2)在群落水平上, 放牧亦显著降低了群落总生物量和茎、叶生物量; 3)过度放牧显著改变了物种的资源分配策略, 使生物量向叶的分配比例增加, 向茎的分配比例减少。资源优先向同化器官分配可能是植物对长期放牧干扰的一种重要适应对策; 4)轻度放牧对物种的资源分配没有显著影响, 单株(丛)生物量和群落茎、叶及总生物量均表现出增加趋势, 这与过度放牧的影响正好相反。过度放牧引起的植物个体小型化改变了生态系统中物种的资源分配策略, 进而对生态系统功能产生重要的影响。     

Subsidies mediate interactions between communities across space

Most spatial ecology focuses on how species dispersal affects community dynamics and coexistence. Ecosystems, however, are also commonly connected by flows of resources. We experimentally tested how neighbouring communities indirectly influence each other in absence of dispersal, via resource exchanges. Using two‐patch microcosm meta‐ecosystems, we manipulated community composition and dynamics, by varying separately species key functional traits (autotroph versus heterotroph species and size of consumer species) and trophic structure of aquatic communities (species growing alone or in presence of competitors or predators). We then analysed the effects of species functional traits and trophic structure on communities connected through spatial subsidies in the absence of actual dispersal. Both functional traits and trophic structure strongly affected dynamics across neighbouring communities. Heterotroph communities connected to autotroph neighbours developed better than with heterotroph neighbours, such that coexistence of competitors was determined by the functional traits of the neighbouring community. Densities in autotroph communities were also strikingly higher when receiving subsidies from heterotroph communities compared to their own subsidies when grown in isolated ecosystems. In contrast, communities connected to predator‐dominated ecosystems collapsed, without any direct contact with the predators. Our results demonstrate that because community composition and structure modify the distribution of biomass within a community, they may also affect communities connected through subsidies through quantitative and qualitative changes of detritus flows. This stresses that ecosystem management should account for such interdependencies mediated by spatial subsidies, given that local community alterations cascade across space onto other ecosystems even if species dispersal is completely absent.     

Grazing response groups among understorey plants in arid rangelands

Abstract. We analysed attributes of the understorey flora in different plant communities along two grazing gradients in arid Australian rangelands. We aimed to determine if there were patterns among species in response to grazing, and easily recognizable ‘indicator response types’ for monitoring grazing-induced change in community composition. Measurements were stratified by vegetation patch type, in woody groves and open patches. Trait selection and analyses followed a hierarchical approach which searched for patterns within major plant life forms. Patch type exerted a dominant influence on both life forms and species attributes, but interacted with grazing. Grazing was associated with loss in differentiation of species composition between patch types, rather than loss in numbers of species overall. Heavy grazing was variously associated with small size, prostrate habit, low meristems, small leaves, coated leaves, high regrowth potential, plasticity in response to grazing, and high fecundity; and light grazing with opposing attributes. Many attributes tended to vary independently of each other and grazing-related attribute syndromes were recognisable only among grasses. This could be because the environmental filters acting on the communities have given rise to many different species with different natural attribute combinations, few of which are closely associated with grazing resistance. For grassy communities ‘large, erect tussocks branching above-ground’ and ‘small, sprawling basal tussocks’ may have potential as response types indicative of light and heavy grazing respectively, but no response types could be identified for herbaceous communities.