草地群落放牧干扰梯度β多样性研究

放牧过程通过牲畜的啃食、践踏作用干扰草场环境,使草地群落的物种组成发生变化,植物种群的优势地位发生更替．结果表明,随放牧干扰强度加重,从盐湿化草甸到典型草原,群落植物种丰富度呈下降趋势．β多样性测度结果显示,盐湿化草甸和羊草杂类草草甸群落物种变化的中度干扰出现在轻牧→中牧阶段,并在整个放牧干扰进程中,表现较低的稳定性;草甸草原和典型草原群落出现在中牧→重牧阶段;而荒漠草原物种变化表现出高度的稳定性,从轻牧到过牧物种替代仅１～３种．各群落放牧干扰植物多样性的稳定性次序是：荒漠草原＞典型草原≥草甸草原＞盐湿化草甸     

草地群落放牧干扰梯度β多样性研究

放牧过程通过牲畜的啃食、践踏作用干扰草场环境,使草地群落的物种组成发生变化,植物种群的优势地位发生更替。结果表明,随放牧干扰强度加重,从盐湿化草甸到典型草原,群落植物种丰富度呈下降趋势。β多样性测度结果显示,盐湿化草甸和羊草杂类草草甸群落物种变化的中度干扰出现在轻牧→中牧阶段,并在整个放牧干扰进程中,表现较低的稳定性;草甸草原和典型草原群落出现在中牧→重牧阶段;而荒漠草原物种变化表现出高度的稳定性,从轻牧到过牧物种替代仅1～3种。各群落放牧干扰植物多样性的稳定性次序是:荒漠草原>典型草原≥草甸草原>盐湿化草甸.     

Deterministic processes have limited impacts on foliar fungal endophyte communities along a savanna-forest successional gradient

Patterns and drivers of succession provide insight into the mechanisms that govern community assembly, but remain poorly understood for microbial communities. We assess whether successional trends of trees are mirrored by foliar endophyte communities of three tree species across a deterministic woody successional gradient. Additionally, we test the relative contribution of abiotic predictors, biotic factors, and spatial distance between sites in predicting composition and richness of endophyte communities. Unlike the tree community, endophyte communities showed no consistent evidence of deterministic succession. Host identity was the most important factor structuring endophyte community composition; within hosts, spatial distance from the indigenous forest and between samples was important, while environmental predictors had small and inconsistent effects. Much variation in endophyte composition remained unexplained. In contrast, endophyte richness was well-explained by predictor variables. Host identity was most important in predicting endophyte richness, while the effect of other predictors on richness differed between host species. We conclude that deterministic succession in trees did not result in deterministic succession in endophyte communities; instead community assembly was most strongly influenced by host identity; while within hosts, neutral processes may be more important for endophyte assembly than deterministic factors.     

Long-term grazing impacts on vegetation diversity,composition, and exotic species presence across an aridity gradient in northern temperate grasslands

Little is known about the specific role of exotic species on measures of grassland plant diversity, including how this may vary with climatic conditions or large mammal herbivory. This study examined vegetation responses to long-term livestock grazing, including plant richness and diversity, as well as the contribution of exotic species to these metrics, across a network of 107 northern temperate grasslands in Alberta, Canada, spanning a broad aridity gradient. Exposure to grazing modestly increased plant richness, but did not alter Shannon’s diversity, Simpson’s diversity, or evenness, suggesting stability in floral diversity relative to grazing. However, grazing did increase grass cover while reducing shrub cover, the latter of which was only apparent in mesic grasslands. Unlike total plant diversity, exotic species richness and cover, together with exotic plant contributions to diversity, varied jointly with grazing and aridity. While long-term grazing increased exotic species, this response was most apparent in wetter areas, and non-grazed grasslands remained more resistant to the presence of exotics. Several exotic species were positive indicators of grazing in wetter grasslands, and coincided with lower native species cover, indicating grazing may be facilitating a shift from native to exotic vegetation under these conditions. Overall, our results indicate that while long-term grazing has altered the composition and cover of certain functional groups, including favoring exotics and minimizing woody vegetation in mesic areas, overall changes to plant diversity were limited. Additionally, these findings suggest that semi-arid northern temperate grasslands remain relatively resistant to grazing effects, including their susceptibility to exotic plant encroachment. These results improve our understanding of how ongoing grazing exposure may impact grassland diversity, including efforts to conserve native vegetation, as well as the important role of climate in altering fundamental grassland responses to grazing.     

Large-scale manipulation of plant litter and fertilizer in a managed successional temperate grassland

Plant litter may play an important role in herbaceous plant communities by limiting primary production and influencing plant species richness. However, it is not known how the effect of litter interacts with fertilization. We tested for the role of litter and fertilization in a large-scale experiment to investigate effects on diversity and biomass of plant species, growth forms, native vs. non-native groups, and abiotic ecosystem components (e.g., soil moisture, PAR). We manipulated plant litter (removed vs. left in situ) and nutrient availability (NPK-fertilized vs. unfertilized) for 4 years in 314-m² plots, replicated six times, in an old-field grassland. While many of our species-level results supported previously published studies and theory, our plant group results generally did not. Specifically, grass species richness and forb biomass was not affected by either fertilization or plant litter. Moreover, plant litter removal significantly increased non-native plant species richness. Relative to native plant species, all of our experimental manipulations significantly increased both the biomass and the species richness of non-native plant species. Thus, this grassland system was sensitive to management treatments through the facilitation of non-native plant species. We coupled biotic and abiotic components within a nonmetric multidimensional scaling (NMS) analysis to investigate treatment effects, which revealed that specific treatments altered ecosystem development. These results suggest that fertilization and plant litter may have larger impacts on plant communities and on ecosystem properties than previously understood, underscoring the need for larger-scale and longer-term experiments.     

Soil seed bank and driftline composition along a successional gradient on a temperate salt marsh

Abstract. This study focuses on the relationship between vegetation succession and soil seed bank composition on the Schiermonnikoog (The Netherlands) salt marsh over 100 yr. The importance of driftline material in seed dispersal and the relationship with succession is also investigated. The results indicate that the majority of species have a transient or short‐term seed persistent bank. Seeds of most species are able to float over the salt marsh and become concentrated in the driftline higher up the marsh. After plants have established a seed bank forms, which disappears when vegetation is replaced by later‐successional species. Exceptions are Spergularia mar‐itima, which is still present in the seed bank of late successional stages, and Juncus gerardi and Glaux maritima, which appear in the seed bank of early successional stages, but are absent in the vegetation. Based on the results of this study constraints and possibilities for salt‐marsh restoration by de‐embankment are discussed.     

Effects of grazing exclusion on plant species richness and phytomass accumulation vary across a regional productivity gradient

Question: Does long‐term grazing exclusion affect plant species diversity? And does this effect vary with long‐term phytomass accumulation across a regional productivity gradient? Location: Lowland grassy ecosystems across the state of Victoria, southeast Australia. Methods: Floristic surveys and phytomass sampling were conducted across a broad‐scale productivity gradient in grazing exclusion plots and adjacent grazed areas. Differences in species richness, evenness and life‐form evenness between grazed and ungrazed areas were analysed. The environmental drivers of long‐term phytomass accumulation were assessed using multiple linear regression analysis. Results: Species richness declined in the absence of grazing only at the high productivity sites (i.e. when phytomass accumulation was >500 g m^?2). Species evenness and life‐form evenness also showed a negative relationship with increasing phytomass accumulation. Phytomass accumulation was positively associated with both soil nitrogen and rainfall, and negatively associated with tree cover. Conclusions: Competitive dominance is a key factor regulating plant diversity in productive grassy ecosystems, but canopy disturbance is not likely to be necessary to maintain diversity in less productive systems. The results support the predictions of models of the effects of grazing on plant diversity, such as the dynamic equilibrium model, whereby the effects of herbivory are context‐dependent and vary according to gradients of rainfall, soil fertility and tree cover.     

Competition at the population level along a standing crop gradient: a field experiment in successional grassland

I measured competitive responses of experimentally-established populations of the perennial grass, Andropogon gerardi, across a complex gradient of standing crop and species composition in the successional grasslands of southwest Michigan. The goal was to assess whether long-term (three year) population-level responses of Andropogon to competition matched the inferences made from a previous phytometer study that examined transplant responses to competition across this same gradient over a single growing season.Replicate experimental populations of Andropogon were established at seven grassland sites by sowing seed into 0.5×0.5 m plots that had been denuded of all vegetation. During the first year of the study, all Andropogon populations were maintained as monocultures by hand weeding. At the end of the first growing season, half of the monocultures were selected for continued weeding and half were left open to invasion by competitors for three years. Invasion of the unweeded populations by neighboring plants varied strongly among sites and was positively correlated with standing crop. Increased susceptibility to invasion and competition resulted in the extinction of the unweeded Andropogon populations at the two most productive sites, supporting the hypothesis that Andropogon is restricted by competition to low productivity sites in these grasslands. The finding that the intensity of competition was positively correlated with standing crop is consistent with the previous transplant study, suggesting that short-term experimental assays of competition on the growth of individual transplants may have predictive value for longer-term outcomes of competition at the population level.     

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.     

Invertebrate herbivory increases along an experimental gradient of grassland plant diversity

Plant diversity is a key driver of ecosystem functioning best documented for its influence on plant productivity. The strength and direction of plant diversity effects on species interactions across trophic levels are less clear. For example, with respect to the interactions between herbivorous invertebrates and plants, a number of competing hypotheses have been proposed that predict either increasing or decreasing community herbivory with increasing plant species richness. We investigated foliar herbivory rates and consumed leaf biomass along an experimental grassland plant diversity gradient in year eight after establishment. The gradient ranged from one to 60 plant species and manipulated also functional group richness (from one to four functional groups—legumes, grasses, small herbs, and tall herbs) and plant community composition. Measurements in monocultures of each plant species showed that functional groups differed in the quantity and quality of herbivory damage they experienced, with legumes being more damaged than grasses or non-legume herbs. In mixed plant communities, herbivory increased with plant diversity and the presence of two key plant functional groups in mixtures had a positive (legumes) and a negative (grasses) effect on levels of herbivory. Further, plant community biomass had a strong positive impact on consumed leaf biomass, but little effect on herbivory rates. Our results contribute detailed data from a well-established biodiversity experiment to a growing body of evidence suggesting that an increase of herbivory with increasing plant diversity is the rule rather than an exception. Considering documented effects of herbivory on other ecosystem functions and the increase of herbivory with plant diversity, levels of herbivory damage might not only be a result, but also a trigger within the diversity–productivity relationship.     

Decomposition and nitrogen transformation rates in a temperate grassland vary among co-occurring plant species

Plant and Soil - Decomposition of organic matter varies depending upon interactions between the composition of the organic matter and the source of the microbial community, with differences in...     

Linkages between root traits,soil fungi and aggregate stability in tropical plant communities along a successional vegetation gradient

Plant and Soil - Determining which abiotic and biotic factors influence soil aggregate stability (MWD) in tropical climates is often confounded by soil type. We aimed to better understand the...     

Methanogen communities and Bacteria along an ecohydrological gradient in a northern raised bog complex

Mires forming an ecohydrological gradient from nutrient-rich, groundwater-fed mesotrophic and oligotrophic fens to a nutrient-poor ombrotrophic bog were studied by comparing potential methane (CH(4)) production and methanogenic microbial communities. Methane production was measured from different depths of anoxic peat and methanogen communities were detected by detailed restriction fragment length polymorphism (RFLP) analysis of clone libraries, sequencing and phylogenetic analysis. Potential CH(4) production changed along the ecohydrological gradient with the fens displaying much higher production than the ombrotrophic bog. Methanogen diversity also decreased along the gradient. The two fens had very similar diversity of methanogenic methyl-coenzyme M reductase gene (mcrA), but in the upper layer of the bog the methanogen diversity was strikingly lower, and only one type of mcrA sequence was retrieved. It was related to the Fen cluster, a group of novel methanogenic sequences found earlier in Finnish mires. Bacterial 16S rDNA sequences from the fens fell into at least nine phyla, but only four phyla were retrieved from the bog. The most common bacterial groups were Deltaproteobacteria, Verrucomicrobia and Acidobacteria.     

Life history variation in an annual plant under two opposing environmental constraints along an aridity gradient

Environmental gradients represent an ideal framework for studying adaptive variation in the life history of plant species. However, on very steep gradients, largely contrasting conditions at the two gradient ends often limit the distribution of the same species across the whole range of environmental conditions. Here, we study phenotypic variation in a winter annual crucifer Biscutella didyma persisting along a steep gradient of increasing rainfall in Israel. In particular, we explored whether the life history at the arid end of the gradient indicates adaptations to drought and unpredictable conditions, while adaptations to the highly competitive environment prevail at the mesic Mediterranean end. We examined several morphological and reproductive traits in four natural populations and in populations cultivated in standard common environment. Plants from arid environments were faster in phenological development, more branched in architecture and tended to maximize reproduction, while the Mediterranean plants invested mainly in vertical vegetative growth. Differences between cultivation and field in diaspore production were very large for arid populations as opposed to Mediterranean ones, indicating a larger potential to increase reproduction under favorable conditions. Our overall findings indicate two strongly opposing selective forces at the two extremes of the aridity gradient, which result in contrasting strategies within the studied annual plant species.     

Evolutionary responses of a dominant plant along a successional gradient in a salt-marsh system

Plant Ecology - The ecological responses of plant populations along a successional gradient have been intensively examined; however, the evolutionary responses received much less attention. Here, I...     

Changes in semi-arid plant species associations along a livestock grazing gradient

In semi-arid ecosystems, vegetation is heterogeneously distributed, with plant species often associating in patches. These associations between species are not constant, but depend on the particular response of each species to environmental factors. Here, we investigated how plant species associations change in response to livestock grazing in a semi-arid ecosystem, Cabo de Gata-Níjar Natural Park in South East Spain. We established linear point-intercept transects at four sites with different grazing intensity, and recorded all species at each point. We investigated plant associations by comparing the number of times that each pair of species occurred at the same spatial point (co-occurrences), with the expected number of times based on species abundances. We also assessed associations for each shrub and grass species by considering all their pairs of associations and for the whole plant community by considering all pairs of associations on each site. At all sites, the plant community had a negative pattern of association, with fewer co-occurrences than expected. Negative association in the plant community increased at maximum grazing intensity. Most species associated as expected, particularly grass species, and positive associations were most important at intermediate grazing intensities. No species changed its type of association along the grazing gradient. We conclude that in the present plant community, grazing-resistant species compete among themselves and segregate in space. Some shrub species act as refuges for grazing-sensitive species that benefit from being spatially associated with shrub species, particularly at intermediate grazing intensities where positive associations were highest. At high grazing intensity, these shrubs can no longer persist and positive associations decrease due to the disappearance of refuges. Spatial associations between plant species and their response to grazing help identify the factors that organize plant communities, and may contribute to improving management of semi-arid ecosystems.     

Adaptation of metabolism and evaporative water loss along an aridity gradient

Broad-scale comparisons of birds indicate the possibility of adaptive modification of basal metabolic rate (BMR) and total evaporative water loss (TEWL) in species from desert environments, but these might be confounded by phylogeny or phenotypic plasticity. This study relates variation in avian BMR and TEWL to a continuously varying measure of environment, aridity. We test the hypotheses that BMR and TEWL are reduced along an aridity gradient within the lark family (Alaudidae), and investigate the role of phylogenetic inertia. For 12 species of lark, BMR and TEWL decreased along a gradient of increasing aridity, a finding consistent with our proposals. We constructed a phylogeny for 22 species of lark based on sequences of two mitochondrial genes, and investigated whether phylogenetic affinity played a part in the correlation of phenotype and environment. A test for serial independence of the data for mass-corrected TEWL and aridity showed no influence of phylogeny on our findings. However, we did discover a significant phylogenetic effect in mass-corrected data for BMR, a result attributable to common phylogenetic history or to common ecological factors. A test of the relationship between BMR and aridity using phylogenetic independent constrasts was consistent with our previous analysis: BMR decreased with increasing aridity.     

西藏高寒草原湿润度梯度下植物群落特征空间格局及其驱动因子

西藏高原是中国高寒生态系统类型和生物多样性均聚集的区域之一,其中以高寒典型草原和高寒草甸草原为主生态系统类型,但其系统活力、组织力和恢复力均较弱,容易受到全球气候变化的影响,表现出极强的脆弱性。目前青藏高原草地生态系统植物群落分布状况与其变化规律以及对各种环境因子的反应研究多集中于站点尺度或样带尺度,但研究结果尚有较大的不确定性。研究于西藏地区沿着不同的湿润度梯度选择14处高寒草原区样地,通过植物群落调查、土壤理化性质分析,探索草本植被群落空间格局对气候因子响应特征及其主要驱动因子,其结果为青藏高原天然草地保护和可持续利用提供基础科学数据支撑。结果表明: (1)研究区内依据湿润度分区可分为干旱区、微干区、微润区和湿润区4类;研究区1971-2021年的气候数据分析发现,北部的干旱程度在逐渐减轻,而南部在1991-2010阶段干旱化趋势逐渐加强,整个研究区的湿润度指数均在下降;(2)植物群落调查发现不同植物功能群的变化各有差异,莎草科地上生物量随湿润度增加而增加,杂类草地上生物量呈现单峰趋势,而其他植物功能群无明显规律;湿润度较高的微润区间植物群落的Shannon-Wiener指数、Simpson指数、丰富度指数、均匀度指数均高于其他湿润度区间;(3)土壤理化性质分析发现高湿润度区域的土壤含水量、全氮、硝态氮、铵态氮、速效磷含量均为最高;对各环境因子与植物群落特征指标进行相关性、主成分分析后构建结构方程模型,发现直接影响西藏高寒草原植物群落特征变化因子为湿润度,土壤含水量和土壤pH,且都表现为显著的负效应(P<0.05),同时湿润度对土壤含水量和土壤全量养分产生了显著的正向影响(P<0.05),进而影响着西藏高原高寒草原不同植物功能群的分布、多样性、地上生物量。     

西藏高寒草原群落植物根系属性在降水梯度下的变异格局

根系功能属性及其变异性能够介导物种共存及环境适应策略, 但强烈的环境约束作用能够引起不同物种间根系属性的趋同性。为了研究西藏高寒草原群落中植物根系属性变异规律, 并阐明不同物种资源获取和适应策略的多样性, 该文对西藏高寒草原不同的环境梯度进行了研究。作者自东向西沿着降水梯度在那曲、班戈和尼玛3个自然草原群落进行群落调查, 并采集了共计22种植物。测定了每种植物的一级根直径、一级侧根长度和根系分支强度3个关键根系属性。结果表明: 在西藏高寒草原群落中, 不同物种根系直径普遍较小, 且种间变异非常小(22.76%), 其中86%的物种一级根直径集中在0.073 mm到0.094 mm之间; 相较于直径较粗的物种, 直径越细的物种分支强度越高, 侧根越短。在群落尺度上, 植物主要通过增加根系直径、侧根长度, 降低分支强度的方式来适应水分的减少; 而在物种尺度上, 植物适应水分变化的策略则呈现多样性。