Impacts of water stress removal and disturbance regimes on Mediterranean dry grasslands diversity and succession

Our goal was to disentangle the effects of stress removal and disturbance on plant communities of a Mediterranean rangeland, La Crau (southeastern France). We compared undisturbed reference steppe vegetation with vegetation impacted by changes in land use such as earlier phases of cultivation (dating back 20 years) and/or current water infiltrations (revealed by the presence of Brachypodium ph?nicoïdes), since the establishment of adjacent hay meadows. We considered plots with and without brambles (Rubus ulmifolius), an indigenous shrub species that had colonized the area after the land-use changes. We monitored the composition and measured the taxonomic richness and richness of functional groups, evenness and similarities of plant communities. The species richness of the undisturbed community was significantly higher than that of all disturbed plant communities. Although cultivation led to the dominance of ruderal species, the removal of water stress had a stronger negative impact, enabling the establishment of herbaceous competitor species such as B. ph?nicoïdes. The dominance of this species resulted in a significant decline in species richness and evenness after water stress removal. The presence of brambles correlating with former cultivation and/or current water infiltration did not have a significant impact on plant species richness in the vicinity of bramble bushes, although it significantly modified the composition of the adjacent herbaceous vegetation. Our study highlights again the low resilience of Mediterranean dry grasslands after disturbance. While both the disturbance and the water stress removal resulted in changes within the plant community, our findings reveal a stronger impact of the water stress removal. Water infiltration led to decreased plant species richness and evenness because the greater availability of water favored competitor species over the stress-tolerant xeric species. Therefore, for restoring the original steppe species richness, the priority will be to control water infiltrations, even before any scrub-clearing is undertaken to control bramble colonization.     

Dynamics of plant and arthropod diversity during old field succession

The successional dynamics of arthropod diversity in 18 abandoned agricultural fields (age 15-54 yr) at Cedar Creek. MN. USA were determined using sweep net sampling (44833 individuals of 618 species). Total arthropod species richness and equitability (J), but not abundance, increased significantly with field successional age. Herbivore and parasite species richness, but not detritivore and predator species richness, also increased significantly with field age. All of these arthropod variables were significantly positively correlated with plant species richness in the fields. When plant species richness was included as a covariate in regressions, there were no longer any significant effects of field age. These results supported the hypothesis that increases in arthropod diversity with field age are influenced by increases in plant diversity. The additional significant positive dependence of herbivore species richness on predator species richness suggests that predator-prey interactions may also influence the successional dynamics of arthropod diversity. Nine of the ten most common arthropod species decreased in abundance with field age, two of them significantly. The abundances of these two generalist forb-feeding species, Melanoplus femurrubrum (Orthoptera: Acrididae) and Scaphytopius acutus (Homoptera: Cicadellidae). each depended significantly on amount of forbs. The average body size of arthropod species (total and herbivores) decreased significantly with field age. An efficiency vs specialization hypothesis predicts such a decrease. Because plants in later secondary succession are generally less palatable, a diversity of smaller, potentially more specialized herbivores may have an advantage over larger and more efficient herbivores in later succession.     

Vole mound effects and disturbance rate in a mediterranean plant community under different grazing and irrigation regimes

A factorial field experiment was used to assess the influence of soil-disturber mammals in the structure of a 9-year-old Mediterranean annual plant community subjected to different sheep grazing and irrigation regimes. We estimated the disturbance rate (mound building activity) by Mediterranean voles, their effects on vegetation and the mechanisms of these effects during a period of vole outbreak. The effects on vegetation were analysed at the levels of species, functional groups and plant community. Disturbance rate was high and voles can disturb the entire soil surface once every four or five years. The availability of certain trophic resources (perennial plants) appeared to drive vole expansion in the experimental plots and it was independent of the irrigation and grazing treatments. Mound building activities largely affected vegetation but conserved plot differences. Total vegetation cover, absolute cover of all functional groups, mean vegetation height and species richness were less on mounds than on undisturbed ground. These effects did not change the relative abundance of annuals, perennials, grasses and forbs. Only the relative abundance of small-seeded species decreased on mounds. As the proportion of these seeds was similar in both types of patches, we suggest that small-seeded species had more difficulties for germinating or emerging when they are buried during mound formation. Irrigation and sheep grazing promoted large changes in the vegetation parameters but these effects were, in general, similar on mounds and undisturbed ground. Our results show that the availability of germinable seeds may be the major limitation for mound revegetation, probably due to the scarcity of seeds existing at the depths from which soils are excavated. Our results also suggested a resource limitation on mounds. The results provide additional evidence that soil disturbances by small herbivore mammals exert relevant ecological effects on abandoned Mediterranean croplands. We discuss the ecological implications of vole mound-building activities for plant succession, plant species conservation and forage resource availability for livestock. This revised version was published online in June 2006 with corrections to the Cover Date.     

Inorganic soil nitrogen under grassland plant communities of different species composition and diversity

We measured aboveground plant biomass and soil inorganic nitrogen pools in a biodiversity experiment in northern Sweden, with plant species richness ranging from 1 to 12 species. In general, biomass increased and nitrate pools decreased with increasing species richness. Transgressive overyielding of mixed plant communities compared to the most productive of the corresponding monocultures occurred in communities with and without legumes. N₂-fixing legumes had a fertilizing function, while non-legumes had a N retaining function. Plant communities with only legumes had a positive correlation between biomass and soil nitrate content, whereas in plant communities without legumes they were negatively correlated. Both nitrate and ammonium soil pools in mixed non-legume communities were approximately equal to the lowest observed in the corresponding monocultures. In mixed legume/non-legume communities, no correlation was found for soil nitrate with either biomass or legume biomass as percentage of total biomass. The idea of complementarity among species in nitrogen acquisition was supported in both pure non-legume and mixed non-legume/legume communities. In the latter, however, facilitation through increased nitrogen availability and retention, was probably dominating. Our results suggest that diversity effects on biomass and soil N pools through resource use complementarity depend on the functional traits of species, especially N₂ fixation or high productivity.     

Patterns of plant species and community diversity at different organization levels in a forested riparian landscape

Abstract. Integrated synusial phytosociology combined with traditional measures of diversity is used to describe the structure of vascular vegetation diversity along the forested riparian landscape of the upper Oise valley (Belgium and France). The two dimensions (longitudinal and lateral) of the geomorphological complex are examined at four scales: synusia, phytocoenosis, tesela and catena. The results support the following hypotheses: (1) the environmental gradients observed, particularly the lateral ones, are very complex; (2) there is a clear lack of coupling between the tree, shrub and herb layers, which indicates a differential response to the underlying influence of environmental controls; (3) moderate flooding‐induced disturbance enhances herb species richness; (4) stressful environments support a low plant species diversity but a high synusial richness; (5) natural factors (substrate, climate, disturbance) are more important at the synusia and phytocoenose scales, but (6) anthropogenic disturbances, mainly through forest and river management, are more important at the landscape level. By considering plant communities as structural‐dynamic entities of ecosystems in a landscape context, integrated synusial phytosociology provides a basis for decomposing a complex system since the different hierarchical levels are both nested and thus strongly relational and process‐based.     

水淹干扰下水库水滨带植物群落谱系结构与物种多样性格局

群落谱系结构将物种系统发育信息整合到群落中,给生物多样性研究提供了全新视角。为探讨水淹干扰下水库水滨带植物群落物种多样性维持机制,以丹江口水库水滨带为研究区域,以南水北调中线一期工程运行前的原有水滨带植被为研究对象,分析不同水淹干扰强度下群落谱系结构和物种多样性及其与环境因素的关系。结果表明：(1)随着水淹干扰强度增加,群落物种丰富度、物种多样性和谱系多样性显著降低。谱系结构指数虽无显著差异,但随着水淹干扰强度增加呈现由发散到聚集的变化趋势。表明水滨带植物群落物种多样性维持机制是生境过滤和生态位分化共同驱动的结果,并且随着水淹干扰强度增加呈现生境过滤作用增加、物种间竞争排斥作用减弱的生态过程影响规律。(2)坡度、土壤质地、速效氮和全氮含量对整个水滨带植物群落物种丰富度、物种多样性和谱系多样性影响显著。此外,土层厚度和速效磷对物种丰富度和土层厚度对谱系多样性也影响显著。而仅有速效氮含量对谱系结构指数影响显著;轻度水淹干扰下坡向、速效磷含量与多样性指数显著相关;中度水淹干扰下坡度、土层厚度、土壤质地、土壤总氮含量与多样性指数显著相关,土壤pH、全氮和全磷含量与谱系结构指数显著相关;重度水淹...     

三江平原不同强度干扰下湿地植物群落的物种组成和多样性变化

以三江平原湿地-农田景观中沟渠的密度作为反映干扰强度的指标,研究了不同沟渠密度下湿地斑块植物群落的物种组成和多样性变化.结果表明:(1)在不同的干扰强度下,湿地植物群落的物种组成差异很大, 随干扰强度增大,原有湿生物种减少,而中湿生、中生物种显著增加;(2)湿地植物群落的多样性随干扰强度的增大,整体上呈降低趋势,群落多样性在中等沟渠密度下(0.6～1.2km/km2)达到最大,但不同沟渠密度下群落间多样性差异基本不显著;(3)位点间的干扰强度差异越大,Whittaker指数值越高;(4)不同的干扰强度下,各湿地斑块内部的物种周转速率同样在沟渠密度中等(0.6～1.2km/km2)时最高.     

Variation in species composition and diversity of wetland communities under different disturbance intensity in the Sanjiang plain

In this study, ditch density was chosen as an indicator of disturbance intensity, and four sites with different ditch density were chosen to compare species composition and diversity among them. Four transects and sixty-seven quadrats were sampled along the ditch density gradient. The results showed that there existed a significant negative relationship between disturbance intensity and species parameters, while species diversity did not differ greatly among sites, but species composition varied considerably. With increasing disturbance intensity, the wetlands tended to be reduced in plant species diversity, and an increasing loss of indigenous wetland species was paralleled with an increasing invasion of upland species. The results also show that the rate of species turnover within communities differed among communities under different disturbance intensities. The communities with intermediate ditch density (0.6-1.2 km/km²) had higher rates than the other communities.     

Predicting invertebrate diversity from disturbance regimes in forest streams

The link between substrate disturbance and stream invertebrate species richness is often complicated by the fact that substrate disturbance removes both invertebrates and periphyton (a potential food source). It is never clear whether disturbance acts directly on species diversity by removing animals or indirectly by reducing one of their food sources. To examine this relationship invertebrate diversity patterns were examined in 25 forest streams in Urewera National Park, New Zealand, where light attenuation from the forest canopy was postulated to limit periphyton biomass and remove the confounding influence of periphyton on the link between substrate disturbance and invertebrate diversity. Invertebrate species richness declined linearly with increasing substrate disturbance. Although periphyton biomass was comparatively low, species richness was more strongly related to periphyton biomass than to any disturbance measure. The highly mobile nature and terrestrial reproductive stage of many lotic invertebrates suggest that colonisation dynamics may have a more important influence on diversity patterns than monopolisation of resources for population growth. Although both the intermediate disturbance hypothesis and the dynamic equilibrium model encompass colonisation as a critical determinant of diversity both models also require a trade-off between the colonising and competitive ability of individual species; a phenomenon which does not appear to occur widely in lotic communities. Rather, it is postulated that resource levels will set an upper limit to the species richness of a benthic community that can be achieved through colonisation of taxa in the absence of disturbance, while disturbance removes taxa and resets the colonisation process.     

Importance,but not intensity of plant interactions relates to species diversity under the interplay of stress and disturbance

The lack of clarity on how the intensity and importance of plant interactions change under the co‐occurrence of stress and disturbance strongly impedes assessing the relative importance of plant interactions for species diversity. We addressed this issue in subalpine grasslands of the French Pyrenees. A natural soil moisture gradient further experimentally stretched at both ends was used and a mowing disturbance treatment was applied at each position along the soil moisture gradient. Changes in intensity and importance of plant interactions were assessed by a neighbour removal experiment using four target ecotypes. A structural equation modelling approach was used to assess the relative impact of stress, disturbance, the intensity and importance of plant interactions on diversity at both the neighbourhood and community scales. Without mowing, changes in intensity and importance of plant interactions only diverged in the dry part of the soil moisture gradient. The intensity of plant interactions linearly shifted from competition to facilitation with increasing stress, while the importance followed a hump‐shaped relationship. Species diversity components were tightly related to the importance of plant interactions only, both the neighbourhood and community scales. Mowing disturbance strongly reduced the importance of facilitation along the soil moisture gradient, and suppressed the relationship between the importance of plant interactions and diversity components. Together, our results highlight that 1) the importance is the best predictor of variations in species diversity in this subalpine herbaceous system, and 2) that fine‐scale processes such as plant interactions can affect the entire plant communities. Finally, our results suggest that high level of constraints due to co‐occurring stress and disturbance can inhibit the effects of plant interactions on species diversity, highlighting their potential role in regulating diversity and the maintenance/extinction of plant communities. Synthesis How plant interactions change along environmental gradients is an unsolved debate, particularly when both stress and disturbance interact. This lack of clarity explains why the relative impact of plant interactions (intensity and importance) on species diversity has been rarely assessed. Using an experimental approach, we found that the importance of plant interactions highly contributed to variation in species diversity, confirming that neighbourhood scale processes such as plant interactions can affect the entire plant communities. The co‐occurrence of stress and disturbance inhibited the effects of plant interactions, highlighting that plant interactions may regulate drops of diversity and the maintenance/extinction of plant communities.     

Natural avalanche disturbance shapes plant diversity and species composition in subalpine forest belt

Background: Disturbances by avalanches have created unique habitats for animals and plants in subalpine ecosystems worldwide, but at the same time avalanches can pose a major threat to humans. Thus, avalanches are suppressed by means of avalanche barriers to protect settlements and infrastructures in populated areas of the European Alps. As a consequence, the disturbance regime in avalanche tracks has fundamentally changed. Methods: In the present study we address ecological consequences of avalanche suppression on plant diversity. We analysed plant diversity and species composition in recent and old avalanche tracks with and without avalanche suppression and in undisturbed adjacent forests at high and low elevations. Results: The number of species was higher in both active and inactive avalanche tracks as compared to undisturbed subalpine forest. The species composition indicated a wider range of ecological niches in active than in inactive avalanche tracks. The vegetation from active tracks showed lower indicator values for temperature and nitrogen availability. The proportion of alpine species was lower in formerly active tracks. Conclusions: The conditions that exist in active avalanche tracks increase plant diversity in relation to undisturbed forest. In the few decades following avalanche suppression, species composition changes in tracks from which avalanches have been excluded. Continued suppression of avalanche disturbance may lead to a decline in plant and habitat diversity. Avalanche disturbance can exert an important influence on the biodiversity of subalpine forests and provide important habitats. Anthropogenic changes in the natural regime of avalanche disturbance are likely to contribute significantly to future landscape changes in subalpine forests.     

Impact of pocket gopher disturbance on plant species diversity in a shortgrass prairie community

Summary We examined the impact of pocket gopher disturbances on the dynamics of a shortgrass prairie community. Through their burrowing activity, pocket gophers (Thomomys bottae) cast up mounds of soil which both kill existing vegetation and create sites for colonization by competitively-inferior plant species. Three major patterns emerge from these disturbances: First, we show that 10 of the most common herbaceous perennial dicots benefit from pocket gopher disturbance; that is, a greater proportion of seedlings are found in the open space created by pocket gopher disturbance than would be expected based on the availability of disturbed habitat. Additionally, these seedlings exhibited higher growth rates than adjacent seedlings of the same species growing in undisturbed habitat. Second, we tested two predictions of the Intermediate Disturbance Hypothesis and found that species diversity was greatest for plots characterized by disturbances of intermediate age. However, we did not detect significant differences in diversity between plots characterized by intermediate and high levels of disturbance, indicating that many species are adapted to or at least tolerant of high levels of disturbance. Third, we noted that the abundance of grasses decreased with increasing disturbance, while the abundance of dicots increased with increasing disturbance.     

Suites of plant traits in species from different stages of a Mediterranean secondary succession

The aim of this study was to detect suites of traits related to whole plant and seed morphology, phenology and resource use – including water – in species differing in successional status. Twenty traits were measured on 55 species representative of 5 successional stages in Mediterranean southern France, including eight pertaining to phenology and five to water economy. Suites of traits that changed along succession in agreement with the acquisition/conservation trade-off were completed by continuous changes in phenology. Early successional species had leaves with a high specific leaf area that were produced and lost continuously through the growing season. Late-successional species were taller with long-lived, high δ¹³C leaves produced during short periods, most of them persisting during summer, and produced large seeds requiring a long ripening period. Replacement of species occurred with change in strategies of drought survival: early successional species escaped drought by dying before summer; later herbaceous species maintained favourable water status in relation to leaf shedding during summer; late successional trees with a large body allowing access to a large pool of resources, produced dense leaves that could tolerate desiccation. These changes occurred concomitantly with a shift in CSR strategies, using traits related to resource use, plant size and flowering phenology: ruderal herbs were replaced by more stress-tolerant herbs and shrubs throughout the succession, with competitive trees dominating the latest successional stage. These results suggest that the breadth of functional variability found in natura is not predicted by the CSR framework, and calls for a more integrated view of whole plant functioning.     

Patterns of population genetic diversity in riparian and aquatic plant species along rivers

Aim The downstream hydrochoric spread of seeds of aquatic and riparian plant species, without upstream compensation, can be expected to result in downstream accumulation of population genetic diversity. This idea has been termed the ‘unidirectional dispersal hypothesis’ and is the genetic equivalent of the more generally known ‘drift paradox’. Our aim was to test this unidirectional diversity hypothesis, and to present a general synthesis of the patterns of population genetic variation across different riparian and aquatic plant species along rivers. Location The Meuse River (Belgium) and rivers world‐wide. Methods First, we used amplified fragment length polymorphism markers to compare patterns of within‐ and between‐population genetic diversity among three riparian plant species (Sisymbrium austriacum, Erysimum cheiranthoides and Rorippa sylvestris), typically occurring in different habitats along a gradient perpendicular to the Meuse River. Second, we performed a meta‐analysis on studies reporting on the population genetic structure of riparian and aquatic plant species along rivers. Results Along the Meuse River, we found significant genetic differentiation among populations of all three riparian species, and significant isolation by distance for one of them (R. sylvestris). There was no clear association between the typical habitat of a species and its population genetic structure. None of the three species provided evidence for the unidirectional dispersal hypothesis. The meta‐analysis, based on 21 data records, did not support the unidirectional dispersal hypothesis either. Average weighted population genetic differentiation across species was significant. Main conclusions Important mechanisms of upstream seed dispersal, probably through zoochory, together with higher seed recruitment opportunities in upstream habitats due to density dependence of recruitment, may explain the absence of downstream accumulation of genetic diversity. Also, it seems difficult to find consistent patterns in genetic variation in species from aquatic and riparian habitats. We argue that this is due to the recurrent extinctions and colonizations characteristic of these habitats, resulting in complex genetic patterns. Our results strongly support previous suggestions that stream ecology should consistently embrace metapopulation theory to be able to understand patterns of genetic diversity, as well as species diversity.     

放牧干扰下高寒草甸物种多样性指数评价与选择

在青藏高原东北缘高寒草甸设置6个放牧强度样地,连续4年研究10个多样性指数(Richness和Abundance 2个实测指数,优势度指数、均匀度指数、丰富度指数和综合指数各2个)对放牧强度和年限影响植物群落的解释能力.结果表明: 相对于重要值,利用多度计算的多样性指数对放牧干扰更敏感.优势度指数(Berger-Parker、Dominance)与放牧强度和年限均无关,不能将放牧干扰对群落优势种的影响有效区分.均匀度指数(Equitability、Evenness)均与放牧强度无关,但Evenness指数与放牧年限呈显著负相关,不受偶见种影响且与物种多度的变异系数呈显著正相关,在基于时间尺度的均匀度比较中可以选择Evenness指数.丰富度指数(Menhinick、Margalef)均与放牧年限无关,但Margalef指数与放牧强度呈显著正相关,且不受偶见种影响.综合指数(Shannon、Simpson)均与放牧强度无关,但Shannon指数与物种丰富度和多度呈显著正相关,且随放牧年限增加而显著升高,不受偶见种影响,Shannon指数可用于在长时间尺度下比较物种多样性变化.在所有多样性指数中,只有实测物种丰富度和多度均与放牧强度呈显著负相关,与放牧年限呈显著正相关,且不受偶见种影响,故实测物种丰富度和多度相结合可作为放牧干扰下多样性比较的首选指标.此外,多样性指数选择须考虑放牧干扰的强度与时间特征、多样性组分和研究目的.     

Functional diversity changes during tropical forest succession

Functional diversity (FD) ‘those components of biodiversity that influence how an ecosystem operates or functions’ is a promising tool to assess the effect of biodiversity loss on ecosystem functioning. FD has received ample theoretical attention, but empirical studies are limited. We evaluate changes in species richness and FD during tropical secondary forest succession after shifting cultivation in Mexico. We also test whether species richness is a good predictor of FD. FD was calculated based on a combination of nine functional traits, and based on two individual traits important for primary production (specific leaf area) and carbon sequestration (wood density). Stand basal area was a good predictor of successional changes in diversity and FD, in contrast to fallow age. Incidence-based FD indices increased logarithmically with stand basal area, but FD weighted by species’ importance values lacked pattern with succession. Species richness and diversity are strong predictors of FD when all traits were considered; linear relationships indicate that all species are equally functionally complementary, suggesting there is little functional redundancy. In contrast, when FD was calculated for individual traits and weighted for abundances, species richness may underestimate FD.Selection of functional trait(s) critically determines FD, with large consequences for studies relating biodiversity to ecosystem functioning. Careful consideration of the traits required to capture the ecosystem process of interest is thus essential.     

黄土高原植被不同演替阶段优势种的光合生理特性

在黄土高原子午岭林区,按照植被的演咎序列,分别选择植被次生演替不同阶段的优势种-白羊草、铁杆蒿、沙棘、狼牙刺、山杨和辽东栎,对其土壤与叶片氮素含量、叶片气体交换参数、叶绿素含量及叶绿素荧光参数进行了观测．结果表明：随着植被演替,0—20cm土壤全氮含量呈增大趋势,植被优势种叶片氮含量先升高后降低,叶绿素含量变化与优势种叶片氮含量相似．灌木群落优势种（沙棘、狼牙刺）叶片氮含量明显高于其它群落优势种（P〈0．05）,草本群落优势种（白羊草、铁杆蒿）和灌木群落优势种的光合速率（Pn）高于早期森林群落优势种（山杨）和顶级群落优势种（辽东栎）,叶片气孔导度（ga）与Pn变化规律相似．草本群落和灌木群落优势种叶片蒸腾速率（Tr）较高,随着演替的进行t逐步下降,顶级群落优势种辽东栎Tr较低．PSⅡ最大光化学量子效率（Fv／Fm）值呈缓慢增长趋势,辽东栎〉山杨〉沙棘〉狼牙刺〉铁杆蒿〉白羊草,而PSⅡ量子效率（ΦPSⅡ）值呈先增加后降低趋势;光化学淬灭系数（qp）在演替过程中呈整体增加趋势,而非光化学淬灭系数（qNP）呈先增加后降低趋势．这说明植被不同演替阶段优势种的光合生理特征以及对微生境的适应性存在明显差异．