Community structure and composition in response to climate change in a temperate steppe

Climate change would have profound influences on community structure and composition, and subsequently has impacts on ecosystem functioning and feedback to climate change. A field experiment with increased temperature and precipitation was conducted to examine effects of experimental warming, increased precipitation and their interactions on community structure and composition in a temperate steppe in northern China since April 2005. Increased precipitation significantly stimulated species richness and coverage of plant community. In contrast, experimental warming markedly reduced species richness of grasses and community coverage. Species richness was positively dependent upon soil moisture (SM) across all treatments and years. Redundancy analysis (RDA) illustrated that SM dominated the response of community composition to climate change at the individual level, suggesting indirect effects of climate change on plant community composition via altering water availability. In addition, species interaction also mediated the responses of functional group coverage to increased precipitation and temperature. Our observations revealed that both abiotic (soil water availability) and biotic (interspecific interactions) factors play important roles in regulating plant community structure and composition in response to climate change in the semiarid steppe. Therefore these factors should be incorporated in model predicting terrestrial vegetation dynamics under climate change.     

Fire frequency influences composition and structure of the shrub layer in an Australian subcoastal temperate grassy woodland

Little is known about the relationship between fire regimes and plant diversity in Australia's temperate grassy woodlands. The effect of fire frequency on shrubs in grassy woodland remnants across Western Sydney's Cumberland Plain was examined. Shrub species richness and composition were compared in sites that had experienced a high, moderate or low frequency of fire over the previous 20 years. Nine sites were surveyed, three in each fire frequency category; most sites, including all low‐fire‐frequency sites, had burnt 9–36 months prior to sampling. Fire frequency had a profound effect on the composition and structure of the shrub layer. Per cent frequency and density of the prickly shrub Bursaria spinosa (Pittosporaceae) was considerably higher in low‐fire‐frequency sites than where fires had occurred at least once a decade. In sites where fire had been absent for decades prior to a recent fire, this species dominated the landscape, while elsewhere it occurred as clumps in a grassy matrix. Per cent frequency of other native shrubs, particularly obligate seeders, was greatest at moderate fire frequencies. Exotic shrubs were recorded most often where fire had been rare. While ordination clearly separated out the low‐fire‐frequency sites, complete separation between high‐ and moderate‐fire‐frequency blocks was not achieved. The increase in Bursaria in the absence of fire mirrors the encroachment of woody plants into a range of grassy ecosystems around the world. The sensitivity of obligate seeder species, many of them short‐lived legumes with fire‐cued seeds, to both very frequent and very infrequent fire shows the vulnerability of these species to extreme fire regimes, despite the safeguards conferred by hard‐seededness. Competition from Bursaria, as well as loss of viable seed in the soil, may have contributed to the low frequency of these species after a long inter‐fire interval.     

Fire season and intensity affect shrub recruitment in temperate sclerophyllous woodlands

The season in which a fire occurs may regulate plant seedling recruitment because of: (1) the interaction of season and intensity of fire and the temperature requirements for seed release, germination and growth; (2) post-fire rainfall and temperature patterns affecting germination; (3) the interaction of post-fire germination conditions and competition from surrounding vegetation; and (4) the interaction of post-fire germination conditions and seed predators and/or seedling herbivores. This study examined the effects of different fire intensities and fire seasons on the emergence and survival of shrubs representing a range of fire response syndromes from a summer rainfall cool climate region. Replicated experimental burns were conducted in two seasons (spring and autumn) in 2 consecutive years and fuel loads were increased to examine the effects of fire intensity (low intensity and moderate intensity). Post-fire watering treatments partitioned the effects of seasonal temperature from soil moisture. Higher intensity fires resulted in enhanced seedling emergence for hard-seeded species but rarely influenced survival. Spring fires enhanced seedling emergence across all functional groups. Reduced autumn recruitment was related to seasonal temperature inhibiting germination rather than a lack of soil moisture or competition. In Mediterranean-type climate regions, seedling emergence has been related to post-fire rainfall and exposure of seeds to seed predators. We think a similar model may operate in temperate summer rainfall regions where cold-induced dormancy over winter exposes seeds to predators for a longer time and subsequently results in recruitment failure. Our results support the theory that the effect of fire season is more predictable where there are strong seasonal patterns in climate. In this study seasonal temperature rather than rainfall appears to be more influential.     

Landscape-scale positive feedbacks between fire and expansion of the large tussock grass, Ampelodesmos mauritanica in Catalan shrublands

In the northern Mediterranean Basin, agricultural land abandonment over the last century has resulted in increasing frequencies of very large, intense fires. In Catalonia (NE Spain) some fires have been locally associated with the expansion of the large, evergreen, resprouting tussock grass Ampelodesmos mauritanica. We tested the hypothesis of a positive feedback between the abundance of A. mauritanica and changing fire regimes. We used permanent plots distributed across a natural gradient of density of A. mauritanica in the Garraf Natural Park near Barcelona. Total aboveground biomass nearly doubled from plots with low to high density through a combination of A. mauritanica replacing the biomass of other components of the community (predominantly native shrubs), and its absolute standing biomass increasing. The quantity of litter also increased. This increase in fuel load and changes in community functional composition resulted from the simultaneous decrease in shrub productivity and an increase in litter accumulation. Litter accumulation was the consequence of A. mauritanica litter decomposing 30% more slowly than that of shrubs. Under standardized conditions, A. mauritanica and its litter were considerably more flammable than any of the shrub species. This resulted in a more than 40‐fold increase in calculated plot flammability from low‐to‐high‐density plots. Feedbacks, at the landscape scale, were then analysed using the landscape simulation platform LAMOS. Invasion success and contribution to community biomass of A. mauritanica increased with decreasing fire return intervals. Total area burnt in the landscape during each fire year was positively and exponentially related to the total biomass of A. mauritanica. Simulations showed that landscapes can abruptly switch from regimes of small localized to extensive fires as a result of the spread of A. mauritanica. Therefore, increases in fires under climate change represent threats not only through their direct impacts on ecosystems, but also by promoting invaders such as A. mauritanica, which have the potential to induce powerful feedforward processes and, thereby, fundamental changes to ecosystems.     

Emergence and survival of herbaceous seedlings in temperate grassy woodlands: Recruitment limitations and regeneration niche

Abstract Field experiments examined herbaceous seedling emergence and survival in temperate grassy woodlands on the New England Tablelands of New South Wales. Effects of intensity of previous grazing, removal of ground cover by fire or clearing, burial of seeds, grazing and seed theft by ants on seedling emergence and survival were studied in two field experiments. Thirteen species with a range of traits were used in the experiments and their cumulative emergence was compared with laboratory germination studies. Field emergence correlated to laboratory germination but all species had lower emergence in the field. Little natural emergence of native species was observed in the field in unsown treatments. Short‐lived forbs had the highest emergence, followed by perennial grasses; rhizomatous graminoids and perennial forbs had the lowest emergence. Soil surface and cover treatments did not markedly enhance emergence suggesting that intertussock spaces were not prerequisites for forb emergence. No consistent pattern of enhanced emergence was found for any treatment combination across all species. Seedling survival varied among species, with perennial grasses and short‐lived forbs having the highest seedling mortality. Low mortality rates in the graminoids and rhizomatous forbs appeared partially to compensate for lower seedling emergence. All perennial grasses and some short‐lived forbs showed increased risk of mortality with grazing. Differences in emergence and survival of species were related to ground cover heterogeneity, soil surfaces and, to some extent, herbivory. The complexity of these patterns when superimposed on temporal variability suggests that no generalizations can be made about the regeneration niche of herbaceous species groups. Strong recruitment limitation and partitioning of resources in the regeneration niche may reduce competition among native species and explain the high species richness of the herbaceous layer in the temperate grassy communities of eastern Australia.     

Soil microbial diversity, community structure and denitrification in a temperate riparian zone

Nitrate (NO ₃ ^– ) removal in riparian zones bordering agricultural areas occurs via plant uptake, microbial immobilisation and bacterial denitrification. Denitrification is a desirable mechanism for removal because the bacterial conversion of NO ₃ ^– to N gases permanently removes NO ₃ ^– from the watershed. A field and laboratory study was conducted in riparian soils adjacent to Carroll Creek, Ontario, Canada, to assess the spatial distribution of denitrification relative to microbial community structure and microbial functional diversity. Soil samples were collected in March, June, and August 1997 at varying soil depths and distances from the stream. Denitrification measurements made using the acetylene block technique on intact soil cores were highly variable and did not show any trends with riparian zone location. Microbial community composition and functional diversity were determined using sole carbon source utilization (SCSU) on Biolog® GN microplates. Substrate richness, evenness and diversity (Shannon index) were greatest within the riparian zone and may also have been influenced by a rhizosphere effect. A threshold relationship between denitrification and measures of microbial community structure implied minimum levels of richness, evenness and diversity were required for denitrification.     

The role of a native tussock grass (Paspalum quadrifarium Lam.) in structuring plant communities in the Flooding Pampa grasslands, Argentina

The patches of Paspalum quadrifarium-dominated grasslands found atpresent could be remnants of the vegetation that defined the Pampas landscape inthe past. The mere presence of such physical structures should lead to systemsin which many other species are dependent upon both the autogenic creation ofsurface area for living space and the autogenic and allogenic modulation ofresources controlled by these structures. We carried out amensurative–comparative study in naturally occurring sites dominated byP. quadrifarium that occupy different positions within thelandscape in the Flooding Pampa. We found different species assemblages in thesestructurally homogeneous stands, related to the edaphic and topographicgradients. The comparison with sites from similar positions in the landscapewhere this native tussock-grass was absent showed that the dominance ofP. quadrifarium is a minor determinant of floristicdifferences, which are mainly controlled by abiotic factors. We assigned thesedominated grassland stands to previously delimited vegetation units for theFlooding Pampa grasslands and we compared them with the vegetation unitsassigned, in terms of composite properties of the community that may beindicators of ecosystem processes, such as biological invasions andproductivity. Our results show that this native tussock grass is associated withhigher floristic richness, higher representation of perennials and grasses andlower presence of alien plants. This homogeneous pattern of variation across thevegetation units would suggest that, despite the absence of dramatic changes incommunity composition, the dominance of this tussock grass plays an importantrole in structuring species diversity patterns.     

Trophic-level differences in functional composition of the Nardus grassland vegetation

In this study, we examined floristic and functional composition of Nardus grassland of the highlands of NE Slovenia. The data-set included 55 relevés, 59 plant species, and 17 plant functional traits (PFT). The TWINSPAN classification resulted in two plant communities; calcifuge species (G1_oligotr) and another group of species characteristic of mesotrophic meadow (G2_mesotr). On the basis of selected PFT 11 out of 17 differ significantly between the groups. Group G1_oligotr had higher community-weighted mean trait values for chamaephytes (G1 = 0.09; G2 = 0.02), geophytes (G1 = 0.03; G2= 0.01), competitors (G1 = 0.43; G2 = 0.41) and plants that start flowering later (G1 = 142.84; G2 = 136.69). On the other side was G2_mesotr represented with more plants that are therophytes (G1 = 0.04; G2 = 0.09), creeping (G1 = 0.01; G2 = 0.12) and short-lived (G1 = 0.04; G2 = 0.11) and have longer flowering period (G1 = 3.24; G2 = 3.60). Differences may reflect the stronger effect of disturbance and eutrophication in G2_mesotr, probably due to intensification of grassland management (grazing) in the region. Our findings are significant in understanding the relative influence of environmental stress and disturbance within Nardus grasslands, and this may have important implications for their conservation management.     

The effects of human disturbance on the species composition,species diversity and functional diversity of a Miombo woodland in northern Malawi

Anthropogenic disturbances have serious impacts on ecosystems across the world. Understanding the effects of disturbance on woodlands, especially in regions where local people depend on these natural resources, is essential for sustainable natural resource management and biodiversity conservation. In this study, we evaluated the effects of anthropogenic disturbance, specifically selective logging of Brachystegia floribunda, on woodlands by comparing species composition, species diversity and functional diversity of woody plants between disturbed and undisturbed woodlands. We combined species data and functional trait data for leaves, fruits and other traits related to resource and disturbance responses to calculate functional indices (functional richness, evenness and divergence) and community‐weighted means of each trait. Shifts in taxonomic species composition were analysed using nonmetric multi‐dimensional scaling. Species composition differed significantly between disturbed and undisturbed woodlands. Tree density was greater in disturbed woodlands, whereas evenness, functional evenness and functional divergence were greater in undisturbed woodlands. In terms of forest cover, selective logging of B. floribunda appeared to have little impact on Miombo woodlands, but some shifts in functional traits, such as the shift from a deciduous to evergreen phenology, may increase the vulnerability of these ecosystems to environmental change, especially drought.     

Disturbance events affecting phytoplankton biomass,composition and species diversity in a shallow,eutrophic, temperate lake

The seasonal changes in phytoplankton biomass and species diversity in a shallow, eutrophic Danish lake are described and related to different disturbance events acting on the phytoplankton community.Both the spring diatom maximum and the summer bloom of the filamentous blue-green alga, Aphanizomenon flos-aquae (L.) Ralfs, coincided with low values of phytoplankton species diversity and equitability. Diatom collapse was mainly due to internal modifications as nutrient depletion (Si, P) caused by rapid growth of phytoplankton, and increased grazing activity from zooplankton. A large population of Daphnia longispina O.F. Müller in June effectively removed smaller algal competitors, thus favouring the development of a huge summer bloom (140 mm³ l^–1) of Aphanizomenon flos-aquae. Heavy rainfall and storms in late July increased the loss of Apahnizomenon by out-flow and disturbed the stratification of the lake. These events caused a marked decline in phytoplankton biomass but had no effect on species diversity. A second storm period in late August circulated the lake completely and was followed by a rapid increase in phytoplankton diversity, and a change in the phytoplankton community structure from dominance of large, slow-growing K-selected species (Aphanizomenon) to small, fast-growing r-selected species (cryptomonads).     

Fire frequency and species associations in perennial grasslands of south-west Ethiopia

Fires play an important role in shaping species composition and associations in East African grasslands. Grassland plains of Omo National Park (ONP), Ethiopia, which are dominated by perennial grass species, exist in a fire-prone environment. Our objective was to determine if the current pattern of plant species composition in ONP's grassland plains was correlated with the historical pattern of fire frequency. Species composition was determined at 160 plots along 30 west-trending transects, approximately 2 km apart. Fire frequency for each plot was estimated using eleven Landsat satellite images that spanned a 23-year period. The Mantel and partial Mantel tests were used to test for correlation between species composition and fire frequency.
Plots in the northern grassland plain appear to burn every other year, while plots in the southern grassland plain burn once every 4–5 years. However, no significant correlation was found between patterns in species composition and fire frequency. Likewise, a selective analysis by functional group (i.e. grass, shrubs) revealed no relationship with fire frequency. It appears that fire does play a role in dictating species composition in ONP, but only in the sense that species that can tolerate the current fire regime persist. Species distribution, however, appears to be under the influence of other factors.     

Fire alters diversity,composition, and structure of dry tropical forests in the Eastern Ghats

Fire is known to have dramatic consequences on forest ecosystems around the world and on the livelihoods of forest‐dependent people. While the Eastern Ghats of India have high abundances of fire‐prone dry tropical forests, little is known about how fire influences the diversity, composition, and structure of these communities. Our study aimed to fill this knowledge gap by examining the effects of the presence and the absence of recent fire on tropical dry forest communities within the Kadiri watershed, Eastern Ghats. We sampled plots with and without evidence of recent fire in the Eswaramala Reserve Forest in 2008 and 2018. Our results indicate that even though stem density increases in the recently burned areas, species richness is lower because communities become dominated by a few species with fire resistance and tolerance traits, such as thick bark and clonal sprouting. Further, in the presence of fire, the size structure of these fire‐tolerant species shifts toward smaller‐sized, resprouting individuals. Our results demonstrate that conservation actions are needed to prevent further degradation of forests in this region and the ecosystem services they provide.     

践踏干扰对碧塔海高寒草甸植被茎叶性状、物种多样性和功能多样性的影响

高寒草甸具有重要的生态服务功能,然而固有脆弱性使其极易遭受气候变化和人为干扰等多重因素的影响。作为滇西北旅游资源中重要的组分之一,高寒草甸吸引了大批游客前往开展徒步旅行活动,但伴随着的践踏干扰作用会不可避免地对高寒草甸生态系统带来负面影响。然而,目前关注践踏干扰对滇西北高寒草甸植被的影响,特别是植被功能性状和功能多样性如何发生变化方面的研究还十分欠缺。以云南省香格里拉市碧塔海自然保护区内的典型高寒草甸生态系统为研究对象,采用实验践踏的方式(一共5种不同强度的践踏处理)来模拟旅游活动对草甸植被的干扰作用,并以草甸植被的茎叶性状特征为切入点,重点探讨践踏干扰对茎叶性状的平均大小和变异程度的影响,以及物种丰富度(以物种形态分类为基础)和功能丰富度(以功能性状为基础)之间的关系。研究结果显示,随着践踏强度的增加,植株高度和叶片大小的平均值,而不是茎叶性状的变异程度,出现明显下降趋势。此外,物种丰富度和功能丰富度均随践踏强度的增强而减小,且两者之间呈现显著正相关关系。然而,较之轻度践踏实验组,重度践踏实验组中的功能均匀度和功能分离度水平均有所增加,表明践踏干扰可能会在短期内打破优势种对资源的绝对占有格局和减少物种间的生态位重叠程度。尽管高寒草甸对人类践踏活动有一定的承受能力,但气候变化和人为干扰等多重因素势必会改变和影响高寒草甸群落的结构和功能可持续性,这也对高寒草甸的保护与管理工作提出了更加紧迫的要求。     

Mass‐ratio and complementarity effects simultaneously drive aboveground biomass in temperate Quercus forests through stand structure

Forests play a key role in regulating the global carbon cycle, a substantial portion of which is stored in aboveground biomass (AGB). It is well understood that biodiversity can increase the biomass through complementarity and mass‐ratio effects, and the contribution of environmental factors and stand structure attributes to AGB was also observed. However, the relative influence of these factors in determining the AGB of Quercus forests remains poorly understood. Using a large dataset retrieved from 523 permanent forest inventory plots across Northeast China, we examined the effects of integrated multiple tree species diversity components (i.e., species richness, functional, and phylogenetic diversity), functional traits composition, environmental factors (climate and soil), stand age, and structure attributes (stand density, tree size diversity) on AGB based on structural equation models. We found that species richness and phylogenetic diversity both were not correlated with AGB. However, functional diversity positively affected AGB via an indirect effect in line with the complementarity effect. Moreover, the community‐weighted mean of specific leaf area and height increased AGB directly and indirectly, respectively; demonstrating the mass‐ratio effect. Furthermore, stand age, density, and tree size diversity were more important modulators of AGB than biodiversity. Our study highlights that biodiversity–AGB interaction is dependent on the regulation of stand structure that can be even more important for maintaining high biomass than biodiversity in temperate Quercus forests.     

Plant sizes mediate mowing‐induced changes in nutrient stoichiometry and allocation of a perennial grass in semi‐arid grassland

While mowing‐induced changes in plant traits and their effects on ecosystem functioning in semi‐arid grassland are well studied, the relations between plant size and nutrient strategies are largely unknown. Mowing may drive the shifts of plant nutrient limitation and allocation. Here, we evaluated the changes in nutrient stoichiometry and allocation with variations in sizes of Leymus chinensis, the dominant plant species in Inner Mongolia grassland, to various mowing frequencies in a 17‐yr controlled experiment. Affected by mowing, the concentrations of nitrogen (N), phosphorus (P), and carbon (C) in leaves and stems were significantly increased, negatively correlating with plant sizes. Moreover, we found significant trade‐offs between the concentrations and accumulation of N, P, and C in plant tissues. The N:P ratios of L. chinensis aboveground biomass, linearly correlating with plant size, significantly decreased with increased mowing frequencies. The ratios of C:N and C:P of L. chinensis individuals were positively correlated with plant size, showing an exponential pattern. With increased mowing frequencies, L. chinensis size was correlated with the allocation ratios of leaves to stems of N, P, and C by the tendencies of negative parabola, positive, and negative linear. The results of structure equation modeling showed that the N, P, and C allocations were co‐regulated by biomass allocation and nutrient concentration ratios of leaves to stems. In summary, we found a significant decoupling effect between plant traits and nutrient strategies along mowing frequencies. Our results reveal a mechanism for how long‐term mowing‐induced changes in concentrations, accumulations, ecological stoichiometry, and allocations of key elements are mediated by the variations in plant sizes of perennial rhizome grass.     

Photosynthetic and growth responses to fire of the subtropical-temperate grass, Spartina argentinensis Parodi

Fire is a common phenomenon in grasslands but there is little experimental evidence of the mechanism involved in the rapid recovery of the plant growth. We tested under controlled conditions the hypothesis that after fire Spartina argentinensis leaves that restart growing have high photosynthetic rates. Photosynthetic rates and emergence of leaves and culms were registered. Initial photosynthetic rates were higher among plants which restarted growth after fire or clipping than in control ones, without effect of different water availability. High photosynthetic rate values remained during two months after fire or clipping treatments though then affected by water availability, and thereafter started declining towards the end of season. From January onwards no differences among treatments were detected except that control plants under water stress conditions had lower photosynthetic rates than all others. Leaf and culm emergence were promoted by both fire and clipping treatments. Though affect-ed by water availability, burned and clipped plants had the highest percentages of living tissues at the end of the experiment. Considering photosynthetic and growth responses after fire it can be concluded that burning is a regular component of the subtropical-temperate S. argentinensis habitat, among the South American grasslands.     

新疆柽柳林地昆虫群落取食功能团结构及物种多样性研究

用网捕和手捕法对不同季节新疆北部大小不同的阜康和呼图壁柽柳林地昆虫和螨进行了采集调查,采到标本52 066头,隶属11个目、50科、近100种.根据营养级和取食方式归类出捕食者、寄生者、剥食者、吸食者、食花和种籽者、造瘿者和游历者等取食功能团.连续分布的阜康林地的总物种数多于孤立斑块的呼图壁林地,其中53种为两林地共有种.两林地各取食功能团物种数在群落中的比例基本相同,剥食、吸食和捕食性昆虫等取食功能团各约占群落总物种数的1/5以上.吸食和造瘿昆虫（螨）等个体数量在各时期均高于其他取食功能团,其次是剥食性的昆虫.在任一季节占昆虫群落总个体数量25%以上的物种有柽柳瘤瘿螨、柽柳白盾蚧、毛尾柽瘿蚊等,占5%～10%的物种有斯氏伞锥象、埃及柽麦蛾和柽柳条叶甲等.阜康林地昆虫群落的均匀度（J）和多样性（H＇）等指数在季节前期和中期均高于呼图壁林地,但在后期则低于呼图壁林地,其优势度指数均低于呼图壁林地.     

Fire frequency and tree canopy structure influence plant species diversity in a forest-grassland ecotone

Disturbances and environmental heterogeneity are two factors thought to influence plant species diversity, but their effects are still poorly understood in many ecosystems. We surveyed understory vegetation and measured tree canopy cover on permanent plots spanning an experimental fire frequency gradient to test fire frequency and tree canopy effects on plant species richness and community heterogeneity within a mosaic of grassland, oak savanna, oak woodland, and forest communities. Species richness was assessed for all vascular plant species and for three plant functional groups: grasses, forbs, and woody plants. Understory species richness and community heterogeneity were maximized at biennial fire frequencies, consistent with predictions of the intermediate disturbance hypothesis. However, overstory tree species richness was highest in unburned units and declined with increasing fire frequency. Maximum species richness was observed in unburned units for woody species, with biennial fires for forbs, and with near-annual fires for grasses. Savannas and woodlands with intermediate and spatially variable tree canopy cover had greater species richness and community heterogeneity than old-field grasslands or closed-canopy forests. Functional group species richness was positively correlated with functional group cover. Our results suggest that annual to biennial fire frequencies prevent shrubs and trees from competitively excluding grasses and prairie forbs, while spatially variable shading from overstory trees reduces grass dominance and provides a wider range of habitat conditions. Hence, high species richness in savannas is due to both high sample point species richness and high community heterogeneity among sample points, which are maintained by intermediate fire frequencies and variable tree canopy cover.