Shifts In Plant Community Structure Research Articles

Soil nutrient availability affects tundra plant community composition and plant–vole interactions

ABSTRACT Climate change in the Arctic is predicted to drastically alter carbon and nutrient pools, plant communities, and plant–animal interactions. We examined how four levels of long-term (16 years) nutrient addition in moist acidic tundra affected plant community structure and the abundance of Eriophorum vaginatum, the preferred forage for tundra voles (Microtus oeconomus). We also explored how simulated press and pulse herbivory for four years affected Eriophorum at these different nutrient levels. Shifts in plant community structure, reductions in species diversity and richness, and decreased Eriophorum abundance were pronounced at the highest levels of nutrient addition. Eriophorum abundance was negatively correlated with deciduous shrub abundance, likely caused by light limitation in fertilized plots. Added nutrients generally increased Eriophorum leaf length, but effects varied among years and simulated herbivory treatments. After being clipped once, Eriophorum leaves regrew in fertilized plots to the same length as leaves that had not been clipped, demonstrating recovery. Our results show how interannual variation in plant growth and animal activity might exacerbate or dampen responses over multiple years. Changes to plant community composition and plant–vole interactions associated with increased soil nutrients resulting from warming could have cascading impacts on arctic ecosystems and carbon cycling.

Precipitation variations, rather than N deposition, determine plant ecophysiological traits in a desert steppe in Northern China

Understanding how plant ecophysiological traits of coexisting species within a community respond to environmental changes could help to predict the shift in plant community structure and function, but it remains limited in the scenarios of co-occurring precipitation variations and N deposition. A two-year field experiment was conducted to explore the effects of large precipitation changes (reduced and increased precipitation amount by 25% and 50% relative to ambient control) and high N deposition (10 g N m−2 yr−1) on a series of leaf ecophysiological traits of three dominant species (Stipa tianschanica, a C3 grass; Cleistogenes squarrosa, a C4 grass; and Artemisia capillaris, a C3 forb) in a desert steppe in Northern China. Increasing precipitation significantly linearly promoted the leaf light-saturated photosynthesis rate (Asat) and N use efficiency of the two C3 species, irrespective of N addition. The rises in Asat of both C3 species were mainly caused by increased soil moisture, which strongly induced increases in leaf stomatal conductance (gs) and declines in quantum yield of photosystem II (ΦPSII). However, the Asat of the two C3 species was weakly correlated with their specific leaf area and leaf N concentration, whereas the Asat of the C4 grass was negatively related to its leaf N. Moreover, the Asat, height, and aboveground biomass of the forbs were much more water-sensitive than those of both grass species, with a consequence of the most dominant species turning from grass to forbs as precipitation increased. Our findings highlight that water limitation, rather than N deficit, is the largest factor controlling plant growth in drylands, and plant species-specific ecophysiological responses to precipitation fluctuations will cause a substantial shift in the production and composition of plant community.

Plant removal across an elevational gradient marginally reduces rates, substantially reduces variation in mineralization.

The loss of aboveground plant diversity alters belowground ecosystem function; yet, the mechanisms underpinning this relationship and the degree to which plant community structure and climate mediate the effects of plant species loss remain unclear. Here, we explored how plant species loss through experimental removal shaped belowground function in ecosystems characterized by different climatic regimes and edaphic properties. We measured plant community composition as well as potential carbon (C) and nitrogen (N) mineralization and microbial extracellular enzyme activity in soils collected from four unique plant removal experiments located along an elevational gradient in Colorado, USA. We found that, regardless of the identity of the removed species or the climate at each site, plant removal decreased the absolute variation in potential N mineralization rates and marginally reduced the magnitude of N mineralization rates. While plant species removal also marginally reduced C mineralization rates, C mineralization, unlike N mineralization, displayed sensitivity to the climatic and edaphic differences among sites, where C mineralization was greatest at the high elevation site that receives the most precipitation annually and contains the largest soil total C pool. Plant removal had little impact on soil enzyme activity. Removal effects were not contingent on the amount of biomass removed annually, and shifts in mineralization rates occurred despite only marginal shifts in plant community structure following plant species removal. Our results present a surprisingly simple and consistent pattern of belowground response to the loss of dominant plant species across an elevational gradient with different climatic and edaphic properties, suggesting a common response of belowground ecosystem function to plant species loss regardless of which plant species are lost or the broader climatic context.

Climate‐mediated shifts toward bunchgrass dominance 40 years after grazing cessation in Saguaro National Park, Arizona, USA

AbstractQuestionsVegetation transects were surveyed in Saguaro National Park, Arizona, in 1976 and 2007 as part of a grazing study. We resurveyed the same transects in 2018 and accessed fire and climate data in order to examine trends in plant community structure. We asked the question: has the plant community shifted in species’ density, canopy cover and diversity in the past 40 years since grazing cessation? And if so, do these shifts favor a specific species, or plant functional guild? We also asked if climate and time since fire influenced plant community structure?LocationSaguaro National Park East is located in the Sonoran Desert and Madrean Sky Island ecoregion interface in Tucson, Arizona, United States.MethodsPlots (10) were 20 m × 50 m, with 21 20‐m transects within each plot used to gather plant canopy cover and density of perennial species with the line‐intercept method. Data were aggregated by plot and analyzed using generalized linear mixed‐effects modeling and non‐metric multi‐dimensional scaling.ResultsOverall canopy cover, density, and Shannon diversity increased significantly between 1976 and 2018. Ferns, forbs, graminoids, subshrubs and vines increased significantly in cover and density between 1976 and 2018. Wildfire recency was significantly negatively correlated with forb density, and density or cover of five dominant species (mostly woody). Shifts in plant community structure between survey years was correlated with increased temperature, precipitation, and vapor pressure differential.ConclusionAfter 40 years of grazing cessation, significant increases in herbaceous cover, and a lack of significant increase in woody encroachment are signs of recovery. Increases in temperature may be responsible for accelerating shifts to warm‐season species‐dominant communities. Fire was unexpectedly not a significant factor in plant community structure; however, park management will likely need to address increased wildfire risk given the increase in herbaceous cover and density.

Different responses of soil element contents and their stoichiometry (C:N:P) to yak grazing and Tibetan sheep grazing in an alpine grassland on the eastern Qinghai – Tibetan Plateau’

The alpine grassland of the Qinghai–Tibetan Plateau has long been subjected to grazing by local livestock, yak and Tibetan sheep. Although there is plenty of evidence that grazing influences the vegetation community structure and functions, researches on how grazing of different livestock affects element contents in soil and their stoichiometry and how they relate to plant community remain inadequate in the region. We quantified the concentration and distribution of carbon (C), nitrogen (N) and phosphorus (P), and investigated the robustness of soil stoichiometry (C:N:P) under yak grazing and Tibetan sheep grazing through a manipulated grazing experiment on an alpine grassland. Our results showed that: (1) Yak grazing increased the soil total carbon content (STC) but decreased the soil total phosphorus content (STP), whereas Tibetan sheep grazing decreased STC but increased STP. Neither of the two livestock species had impact on the soil total nitrogen content (STN). (2) Yak grazing and sheep grazing had no impact on soil stoichiometry except for C:P. (3) Only STP and C:P were affected by the mixing ratio of yak to sheep. The plant biomass loss and the shifts in plant community structure may account for the opposite responses of STC and STP to yak grazing and sheep grazing, whereas the dung chemical property of different livestock species might also contribute to the response of STP. The higher soil C:N and lower soil N:P ratios indicated that plant growth may be limited more by N than by P in the region.

Precipitation reduction alters herbaceous community structure and composition in a savanna

AbstractQuestionsIn the changing climate scenario, the decline in precipitation is expected to alter water availability for plants, which in turn affects plant community structure and composition. The responses of community composition and structure to declines in precipitation are well documented in other biomes but remain understudied in water‐limited savannas.LocationA savanna ecosystem in southwest China.MethodsWe used a four‐year (2014–2017) precipitation manipulation experiment to examine changes in herbaceous community composition and structure across the species, functional group and community levels under precipitation reduction.ResultsPrecipitation reduction significantly decreased the average height and percentage cover of the herbaceous community, while increasing species richness and the Pielou evenness index. Precipitation reduction significantly decreased average height, percentage cover and relative abundance of graminoids and perennials, but increased those of forbs and annuals. Precipitation reduction prompted a shift in the dominant species of the herbaceous community towards Fimbristylis monostachya.ConclusionsThe results show that precipitation reduction changed the composition and structure of the herbaceous community of this savanna. Furthermore, they provide strong evidence that changes in herbaceous community structure and composition in response to the intensity and duration of precipitation reduction in this savanna exhibited relatively low thresholds, which suggests that the herbaceous community's response to a decline in precipitation was essentially nonlinear. These findings imply that even relatively small declines in precipitation may stimulate shifts in plant community structure and composition and affect the function and stability of savanna ecosystems.

The decline in plant biodiversity slows down soil carbon turnover under increasing nitrogen deposition in a temperate steppe

Abstract Nitrogen (N) deposition not only alters the physiological processes of individual plant, but also leads to world‐wide biodiversity loss. However, little is known about how the hierarchical responses from individual physiological processes to plant community structure would have cascading effects on soil carbon (C) cycling. Here, we assessed whether changes in plant chemical composition and community composition under increasing N input would affect the turnover rate of litter layer and soil C loss via heterotrophic respiration (Rh) in a temperate grassland. We showed that more than a decade’s N addition significantly decreased plant species richness, litter layer turnover rate and Rh. The 13C‐NMR results showed that, for individual species, N addition either increased the abundance of recalcitrant C groups such as alkyl and methoxyl, or decreased labile C groups such as carbohydrate, resulting in decreases in carbohydrate C‐to‐methoxyl C ratio (CC/MC) for most species. Our data also showed that with the increase in N deposition, the abundance of relatively high degradable dominant species, such as Agropyron cristatum and Artimesia frigida declined rapidly, and the relatively recalcitrant species such as Potentilla bifurca and Leymus chinensis become dominant. Changes in individual species’ chemical composition and plant community composition significantly decreased litter quality at community level, as indicated by the lower community‐level CC/MC at higher N addition rates. The result of step‐AIC model selection further showed that plant diversity loss and the decrease in community‐level CC/MC jointly explained the decrease in Rh after N addition best, and further relative importance partition result showed that these two factors respectively contributed 65.1% and 34.9% of the explained variation. Overall, we demonstrated that changes in plant chemical composition and diversity loss due to N addition reduced the quality of plant C input to soil, which further slowed down litter layer turnover rate and inhibited soil heterotrophic respiration. Our study complements the intermediate links of how shifts in plant community structure regulate soil C cycle under global changes. A plain language summary is available for this article.

Post-fire recovery of the plant community in Pinus brutia forests: active vs. indirect restoration techniques after salvage logging

Abstract: Although reforestation is frequently utilized in many Mediterranean Basin countries to restore burned Mediterranean pine woodlands, post-fire recovery of the plant community is often neglected. To compare the post-fire recovery of the plant community following active and indirect post-fire restoration techniques, we studied three post-fire regeneration treatments in a salvage-logged Pinus brutia forest, including two active (plantation and seeding) restoration techniques and one indirect (natural regeneration). An unburned pine stand was also included in the study. We applied the point-intercept method to obtain data on the presence and cover of individual species and functional groups in six replicate one-hectare plots for each treatment. We found no significant differences in plant species richness among post-fire treatments; however, plant community composition and vegetation structure were significantly different between treatments. There was a shift in plant community structure when active restoration techniques were applied, from the woody- and resprouter-dominated plant community of the unburned site to an annual herbaceous- and non-resprouter-dominated one. Our results suggest that active restoration by planting tree saplings in Mediterranean pine forests after a fire may decrease the plant community’s resilience and provide empirical evidence that pine plantation treatments change the plant species composition of these forests. These results have important implications for post-fire management of Mediterranean Basin pine forests.

Responses of Native and Invasive Floating Aquatic Plant Communities to Salinity and Desiccation Stress in the Southeastern US Coastal Floodplain Forests

Low-lying coastal ecosystems along the northern Gulf of Mexico are already experiencing the effects of elevated salinity from sea-level rise and are predicted to face extreme events such as extended saltwater inundation, intense Atlantic hurricanes, and episodic drought. The ability of coastal plant communities to survive stresses from these events depends largely on how these communities respond to the stresses. Our understanding of how plant communities dominated by native vs. invasive plants respond to extreme events is limited. Utilizing controlled greenhouse experiments, we assessed the responses of floating aquatic macrophyte communities, dominated by native or invasive plants, of the coastal floodplains, Louisiana, USA, to a gradient of chronic salinity, mimicking sea-level rise; a gradient of acute salinity, mimicking hurricane storm surges; and a gradient of desiccation stress, mimicking episodic drought. We found that salinity and desiccation stress effects on plant communities depended on the degree of plant invasion; plant community cover decreased precipitously as severity of stress increased. Specifically, extreme salinity led to a decrease in plant cover of > 90% when communities were dominated by invasive plants, whereas increased desiccation stress led to decreased plant cover of 100% when communities were dominated by native species. At low to moderate salinity, invasive dominated plant communities performed better than native dominated. These responses to salinity and desiccation stress may drive large-scale shifts in plant community structure, including loss of species. Our results underscore the importance of evaluating plant community responses to environmental extremes to determine the potential for future effects on dynamics and functioning of low-lying coastal floodplain ecosystems experiencing effects of climate change.

Grassland Community Composition Response to Grazing Intensity Under Different Grazing Regimes

Grazing plays a key role in many ecosystems worldwide and can affect the structure and composition of terrestrial plant communities. Nonetheless, how grazing management, especially grazing regime (yearlong continuous and seasonal grazing), affects the relationship between grazing and vegetation on alpine grasslands has not been extensively studied. Here, we performed a grazing experiment in Gangcha county of Qinghai province of the Qinghai-Tibetan Plateau to test the effects of different stocking rates and grazing regimes on grassland biomass and plant structure and composition. Six stocking rates (ranging from 0 to 5.62 sheep ha−1) were used for continuous grazing, and three grazing intensities (1.72, 2.87, and 5.62 sheep ha−1) were used for seasonal grazing (grazed only in the growing season, from June to October) at the study sites. Plant biomass and grass functional community composition were characterized in two different yr (2011 and 2012). Additionally, species richness and plant diversity indexes were estimated to quantify the impacts of grazing on plant community composition. Our results indicated that grazing intensity best explained the plant biomass decrease in low-productivity environments, and different grazing regimes also influenced these results. The shifts in plant community structure and composition in response to increased grazing intensity differed considerably between continuous grazing and seasonal grazing. Seasonal grazing maintained greater amounts of palatable plant species, and fewer undesirable species in plant communities when compared with the composition after continuous grazing. Our results emphasize the importance of grazing regime in regulating the effects of grazing on plant communities and the importance of seasonal grazing for ecosystem maintenance, especially in the Qinghai-Tibetan Plateau. This suggests that periodic resting of grasslands could be a good management strategy to keep palatable species, thereby minimizing undesirable species in the overall species composition.

Circumpolar arctic tundra biomass and productivity dynamics in response to projected climate change and herbivory.

Satellite remote sensing data have indicated a general 'greening' trend in the arctic tundra biome. However, the observed changes based on remote sensing are the result of multiple environmental drivers, and the effects of individual controls such as warming, herbivory, and other disturbances on changes in vegetation biomass, community structure, and ecosystem function remain unclear. We apply ArcVeg, an arctic tundra vegetation dynamics model, to estimate potential changes in vegetation biomass and net primary production (NPP) at the plant community and functional type levels. ArcVeg is driven by soil nitrogen output from the Terrestrial Ecosystem Model, existing densities of Rangifer populations, and projected summer temperature changes by the NCAR CCSM4.0 general circulation model across the Arctic. We quantified the changes in aboveground biomass and NPP resulting from (i) observed herbivory only; (ii) projected climate change only; and (iii) coupled effects of projected climate change and herbivory. We evaluated model outputs of the absolute and relative differences in biomass and NPP by country, bioclimate subzone, and floristic province. Estimated potential biomass increases resulting from temperature increase only are approximately 5% greater than the biomass modeled due to coupled warming and herbivory. Such potential increases are greater in areas currently occupied by large or dense Rangifer herds such as the Nenets-occupied regions in Russia (27% greater vegetation increase without herbivores). In addition, herbivory modulates shifts in plant community structure caused by warming. Plant functional types such as shrubs and mosses were affected to a greater degree than other functional types by either warming or herbivory or coupled effects of the two.

Long-term response of plant communities to herbivore exclusion at high elevation grasslands

Understanding the effects of herbivores on plant communities is needed for anticipating how variation in grazing regimes will impact natural and semi-natural ecosystems. Prominent ecological hypotheses predict that drastic reductions of herbivory, as have occurred in many European mountains, will trigger fast diversity loss and structural changes in grasslands, because grazing tolerant species are expected to be rapidly replaced by taller and more competitive species. The aims of this study were to test generalizations of herbivory effects (mostly from lowlands) in high elevation grasslands over ecological relevant times scales, and to unravel the contribution of climatic conditions to observed changes. Species richness, canopy height, plant community structure, and transition probabilities among ecological groups were monitored throughout 19 years in herbivore exclusions and control plots in the Central Pyrenees. We used ordination analyses to track the long-term community response to herbivore exclusion, and generalized additive models to assess the non-linear effects of herbivore exclusion and climatic conditions on measured variables. Contrary to expectations, herbivore exclusion did not significantly affect species richness, and although canopy height increased, it was not drastic enough to suppress shade intolerant species. The strongest shifts in plant community structure and transition probabilities between ecological groups occurred during a sequence of warm and dry growing seasons, whereas in control plots, these changes were smaller, and largely reversed after cooler and wetter climatic conditions returned. Our results suggest that long-term effects of grazing cessation in high elevation grasslands can be weaker and slower than predicted. However, these effects can act synergistically with dry and warm events. Therefore, the maintenance of past grazing activities can be key in the face of ongoing climatic warming.

Changes in southern Piedmont grassland community structure and nutritive quality with future climate scenarios of elevated tropospheric ozone and altered rainfall patterns.

Forage species common to the southern USA Piedmont region, Lolium arundinacea, Paspalum dilatatum, Cynodon dactylon and Trifolium repens, were established in a model pasture system to test the future climate change scenario of increasing ozone exposure in combination with varying rainfall amounts on community structure and nutritive quality. Forages were exposed to two levels of ozone [ambient (non-filtered; NF) and twice ambient (2×) concentrations] with three levels of precipitation (average or ±20% of average) in modified open-top chambers (OTCs) from June to September 2009. Dry matter (DM) yield did not differ over the growing season between forage types, except in primary growth grasses where DM yield was higher in 2× than NF treatment. Primary growth clover decreased in nutritive quality in 2× ozone because of increased concentrations of neutral detergent fibre (NDF), acid detergent fibre (ADF) and acid detergent lignin (ADL). Re-growth clover exhibited the largest decrease in nutritive quality, whereas grasses were not adversely affected in 2× ozone. Re-growth grasses responded positively to 2× ozone exposure, as indicated in increased relative food value (RFV) and percentage crude protein (CP) than NF-exposed re-growth grasses. Effects of precipitation were not significant over the growing season for primary or re-growth forage, except in primary growth grasses where DM yield was higher in chambers with above average (+20%) precipitation. Total canopy cover was significantly higher over the growing season in chambers receiving above average precipitation, but no significant effects were observed with ozone. Results indicate shifts in plant community structure and functioning related to mammalian herbivore herbivory in future climate change scenarios.

Bird Assemblages inPhragmitesDominated and Non-PhragmitesHabitats in Two Lake Erie Coastal Marshes

A decline in Lake Erie water levels in 2000 from historic high levels of the 1990s has facilitated a shift in coastal wetland vegetation from open-water floating-leaf plant communities to emergent communities often dominated by the invasive perennial grass, Phragmites australis. Dense, near monotypic stands of this grass may lead to the loss of native plants and reduce suitable habitat for waterfowl and other wetland birds. To assess avian response to this shift in plant community structure, we conducted bird surveys (June–August, 2007) across four vegetation types in two coastal wetlands in the western basin of Lake Erie. Phragmites habitat had higher overall bird abundance but contained the lowest species diversity (H' = 0.71) of the four habitat types. Of the 35 species observed across habitat types, 4 species: (1) red-winged blackbird (Agelaius phoeniceus); (2) tree swallow (Tachycineta bicolor); (3) barn swallow (Hirundo rustica); and (4) bank swallow (Riparia riparia), accounted for 94% of total bird abundance. Ninety-four percent of all birds observed in sampled plots of Phragmites were red-winged blackbirds, and 73% of the total bird abundance (all species) across habitats occurred in Phragmites. This was mostly attributed to the large roosts (>500 birds/50-m radius plot) of red-winged blackbirds in sampled plots of Phragmites. Phragmites community overlap (Ro), represented by Horn's index, varied from a low of 0.30 with floating-leaved vegetation to a high of 0.69 within the Typha (cattail) community. Our results suggest that Phragmites does influence bird abundance and species diversity, but caution is warranted without additional data on nest success and survival.

Love thy neighbor? reciprocal impacts between plant community structure and insect herbivory in co‐occurring Asteraceae

Patterns of herbivory may vary with fine‐scale plant community structure: the degree of damage plants experience may depend on their neighbors (i.e., associational resistance or susceptibility). Differential herbivory, in turn, may facilitate a shift in plant community structure. We investigated these reciprocal effects of plant community structure and insect herbivory in a field mesocosm experiment with closely related, native Asteraceae that co‐occur in early‐successional habitats (old fields). After one year of establishment, we excluded or augmented insect herbivores for two years in equal‐density communities of three types: goldenrod‐dominated (Solidago spp.) or aster‐dominated (Symphyotrichum spp.) congeneric communities and mixtures of the two genera. In manipulated outbreaks, overall and species‐specific patterns of herbivory by the main herbivore, the leaf beetle Trirhabda virgata, varied dramatically with community composition. In both years, the preferred goldenrods suffered 25−70% higher defoliation in mixtures with the less‐preferred asters (i.e., associational susceptibility), compared to when growing with congeners; in contrast, asters experienced lower damage in mixtures (i.e., associational resistance). Insect herbivory consistently reduced overall plant productivity, and promoted colonization by other old‐field species. Importantly, herbivory also initiated a shift in the structure of the plant communities, and this effect depended on the starting community composition, implying potential reciprocal effects. For instance, only in mixtures did elevated herbivory reduce the proportional abundance of the preferred host, and the old‐field dominant, Solidago altissima. Our findings underscore the importance of plant community composition for variation in and impacts of herbivory and suggest the possibility of feedbacks between herbivory and local community structure as one of the mechanisms contributing to the maintenance of vegetation heterogeneity.

Plant community feedbacks and long-term ecosystem responses to multi-factored global change.

While short-term plant responses to global change are driven by physiological mechanisms, which are represented relatively well by models, long-term ecosystem responses to global change may be determined by shifts in plant community structure resulting from other ecological phenomena such as interspecific interactions, which are represented poorly by models. In single-factor scenarios, plant communities often adjust to increase ecosystem response to that factor. For instance, some early global change experiments showed that elevated CO2 favours plants that respond strongly to elevated CO2, generally amplifying the response of ecosystem productivity to elevated CO2, a positive community feedback. However, most ecosystems are subject to multiple drivers of change, which can complicate the community feedback effect in ways that are more difficult to generalize. Recent studies have shown that (i) shifts in plant community structure cannot be reliably predicted from short-term plant physiological response to global change and (ii) that the ecosystem response to multi-factored change is commonly less than the sum of its parts. Here, we survey results from long-term field manipulations to examine the role community shifts may play in explaining these common findings. We use a simple model to examine the potential importance of community shifts in governing ecosystem response. Empirical evidence and the model demonstrate that with multi-factored change, the ecosystem response depends on community feedbacks, and that the magnitude of ecosystem response will depend on the relationship between plant response to one factor and plant response to another factor. Tradeoffs in the ability of plants to respond positively to, or to tolerate, different global change drivers may underlie generalizable patterns of covariance in responses to different drivers of change across plant taxa. Mechanistic understanding of these patterns will help predict the community feedbacks that determine long-term ecosystem responses.

Drivers of Decadal-Scale Change in Southern Everglades Wetland Macrophyte Communities of the Coastal Ecotone

Understanding the relationships between hydrology and salinity and plant community structure and production is critical to allow predictions of wetland responses to altered water management, changing precipitation patterns and rising sea-level. We addressed how salinity, water depth, hydroperiod, canal inflows, and local precipitation control marsh macrophyte aboveground net primary production (ANPP) and structure in the coastal ecotone of the southern Everglades. We contrasted responses in two watersheds - Taylor Slough (TS) and C-111 - systems that have and will continue to experience changes in water management. Based on long-term trajectories in plant responses, we found continued evidence of increasing water levels and length of inundation in the C-111 watershed south of the C-111 canal. We also found strong differentiation among sites in upper TS that was dependent on hydrology. Finally, salinity, local precipitation and freshwater discharge from upstream explained over 80 % of the variance in Cladium ANPP at a brackish water site in TS. Moreover, our study showed that, while highly managed, the TS and C-111 watersheds maintain legacies in spatial pattern that would facilitate hydrologic restoration. Based on the trajectories in Cladium and Eleocharis, shifts in plant community structure could occur within 5–10 years of sustained water management change.

Effects of mammalian herbivore declines on plant communities: observations and experiments in an African savanna

1. Herbivores influence the structure and composition of terrestrial plant communities. However, responses of plant communities to herbivory are variable and depend on environmental conditions, herbivore identity and herbivore abundance. As anthropogenic impacts continue to drive large declines in wild herbivores, understanding the context dependence of herbivore impacts on plant communities becomes increasingly important.2. Exclosure experiments are frequently used to assess how ecosystems reorganize in the face of large wild herbivore defaunation. Yet in many landscapes, declines in large wildlife are often accompanied by other anthropogenic activities, especially land conversion to livestock production. In such cases, exclosure experiments may not reflect typical outcomes of human-driven extirpations of wild herbivores.3. Here, we examine how plant community responses to changes in the identity and abundance of large herbivores interact with abiotic factors (rainfall and soil properties). We also explore how effects of wild herbivores on plant communities differ between large-scale herbivore exclosures and landscape sites where anthropogenic activity has caused wildlife declines, often accompanied by livestock increases.4. Abiotic context modulated the responses of plant communities to herbivore declines with stronger effect sizes in lower-productivity environments. Also, shifts in plant community structure, composition and species richness following wildlife declines differed considerably between exclosure experiments and landscape sites in which wild herbivores had declined and were often replaced by livestock. Plant communities in low wildlife landscape sites were distinct in both composition and physical structure from both exclosure and control sites in experiments. The power of environmental (soil and rainfall) gradients in influencing plant response to herbivores was also greatly dampened or absent in the landscape sites. One likely explanation for these observed differences is the compensatory effect of livestock associated with the depression or extirpation of wildlife.5. Synthesis. Our results emphasize the importance of abiotic environmental heterogeneity in modulating the effects of mammalian herbivory on plant communities and the importance of such covariation in understanding effects of wild herbivore declines. They also suggest caution when extrapolating results from exclosure experiments to predict the consequences of defaunation as it proceeds in the Anthropocene.

Four years of simulated climate change reduces above‐ground productivity and alters functional diversity in a grassland ecosystem

AbstractQuestionsHow does above‐ground grassland biomass production respond to change in multiple climate drivers over a 4‐yr period? Can climate‐induced patterns of biomass response be explained by shifts in plant community structure? Does sustained climate change affect the relationships between abundance of functional groups, community‐scale leaf traits and above‐ground production?LocationPerennial grassland in the French Massif Central.MethodsMonoliths extracted from the study grassland were exposed to a simulated climate change corresponding to the air temperature, atmospheric CO 2 and summer rainfall conditions projected for 2080. We examined impacts of climate treatments on above‐ground biomass and community structure for 4 yr, and investigated the relationship between biomass production, species diversity and three key functional traits: specific leaf area, leaf dry matter content and leaf N content.ResultsBoth warming and simultaneous application of warming, summer drought and elevated CO 2 were associated with an increase in annual above‐ground biomass at the start of the study, but biomass responses became progressively negative over the course of the experiment. Decreases in vegetation N exports were also observed over time, possibly due to reduced soil N availability under climate change. Taxonomic diversity showed no response to climate treatments, but the relative abundance of grasses decreased under both warming and simultaneous application of warming, summer drought and elevated CO 2 after 3 yr. In parallel, legume relative abundance increased in all warmed treatments. Functional diversity responses varied depending on climate treatment and leaf trait. In the control treatment, patterns of variation in annual plant biomass were best explained by functional diversity during the study period. However, in warmed treatments, variation in annual plant biomass was more closely linked to the functional traits of dominant species.ConclusionsContinuous, multi‐year exposure to projected climate conditions has a negative impact on above‐ground biomass in our grassland study system. Our data suggest that climate‐induced decreases in above‐ground biomass may be driven by changes in the relative abundance of plant functional groups, and could also reflect changes in soil nutrient availability. Unlike species diversity, community‐level leaf traits and functional diversity appear to play an important role for above‐ground biomass production, and may have indirect effects on ecosystem stability in changing climates.

Experimental changes in disturbance type do not induce short‐term shifts in plant community structure in three semi‐arid grasslands of the Victorian Riverine Plain managed for nature conservation

Summary Since the mid 1990s, there has been a significant increase in the area of semi‐arid grasslands included in the National Reserve Systems in the Victorian Riverine Plain. This expansion has not been matched by an improved understanding of the alternate disturbance regimes that might produce better outcomes for native ecosystem conservation. Over the past 150 years, stock grazing has completely replaced fire in these grasslands. As a result, the impact of fire on native (and exotic) plant biodiversity is little understood. This study compared the current grazing regime (i.e. ‘status quo’) with burning and the removal of grazing (‘deferred’ management) across three grasslands in the Victorian Riverine Plain to determine the effects of short‐term exposure to alternate disturbances on community structure. Our results showed little change in species density, composition or abundance under the three disturbance treatments. A long exposure to stock grazing may have reduced the abundance of species likely to respond positively to burning. The cover of the biological soil crust responded positively to fire; such changes are known to significantly influence establishment and the functional composition of communities. As such, further investigation of the functional attributes of these communities may broaden our understanding of short‐term responses to alternate disturbance events. To better understand the utility of fire as a management tool, a long‐term commitment to expanding the implementation of this regime from its current extent will greatly increase the understanding of alternate disturbances in this landscape.