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The US National Science Foundation—funded Long Term Ecological Research (LTER) Network supports a large (around 240) and diverse portfolio of long-term ecological experiments. Collectively, these long-term experiments have (a) provided unique insights into ecological patterns and processes, although such insight often became apparent only after many years of study; (b) influenced management and policy decisions; and (c) evolved into research platforms supporting studies and involving investigators who were not part of the original design. Furthermore, this suite of long-term experiments addresses, at the site level, all of the US National Research Council's Grand Challenges in Environmental Sciences. Despite these contributions, we argue that the scale and scope of global environmental change requires a more-coordinated multisite approach to long-term experiments. Ideally, such an approach would include a network of spatially extensive multifactor experiments, designed in collaboration with ecological mod...

Precipitation and temperature drive many aspects of terrestrial ecosystem function. Climate change scenarios predict increasing precipitation variability and temperature, and long term experiments are required to evaluate the ecosystem consequences of interannual climate variation, increased growing season (intra-annual) rainfall variability, and warming. We present results from an experiment applying increased growing season rainfall variability and year round warming in native tallgrass prairie. During ten years of study, total growing season rainfall varied 2-fold, and we found ~50–200% interannual variability in plant growth and aboveground net primary productivity (ANPP), leaf carbon assimilation (ACO2), and soil CO2 efflux (JCO2) despite only ~40% variation in mean volumetric soil water content (0–15 cm, Θ15). Interannual variation in soil moisture was thus amplified in most measures of ecosystem response. Differences between years in Θ15 explained the greatest portion (14–52%) of the variation in these processes. Experimentally increased intra-annual season rainfall variability doubled the amplitude of intra-annual soil moisture variation and reduced Θ15 by 15%, causing most ecosystem processes to decrease 8–40% in some or all years with increased rainfall variability compared to ambient rainfall timing, suggesting reduced ecosystem rainfall use efficiency. Warming treatments increased soil temperature at 5 cm depth, particularly during spring, fall, and winter. Warming advanced canopy green up in spring, increased winter JCO2, and reduced summer JCO2 and forb ANPP, suggesting that the effects of warming differed in cooler versus warmer parts of the year. We conclude that (1) major ecosystem processes in this grassland may be substantially altered by predicted changes in interannual climate variability, intra-annual rainfall variability, and temperature, (2) interannual climate variation was a larger source of variation in ecosystem function than intra-annual rainfall variability and warming, and (3) effects of increased growing season rainfall variability and warming were small, but ecologically important. The relative effects of these climate drivers are likely to vary for different ecosystem processes and in wetter or drier ecosystems.

Summary 1. Growing recognition of the importance of climate extremes as drivers of contemporary and future ecological dynamics has led to increasing interest in studying these locally and globally important phenomena. 2. Many ecological studies examining the impacts of what are deemed climate extremes, such as heat waves and severe drought, do not provide a definition of extremity, either from a statistical context based on the long-term climatic record or from the perspective of the response of the system – are the effects extreme (unusual or profound) in comparison to normal variability? 3. A synthetic definition of an extreme climatic event (ECE) is proposed that includes ‘extremeness’ in both the driver and the response: an ECE is as an episode or occurrence in which a statistically rare or unusual climatic period alters ecosystem structure and/or function well outside the bounds of what is considered typical or normal variability. This definition is accompanied by a mechanistic framework based on the concept that extreme response thresholds associated with significant community change and altered ecosystem function must be crossed in order for an ECE to occur. 4. Synthesis. A definition and mechanistic framework for ECEs is used to identify priorities for future research that will enable ecologists to more fully assess the ecological consequences of climate extremes for ecosystem structure and function today and in a future world where their frequency and intensity are expected to increase.

Summary 1. Climate extremes, such as severe drought, heat waves and periods of heavy rainfall, can have profound consequences for ecological systems and for human welfare. Global climate change is expected to increase both the frequency and the intensity of climate extremes and there is an urgent need to understand their ecological consequences. 2. Major challenges for advancing our understanding of the ecological consequences of climate extremes include setting a climatic baseline to facilitate the statistical determination of when climate conditions are extreme, having sufficient knowledge of ecological systems so that extreme ecological responses can be identified, and finally, being able to attribute a climate extreme as the driver of an extreme ecological response, defined as an extreme climatic event (ECE). 3. The papers in this issue represent a cross-section of the emerging field of climate extremes research, including an examination of the palaeo-ecological record to assess patterns and drivers of extreme ecological responses in the late Quaternary, experiments in grasslands assessing a range of ecological responses and the role of ecotypic variation in determining responses to climate extremes, and the quantification of the ecological consequences of a recent ECE in the desert Southwest of the USA. 4. Synthesis. The papers in this Special Feature suggest that although the occurrence of ECEs may be common in palaeo-ecological and observational studies, studies in which climate extremes have been experimentally imposed often do not result in ecological responses outside the bounds of normal variability of a system. Thus, ECEs occur much less frequently than their potential drivers and even less frequently than observational studies suggest. Future research is needed to identify the types and time-scales of climate extremes that result in ECEs, the potential for interactions among different types of climate changes and extremes, and the role of genetic, species and trait diversity in determining ecological responses and their evolutionary consequences. These research priorities require the development of alternative research approaches to impose realistic climate extremes on a broad range of organisms and ecosystems.

Genotypic diversity of dominant species has been shown to have important consequences for community and ecosystem processes at a fine spatial scale. We examined the fine-scale (i.e., plant neighborhood scale

Many serious ecosystem consequences of climate change will take decades or even centuries to emerge. Long-term ecological responses to global change are strongly regulated by slow processes, such as changes in species composition, carbon dynamics in soil and by long-lived plants, and accumulation of nutrient capitals. Understanding and predicting these processes require experiments on decadal time scales. But decadal experiments by themselves may not be adequate because many of the slow processes have characteristic time scales much longer than experiments can be maintained. This article promotes a coordinated approach that combines long-term, large-scale global change experiments with process studies and modeling. Long-term global change manipulative experiments, especially in high-priority ecosystems such as tropical forests and high-latitude regions, are essential to maximize information gain concerning future states of the earth system. The long-term experiments should be conducted in tandem with complementary process studies, such as those using model ecosystems, species replacements, laboratory incubations, isotope tracers, and greenhouse facilities. Models are essential to assimilate data from long-term experiments and process studies together with information from long-term observations, surveys, and space-for-time studies along environmental and biological gradients. Future research programs with coordinated long-term experiments, process studies, and modeling have the potential to be the most effective strategy to gain the best information on long-term ecosystem dynamics in response to global change.

The global reach of human activities affects all natural ecosystems, so that the environment is best viewed as a social–ecological system. Consequently, a more integrative approach to environmental science, one that bridges the biophysical and social domains, is sorely needed. Although models and frameworks for social–ecological systems exist, few are explicitly designed to guide a long-term interdisciplinary research program. Here, we present an iterative framework, “Press–Pulse Dynamics” (PPD), that integrates the biophysical and social sciences through an understanding of how human behaviors affect “press” and “pulse” dynamics and ecosystem processes. Such dynamics and processes, in turn, influence ecosystem services –thereby altering human behaviors and initiating feedbacks that impact the original dynamics and processes. We believe that research guided by the PPD framework will lead to a more thorough understanding of social–ecological systems and generate the knowledge needed to address pervasive environmental problems.

Summary 1. If we are to understand the mechanisms underlying species responses to climate change in natural systems, studies are needed that focus on responses of non-model species under field conditions. We measured transcriptional profiles of individuals of Andropogon gerardii ,aC 4 grass native to North American grasslands, in a field experiment in which both temperature and precipitation were manipulated to simulate key aspects of forecasted climate change. 2. By using microarrays developed for a closely related model species, Zea mays ,w e were able to compare the relative influence of warming versus altered soil moisture availability on expression levels of over 7000 genes, identify responsive functional groups of genes and correlate changes in gene transcription with physiological responses. 3. We observed more statistically significant shifts in transcription levels of genes in response to thermal stress than in response to water stress. We also identified candidate genes that demonstrated transcription levels closely associated with physiological variables, in particular chlorophyll fluorescence. 4. Synthesis. These results suggest that an ecologically important species responds differently to different environmental aspects of forecast climate change. These translational changes have the potential to influence phenotypic characters and ultimately adaptive responses.

In contrast to pulses in resource availability following disturbance events, many of the most pressing global changes, such as elevated atmospheric carbon dioxide concentrations and nitrogen deposition, lead to chronic and often cumulative alterations in available resources. Therefore, predicting ecological responses to these chronic resource alterations will require the modification of existing disturbance-based frameworks. Here, we present a conceptual framework for assessing the nature and pace of ecological change under chronic resource alterations. The "hierarchical-response framework" (HRF) links well-documented, ecological mechanisms of change to provide a theoretical basis for testing hypotheses to explain the dynamics and differential sensitivity of ecosystems to chronic resource alterations. The HRF is based on a temporal hierarchy of mechanisms and responses beginning with individual (physiological/metabolic) responses, followed by species reordering within communities, and finally species loss and immigration. Each mechanism is hypothesized to differ in the magnitude and rate of its effects on ecosystem structure and function, with this variation depending on ecosystem attributes, such as longevity of dominant species, rates of biogeochemical cycling, levels of biodiversity, and trophic complexity. Overall, the HRF predicts nonlinear changes in ecosystem dynamics, with the expectation that interactions with natural disturbances and other global-change drivers will further alter the nature and pace of change. The HRF is explicitly comparative to better understand differential sensitivities of ecosystems, and it can be used to guide the design of coordinated, cross-site experiments to enable more robust forecasts of contemporary and future ecosystem dynamics.

Grassland ecosystems are an important terrestrial component of the global biogeochemical silicon cycle. Although the structure and ecological functioning of grasslands are strongly influenced by fire and grazing, the role of these key ecological drivers in the production and storage of silicon represents a significant knowledge gap, particularly since they are being altered worldwide by human activities. We evaluated the effects of fire and grazing on the range and variability of plant derived biogenic silica stored in plant biomass and soils by sampling plants and soils from long-term experimental plots with known fire and grazing histories. Overall, plants and soils from grazed sites in the South African ecosystems had up to 76 and 54% greater biogenic silica totals (kg ha−1), respectively, than grazed North American sites. In North American soils, the combination of grazing and annual fire resulted in the greatest abundance of biogenic silica, whereas South African soils had the highest biogenic silica content where grazed regardless of burn frequency. These results as well as those that show greater Si concentrations in grazed South African plants indicate that South African plants and soils responded somewhat differently to fire and grazing with respect to silicon cycling, which may be linked to differences in the evolutionary history and in the grazer diversity and grazing intensity of these ecosystems. We conclude that although fire and grazing (as interactive and/or independent factors) do not affect the concentration of Si taken up by plants, they do promote increased silicon storage in aboveground biomass and soil as a result of directly affecting other site factors such as aboveground net primary productivity. Therefore, as management practices, fire and grazing have important implications for assessing global change impacts on the terrestrial biogeochemical cycling of silicon.

Patterns and controls of annual aboveground net primary productivity (ANPP) are fundamental metrics of ecosystem functioning. It is generally assumed, but rarely tested, that determinants of ANPP in one region within a biome will operate similarly throughout that biome, as long as physiognomy and climate are broadly consistent. We tested this assumption by quantifying ANPP responses to fire, grazing history, and nitrogen (N) addition in North American (NA) and South African (SA) savanna grasslands. We found that total ANPP responded in generally consistent ways to fire, grazing history, and N addition on both continents. Annual fire in both NA and SA consistently stimulated total ANPP (28–100%) relative to unburned treatments at sites with deep soils, and had no effect on ANPP in sites with shallow soils. Fire did not affect total ANPP in sites with a recent history of grazing, regardless of whether a single or a diverse suite of large herbivores was present. N addition interacted strongly and consistently with fire regime in both NA and SA. In annually burned sites that were not grazed, total ANPP was stimulated by N addition (29–39%), but there was no effect of N fertilization in the absence of fire. In contrast, responses in forb ANPP to fire and grazing were somewhat divergent across this biome. Annual fire in NA reduced forb ANPP, whereas grazing increased forb ANPP, but neither response was evident in SA. Thus, despite a consistent response in total ANPP, divergent responses in forb ANPP suggest that other aspects of community structure and ecosystem functioning differ in important ways between these mesic savanna grasslands.

Plant-herbivore interactions mediate the trophic structure of ecosystems. We use a comprehensive data set extracted from the literature to test the relative explanatory power of two contrasting bodies of ecological theory, the metabolic theory of ecology (MTE) and ecological stoichiometry (ES), for per-capita and population-level rates of herbivory across ecosystems. We found that ambient temperature and herbivore body size (MTE) as well as stoichiometric mismatch (ES) both constrained herbivory, but at different scales of biological organization. Herbivore body size, which varied over 11 orders of magnitude, was the primary factor explaining variation in per-capita rates of herbivory. Stoichiometric mismatch explained more variation in population-level herbivory rates and also in per-capita rates when we examined data from within functionally similar trophic groups (e.g. zooplankton). Thus, predictions from metabolic and stoichiometric theories offer complementary explanations for patterns of herbivory that operate at different scales of biological organization.

Ecosystem responses to climate change will largely be driven by responses of the dominant species. However, if co-dominant species have traits that lead them to differential responses, then predicting how ecosystem structure and function will be altered is more challenging. We assessed differences in response to climate change factors for the two dominant C4 grass species in tallgrass prairie, Andropogon gerardii and Sorghastrum nutans, by measuring changes in a suite of plant ecophysiological traits in response to experimentally elevated air temperatures and increased precipitation variability over two growing seasons. Maximum photosynthetic rates, stomatal conductance, water-use efficiency, chlorophyll fluorescence, and leaf water potential varied with leaf and canopy temperature as well as with volumetric soil water content (0–15 cm). Both species had similar responses to imposed changes in temperature and water availability, but when differences occurred, responses by A. gerardii were more closely linked with changes in air temperature whereas S. nutans was more sensitive to changes in water availability. Moreover, S. nutans was more responsive overall than A. gerardii to climate alterations. These results indicate both grass species are responsive to forecast changes in temperature and precipitation, but their differential sensitivity to temperature and water availability suggest that future population and community structure may vary based on the magnitude and scope of an altered climate.

The use of genomic techniques to address ecological questions is emerging as the field of genomic ecology. Experimentation under environmentally realistic conditions to investigate the molecular response of plants to meaningful changes in growth conditions and ecological interactions is the defining feature of genomic ecology. Because the impact of global change factors on plant performance are mediated by direct effects at the molecular, biochemical, and physiological scales, gene expression analysis promises important advances in understanding factors that have previously been consigned to the ‘black box’ of unknown mechanism. Various tools and approaches are available for assessing gene expression in model and nonmodel species as part of global change biology studies. Each approach has its own unique advantages and constraints. A first generation of genomic ecology studies in managed ecosystems and mesocosms have provided a testbed for the approach and have begun to reveal how the experimental design and data analysis of gene expression studies can be tailored for use in an ecological context.

Loss of biodiversity poses one of the greatest threats to natural ecosystems throughout the world. However, a comprehensive understanding of the impacts of species losses from upper trophic levels is still emerging. Here we compare the impacts of large mammalian herbivore species loss on grassland plant community structure and composition in a South African and North American grassland. Herbaceous plant communities were surveyed at sites without large mammalian herbivores present and at sites with a single species of herbivore present in both locations, and additionally at one site in South Africa with multiple herbivore species. At both the North American and South African locations, plant communities on sites with a single herbivore species were more diverse and species rich than on sites with no herbivores. At the multi-herbivore site in South Africa, plant diversity and richness were comparable to that of the single herbivore site early in the growing season and to the no herbivore site late in the growing season. Analyses of plant community composition, however, indicated strong differences between the multi-herbivore site and the single and no herbivore sites, which were more similar to each other. In moderate to high-productivity ecosystems with one or a few species of large herbivores, loss of herbivores can cause a significant decrease in plant diversity and richness, and can have pronounced impacts on grassland plant community composition. In ecosystems with higher herbivore richness, species loss may also significantly alter plant community structure and composition, although standard metrics of community structure may obscure these differences.

Amplification of the hydrological cycle as a consequence of global warming is forecast to lead to more extreme intra-annual precipitation regimes characterized by larger rainfall events and longer intervals between events. We present a conceptual framework, based on past investigations and ecological theory, for predicting the consequences of this underappreciated aspect of climate change. We consider a broad range of terrestrial ecosystems that vary in their overall water balance. More extreme rainfall regimes are expected to increase the duration and severity of soil water stress in mesic ecosystems as intervals between rainfall events increase. In contrast, xeric ecosystems may exhibit the opposite response to extreme events. Larger but less frequent rainfall events may result in proportional reductions in evaporative losses in xeric systems, and thus may lead to greater soil water availability. Hydric (wetland) ecosystems are predicted to experience reduced periods of anoxia in response to pr...

We surveyed the prevalence and amount of leaf damage related to herbivory and pathogens on 12 pairs of exotic (invasive and noninvasive) and ecologically similar native plant species in tallgrass prairie to examine whether patterns of damage match predictions from the enemy release hypothesis. We also assessed whether natural enemy impacts differed in response to key environmental factors in tallgrass prairie by surveying the prevalence of rust on the dominant C4 grass, Andropogon gerardii, and its congeneric invasive exotic C4 grass, A. bladhii, in response to fire and nitrogen fertilization treatments. Overall, we found that the native species sustain 56.4% more overall leaf damage and 83.6% more herbivore-related leaf damage when compared to the exotic species. Moreover, we found that the invasive exotic species sustained less damage from enemies relative to their corresponding native species than the noninvasive exotic species. Finally, we found that burning and nitrogen fertilization both significantly increased the prevalence of rust fungi in the native grass, while rust fungi rarely occurred on the exotic grass. These results indicate that reduced damage from enemies may in part explain the successful naturalization of exotic species and the spread of invasive exotic species in tallgrass prairie.

In grasslands worldwide, grazing by ungulates and periodic fires are important forces affecting resource availability and plant community structure. It is not clear, however, whether changes in community structure are the direct effects of the disturbance (i.e. fire and grazing) or are mediated indirectly through changes in resource abundance and availability. In North American tallgrass prairies, fire and grazing often have disparate effects on plant resources and plant diversity, yet, little is known about the individual and interactive effects of fire and grazing on resource variability and how that variability relates to heterogeneity in plant community structure, particularly at small scales. We conducted a Field study to determine the interactive effects of different long-term fire regimes (annual vs four-year fire frequency) and grazing by native ungulates (Bos bison) on small-scale plant community structure and resource variability (N and light) in native tallgrass prairie. Grazing enhanced light and nitrogen availability, but did not affect small-scale resource variability. In addition, grazing reduced the dominance of C-4 grasses which enhanced species richness, diversity and community heterogeneity. In contrast, annual fire increased community dominance and reduced species richness and diversity, particularly in the absence of grazing, but had no effect on community heterogeneity, resource availability and resource variability. Variability in the abundance of resources showed no relationship with community heterogeneity at the scale measured in this study, however we found a relationship between community dominance and heterogeneity. Therefore, we conclude that grazing generated small-scale community heterogeneity in this mesic grassland by directly affecting plant community dominance, rather than indirectly through changes in resource variability.

Models predict that precipitation variability will increase with climate change. We used a 15-year precipitation manipulation experiment to determine if altering the timing and amount of growing season rainfall will impact plant community structure in annually burned, native tallgrass prairie. The altered precipitation treatment maintained the same total growing season precipitation as the ambient precipitation treatment, but received a rainfall regime of fewer, larger rain events and longer intervals between events each growing season. Although this change in precipitation regime significantly lowered mean soil water content, overall this plant community was remarkably resistant to altered precipitation with species composition relatively stable over time. However, we found significantly higher forb cover and richness and slightly lower grass cover on average with altered precipitation, but the forb responses were manifest only after a ten-year lag period. Thus, although community structure in this grassland is relatively resistant to this type of altered precipitation regime, forb abundance in native tallgrass prairie may increase in a future characterized by increased growing season precipitation variability.

Theoretical analyses and experimental studies of synthesized assemblages indicate that under particular circumstances species diversity can enhance community productivity through niche complementarity. It remains unclear whether this process has important effects in mature natural ecosystems where competitive feedbacks and complex environmental influences affect diversity-productivity relationships. In this study, we evaluated diversity-productivity relationships while statistically controlling for environmental influences in 12 natural grassland ecosystems. Because diversity-productivity relationships are conspicuously nonlinear, we developed a nonlinear structural equation modeling ( SEM) methodology to separate the effects of diversity on productivity from the effects of productivity on diversity. Meta-analysis was used to summarize the SEM findings across studies. While competitive effects were readily detected, enhancement of production by diversity was not. These results suggest that the influence of small-scale diversity on productivity in mature natural systems is a weak force, both in absolute terms and relative to the effects of other controls on productivity.

Recent reviews have emphasized the need to incorporate genomics into ecological field studies to further understand how species respond to changing environmental conditions. Genomic tools, such as cDNA (complementary DNA) microarrays, allow for the simultaneous analysis of gene expression of thousands of genes from all or part of an organism's genome (the transcription profile), thereby revealing the genetic mechanisms that underlie species' responses to environmental change. However, despite their potential, two major limitations have hindered the incorporation of microarrays and other genomic tools into field studies: (1) the limited availability of microarrays for ecologically relevant, non-model species and limited financial resources for developing new microarrays; and (2) concern that high sensitivity of gene expression to even subtle alterations in environmental conditions will hinder detection of relevant changes in field measures of transcription profiles. Here, we show that with cross-species hy...

Why some species are consistently more abundant than others, and predicting how species will respond to global change, are fundamental questions in ecology. Long-term observations indicate that plant species with high stoichiometric homeostasis for nitrogen (HN), i.e., the ability to decouple foliar N levels from variation in soil N availability, were more common and stable through time than low-HN species in a central U.S. grassland. However, with nine years of nitrogen addition, species with high H(N) decreased in abundance, while those with low H(N) increased in abundance. In contrast, in climate change experiments simulating a range of forecast hydrologic changes, e.g., extreme drought (two years), increased rainfall variability (14 years), and chronic increases in rainfall (21 years), plant species with the highest H(N) were least responsive to changes in soil water availability. These results suggest that H(N) may be predictive of plant species success and stability, and how plant species and ecosystems will respond to global-change-driven alterations in resource availability.

To better understand the mechanisms underlying plant species responses to climate change, we compared transcriptional profiles of the co-dominant C4 grasses, Andropogon gerardii Vitman and Sorghastrum nutans (L.) Nash, in response to increased temperatures and more variable precipitation regimes in a long-term field experiment in native tallgrass prairie. We used microarray probing of a closely related model species (Zea mays) to assess correlations in leaf temperature (Tleaf) and leaf water potential (LWP) and abundance changes of ~10,000 transcripts in leaf tissue collected from individuals of both species. A greater number of transcripts were found to significantly change in abundance levels with Tleaf and LWP in S. nutans than in A. gerardii. S. nutans also was more responsive to short-term drought recovery than A. gerardii. Water flow regulating transcripts associated with stress avoidance (e.g., aquaporins), as well as those involved in the prevention and repair of damage (e.g., antioxidant enzymes, HSPs), were uniquely more abundant in response to increasing Tleaf in S. nutans. The differential transcriptomic responses of the co-dominant C4 grasses suggest that these species may cope with and respond to temperature and water stress at the molecular level in distinct ways, with implications for tallgrass prairie ecosystem function.

Timing of precipitation is equally important as amount for determining ecosystem function, especially aboveground net primary productivity (ANPP), in a number of ecosystems. In tallgrass prairie of the Central Plains of North America, grass flowering stalks of dominant C4 grasses, such as Andropogon gerardii, can account for more than 70 % of ANPP, or almost none of it, as the number of flowering stalks produced is highly variable. Although growing season precipitation amount is important for driving variation in flowering stalk production, it remains unknown whether there are critical periods within the growing season in which sufficient rainfall must occur to allow for flowering. The effect of timing of rainfall deficit (drought) on flowering of A. gerardii, was tested by excluding rainfall during three periods within the growing season (starting in mid-April, mid-May and mid-June). Mid-summer drought (starting in mid-June) strongly reduced the flowering rate (e.g., density and biomass) of A. gerardii (e.g., as high as 94 % compared to the control), suggesting flowering is highly sensitive to precipitation at this time. This effect appeared to be related to plant water status at the time of flowering stalk initiation, rather than an indirect consequence of reduced C assimilation. Our results suggest that increased frequency of growing season drought forecast with climate change could reduce sexual reproduction in this dominant grass species, particularly if it coincides with timing of flowering stalk initiation, with important implications for ecosystem functioning.

Exotic species dominate many communities; however the functional significance of species' biogeographic origin remains highly contentious. This debate is fuelled in part by the lack of globally replicated, systematic data assessing the relationship between species provenance, function and response to perturbations. We examined the abundance of native and exotic plant species at 64 grasslands in 13 countries, and at a subset of the sites we experimentally tested native and exotic species responses to two fundamental drivers of invasion, mineral nutrient supplies and vertebrate herbivory. Exotic species are six times more likely to dominate communities than native species. Furthermore, while experimental nutrient addition increases the cover and richness of exotic species, nutrients decrease native diversity and cover. Native and exotic species also differ in their response to vertebrate consumer exclusion. These results suggest that species origin has functional significance, and that eutrophication will lead to increased exotic dominance in grasslands., It remains unclear whether exotic and native species are functionally different. Using a global grassland experiment, Seabloom et al. show that native and exotic species respond differently to two globally pervasive environmental changes, addition of mineral nutrients and alteration of herbivore density.

© 2015 John Wiley & Sons Ltd. Climate change is intensifying the hydrologic cycle and is expected to increase the frequency of extreme wet and dry years. Beyond precipitation amount, extreme wet and dry years may differ in other ways, such as the number of precipitation events, event size, and the time between events. We assessed 1614 long-term (100 year) precipitation records from around the world to identify key attributes of precipitation regimes, besides amount, that distinguish statistically extreme wet from extreme dry years. In general, in regions where mean annual precipitation (MAP) exceeded 1000 mm, precipitation amounts in extreme wet and dry years differed from average years by ~40% and 30%, respectively. The magnitude of these deviations increased to >60% for dry years and to >150% for wet years in arid regions (MAP99th percentile of all events); these occurred twice as often in extreme wet years compared to average years. In contrast, these large precipitation events were rare in extreme dry years. Less important for distinguishing extreme wet from dry years were mean event size and frequency, or the number of dry days between events. However, extreme dry years were distinguished from average years by an increase in the number of dry days between events. These precipitation regime attributes consistently differed between extreme wet and dry years across 12 major terrestrial ecoregions from around the world, from deserts to the tropics. Thus, we recommend that climate change experiments and model simulations incorporate these differences in key precipitation regime attributes, as well as amount into treatments. This will allow experiments to more realistically simulate extreme precipitation years and more accurately assess the ecological consequences.

Identifying key ecophysiological traits that differ among dominant plant species and can be linked to species‐specific responses to drought would improve our ability to forecast community and ecosystem responses to global climate change. The mesic grasslands of the central plains of North America are dominated by two C4 grass species, Andropogon gerardii and Sorghastrum nutans, which purportedly differ in their tolerance of water stress. Individuals of these two species were grown in the field under rain‐out shelters and subjected to wet (watered every 2–3 d) or dry (repeatedly subjected to wilting before watering) soil moisture regimes. A range of ecophysiological traits potentially important for tolerating water stress were concurrently measured. Although few traits differed between the species in the wet treatment, several traits were identified in the dry treatment that may enable A. gerardii to better tolerate drought. These were greater allocation to roots, reduced allocation to flowering, more rapi...

Natural disturbances affect spatial and temporal heterogeneity in plant communities, but effects vary depending on type of disturbance and scale of analysis. In this study, we examined the effects of fire frequency (1-, 4-, and 20-yr intervals) and grazing by bison on spatial and temporal heterogeneity in species composition in tallgrass prairie plant communities. Compositional heterogeneity was estimated at 10-, 50-, and 200-m2 scales. For each measurement scale, we used the average Euclidean Distance (ED) between samples within a year (2000) to measure spatial heterogeneity and between all time steps (1993-2000) for each sample to measure temporal heterogeneity. The main effects of fire and grazing were scale independent. Spatial and temporal heterogeneity were lowest on annually burned sites and highest on infrequently burned (20-yr) sites at all scales. Grazing reduced spatial heterogeneity and increased temporal heterogeneity at all scales. The rate of community change over time decreased as fire frequency increased at all scales, whereas grazing had no effect on rate of community change over time at any spatial scale. The interactive effects of fire and grazing on spatial and temporal heterogeneity differed with scale. At the 10-m2 scale, grazing increased spatial heterogeneity in annually burned grassland but decreased heterogeneity in less frequently burned areas. At the 50-m2 scale, grazing decreased spatial heterogeneity on 4-yr burns but had no effect at other fire frequencies. At the 10-m scale, grazing increased temporal heterogeneity only on 1- and 20-yr burn sites. Our results show that the individual effects of fire and grazing on spatial and temporal heterogeneity in mesic prairie are scale independent, but the interactive effects of these disturbances on community heterogeneity change with scale of measurement. These patterns reflect the homogenizing impact of fire at all spatial scales, and the different frequency, intensity, and scale of patch grazing by bison in frequently burned vs. infrequently burned areas.

The species-time relationship (STR) describes how the species richness of a community increases with the time span over which the community is observed. This pattern has numerous implications for both theory and conservation in much the same way as the species-area relationship (SAR). However, the STR has received much less attention and to date only a handful of papers have been published on the pattern. Here we gather together 984 community time-series, representing 15 study areas and nine taxonomic groups, and evaluate their STRs in order to assess the generality of the STR, its consistency across ecosystems and taxonomic groups, its functional form, and its relationship to local species richness. In general, STRs were surprisingly similar across major taxonomic groups and ecosystem types. STRs tended to be well fit by both power and logarithmic functions, and power function exponents typically ranged between 0.2 and 0.4. Communities with high richness tended to have lower STR exponents, suggesting that factors increasing richness may simultaneously decrease turnover in ecological systems. Our results suggest that the STR is as fundamental an ecological pattern as the SAR, and raise questions about the general processes underlying this pattern. They also highlight the dynamic nature of most species assemblages, and the need to incorporate time scale in both basic and applied research on species richness patterns.

Hyperspectral imagery of the Konza Prairie Biological Station in northeastern Kansas was used to evaluate upwelling spectral radiance, prairie spectral reflectance and band ratios of each as potential indicators of vascular plant species richness in a mesic grassland. The extent to which spatial variability in these parameters related to plant species richness also was investigated. A 224 channel hyperspectral data cube acquired in June 2000 by the Airborne Visible and Infrared Imaging Spectrometer (AVIRIS) provided complete coverage of the 400–2500 nm range at approximately 10 nm per channel. After band deletions accounted for detector overlap and strong atmospheric attenuation features, 176 bands were retained for analysis and spanned the 404–2400 nm range. Prairie reflectance was estimated via radiative transfer modeling and scaling to a library spectrum of highway construction material. Data were sampled from pixels having a 19 m ground sample distance (GSD) to represent each of 93 vegetation sampling transects. Reflectance and radiance at mid-infrared wavelengths (e.g., 1553 nm), and band ratios that were based on atmospheric windows in the red, near-infrared and mid-infrared spectra estimated species richness to within 6 to 7 species per transect. The 856 to 780 nm radiance or reflectance ratio yielded maximum adjusted coefficients of determination (r2) of approximately 0.4 in regressions with richness when data from bison-grazed and ungrazed areas were combined. These regressions remained significant (p ≤ 0.001) when only ungrazed areas were assessed although r2 reduced to approximately 0.2. Richness was related significantly also to the 433 to 674 nm reflectance ratio for grazed-plus-ungrazed and ungrazed-only areas. In contrast, the effectiveness of the 433 to 674 nm radiance ratio was reduced by atmospheric backscatter. Species richness did not correlate strongly or consistently with transect spatial variability (coefficient of variation or range) in radiance, reflectance or band ratio value, apparently as a consequence of the relatively small area sampled for each transect (approximately 0.5 ha). Relationships between richness and prairie spectral features were explained by the influence of soil exposure on both parameters. Richness and estimated soil exposure tended to increase from ungrazed lowlands, to ungrazed slopes, to ungrazed uplands to grazed areas. Remotely sensed estimates of soil exposure may be particularly useful in addressing plant species richness on grazed grasslands owing to an overall similarity in spectral reflectance among dominant plant species.

Quantifying patterns of temporal or spatial change in community structure is critical for assessing the impact of disturbances on biological systems and the stability of ecosystems. Detecting change in communities can be problematic, however, because of the inherent variability of systems and limitations of commonly used methods, such as similarity indices and ordination that do not explicitly test a hypothesis. Here we present empirical data to show a strong relationship between species mean abundance and variability across three broad taxonomic groups (plants, zooplankton, and fish). These statistical re- lationships were then used to construct null models of expected community variability that were used to test against the observed temporal change of these communities. We evaluated the ability of this approach to detect significant temporal change above that associated with random variation with nine communities (three Midwestern stream fish, three north tem- perate zooplankton, and three tallgrass prairie plant), each having long-term data sets and different expected levels of change. Nonrandom change was detected in 21.3% of samples from the expected low-change communities, 52.6% in moderately changed communities, and 60.4% in high-change communities. Thus, this approach was effective in detecting change over time in those communities expected to change most. By using empirical re- lationships between species abundance and variability, this null model approach provides ecologists and resource managers an objective tool, which can be used along with existing community indices and statistical techniques to assess the type and magnitude of community change with limited data sets.

Ecology has emerged as a global science, and there is a pressing need to identify ecological rules – general principles that will improve its predictive capability for scientists and its usefulness for managers and policy makers. Ideally, the generality and limits of these ecological rules should be assessed using extensive, coordinated experiments that ensure consistency in design and comparability of data. To improve the design of these large-scale efforts, existing data should be used to test prospective ecological rules and to identify their limits and contingencies. As an example of this approach, we describe prospective rules for grassland responses to fire and rainfall gradients, identified from long-term studies of North American grasslands and tested with existing data from long-term experiments in South African savanna grasslands. Analyses indicated consistent effects of fire on the abundance of the dominant (grasses) and subdominant (forbs) flora on both continents, but no common response of gr...

Recent research in aquatic systems suggests that productivity-richness re- lationships change with spatial scale and that species turnover (i.e., spatial and temporal variation in species composition) plays an important role in generating this scale depen- dence. The generality of such scale dependence and the effects of variation in temporal scale remain unknown. We examined the extent to which the richness-productivity rela- tionship in terrestrial plant communities depends on spatial or temporal scale and evaluated how spatial and temporal turnover (i.e., species turnover in space and time) generates scale dependence in these relationships using data from two Long-Term Ecological Research (LTER) sites (Jornada and Konza). We found a weak hump-shaped relationship (Jornada) and no relationship (Konza) between richness and productivity at the smallest focal scale (1 m 2 at Jornada and 50 m 2 at Konza) at each site, but strong hump-shaped relationships at the largest focal scale (49 m 2 at Jornada and 200 m 2 at Konza) for each site. Relationships between spatial turnover and productivity at each site mirrored the productivity-richness relationships that emerged at the larger spatial scale (i.e., a significant hump-shaped pattern). In contrast, temporal turnover was unrelated to productivity, and hence increasing temporal scale did not appreciably change the form of the productivity-richness relationship. Our study suggests that the way in which productivity-richness relationships change with spatial or temporal scale depends on the form and strength of the underlying relationship between species turnover and productivity. Moreover, we contend that a dominant effect of increasing productivity is the generation of dissimilarity in species composition among localities that comprise a region, rather than increasing the number of species that occur within local communities. Thus, understanding the mechanisms that cause species turnover to vary with productivity is critical to understanding scale dependence in richness-productivity rela- tionships.

Ecologists have long sought to understand the relationships among species diversity, community productivity and invasion by non-native species. Here, four long-term observational datasets were analyzed using repeated measures statistics to determine how plant species richness and community resource capture (i.e. productivity) influenced invasion. Multiple factors influenced the results, including the metric used to quantify invasion, interannual variation and spatial scale. Native richness was positively correlated with non-native richness, but was usually negatively correlated with non-native abundance, and these patterns were stronger at the larger spatial scale. Logistic regressions indicated that the probability of invasion was reduced both within and following years with high productivity, except at the desert grassland site where high productivity was associated with increased invasion. Our analysis suggests that while non-natives were most likely to establish in species rich communities, their success was diminished by high resource capture by the resident community.

Many recent studies suggest that more diverse communities are more resistant to invasion. Community characteristics that most strongly influence invasion are uncertain, however, due to covariation of diversity with competition and crowding. We examined separately the effects of species richness and dominance on invasion by an exotic legume, Melilotus officinalis, in intact, native Kansas grassland. We manipulated dominance of C4 grasses by reducing their abundance (i.e. ramet densities) by � /25 and 50%. In addition, richness was reduced by removing species that were mainly rare and uncommon as might be expected with environmental changes such as drought and fragmentation. In both years of the study (2001/2002), invasibility, measured as peak establishment of Melilotus, was not affected by a 3fold reduction in species richness, nor was there an interaction between loss of species and reduced dominance on invasion. In contrast, reductions in abundance of the dominants significantly reduced invasibility of the grassland plots in both years. Because the abundance of dominants was highly correlated with measures of competition (i.e. ratio of dominant biomass to total biomass) and crowding (total stem densities), this pattern was opposite to that expected if competition were indeed limiting invasion. Rather, invasion appeared to be facilitated by the dominant species, most likely because reduced dominance increased environmental stress. Our results suggest that dominance is the key community characteristic determining invasibility, because highly competitive and space-filling species can either enhance or reduce susceptibility to invasion depending on whether dominants create a more competitive environment or alleviate stressful conditions.

Water availability limits plant growth and production in almost all terrestrial ecosystems. However, biomes differ substantially in sensitivity of aboveground net primary production (ANPP) to between-year variation in precipitation. Average rain-use efficiency (RUE; ANPP/precipitation) also varies between biomes, supposedly because of differences in vegetation structure and/or biogeochemical constraints. Here we show that RUE decreases across biomes as mean annual precipitation increases. However, during the driest years at each site, there is convergence to a common maximum RUE (RUE(max)) that is typical of arid ecosystems. RUE(max) was also identified by experimentally altering the degree of limitation by water and other resources. Thus, in years when water is most limiting, deserts, grasslands and forests all exhibit the same rate of biomass production per unit rainfall, despite differences in physiognomy and site-level RUE. Global climate models predict increased between-year variability in precipitation, more frequent extreme drought events, and changes in temperature. Forecasts of future ecosystem behaviour should take into account this convergent feature of terrestrial biomes.

A new analysis of the nearly century-old Lotka-Volterra theory allows us to link species interactions to biodiversity patterns, including: species abundance distributions, estimates of total community size, patterns of community invasibility, and predicted responses to disturbance. Based on a few restrictive assumptions about species interactions, our calculations require only that the community is sufficiently large to allow a mean-field approximation. We develop this analysis to show how an initial assemblage of species with varying interaction strengths is predicted to sort out into the final community based on the species' predicted target densities. The sorting process yields predictions of covarying patterns of species abundance, community size, and species interaction strengths. These predictions can be tested using enrichment experiments, examination of latitudinal and productivity gradients, and features of community assembly. (Less)

Loss of species caused by widespread stressors, such as drought and fragmentation, is likely to be non-random depending on species abundance in the community. We experimentally reduced the number of rare and uncommon plant species while independently reducing only the abundance of dominant grass species in intact, native grassland. This allowed us to simulate a non-random pattern of species loss, based on species abundances, from communities shaped by natural ecological interactions and characterized by uneven species abundance distributions. Over two growing seasons, total above-ground net primary productivity (ANPP) declined with reductions in abundance of the dominant species but was unaffected by a threefold decline in richness of less common species. In contrast, productivity of the remaining rare and uncommon species decreased with declining richness, in part due to loss of complementary interactions among these species. However, increased production of the dominant grasses offset the negative effects of species loss. We conclude that the dominant species, as controllers of ecosystem function, can provide short-term resistance to reductions in ecosystem function when species loss is nonrandom. However, the concurrent loss of complementary interactions among rare and uncommon species, the most diverse component of communities, may contribute to additional species loss and portends erosion of ecosystem function in the long term.

Limitation of disturbances, such as grazing and fire, is a key tool for nature reserve management and ecological restoration. While the role of these disturbances in shaping ecosystem structure and functioning has been intensively studied, less is known about the consequences of long-term prevention of grazing and fire. Based on a 31-year study, we show that relative biomass of the dominant grass, Leymus chinensis, of grasslands in northern China declined dramatically, but only after 21 years of exclusion of fire and grazing. However, aboveground net primary productivity (ANPP) did not decline accordingly due to compensatory responses of several subdominant grass species. The decline in dominance of L. chinensis was not related to gradually changing climate during the same period, whereas experimentally imposed litter removal (simulating fire), mowing (simulating grazing), fire and moderate grazing enhanced dominance of L. chinensis significantly. Thus, our findings show that disturbances can be critical to maintain the dominance of key grass species in semiarid grassland, but that the collapse of a dominant species does not necessarily result in significant change in ANPP if there are species in the community capable of compensating for loss of a dominant.

It remains unclear whether biodiversity buffers ecosystems against climate extremes, which are becoming increasingly frequent worldwide1. Early results suggested that the ecosystem productivity of diverse grassland plant communities was more resistant, changing less during drought, and more resilient, recovering more quickly after drought, than that of depauperate communities2. However, subsequent experimental tests produced mixed results3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13. Here we use data from 46 experiments that manipulated grassland plant diversity to test whether biodiversity provides resistance during and resilience after climate events. We show that biodiversity increased ecosystem resistance for a broad range of climate events, including wet or dry, moderate or extreme, and brief or prolonged events. Across all studies and climate events, the productivity of low-diversity communities with one or two species changed by approximately 50% during climate events, whereas that of high-diversity communities with 16–32 species was more resistant, changing by only approximately 25%. By a year after each climate event, ecosystem productivity had often fully recovered, or overshot, normal levels of productivity in both high- and low-diversity communities, leading to no detectable dependence of ecosystem resilience on biodiversity. Our results suggest that biodiversity mainly stabilizes ecosystem productivity, and productivity-dependent ecosystem services, by increasing resistance to climate events. Anthropogenic environmental changes that drive biodiversity loss thus seem likely to decrease ecosystem stability14, and restoration of biodiversity to increase it, mainly by changing the resistance of ecosystem productivity to climate events.

Ecosystem responses to increased variability in rainfall, a prediction of general circulation models, were assessed in native grassland by reducing storm frequency and increasing rainfall quantity per storm during a 4-year experiment. More extreme rainfall patterns, without concurrent changes in total rainfall quantity, increased temporal variability in soil moisture and plant species diversity. However, carbon cycling processes such as soil CO 2 flux, CO 2 uptake by the dominant grasses, and aboveground net primary productivity (ANPP) were reduced, and ANPP was more responsive to soil moisture variability than to mean soil water content. Our results show that projected increases in rainfall variability can rapidly alter key carbon cycling processes and plant community composition, independent of changes in total precipitation.

Resource availability may influence invertebrate communities, with important consequences for ecosystem function, such as biomass production. We assessed: (1) the effects of experimental soil nutrient additions on invertebrate abundances and feeding rates and (2) the resultant changes in the effects of invertebrates on aboveground plant biomass at three grassland sites spanning the North American Central Plains, across which plant tissue chemistry and biomass vary. Invertebrate communities and rates of herbivory were sampled within a long-term nutrient-addition experiment established at each site along the broad Central Plains precipitation gradient. Additionally, the effects of invertebrates on aboveground plant biomass were determined under ambient and elevated nutrient conditions. At the more mesic sites, invertebrate herbivore abundances increased and their per capita rate of herbivory decreased with nutrient additions. In contrast, at the semi-arid site where plant biomass is low and plant nutrient concentrations are high, invertebrate herbivore abundances did not vary and per capita rates of herbivory increased with nutrient additions. No change in the effect of invertebrate herbivores on aboveground plant biomass was observed at any of the sites. In sum, nutrient additions induced shifts in both plant biomass and leaf nutrient content, which altered invertebrate abundances and feeding rate. However, due to the inverse relationship between changes in herbivore abundance and per capita rates of herbivory, nutrient additions did not alter the effect of invertebrates on aboveground biomass. Overall, we suggest that this inverse response of herbivore abundance and per capita feeding rate may buffer ecosystems against changes in invertebrate damage in response to fluctuations in nutrient levels.

Human alterations to nutrient cycles and herbivore communities are affecting global biodiversity dramatically. Ecological theory predicts these changes should be strongly counteractive: nutrient addition drives plant species loss through intensified competition for light, whereas herbivores prevent competitive exclusion by increasing ground-level light, particularly in productive systems. Here we use experimental data spanning a globally relevant range of conditions to test the hypothesis that herbaceous plant species losses caused by eutrophication may be offset by increased light availability due to herbivory. This experiment, replicated in 40 grasslands on 6 continents, demonstrates that nutrients and herbivores can serve as counteracting forces to control local plant diversity through light limitation, independent of site productivity, soil nitrogen, herbivore type and climate. Nutrient addition consistently reduced local diversity through light limitation, and herbivory rescued diversity at sites where it alleviated light limitation. Thus, species loss from anthropogenic eutrophication can be ameliorated in grasslands where herbivory increases ground-level light. © 2014 Macmillan Publishers Limited. Fil: Borer, Elizabeth. University of Minnesota; Estados Unidos Fil: Seabloom, Eric. University of Minnesota; Estados Unidos Fil: Gruner, Daniel. University of Maryland; Estados Unidos Fil: Harpole, Stanley. Iowa State University; Estados Unidos Fil: Hillebrand, Helmut. Carl-von- Ossietzky University; Alemania Fil: Lind, Eric M.. University of Minnesota; Estados Unidos Fil: Adler, Peter B.. Utah State University; Estados Unidos Fil: Alberti, Juan. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Anderson, Michael. Wake Forest University; Estados Unidos Fil: Bakker, Jonathan D.. University of Washington; Estados Unidos Fil: Biederman, Lori. Iowa State University; Estados Unidos Fil: Blumenthal, Dana. United States Department of Agriculture. Agricultural Research Service; Estados Unidos Fil: Brown, Cynthia S.. State University of Colorado - Fort Collins; Estados Unidos Fil: Brudvig, Lars A.. University of Minnesota; Estados Unidos Fil: Buckley, Yvonne M.. The University Of Queensland; Australia Fil: Cadotte, Marc. University of Toronto Scarborough; Canadá Fil: Chu, Chengjin. Lanzhou University; China Fil: Cleland, Elsa. University of California; Estados Unidos Fil: Crawley, Michael. Imperial College at Silwood Park; Reino Unido Fil: Daleo, Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Damschen, Ellen. University of Wisconsin; Estados Unidos Fil: Davies, Kendi. State University of Colorado - Fort Collins; Estados Unidos Fil: DeCrappeo, Nicole. Forest and Rangeland Ecosystem Science Center; Estados Unidos Fil: Du, Guozhen. Lanzhou University; Estados Unidos Fil: Firn, Jennifer. Queensland University of Technology; Australia Fil: Hautier, Yann. University of Minnesota; Estados Unidos Fil: Heckman, Robert. University of North Carolina at Chapel Hill; Estados Unidos Fil: Hector, Andy. University of Oxford; Reino Unido Fil: Hillerislambers, Janneke. University of Washington; Estados Unidos Fil: Iribarne, Oscar Osvaldo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Klein, Julia. State University of Colorado - Fort Collins; Estados Unidos Fil: Knops, Johannes M. H.. University of Nebraska; Estados Unidos Fil: La Pierre, Kimberly. University of California; Estados Unidos Fil: Leakey, Andrew D. B.. University of Illinois at Urbana-Champaign; Estados Unidos Fil: Li, Wei. Iowa State University; Estados Unidos Fil: MacDougall, Andrew S.. University of Guelph; Canadá Fil: McCulley, Rebecca. University Of Kentucky; Estados Unidos Fil: Melbourne, Brett. State University of Colorado - Fort Collins; Estados Unidos Fil: Mitchell, Charles. University of North Carolina at Chapel Hill; Estados Unidos Fil: Moore, Joslin. University of Melbourne; Australia Fil: Mortensen, Brent. Iowa State University; Estados Unidos Fil: O'Halloran, Lydia. Oregon State University; Estados Unidos Fil: Orrock, John. University of Wisconsin; Estados Unidos Fil: Pascual, Jesus Maria. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mar del Plata. Instituto de Investigaciones Marinas y Costeras. Universidad Nacional de Mar del Plata. Facultad de Ciencia Exactas y Naturales. Instituto de Investigaciones Marinas y Costeras; Argentina Fil: Prober, Suzanne. CSIRO Ecosystem Sciences; Australia Fil: Pyke, David. Forest and Rangeland Ecosystem Science Center; Canadá Fil: Risch, Anita. Swiss Federal Institute For Forest, Snow And Landscape Research; Suiza Fil: Schuetz, Martin. Swiss Federal Institute For Forest, Snow And Landscape Research; Suiza Fil: Smith, Melinda D.. State University of Colorado - Fort Collins; Estados Unidos Fil: Stevens, Carly. Lancaster University; Reino Unido Fil: Sullivan, Lauren. Iowa State University; Estados Unidos Fil: Williams, Ryan. Iowa State University; Estados Unidos Fil: Wragg, Peter. University of Minnesota; Estados Unidos Fil: Yang, Louie. University of California; Estados Unidos

The effects of arbuscular mycorrhizal (AM) symbiosis on ramet and genet densities, vegetative growth rates, and flowering of three forb species were studied in native tallgrass prairie in northeastern Kansas. Mycorrhizal activity was experimentally suppressed for six growing seasons on replicate plots in an annually burned and an infrequently burned watershed with the fungicide benomyl. Benomyl reduced mycorrhizal root colonization to an average of 4.2%, approximately a two-thirds reduction relative to controls (13.7% colonization). Mycorrhizae influenced the population structure of these forbs. Although mycorrhizal suppression had no long-term effect on genet densities and no effect on ramet survivorship throughout the growing season, the number of ramets per individual was significantly increased such that ramet densities of all three species were approximately doubled in response to long-term mycorrhizal suppression. Effects of mycorrhizae on ramet growth and reproduction varied among species. Ramet growth rates, biomass, and flowering of Salvia azurea were greater in plots with active mycorrhizal symbiosis, whereas mycorrhizae reduced ramet growth rates and biomass of Artemesia ludoviciana. Aster sericeus ramet growth rates and biomass were unaffected by the fungicide applications, but its flowering was reduced. The pattern of responses of these three species to mycorrhizae differed considerably between the two sites of contrasting fire regime, indicating that the interaction of fire-induced shifts in resource availability and mycorrhizal symbiosis together modulates plant responses and the intensity and patterns of interspecific competition between and among tallgrass prairie grass and forb species. Further, the results indicate that effects of mycorrhizae on community structure are a result of interspecific differences in the balance between direct positive effects of the symbiosis on host plant performance and indirect negative effects mediated through altered competitive interactions.

We compared 13 traits of invasive exotic, noninvasive exotic, and ecologically similar native species to determine if there are generalizable differences among these groups that relate to persistence and spread of exotic species in tallgrass prairie plant communities. When species were grouped as invasive (two species), noninvasive (five species), and native (six species), no differences were found for the suite of traits examined, likely because of the high variability within and between groups. However, when exotic species, regardless of invasiveness, were compared with the native species, specific leaf area was ca. 40% higher for the exotic species, a result that is consistent with that of other studies. This pattern was also observed for five of seven pairwise comparisons of exotic and native species with similar life history traits. In contrast, total end‐of‐season biomass was as much as three times higher for the native species in five of seven of the native‐exotic species pairs. For other traits, d...

Interannual variability in aboveground net primary production (ANPP) was assessed with long-term (mean = 12 years) data from 11 Long Term Ecological Research sites across North America. The greatest interannual variability in ANPP occurred in grasslands and old fields, with forests the least variable. At a continental scale, ANPP was strongly correlated with annual precipitation. However, interannual variability in ANPP was not related to variability in precipitation. Instead, maximum variability in ANPP occurred in biomes where high potential growth rates of herbaceous vegetation were combined with moderate variability in precipitation. In the most dynamic biomes, ANPP responded more strongly to wet than to dry years. Recognition of the fourfold range in ANPP dynamics across biomes and of the factors that constrain this variability is critical for detecting the biotic impacts of global change phenomena.

We used data from a 15-year experiment in a C4-dominated grassland to address the effects of community structure (i.e., plant species richness, dominance) and disturbance on invasibility, as measured by abundance and richness of exotic species. Our specific objectives were to assess the temporal and spatial patterns of exotic plant species in a native grassland in Kansas (USA) and to determine the factors that control exotic species abundance and richness (i.e., invasibility). Exotic species (90% C3 plants) comprised approximately 10% of the flora, and their turnover was relatively high (30%) over the 15-year period. We found that disturbances significantly affected the abundance and richness of exotic species. In particular, long-term annually burned watersheds had lower cover of exotic species than unburned watersheds, and fire reduced exotic species richness by 80–90%. Exotic and native species richness were positively correlated across sites subjected to different fire (r = 0.72) and grazing (r = 0.67) treatments, and the number of exotic species was lowest on sites with the highest productivity of C4 grasses (i.e., high dominance). These results provide strong evidence for the role of community structure, as affected by disturbance, in determining invasibility of this grassland. Moreover, a significant positive relationship between exotic and native species richness was observed within a disturbance regime (annually burned sites, r = 0.51; unburned sites, r = 0.59). Thus, invasibility of this C4-dominated grassland can also be directly related to community structure independent of disturbance.

Over the project period, we have addressed the following objectives: 1) assess the effects of altered precipitation patterns (i.e., increased variability in growing season precipitation) on genetic diversity of the dominant C4 grass species, Andropogon gerardii, and 2) experimentally assess the impacts of extreme climatic events (heat wave, drought) on responses of the dominant C4 grasses, A. gerardii and Sorghastrum nutans, and the consequences of these response for community and ecosystem structure and function. Below is a summary of how we have addressed these objectives. Objective 1 After ten years of altered precipitation, we found the number of genotypes of A. gerardii was significantly reduced compared to the ambient precipitation treatments (Avolio et al., 2013a). Although genotype number was reduced, the remaining genotypes were less related to one another indicating that the altered precipitation treatment was selecting for increasingly dissimilar genomes (based on mean pairwise Dice distance among individuals). For the four key genotypes that displayed differential abundances depending on the precipitation treatment (G1, G4, and G11 in the altered plots and G2 in the ambient plots), we identified phenotypic differences in the field that could account for ecological sorting (Avolio & Smith, 2013a). The three altered rainfall genotypes alsomore » have very different phenotypic traits in the greenhouse in response to different soil moisture availabilities (Avolio and Smith, 2013c). Two of the genotypes that increased in abundance in the altered precipitation plots had greater allocation to root biomass (G4 and G11), while G1 allocated more biomass aboveground. These phenotypic differences among genotypes suggests that changes in genotypic structure between the altered and the ambient treatments has likely occurred via niche differentiation, driven by changes in soil moisture dynamics (reduced mean, increased variability and changes in the depth distribution of soil moisture) under a more variable precipitation regime, rather than reduced population numbers (A. gerardii tiller densities did not differ between altered and ambient treatments; p = 0.505) or a priori differences in genotype richness (Avolio et al.2013a). This ecological sorting of genotypes, which accounts for 40% of all sampled individuals in the altered plots, is an important legacy of the press chronic climate changes in the RaMPs experiment. Objective 2 In May 2010, we established the Climate Extremes Experiment at the Konza Prairie Biological Station. For the experiment, a gradient of temperatures, ranging from ambient to extreme, were imposed in 2010 and 2011 as a mid-season heat wave under well-watered or severe drought conditions. This study allowed us for the first time to examine species-specific thresholds of responses to climate extremes and assess how these phenotypic responses may impact selection of particular genotypes, with the ultimate goal of linking alterations in individual performance and genetic diversity to ecosystem structure and functioning. We found that tallgrass prairie was resistant to heat waves, but it was not resistant to extreme drought, which reduced aboveground net primary productivity (ANPP) below the lowest level measured in this grassland in almost thirty years (Hoover et al. in press(a)). This extreme reduction in ecosystem function was a consequence of reduced productivity of both C4 grasses and C3 forbs. This reduction in biomass of the C4 grasses (Andropogon gerardii and Sorghastrum nutans) was, in part, due to significant reductions in photosynthesis, leaf water potential and productivity with drought in the dominant grasses species, with S. nutans was more sensitive than A. gerardii to drought (Hoover et al. in press(b)). However, the dominant forb was negatively impacted by the drought more than the dominant grasses, and this led to a reordering of species abundances within the plant community. Although this change in community composition persisted post-drought, ANPP recovered completely the year after drought due to rapid demographic responses by the dominant grass, compensating for loss of the dominant forb. Overall, our results show that an extreme reduction in ecosystem function attributable to a climate extreme (e.g., low resistance) does not preclude rapid ecosystem recovery. Given that dominance by a few species is characteristic of most ecosystems, knowledge of the traits of these species and their responses to climate extremes will be key for predicting future ecosystem dynamics and function. In addition, our research suggests that water stress will dominate photosynthetic and productivity responses caused by discrete drought and heat wave events, rather than direct or additive effects of heat stress, with differential sensitivity in these grasses altering future ecosystem function.« less