Search

Environmental circumstances shaping soil microbial communities have been studied extensively. However, due to disparate study designs, it has been difficult to resolve whether a globally consistent set of predictors exists, or context‐dependency prevails. Here, we used a network of 18 grassland sites (11 of those containing regional plant productivity gradients) to examine (i) if similar abiotic or biotic factors predict both large‐scale (across sites) and regional‐scale (within sites) patterns in bacterial and fungal community composition, and (ii) if microbial community composition differs consistently at two levels of regional plant productivity (low vs. high). Our results revealed that bacteria were associated with particular soil properties (such as base saturation) and both bacteria and fungi were associated with plant community composition across sites and within the majority of sites. Moreover, a discernible microbial community signal emerged, clearly distinguishing high and low‐productivity soils across different grasslands independent of their location in the world. Hence, regional productivity differences may be typified by characteristic soil microbial communities across the grassland biome. These results could encourage future research aiming to predict the general effects of global changes on soil microbial community composition in grasslands and to discriminate fertile from infertile systems using generally applicable microbial indicators. [ABSTRACT FROM AUTHOR]

Competition is often highlighted as a major force influencing plant species diversity. However, there are multiple facets of competition (e.g., strength, intransitivity, and size asymmetry) that may have independent and differential impacts on diversity, making understanding the degree to which competition structures communities difficult. Unfortunately, field‐based experiments that decouple multiple facets of competition are lacking, limiting our ability to test theoretical frameworks and reducing understanding of the actual linkages among competition and coexistence. Here, we experimentally manipulate the size structure of local grassland communities to examine the relative impacts of competitive size asymmetry (i.e., competitive advantage based on relative size) and intensity (i.e., mean effect of neighbors on plant growth) on species loss and gain. Increased competitive size asymmetry was associated with increased species loss and decreased species gain, while no relationship was found between competitive intensity and species loss and gain. Furthermore, the probability of loss was not dependent on a species initial size, suggesting that small species may not always be the losers of size‐asymmetric interactions. Instead, loss was dependent on species rarity, where loss was higher for rare species. Overall, these results suggest that competitive size asymmetry may be more important for species loss than intensity in some plant communities and demonstrates the importance of decoupling different aspects of competition to better understand their drivers and ecological consequences. [ABSTRACT FROM AUTHOR]

The occurrence of multi‐year drought is predicted to increase globally with climate change. However, it is unclear whether drought effects on ecosystems are progressive through time.Here, we experimentally reduced growing season precipitation (GSP) by 45% at seven North American temperate grasslands for four consecutive years to determine the following: (a) whether the effects of reduced precipitation on plant community structure and biomass components (shoot, root, litter) are compounding over time; (b) whether prior year climatic and soil conditions influence subsequent drought impacts on plant community structure and biomass components; and (c) whether the effects of reduced precipitation on individual ecosystem components are related to one another.Across the seven field sites, we observed neither consistent nor progressive effects of reduced precipitation on any biomass component during the experiment, despite having extreme drought conditions imposed for four consecutive years. Relative to the ambient treatment, above‐ground net primary productivity (ANPP) declined in response to drought during the early years of the experiment but increased above the ambient treatment in the fourth year, while root and litter biomass were stable across the sites throughout the study. Similarly, graminoid cover decreased initially but recovered by the final year of the experiment, contributing to observed differences in species composition between treatments across sites. Compositional changes were not associated with any declines in species richness or evenness. Divergent responses among years were not driven by lag effects based on prior year climatic and soil conditions. Furthermore, precipitation effects on ecosystem components were largely independent as we found only two positive links: between ANPP and plant species richness, and between species evenness and composition.Synthesis. Overall, our results suggest that these northern grasslands are relatively resistant to short‐term multi‐year drought in the context of supporting plant diversity and biomass production. [ABSTRACT FROM AUTHOR]

Questions: Woody plant encroachment and invasive plants are two critical factors that negatively impact grass‐dominated ecosystems. While studies have extensively investigated these factors individually, research on how invasive species impact the susceptibility of grasslands to encroachment is less common. Using the aspen parkland, an endangered savannah‐type ecosystem, we asked how the presence of a widespread invasive grass in North America, smooth brome (Bromus inermis), impacted the growth and survival of one‐year old trembling aspen (Populus tremuloides) seedlings. We also asked if plant litter was a potential mechanism of inhibition (as has been proposed for brome litter) or coexistence (as has been hypothesized for aspen litter) in this system. Location: Alberta, Canada. Methods: We used a manipulative experimental approach to determine if the survival and subsequent growth of planted aspen seedlings was impacted by the presence of smooth brome and manipulation of litter amount and type. Results: We found that the presence of smooth brome reduced aspen survival by 57% compared to uninvaded habitats, likely mediated by reduced soil moisture, while litter manipulation had no effect on survival. For surviving seedlings, local context had complex impacts on growth; the addition of aspen litter to brome‐invaded communities increased seedling growth while aspen litter additions to native communities resulted in decreased growth. Conclusion: These results suggest that invasion by smooth brome will alter the dynamics of aspen establishment in this system, potentially leading to significant changes to this already endangered landscape. Though smooth brome may serve as a barrier to aspen establishment, accumulation of aspen litter from nearby stands to brome patches could lead to faster growth of seedlings in invaded areas at the edge of existing aspen stands. Our results also suggest more generally that the impact of invasive plants on the establishment of native woody plants can be dependent on litter inputs. [ABSTRACT FROM AUTHOR]

Ecosystems are spatially heterogenous in plant community composition and function. Shrub occurrence in grasslands is a visually striking example of this, and much research has been conducted to understand the functional implications of this pattern. Within savannah ecosystems, the presence of tree and shrub overstories can have significant impacts on the understory herbaceous community. The exact outcomes, however, are likely a function of the spatial arrangement and traits of the overstory species. Here we test whether there are functional linkages between the spatial patterning of a native shrub and the standing biomass, community composition, and overall nutrient cycling of a neighbouring grassland understory communities within the Aspen Parkland of central Alberta, Canada. In a paired grassland-shrub stand study, we found the native shrub, Elaeagnus commutata , has relatively few stand-level impacts on the composition and standing biomass of the ecosystem. One factor contributing to these limited effects may be the overdispersion of shrub stems at fine spatial scales, preventing areas of deep shade. When we looked across a shrub density gradient and incorporated shrub architecture into our analyses, we found these shrub traits had significant associations with species abundance and root biomass in the understory community. These results suggest that stem dispersion patterns, as well as local stand architecture, are influential in determining how shrubs may affect their herbaceous plant understory. Thus, it is important to incorporate shrub spatial and architectural traits when assessing shrub-understory interactions. [ABSTRACT FROM AUTHOR]

Grassland plants, especially their root systems, are dynamic and can buffer changes resulting from exposure to multiple stressors; however, the interactive stressor effects on root dynamics and associated above‐ground growth are poorly understood.Here, we examine the effects of changed precipitation and air temperature, and defoliation intensity on root length dynamics and above‐ground biomass using the third year data from a multifactor experiment conducted across three northern temperate grasslands.We found that root length was more sensitive to the changes in environmental and management conditions than root mass, demonstrating the importance of root length as an indicator of rapid root system changes. Across all sites, warming, altered precipitation and defoliation intensity interacted to affect root length while above‐ground biomass was only affected by defoliation intensity, indicating that the root system was more responsive than above‐ground biomass when climatic conditions change. Overall, drought reduced root length, particularly under low defoliation intensity, as well as in combination with warming and heavy defoliation, highlighting the risk of additive effects of such environmental stresses. Across unclipped plots, above‐ground biomass was positively associated with total root length, the latter of which further interacted with precipitation, to affect above‐ground biomass. Compared to defoliated communities, non‐defoliated plant communities exhibited a greater ability to maintain above‐ground biomass under drought conditions via increases in root system efficiency (the amount of above‐ground biomass produced per unit of root length invested).Our results highlight the rapid change of root length in the face of interactive stressors. We postulate that the degree of stability in above‐ground biomass is driven by the altered root system dynamics or species turnover. Future studies are warranted that more directly assess how root length responses under climate change impact other important plant traits in grasslands. A free Plain Language Summary can be found within the Supporting Information of this article. [ABSTRACT FROM AUTHOR]

Questions: Small‐scale disturbances have been linked to the maintenance of plant diversity by promoting the coexistence of competitive and subdominant species. Many proposed mechanisms for this coexistence involve changes to the competitive dynamics or abiotic environment. However, another possible mechanism could be a change to seed bank dynamics, ultimately impacting the species available for re‐establishment. Here, we test: (a) how gap creation alters seed bank composition, seed inputs (i.e. seed rain) and outputs (i.e. seedling establishment and predation); and (b) whether these changes are due to total vegetation removal or an indirect consequence of the removal of particularly influential species or traits. Location: Rough fescue grassland in Alberta, Canada. Methods: To test the impact of gap creation on seed bank dynamics, we experimentally removed or kept the standing vegetation intact in a paired‐plot design. Within each pair, we combined field sampling with greenhouse emergence studies to measure seed bank composition throughout the growing season. To examine inputs, we added seed trays that collected seed rain, and to examine outputs, we measured germination in seed trays with or without seed predation. Results: Seed bank composition within gaps was significantly different from the original standing vegetation before removal and the seed bank where standing vegetation was kept intact. Gap creation increased net seed rain and seedling establishment but had no impact on seed predation. These effects of gap creation on seed outputs seem to be reliant on the total removal of vegetation, while seed inputs are also dependent on the community and trait composition of the standing vegetation. Conclusions: The creation of gaps through small‐scale disturbances leading to a distinct seed bank community is another possible explanation for why we see increased β‐diversity with disturbance. Further, these gaps seem to be more important than standing vegetation compositional changes for preserving the local seed bank and promoting coexistence. [ABSTRACT FROM AUTHOR]

Questions: Local size structure is frequently measured in plant populations and forested systems due to its association with population‐ and community‐level processes. In contrast, size structure is rarely examined within herbaceous communities despite evidence for size‐dependent processes in these systems, suggesting an important functional vegetation property may be widely overlooked. Here, we test whether vertical size structure (a) varies within a short‐stature grassland, (b) varies along a productivity gradient, and (c) enhances the understanding of a foundational ecological pattern: the diversity–productivity relationship. Location: Rough fescue grassland in Alberta, Canada. Methods: We measured the heights of individuals within 32 plots and used average height, the Gini coefficient, and a two‐parameter Weibull distribution to estimate community‐level vertical size structure. Linear mixed models were used to test whether size structure parameters were significantly associated with productivity, species richness, and species evenness. Results: Vertical size structure varied significantly among local communities, such that some were dominated by short individuals while others were comprised of a more equal number of taller individuals. With increasing productivity, there was an increase in average plant height and a decrease in height inequality. Decreased height inequality was associated with reduced species richness with evidence that loss was not size‐dependent. The inclusion of vertical size structure parameters increased the explained variance of the diversity–productivity relationship by ~35% while also increasing its parsimony. Conclusions: Even within a short‐stature grassland, there is substantial functional variation in vertical size structure. Productivity is strongly associated with size structure patterns, and the inclusion of height inequalities greatly enhances productivity–diversity relationships, likely because they are the product of assembly mechanisms such as size‐asymmetric competition, assemblage‐level thinning, and niche complementarity. Overall, this demonstrates that vertical size structure has been a missing mechanism in most community assembly theories and models and should be included in the future. [ABSTRACT FROM AUTHOR]

Climatic changes are altering Earth's hydrological cycle, resulting in altered precipitation amounts, increased interannual variability of precipitation, and more frequent extreme precipitation events. These trends will likely continue into the future, having substantial impacts on net primary productivity ( NPP) and associated ecosystem services such as food production and carbon sequestration. Frequently, experimental manipulations of precipitation have linked altered precipitation regimes to changes in NPP. Yet, findings have been diverse and substantial uncertainty still surrounds generalities describing patterns of ecosystem sensitivity to altered precipitation. Additionally, we do not know whether previously observed correlations between NPP and precipitation remain accurate when precipitation changes become extreme. We synthesized results from 83 case studies of experimental precipitation manipulations in grasslands worldwide. We used meta-analytical techniques to search for generalities and asymmetries of aboveground NPP ( ANPP) and belowground NPP ( BNPP) responses to both the direction and magnitude of precipitation change. Sensitivity (i.e., productivity response standardized by the amount of precipitation change) of BNPP was similar under precipitation additions and reductions, but ANPP was more sensitive to precipitation additions than reductions; this was especially evident in drier ecosystems. Additionally, overall relationships between the magnitude of productivity responses and the magnitude of precipitation change were saturating in form. The saturating form of this relationship was likely driven by ANPP responses to very extreme precipitation increases, although there were limited studies imposing extreme precipitation change, and there was considerable variation among experiments. This highlights the importance of incorporating gradients of manipulations, ranging from extreme drought to extreme precipitation increases into future climate change experiments. Additionally, policy and land management decisions related to global change scenarios should consider how ANPP and BNPP responses may differ, and that ecosystem responses to extreme events might not be predicted from relationships found under moderate environmental changes. [ABSTRACT FROM AUTHOR]

The spatial arrangement of nutrients and neighbours in soil influences plant growth and reproduction. Plants often respond to such stimuli through plasticity in root proliferation (root mass per soil volume), or the breadth of their root system., Here, we asked how plants adjust nutrient foraging strategies when grown alone or with neighbours. We asked (i) Does root proliferation into nutrient-rich patches when plants are grown alone predict root proliferation when plants are grown with neighbours? (ii) What factors (nutrients or neighbours) best predict the probability of root placement at different soil locations? (iii) How does the spatial distribution of nutrients alter the degree to which neighbours suppress plant growth?, To answer these questions, we grew four grassland species either as individual plants or in competition, in patchy or patch-free soil, in a factorial design. We used genomic DNA to identify the spatial distribution of roots of each species when plants were grown in mixtures., The root foraging behaviour of individuals grown alone did not consistently predict behaviour in mixture. Specifically, (i) the behaviour of individually grown plants predicted behaviour of competing plants inside patches, but not in background soil. We observed over-proliferation of roots in background soil relative to what was expected from plants grown alone. (ii) Neighbours were consistently the most important variable for predicting the placement of roots in soil and caused either an increase in root system breadth, or no change relative to alone. (iii) If a species experienced growth suppression when grown in competition, individuals experienced this more severely in patchy soil compared to patch-free soil., Synthesis. Game theoretic models have predicted that under interspecific competition, over-proliferation of roots in the presence of neighbours might occur for some species but not others. Our data are consistent with these predictions but more work is needed. Nutrient foraging studies have primarily focused on plants grown alone or assumed that plants do not respond separately to neighbours and nutrients. Our data call these practices into question and contribute to a growing understanding that plants integrate information about both nutrients and neighbours when placing roots in soil. [ABSTRACT FROM AUTHOR]

Abstract: Whether or not niche conservatism is common is widely debated. Despite this uncertainty, closely related species are often assumed to be ecologically similar. This principle has led to the proposed use of phylogenetic information in forecasting species responses to environmental change. Tests of niche conservatism often focus on ‘functional traits’ and environmental tolerances, but there have been limited tests for conservatism in species’ responses to changes in the environment, especially in the field. The prevalence of functional convergence and the likelihood of functional trade-offs in a heterogeneous environment suggest that conservatism of the response niche is unlikely to be detectable under natural conditions. To test the relevance of evolutionary information in predicting ecological responses, we tested for conservatism (measured as phylogenetic signal) of grassland plant population responses to 14 treatments (e.g. light, nutrients, water, enemies, mutualists), each manipulated for 2–3 years, and 4 treatment categories (aboveground, belowground, resource, and herbivory) at a single site. Individual treatment responses showed limited evidence of conservatism, with only weak conservatism in plant responses to mycorrhizae and grazing. Aspects of the response niche were conserved among monocots both aboveground and belowground, although the pattern varied. Conservatism was limited to grazing aboveground, but belowground responses were conserved as a group, suggesting fundamental differences in how selection has led to niche conservatism in aboveground and belowground environments. Overall, our results suggest that conservatism of the response niche is not common, but is actually rare. As such, evolutionary relationships are likely to be of limited relevance for predicting species responses under field conditions, at least over the short time scales used in this study. [Copyright &y& Elsevier]

That competition is stronger among closely related species and leads to phylogenetic overdispersion is a common assumption in community ecology. However, tests of this assumption are rare and field-based experiments lacking. We tested the relationship between competition, the degree of relatedness, and overdispersion among plants experimentally and using a field survey in a native grassland. Relatedness did not affect competition, nor was competition associated with phylogenetic overdispersion. Further, there was only weak evidence for increased overdispersion at spatial scales where plants are likely to compete. These results challenge traditional theory, but are consistent with recent theories regarding the mechanisms of plant competition and its potential effect on phylogenetic structure. We suggest that specific conditions related to the form of competition and trait conservatism must be met for competition to cause phylogenetic overdispersion. Consequently, overdispersion as a result of competition is likely to be rare in natural communities. [ABSTRACT FROM AUTHOR]

The continued increase in global temperatures is widely expected to influence soil greenhouse gas emissions (GHG) in grazed grassland ecosystems. This study was conducted to quantify the effects of experimental warming and simulated grazing on greenhouse gas emissions in a northern temperate grassland, a system susceptible to climatic fluctuations, during two consecutive growing seasons in 2006 and 2007. Soil greenhouse gas emissions including N2O, CO2 and CH4 were studied. Soil nitrous oxide (N2O) emission at the soil surface increased due to warming, changing the system from an N2O sink to a source in 2006. However, in the following year (2007), the effects of warming on the above-mentioned N fluxes differed with or without defoliation. Soil CO2 and CH4 effluxes showed transient and weak responses to defoliation and warming in both years. In addition, soil respiration was temporarily influenced by a warming x defoliation interaction early in the study, with a 45% decrease in defoliated plots and a 146% increase in non-defoliated plots due to warming. The transient and weak responses of soil N2O, CO2 and CH4 emissions suggest that this temperate grassland may be able to acclimatize fast from warming for the short-term. [ABSTRACT FROM AUTHOR]

The passenger, driver, and opportunist models are conceptual models of the invasion process used to describe alternative invasion scenarios. In the passenger model, both the invasive species and native community respond independently to environmental changes. In the driver model, changes to the native community are driven by the invasive species, while in the opportunist model invasion occurs in response to changes in the native community. In any given invasion scenario, however, it is possible that the relationships between the invasive, the native community, and the environment correspond to some combination of these invasion models acting simultaneously. We study invasion by Poa pratensis in a grassland in Alberta, Canada. Poa pratensis is a non-native plant implicated with loss of plant diversity in the region. In a three year field experiment, we manipulate the environment though defoliation, water addition, and nitrogen addition, and measure responses of P. pratensis cover, and cover and richness of the native community. We use structural equation modelling to describe the relationships between the invasive, the native community, and the environmental changes, and then interpret these relationships using the three invasion models. We found that P. pratensis predominantly invaded via the driver model, with subsequent reductions in native plant cover, but not in species richness. Positive effects of the environmental changes on P. pratensis also aided its ability to drive native cover. As well, we found some involvement of the opportunist model, through a negative relationship between the native community and the invasive. As invasion mainly proceeded via the driver model, management actions to limit invasion should focus on efforts to control abundance of P. pratensis itself. [ABSTRACT FROM AUTHOR]

We examined how water and nitrogen addition and water–nitrogen interactions affect root and shoot competition intensity and competition–productivity relationships in a native rough fescue grassland in central Alberta, Canada. Water and nitrogen were added in a factorial design to plots and root exclusion tubes and netting were used to isolate root and shoot competition on two focal species ( Artemisia frigida and Chenopodium leptophyllum). Both water and nitrogen were limiting to plant growth, and focal plant survival rates increased with nitrogen but not water addition. Relative allocation to root biomass increased with water addition. Competition was almost entirely belowground, with focal plants larger when released from root but not shoot competition. There were no significant relationships between productivity and root, shoot, or total competition intensity, likely because in this system shoot biomass was too low to cause strong shoot competition and root biomass was above the levels at which root competition saturates. Water addition had few effects on the intensity of root competition suggesting that root competition intensity is invariant along soil moisture gradients. Contrary to general expectation, the strength of root competition increased with nitrogen addition demonstrating that the relationship between root competition intensity and nitrogen is more complex than a simple monotonic decline as nitrogen increases. Finally, there were few interactions between nitrogen and water affecting competition. Together these results indicate that the mechanisms of competition for water and nitrogen likely differ. [ABSTRACT FROM AUTHOR]

Summary A field experiment in a native grassland in Central Alberta, Canada, tested whether plants alter relative allocation to roots vs. shoots in response to below-ground competition, and whether the mass of a species’ root system accounts for interspecific differences in below-ground competitive response. 2 Seedlings of each of 10 native species were transplanted into the naturally occurring vegetation in the field at the start of the growing season. Root interactions between the target plants and their neighbours were manipulated through the use of PVC root exclusion tubes, with target plants grown with or without potential root interactions with their neighbours. Neighbour shoots were also tied back, forcing any target–neighbour interactions to be below ground. 3 Below-ground competition generally reduced plant growth, with its relative magnitude varying among species. 4 An allometric analysis indicated that competition below ground did not result in an increase in the relative biomass allocated to roots for any of the 10 target species. This is counter to the growth-balance hypothesis (and optimal foraging theory). Below-ground competition did increase root : shoot ratios, but this was due to reduced plant size (small plants have larger root : shoot ratios), rather than adaptive plasticity. 5 A species’ below-ground competitive ability was not related to its root system size. Although this finding is counter to commonly made assumptions, it is supported by other work demonstrating below-ground competition to be generally size-symmetric. 6 Despite the majority of plant–plant interactions in grasslands being below ground, our understanding of plant competition above ground is significantly more robust. Several wide-spread assumptions regarding below-ground competition are suspect, and more multispecies studies such as this are required to provide a fuller picture of how plants respond to, and compete for, soil resources. [ABSTRACT FROM AUTHOR]

Abstract The diversity-invasion resistance theory argues that increased diversity results in increased competitive suppression of establishing plants. Although there is support for the pattern of decreased invasion with increased diversity, empirical demonstrations of increased competition are limited. An experiment was conducted during a severe drought in a native grassland community. The drought resulted in minimal shading among neighbours, and in contrast to prior studies, competition here was exclusively belowground. Neither diversity nor community composition influenced root crowding or competition. It appears that when competition is belowground, it is independent of diversity, likely because of fundamental differences in the mechanisms of above- and belowground competition. This suggests that even at the neighbourhood scale, there is no inherently negative relationship between competition and diversity, and lends support to alternative theories which suggest factors of than diversity may more strongly influence community invasibility. [ABSTRACT FROM AUTHOR]

Copyright of Canadian Journal of Animal Science is the property of Canadian Science Publishing and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. This abstract may be abridged. No warranty is given about the accuracy of the copy. Users should refer to the original published version of the material for the full abstract. (Copyright applies to all Abstracts.)

Invasive species can alter the structure and function of the communities they invade, as well as lead to biotic homogenization across their invasive range, thus affecting large‐scale diversity patterns. The mechanisms by which invasive species can lead to biotic homogenization are poorly understood. We argue that invasive species acting as strong, deterministic, and consistent filters within and across invaded communities are likely to cause biotic homogenization at multiple spatial scales. We studied Bromus inermis Leyss. invasion into eight grasslands covering most of the grassland and parkland natural regions of Alberta (western Canada). Specifically, we tested whether B. inermis (1) has a strong impact on species richness and composition, (2) consistently alters resources (nutrients, light, and soil moisture), imposing the same ecological filter to species establishment and persistence across sites, and hence (3) whether it leads to biotic homogenization within and across sites. We recorded plant cover and resources across native‐to‐invaded transition areas combining space‐for‐time substitutions with time‐series data analyses. Bromus inermis invasion was associated with rapid biotic homogenization of communities, within and among the eight grasslands. The sharp changes in species relative abundances following invasion was the initial driver of biotic homogenization, and species loss was delayed. Supporting the idea that biotic homogenization can occur when an invasive species presents a broad and consistent filter, resources modified by B. inermis invasion (particularly light and certain nutrients) were altered rather consistently within and across sites. The 50% reduction in light was likely the initial driver of biotic homogenization, and the increase in nutrient availability probably facilitates the displacement of species from the invaded areas and could lead to the establishment of self‐reinforcing dynamics. Overall, our results support the idea that invaders acting as strong, deterministic, and consistent ecological filters are likely to cause biotic homogenization of the communities they invade. [ABSTRACT FROM AUTHOR]

Multi-year droughts combined with other stressors such as defoliation are likely to have substantial ecological impacts. However, grasslands vary significantly in their response to drought and defoliation, and interpretations rarely address the importance of roots despite evidence showing roots are likely to respond differently to drought and defoliation than shoots. Here, we conducted a three-year manipulative experiment, combining a reduction in rainfall treatment (drought, 45% of ambient) with each of a time-of-defoliation (early vs. late) and an intensity-of-defoliation (none, light, heavy) treatment at seven temperate grassland sites to test three a priori hypotheses: 1) defoliation early relative to late in the season would exacerbate drought impacts on aboveground net primary productivity (ANPP) and root biomass; 2) ANPP and root biomass would progressively decrease with increasing intensity of defoliation early in the growing season under drought; 3) drought and defoliation effects on ANPP and root biomass would vary along a gradient of long-term mean annual precipitation of study sites. Across all seven sites, the combined effects of drought and defoliation led to greater changes in root biomass than ANPP. Under drought conditions specifically, defoliation early in the growing season reduced root biomass, but not ANPP, compared to defoliation late in the growing season. The decline in root biomass under drought was more substantial when the intensity of early-season defoliation increased from none to heavy. Furthermore, variation in the timing and intensity of defoliation altered root biomass more in mesic than arid grasslands, while their effects on ANPP remained unchanged across the seven sites. These results indicate that root systems may be more responsive than ANPP to simultaneous drought and defoliation, and could lead to long-term changes in plant productivity. • Drought and defoliation together affected root biomass more than shoot biomass. • Early season defoliation reduced root biomass but not shoot biomass under drought. • Varying defoliation regimes altered root biomass more in mesic than arid grasslands. [ABSTRACT FROM AUTHOR]

• We studied in-situ responses of GHGs to warming, altered precipitation and defoliation. • The temperate grassland was a source for CO 2 , N 2 O but a sink for CH 4 during the study. • GHG fluxes were decoupled from microbial biomass in the study. • Precipitation was the primary driver directly regulating CO 2 and N 2 O fluxes. Grasslands store large quantities of organic carbon (C) and thus can markedly impact global climate change if they are disturbed. How practices such as grazing intensity and climate change interact to affect soil greenhouse gas (GHG) fluxes within native northern temperate grasslands is unclear. We investigated the responses of three major GHGs (CO 2 , N 2 O and CH 4) to temperature (ambient vs. warmed), precipitation (ambient vs. reduced or increased) and defoliation (no defoliation vs. low- or high-intensity defoliation) treatments in a factorial experiment over a two-year period in a native northern temperate grassland in Alberta, Canada. Growing season mean soil gas fluxes across all treatments were 93.8 mg CO 2 m−2 h−1, −0.025 mg CH 4 m−2 h−1, and 0.003 mg N 2 O m−2 h−1, suggesting that this grassland was a source for CO 2 and N 2 O, but a sink for CH 4 over the study period. Structural equation modeling showed that increased precipitation, rather than warming or defoliation, was the primary driver directly regulating fluxes of CO 2 and N 2 O. While increased precipitation also increased microbial biomass C (MBC), the latter had little impact on GHG fluxes, suggesting that MBC was not a critical factor regulating the precipitation-GHG flux relationship. Warming and precipitation effects on CH 4 fluxes depended on the month of sampling during the growing season. Warming decreased CH 4 uptake but only in mid-July, while increased precipitation reduced CH 4 uptake across all sampling dates except early August, at which time increased precipitation increased CH 4 uptake. Our study shows that short-term soil GHG fluxes in this native grassland are relatively resilient to moderate warming and defoliation, but sensitive to altered precipitation. Increased precipitation was the main contributor to short-term GHG fluxes in this temperate grassland via changes in CO 2 efflux. [ABSTRACT FROM AUTHOR]

The search for predictions of species diversity across environmental gradients has challenged ecologists for decades. The humped-back model (HBM) suggests that plant diversity peaks at intermediate productivity; at low productivity few species can tolerate the environmental stresses, and at high productivity a few highly competitive species dominate. Over time the HBM has become increasingly controversial, and recent studies claim to have refuted it. Here, by using data from coordinated surveys conducted throughout grasslands worldwide and comprising a wide range of site productivities, we provide evidence in support of the HBM pattern at both global and regional extents. The relationships described here provide a foundation for further research into the local, landscape, and historical factors that maintain biodiversity. [ABSTRACT FROM AUTHOR]