Anthropogenic degradation of wetlands often leads to regional biotic homogenization and reduced plant diversity. This reduction is often attributed to the proliferation and dominance of a few generalist, often non‐native, species. Biotic resistance from natives can sometimes impede the growth and spread of colonizers, but its dependence on environmental conditions is poorly understood. Based on field and modeling studies, we tested the predictions that (1) biotic resistance declines at higher nitrogen loading and (2) size influences colonization success. In a five‐growing season mesocosm experiment, we grew three cattail taxa: Typha latifolia (native, large), Typha angustifolia (non‐native, invasive, smallest), and Typha × glauca (hybrid, most invasive, large) as potential colonizers in the presence or absence of pre‐established resident vegetation. At two sites differing in climate and growing season length, biotic resistance treatments were crossed with 12 nitrogen levels (inflows 0–45 g N m −2 year −1 ). Each treatment combination was replicated twice, totaling 48 mesocosms per site. Without residents, colonizers (as total biomass of all three cattail taxa) persisted and expanded clonally across all nitrogen levels. However, their expansion was generally lower when colonizing a pre‐established resident community compare to bare ground. The magnitude of biotic resistance, measured as the effect of residents on colonizers' biomass, and its interaction with nitrogen differed between sites. As predicted, biotic resistance decreased with high nitrogen at the northern site, but at the southern site, residents nearly eliminated colonizers. As anticipated, smaller T. angustifolia was a poorer colonizer than the other taxa, while T. × glauca was the strongest colonizer, especially under high nitrogen conditions where biotic resistance was minimal. Our findings partially support the hypothesis that biotic resistance declines with nitrogen loading, indicating that additional research on the factors influencing the magnitude of biotic resistance is needed. Importantly, when combined with our finding that Typha can persist at all nutrient levels when natives are absent, this information could help identify wetlands particularly vulnerable to invasion, especially in environments experiencing concurrent nutrient enrichment and disturbances that expose bare ground.

Human activities are altering ecological communities around the globe. Understanding the implications of these changes requires that we consider the composition of those communities. However, composition can be summarized by many metrics which in turn are influenced by different ecological processes. For example, incidence-based metrics strongly reflect species gains or losses, while abundance-based metrics are minimally affected by changes in the abundance of small or uncommon species. Furthermore, metrics might be correlated with different predictors. We used a globally distributed experiment to examine variation in species composition within 60 grasslands on six continents. Each site had an identical experimental and sampling design: 24 plots x 4 years. We expressed compositional variation within each site-not across sites-using abundance- and incidence-based metrics of the magnitude of dissimilarity (Bray-Curtis and Sorensen, respectively), abundance- and incidence-based measures of the relative importance of replacement (balanced variation and species turnover, respectively), and species richness at two scales (per plot-year [alpha] and per site [gamma]). Average compositional variation among all plot-years at a site was high and similar to spatial variation among plots in the pretreatment year, but lower among years in untreated plots. For both types of metrics, most variation was due to replacement rather than nestedness. Differences among sites in overall within-site compositional variation were related to several predictors. Environmental heterogeneity (expressed as the CV of total aboveground plant biomass in unfertilized plots of the site) was an important predictor for most metrics. Biomass production was a predictor of species turnover and of alpha diversity but not of other metrics. Continentality (measured as annual temperature range) was a strong predictor of Sorensen dissimilarity. Metrics of compositional variation are moderately correlated: knowing the magnitude of dissimilarity at a site provides little insight into whether the variation is driven by replacement processes. Overall, our understanding of compositional variation at a site is enhanced by considering multiple metrics simultaneously. Monitoring programs that explicitly incorporate these implications, both when designing sampling strategies and analyzing data, will have a stronger ability to understand the compositional variation of systems and to quantify the impacts of human activities.

Ecological disturbance regimes across the globe are being altered via direct and indirect human influences. Biodiversity loss at multiple scales can be a direct outcome of these shifts. Fire, especially in dry forests, is an ecological disturbance that is experiencing dramatic changes due to climate change, fire suppression, increased human population in fire-prone areas, and alterations to vegetation composition and structure. Dry western conifer forests that historically experienced frequent, low-severity fires are now increasingly burning at high severity. Relatively little work has been done looking at the effects of this novel disturbance type on affected plant communities, and little is known about how these impacts change over time. To fill in these knowledge gaps, we examined a fire that burned in a yellow pine and mixed conifer forest in the central Sierra Nevada in California, USA. We sampled at five time steps across the nine years following the fire (1, 3, 5, 8, and 9 years postfire). We found a generally unimodal relationship between fire severity and plant alpha and gamma diversity, but found that areas that burned at high severity supported progressively lower plant diversity as time since fire increased. Similarly, beta diversity decreased drastically through time for the high-severity areas, while remaining more static in the other severity classes. The combination of these findings indicates that significant floristic homogenization can result from high-severity fire in this ecosystem type. We also saw consistently lower diversity in unburned areas in comparison to area burned at low and moderate severity, underlining that both lack of fire and high-severity fire can have negative impacts on postfire plant diversity. Unburned areas that experienced forest thinning after the first sample year saw an increase in plant diversity over time, suggesting that some-but not all-of the effects of fire on plant diversity can be approximated through forest management.

Disturbance often increases local-scale (α) diversity by suppressing dominant competitors. However, widespread disturbances may also reduce biotic heterogeneity (β diversity) by making the identities and abundances of species more similar among patches. Landscape-scale (γ) diversity may also decline if disturbance-sensitive species are lost. California's vernal pool plant communities are species rich, in part because of two scales of β diversity: (1) within pools, as species composition changes with depth (referred to here as vertical β diversity), and (2) between pools, in response to dispersal limitation and variation in pool attributes (referred to here as horizontal β diversity). We asked how grazing by livestock, a common management practice, affects vernal pool plant diversity at multiple hierarchical spatial scales. In terms of abundance-weighted diversity, grazing increased α both within local pool habitat zones and at the whole-pool scale, as well as γ at the pasture scale without influencing horizontal or vertical β diversity. In terms of species richness, increases in α diversity within habitat zones and within whole pools led to small decreases in horizontal β diversity as species occupancy increased. This had a dampened effect on species richness at the γ (pasture) scale without any loss of disturbance-sensitive species. We conclude that grazing increases species richness and evenness (α) by reducing competitive dominance, without large disruptions to the critical spatial heterogeneity (β) that generates high landscape-level diversity (γ).

The effects of altered nutrient supplies and herbivore density on species diversity vary with spatial scale, because coexistence mechanisms are scale dependent. This scale dependence may alter the shape of the species-area relationship (SAR), which can be described by changes in species richness (S) as a power function of the sample area (A): S = cAz , where c and z are constants. We analysed the effects of experimental manipulations of nutrient supply and herbivore density on species richness across a range of scales (0.01-75 m2 ) at 30 grasslands in 10 countries. We found that nutrient addition reduced the number of species that could co-occur locally, indicated by the SAR intercepts (log c), but did not affect the SAR slopes (z). As a result, proportional species loss due to nutrient enrichment was largely unchanged across sampling scales, whereas total species loss increased over threefold across our range of sampling scales.

Nutrient enrichment can simultaneously increase and destabilise plant biomass production, with co-limitation by multiple nutrients potentially intensifying these effects. Here, we test how factorial additions of nitrogen (N), phosphorus (P) and potassium with essential nutrients (K+) affect the stability (mean/standard deviation) of aboveground biomass in 34 grasslands over 7 years. Destabilisation with fertilisation was prevalent but was driven by single nutrients, not synergistic nutrient interactions. On average, N-based treatments increased mean biomass production by 21-51% but increased its standard deviation by 40-68% and so consistently reduced stability. Adding P increased interannual variability and reduced stability without altering mean biomass, while K+ had no general effects. Declines in stability were largest in the most nutrient-limited grasslands, or where nutrients reduced species richness or intensified species synchrony. We show that nutrients can differentially impact the stability of biomass production, with N and P in particular disproportionately increasing its interannual variability.

For clonal plants, spatial spread within a site is primarily determined by production of new ramets, which suggests that allocation to this function is a key component of the population and community dynamics of clonal plant species. However, surprisingly few studies, either theoretical or empirical, quantify the amount of resources that is or should be allocated to new daughters. Instead, the focus of clonal plant research has been on two unique features of clonal growth: architecture of new ramet deployment and the degree of physiological integration among ramets. We argue that more attention should be paid to resource allocation to new ramets as an important clonal trait and present an operational definition of allocation to clonal growth on a ramet level. Specifically, we propose that clonal allocation be measured as the biomass of a new spacer from its point of origin on an old stem/node plus biomass of a new aphotosynthetic stem arising from the terminal node of the new spacer. Using this definition, we develop a set of important questions that future studies in clonal allocation should address. We then describe protocols to answer these questions and a developing international network to collect data using these protocols.

The evolution of clonal growth is a widespread phenomenon among plant species, characterized by the production of genetically identical clonal fragments (ramets) via rhizomes or stolons that form an interconnected clonal organism (genet). Clonal plant species are known to differ in their investment into ramet production, and exhibit considerable variation in ramet morphology both within and among species. While patterns of resource allocation are thought to be linked to a number of plant characteristics, many analyses are limited by uncertainty in how clonal plants determine the morphology and resources allocated to new ramets. In this study, we attempted to discern what aspects of parent ramets best predicted resource allocation to new daughter ramets, and the relationship between resource allocation and daughter ramet rhizome morphology. We grew two sedge species, Schoenoplectus tabernaemontani and Eleocharis elliptica , in a greenhouse under two levels of fertilizer addition. By harvesting daughter ramets that had initiated stem production, yet remained aphotosynthetic, we were able to isolate parental investment into non-independent daughter ramets at a point where daughter ramet spacer length became fixed. Our results indicate that parent ramets allocated a non-linear proportion of parent rhizome biomass to the production of daughter ramets. Moreover, this relationship was unaffected by environmental nutrient availability. Daughter ramet biomass, in turn, was strongly correlated with daughter ramet spacer length. These observations shed light on key processes governing clonal growth in plants, and their potential application in unifying allocational and morphological perspectives to explore the fitness implications of variability in clonal growth.