Katharine N. Suding

Plant community responses to experimental warming across the tundra biome

Recent observations of changes in some tundra ecosystems appear to be responses to a warming climate. Several experimental studies have shown that tundra plants and ecosystems can respond strongly to environmental change, including warming; however, most studies were limited to a single location and were of short duration and based on a variety of experimental designs. In addition, comparisons among studies are difficult because a variety of techniques have been used to achieve experimental warming and different measurements have been used to assess responses. We used metaanalysis on plant community measurements from standardized warming experiments at 11 locations across the tundra biome involved in the International Tundra Experiment. The passive warming treatment increased plant-level air temperature by 1-3°C, which is in the range of predicted and observed warming for tundra regions. Responses were rapid and detected in whole plant communities after only two growing seasons. Overall, warming increased height and cover of deciduous shrubs and graminoids, decreased cover of mosses and lichens, and decreased species diversity and evenness. These results predict that warming will cause a decline in biodiversity across a wide variety of tundra, at least in the short term. They also provide rigorous experimental evidence that recently observed increases in shrub cover in many tundra regions are in response to climate warming. These changes have important implications for processes and interactions within tundra ecosystems and between tundra and the atmosphere.

Don't judge species on their origins

Conservationists should assess organisms on environmental impact rather than on whether they are natives, argue Mark Davis and 18 other ecologists.

Management of novel ecosystems: are novel approaches required?

Most ecosystems are now sufficiently altered in structure and function to qualify as novel systems, and this recognition should be the starting point for ecosystem management efforts. Under the emerging biogeochemical configurations, management activities are experiments, blurring the line between basic and applied research. Responses to specific management manipulations are context specific, influenced by the current status or structure of the system, and this necessitates reference areas for management or restoration activities. Attempts to return systems to within their historical range of biotic and abiotic characteristics and processes may not be possible, and management activities directed at removing undesirable features of novel ecosystems may perpetuate or create such ecosystems. Management actions should attempt to maintain genetic and species diversity and encourage the biogeochemical characteristics that favor desirable species. Few resources currently exist to support the addition of proactive measures and rigorous experimental designs to current management activities. The necessary changes will not occur without strong input from stakeholders and policy makers, so rapid information transfer and proactive research–management activities by the scientific community are needed.

Threshold models in restoration and conservation: a developing framework

The recognition that a system can appear resilient to changes in the environment, only to reach a critical threshold of rapid and unexpected change, is spurring work to apply threshold models in conservation and restoration. Here we address the relevance of threshold models to habitat management. Work to date indicates these concepts are highly applicable: human impacts can widen the range of habitats where threshold dynamics occur and shift communities into new states that are difficult to reverse. However, in many applied settings, threshold concepts are being adopted without evaluation of evidence and uncertainty. We suggest a framework for incorporating threshold models that reflects an emphasis on applicability to decision making and management on relatively short timescales and in human-impacted systems.

Niche complementarity due to plasticity in resource use: plant partitioning of chemical N forms

Niche complementarity, in which coexisting species use different forms of a resource, has been widely invoked to explain some of the most debated patterns in ecology, including maintenance of diversity and relationships between diversity and ecosystem function. However, classical models assume resource specialization in the form of distinct niches, which does not obviously apply to the broadly overlapping resource use in plant communities. Here we utilize an experimental framework based on competition theory to test whether plants partition resources via classical niche differentiation or via plasticity in resource use. We explore two alternatives: niche preemption, in which individuals respond to a superior competitor by switching to an alternative, less-used resource, and dominant plasticity, in which superior competitors exhibit high resource use plasticity and shift resource use depending on the competitive environment. We determined competitive ability by measuring growth responses with and without neighbors over a growing season and then used 15N tracer techniques to measure uptake of different nitrogen (N) forms in a field setting. We show that four alpine plant species of differing competitive abilities have statistically indistinguishable uptake patterns (nitrate > ammonium > glycine) in their fundamental niche (without competitors) but differ in whether they shift these uptake patterns in their realized niche (with competitors). Competitively superior species increased their uptake of the most available N form, ammonium, when in competition with the rarer, competitively inferior species. In contrast, the competitively inferior species did not alter its N uptake pattern in competition. The existence of plasticity in resource use among the dominant species provides a mechanism that helps to explain the manner by which plant species with broadly overlapping resource use might coexist.

Competitive impacts and responses of an invasive weed: dependencies on nitrogen and phosphorus availability

Changes in competitive interactions under conditions of enhanced resource availability could explain the invasion success of some problematic plant species. For one invader of North American grasslands, Centaurea diffusa (diffuse knapweed), we test three hypotheses: (1) under ambient (high resource) conditions, C. diffusa is better able to tolerate competition from the resident community (competitive response), (2) under ambient conditions, C. diffusa strong impacts the competitive environment (competitive effect), and (3) reductions in nitrogen and/or phosphorus availability diminish these advantages. In support of our first hypothesis, C. diffusa was the most tolerant to neighbor competition of the four focal species under current resource conditions. In opposition to our second hypothesis, however, neighborhoods that contained C. diffusa and those where C. diffusa had been selectively removed did not differ in their impact on the performance of target transplant individuals or on resource conditions. Reduction in resource availability influenced competitive tolerance but not competitive impact, in partial support of our last hypothesis. Reduction in soil nitrogen (via sucrose carbon addition) enhanced the degree of neighbor competition experienced by all species but did not change their relative rankings; C. diffusa remained the best competitor under low nitrogen conditions. Reduction of soil phosphorus (via gypsum addition) weakened the ability of C. diffusa to tolerate neighbor competition proportionately more than the other focal species. Consequently, under low phosphorus conditions, C. diffusa lost its competitive advantage and tolerated neighbor competition similarly to the other focal species. We conclude that C. diffusa invasion may be double-edged: C. diffusa is less limited by nitrogen than the other focal species and is better able to utilize phosphorus to its competitive advantage.

Committing to ecological restoration

Efforts around the globe need legal and policy clarification At the September 2014 United Nations Climate Summit, governments rallied around an international agreement—the New York Declaration on Forests—that underscored restoration of degraded ecosystems as an auspicious solution to climate change. Ethiopia committed to restore more than one-sixth of its land. Uganda, the Democratic Republic of Congo, Guatemala, and Colombia pledged to restore huge areas within their borders. In total, parties committed to restore a staggering 350 million hectares by 2030.

Leaf dry matter content and lateral spread predict response to land use change for six subalpine grassland species

Abstract Question: Land-use change has a major impact on terrestrial plant communities by affecting fertility and disturbance. We test how particular combinations of plant functional traits can predict species responses to these factors and their abundance in the field by examining whether trade-offs at the trait level (fundamental trade-offs) are linked to trade-offs at the response level (secondary trade-offs). Location: Central French Alps. Methods: We conducted a pot experiment in which we characterized plant trait syndromes by measuring whole plant and leaf traits for six dominant species, originating from contrasting subalpine grassland types. We characterized their response to nutrient availability, shading and clipping. We quantified factors linked with different land usage in the field to test the relevance of our experimental treatments. Results: We showed that land management affected nutrient concentration in soil, light availability and disturbance intensity. We identified particular suites of traits linked to plant stature and leaf structure which were associated with species responses to these environmental factors. Leaf dry matter content separates fast and slow growing species. Height and lateral spread separated tolerant and intolerant species to shade and clipping. Discussion and Conclusion: Two fundamental trade-offs based on stature traits and leaf traits were linked to two secondary trade-offs based on response to fertilization shade and mowing. Based on these trade-offs, we discuss four different species strategies which could explain and predict species distributions and traits syndrome at community scale under different land-uses in subalpine grasslands. Nomenclature: Tutin et al. (1964–1980).

Consequences of plant-soil feedbacks in invasion.

1. Plant species can influence soil biota, which in turn can influence the relative performance of plant species. These plant–soil feedbacks (PSFs) have been hypothesized to affect many community-level dynamics including species coexistence, dominance and invasion. 2. The importance of PSFs in exotic species invasion, although widely hypothesized, has been difficult to determine because invader establishment necessarily precedes invader-mediated PSFs. Here, we combine a spatial simulation model of invasion that incorporates PSFs with a meta-analysis that synthesizes published case studies describing feedbacks between pairs of native and exotic species. 3. While our spatial model confirmed the link between positive soil feedbacks (‘home’ advantage) for exotic species and exotic species spread, results were dependent on the initial abundance of the exotic species and the equivalence of dispersal and life history characteristics between exotic and native species. 4. The meta-analysis of 52 native–exotic pairwise feedback comparisons in 22 studies synthesized measures of native and exotic performance in soils conditioned by native and exotic species. The analysis indicated that the growth responses of native species were often greater in soil conditioned by native species than in soil conditioned exotic species (a ‘home’ advantage). The growth responses of exotic species were variable and not consistently related to species soil-conditioning effects. 5. Synthesis. Overlaying empirical estimates of pairwise PSFs with spatial simulations, we conclude that the empirically measured PSFs between native and exotic plant species are often not consistent with predictions of the spread of exotic species and mono-dominance. This is particularly the case when exotic species are initially rare and share similar dispersal and average fitness characteristics with native species. However, disturbance and other processes that increase the abundance of exotic species as well as the inclusion of species dispersal and life history differences can interact with PSF effects to explain the spread of invasive species

Single‐trait functional indices outperform multi‐trait indices in linking environmental gradients and ecosystem services in a complex landscape

Summary Functional traits can be used to describe the composition of communities through indices that seek to explain the factors that drive community assembly, biotic effects on ecosystem processes or both. Appropriately representing functional composition is therefore essential for predicting the consequences of environmental context and management actions for the provisioning of multiple ecosystem services (ESs) in heterogeneous landscapes. Functional indices can be constructed from single or multiple traits; however, it is not clear how they differ in information content or ability to predict biodiversity – ecosystem function relationships in complex landscapes. Here, we compare the utility of analogous single- and multi-trait indices in linking environmental variation and functional composition to ESs in a heterogeneous landscape, relating functional indices based on three plant traits [height, relative growth rate and root density (RD)] to variation in the physical environment and to two ESs (forage production and soil carbon) and their net ES level. Two orthogonal gradients, elevation and soil bulk density (BD), explained significant variation in several dimensions of functional composition comprised of single traits. These traits in turn significantly predicted variation in ESs and their net values. Only one index measured with multiple traits (functional richness) varied with the physical environment, while none predicted variation in ES or net ES levels. One ES, soil carbon, increased with the community-average value of RD, while the other, forage production, was related to the range and community-average value of height. In turn, average RD increased with soil BD while the average and range of height declined with elevation. Due to these environmental patterns, soil carbon and forage production did not covary strongly, leading to moderate net ES levels across the landscape. Synthesis: Single-trait indices of functional composition best linked variation in environmental gradients with productivity and soil carbon. Because the environment–trait functioning relationships were independent of one another, the ESs were independently distributed across the landscape, providing little evidence of synergies or trade-offs. Single- and multi-trait indices contained unique information about functional composition of these communities, and both are likely to have a place in predicting variation in ESs under different scenarios.