Elise S. Gornish

Climate-driven diversity loss in a grassland community

Significance Whereas a dominant conservation paradigm proposes that species are being lost from ecological communities with a consequent loss of ecosystem function, recent analyses have concluded there is no globally consistent trend toward lower community diversity. In a study of Californian grassland communities, we show that 15 years of climatic drying—consistent with the forecasts for this and other semiarid regions under climate change—have led to directional losses of plant species richness, especially of native annual forb (“wildflower”) species with traits indicative of low drought tolerance. Although many anthropogenic impacts may increase or not affect community diversity, our result underlines that declining plant community diversity may be especially likely in climates that are becoming more arid and less productive. Local ecological communities represent the scale at which species coexist and share resources, and at which diversity has been experimentally shown to underlie stability, productivity, invasion resistance, and other desirable community properties. Globally, community diversity shows a mixture of increases and decreases over recent decades, and these changes have relatively seldom been linked to climatic trends. In a heterogeneous California grassland, we documented declining plant diversity from 2000 to 2014 at both the local community (5 m2) and landscape (27 km2) scales, across multiple functional groups and soil environments. Communities became particularly poorer in native annual forbs, which are present as small seedlings in midwinter; within native annual forbs, community composition changed toward lower representation of species with a trait indicating drought intolerance (high specific leaf area). Time series models linked diversity decline to the significant decrease in midwinter precipitation. Livestock grazing history, fire, succession, N deposition, and increases in exotic species could be ruled out as contributing causes. This finding is among the first demonstrations to our knowledge of climate-driven directional loss of species diversity in ecological communities in a natural (nonexperimental) setting. Such diversity losses, which may also foreshadow larger-scale extinctions, may be especially likely in semiarid regions that are undergoing climatic trends toward higher aridity and lower productivity.

Community shifts under climate change: Mechanisms at multiple scales

PREMISE OF THE STUDY Processes that drive ecological dynamics differ across spatial scales. Therefore, the pathways through which plant communities and plant-insect relationships respond to changing environmental conditions are also expected to be scale-dependent. Furthermore, the processes that affect individual species or interactions at single sites may differ from those affecting communities across multiple sites. METHODS We reviewed and synthesized peer-reviewed literature to identify patterns in biotic or abiotic pathways underpinning changes in the composition and diversity of plant communities under three components of climate change (increasing temperature, CO2, and changes in precipitation) and how these differ across spatial scales. We also explored how these changes to plants affect plant-insect interactions. KEY RESULTS The relative frequency of biotic vs. abiotic pathways of climate effects at larger spatial scales often differ from those at smaller scales. Local-scale studies show variable responses to climate drivers, often driven by biotic factors. However, larger scale studies identify changes to species composition and/or reduced diversity as a result of abiotic factors. Differing pathways of climate effects can result from different responses of multiple species, habitat effects, and differing effects of invasions at local vs. regional to global scales. Plant community changes can affect higher trophic levels as a result of spatial or phenological mismatch, foliar quality changes, and plant abundance changes, though studies on plant-insect interactions at larger scales are rare. CONCLUSIONS Climate-induced changes to plant communities will have considerable effects on community-scale trophic exchanges, which may differ from the responses of individual species or pairwise interactions.

Managing Medusahead (Taeniatherum caput-medusae) on Rangeland: A Meta-Analysis of Control Effects and Assessment of Stakeholder Needs☆

ABSTRACT Invasive plant response to control efforts is strongly modified by site-specific factors, treatment timing, and environmental conditions following treatment, making management outcomes challenging to predict. Systematic reviews, which involve quantitative synthesis of data, can address this challenge by identifying general patterns of treatment effects across studies and quantifying the degree to which these effects vary. We conducted a systematic review of medusahead (Taeniatherum caput-medusae [L.] Nevski) control treatments that couples a meta-analysis on control data with an assessment of stakeholder needs to identify critical medusahead management knowledge gaps. With the meta-analysis we generated effect size estimates of how combinations of herbicide, burning, seeding, and grazing impacted medusahead on rangeland dominated by either annual or perennial vegetation. All combinations of treatments in both rangeland systems provided significant short-term control of medusahead, although treatment effects were highly transient on perennial rangeland, particularly for seeding treatments. Stakeholders listed grazing as a preferred management tool, and on annual rangeland an almost twofold reduction in medusahead abundance was achieved by timing high stocking rates to match phenological stages when medusahead was most susceptible to defoliation. Insufficient data were available to evaluate effects of grazing on medusahead on perennial rangeland. On the basis of these data and our stakeholder survey, four major information needs emerged, including the need to better understand 1) seedbank response to burning and herbicide treatments, 2) how to optimize grazing animal impacts on medusahead given ranch enterprise constraints, 3) costs and benefits of control and risk of practice failure, and 4) impacts of adaptive management treatments conducted on larger scales and at longer time intervals. Addressing these knowledge gaps should help overcome key ecological and economic barriers inhibiting implementation of medusahead and other invasive plant management programs on rangeland and provide a positive step toward conserving the critical ecosystem services these systems provide.

Interdisciplinary Climate Change Collaborations Are Essential for Early‐Career Scientists

Climate change research is an interdisciplinary field, and understanding its social, political, and environmental implications requires integration across fields of research where different tools may be used to address common concerns [Baerwald, 2010]. One of the many advantages of interdisciplinary approaches is that they open communication between complementary fields, filling knowledge gaps and facilitating progression within both individual fields and the broader field of climate change research [Ludwig et al., 2011].

Positive Species Interactions as Drivers of Vegetation Change on a Barrier Island

ABSTRACT Monge, J.A. and Gornish, E.S., 2015. Positive species interactions as drivers of vegetation change on a barrier island. Positive species interactions have been shown to occur in a variety of plant systems, and the importance of these interactions is expected to vary with resource availability and abiotic stress. The processes by which these types of relationships operate and influence plant communities in coastal environments, however, are not fully understood. Positive species interactions were observed in areas of St. George Island, Florida, shortly after transplanting dune species for a restoration experiment designed to encourage the growth of foredune, interdune, and backdune vegetation. The dune habitats in St. George are subject to abiotic stresses that vary in type and magnitude, and the environmental factors responsible for ameliorating these conditions and encouraging positive vegetation change operate differently across these areas. We (1) investigated if transplants encouraged positive changes in vegetation across dune habitats, (2) determined whether disturbance (through transplanting) played a role, and (3) tested environmental factors involved in positive interactions to explain the changes in vegetation observed across dune habitats. The presence of transplants positively modified vegetation (e.g., species richness or cover) across all dune habitats. Experimental disturbance had no strong overall positive effect on vegetation change. Shading and soil moisture redistribution had differential effects on vegetation change among habitats, suggesting that these environmental factors interact with the abiotic characteristics unique to each dune habitat in complex ways. Our results suggest that experimentation over a longer time scale might be required to fully understand the extent at which positive interactions affect vegetation patterns along stressful environmental gradients.

Interactions among habitat, management, and demography for an invasive annual grass

Effective long-term management of noxious plant species depends on our ability to identify and manage ecological processes-driving invasion and to use site-specific information to design realistic management strategies and goals. However, there is a surprising lack of research that investigates relationships among habitat characteristics, weed demography, and management efforts. We investigated the interactions among these factors using the invasive annual grass medusahead (Elymus caput-medusae). In Yuba County, California, we seeded medusahead in densities from 0 to 50,000 seeds/m2 in open grassland and oak woodland habitats. We also exposed plots to defoliation none, once, or twice in a season and captured how medusahead germination, establishment, and persistence responded to experimental treatments. We found that medusahead establishment was higher in the grassland, compared to the woodland habitat, likely due, in part, to the presence of litter under oak canopies. After a single defoliation event, medusahead persistence was higher in the oak, but not the grassland plots. We also found that defoliation once or twice reduced medusahead density. However, a single clipping treatment actually resulted in increased seed production. This was likely a compensatory response by a younger cohort, and provides an explanation for why single control efforts do not generally result in successful long-term outcomes. This work highlights the complex relationships between density-dependent and density-independent processes that may influence invasion dynamics.

Demographic effects of warming, elevated soil nitrogen and thinning on the colonization of a perennial plant

Global change is causing significant modifications to native plant communities. These effects can be direct through changes in productivity, or indirect through the spread of invading species. Identifying vital traits important for individual species’ response to environmental variation could be useful for making predictions about how entire communities may respond to global change. I studied the effects of factors associated with global change on the demography of an experimentally introduced species, Pityopsis aspera. In a Florida old-field, I investigated how warming, increased soil nitrogen and thinning of the extant plant community affected survival, growth and reproduction of P. aspera using a life table response experiment. The estimated population growth rate (λ) of P. aspera was reduced by nitrogen addition, as a result of decreased fecundity. However, λ increased in response to the warming treatment, as a result of increased fecundity. In the presence of thinning, both warming and nitrogen served to increase λ as a result of an increase in the growth of young individuals. This experiment illustrates how different vital rates contribute to the population level responses of an experimentally introduced plant to warming, and nitrogen deposition. Results also show how these demographic responses may occur via indirect effects through established species. This work highlights the importance of studying interactions among temperature, soil nitrogen and demography across the entire life cycle in order to capture the complex and, often, non-additive relationships mediating global change effects.

Testing the competition–colonization trade-off with a 32-year study of a saxicolous lichen community

Competition-colonization trade-offs are theorized to be a mechanism of coexistence in communities structured by environmental fluctuations. But many studies that have tested for the trade-off have failed to detect it, likely because a spatiotemporally structured environment and many species assemblages are needed to adequately test for a competition-colonization trade-off. Here, we present a unique 32-year study of rock-dwelling lichens in New Mexico, USA, in which photographs were used to quantify lichen life history traits and interactions through time. These data allowed us to determine whether there were any trade-offs between traits associated with colonization and competition, as well as the relationship between diversity and disturbance in the community. We did not find evidence for a trade-off between competitive ability and colonization rate or any related life history traits. Interestingly, we did find a peak in all measures of species diversity at intermediate levels of disturbance, consistent with the intermediate disturbance hypothesis pattern. We suggest that the coexistence of the dominant species in this system is regulated by differences in persistence and growth rate mediating overgrowth competition rather than a competition-colonization trade-off.

Associations between an Invasive Plant (Taeniatherum caput-medusae, Medusahead) and Soil Microbial Communities

Understanding plant-microbe relationships can be important for developing management strategies for invasive plants, particularly when these relationships interact with underlying variables, such as habitat type and seedbank density, to mediate control efforts. In a field study located in California, USA, we investigated how soil microbial communities differ across the invasion front of Taeniatherum caput-medusae (medusahead), an annual grass that has rapidly invaded most of the western USA. Plots were installed in habitats where medusahead invasion is typically successful (open grassland) and typically not successful (oak woodland). Medusahead was seeded into plots at a range of densities (from 0–50,000 seeds/m2) to simulate different levels of invasion. We found that bacterial and fungal soil community composition were significantly different between oak woodland and open grassland habitats. Specifically, ectomycorrhizal fungi were more abundant in oak woodlands while arbuscular mycorrhizal fungi and plant pathogens were more abundant in open grasslands. We did not find a direct effect of medusahead density on soil microbial communities across the simulated invasion front two seasons after medusahead were seeded into plots. Our results suggest that future medusahead management initiatives might consider plant-microbe interactions.

A systematic review of management efforts on goatgrass (Aegilops spp) dominance

Goatgrass (Aegilops) species are some of the most aggressive invasive plants in the Western U.S. Despite intense management efforts, goatgrass continues to reduce the ecological and economic integrity of natural and agroecological systems. The mismatch between current research outcomes and practical needs of land managers is likely a result of limitations associated with generalizing from single location, treatment, or season studies. We conducted a systematic review of experiments testing control of two dominant goatgrass invaders (A. cylindrica and A. triuncialis) to identify general patterns in treatment efficiency. Using data from 391 separate experiments, we found that experimental treatments were more successful at controlling the dominance of A. cylindrica compared to A. triuncialis. For A. cylindrica, no treatment demonstrated particular utility for control. Treatment of A. cylindrica in the vegetative stage was more effective for control than treatment at other stages. For A. triuncialis, burning and grazing demonstrated effective overall control among all treatments, although grazing produced variable results. Treatment in the fruiting stage of A. triuncialis was more effective for control than treatment at other stages. For both species, multiple applications of a management treatment within a year resulted in no better control than a single application within a year. Additionally, treatments deployed in two consecutive years resulted in better control of both species, than a treatment deployed in a single year. This work highlights promising avenues for more intensive research on goatgrass weed control and suggests that management funding is most effectively utilized when employed across years rather than focused on a single year.