Introduced annuals mediate climate-driven community change in Mediterranean prairies of the Pacific Northwest, USA

12 pagesAim: How climate change will alter plant functional group composition is a critical question given the well-recognized effects of plant functional groups on ecosystem services. While climate can have direct effects on different functional groups, indirect effects mediated through changes in biotic interactions have the potential to amplify or counteract direct climatic effects. As a result, identifying the underlying causes for climate effects on plant communities is important to conservation and restoration initiatives. Location: Western Pacific Northwest (Oregon and Washington), USA. Methods: Utilizing a 3-year experiment in three prairie sites across a 520-km latitudinal climate gradient, we manipulated temperature and precipitation and recorded plant cover at the peak of each growing season. We used structural equation models to examine how abiotic drivers (i.e. temperature, moisture and soil nitrogen) controlled functional group cover, and how these groups in turn determined overall plant diversity. Results: Warming increased the cover of introduced annual species, causing subsequent declines in other functional groups and diversity. While we found direct effects of temperature and moisture on extant vegetation (i.e. native annuals, native perennials and introduced perennials), these effects were typically amplified by introduced annuals. Competition for moisture and light or space, rather than nitrogen, were critical mechanisms of community change in this seasonally water-limited Mediterranean-climate system. Diversity declines were driven by reductions in native annual cover and increasing dominance by introduced annuals. Main conclusions: A shift towards increasing introduced annual dominance in this system may be akin to that previously experienced in California grasslands, resulting in the “Californication” of Pacific Northwest prairies. Such a phenomenon may challenge local land managers in their efforts to maintain species-rich and functionally diverse prairie ecosystems in the future

Hydrogenation of Organic Matter as a Terminal Electron Sink Sustains High CO2:CH4 Production Ratios During Anaerobic Decomposition

Once inorganic electron acceptors are depleted, organic matter in anoxic environments decomposes by hydrolysis, fermentation, and methanogenesis, requiring syntrophic interactions between microorganisms to achieve energetic favorability. In this classic anaerobic food chain, methanogenesis represents the terminal electron accepting (TEA) process, ultimately producing equimolar CO2 and CH4 for each molecule of organic matter degraded. However, CO2:CH4 production in Sphagnum-derived, mineral-poor, cellulosic peat often substantially exceeds this 1:1 ratio, even in the absence of measureable inorganic TEAs. Since the oxidation state of C in both cellulose-derived organic matter and acetate is 0, and CO2 has an oxidation state of +4, if CH4 (oxidation state -4) is not produced in equal ratio, then some other compound(s) must balance CO2 production by receiving 4 electrons. Here we present evidence for ubiquitous hydrogenation of diverse unsaturated compounds that appear to serve as organic TEAs in peat, thereby providing the necessary electron balance to sustain CO2:CH4 \u3e1. While organic electron acceptors have previously been proposed to drive microbial respiration of organic matter through the reversible reduction of quinone moieties, the hydrogenation mechanism that we propose, by contrast, reduces C-C double bonds in organic matter thereby serving as 1) a terminal electron sink, 2) a mechanism for degrading complex unsaturated organic molecules, 3) a potential mechanism to regenerate electron-accepting quinones, and, in some cases, 4) a means to alleviate the toxicity of unsaturated aromatic acids. This mechanism for CO2 generation without concomitant CH4 production has the potential to regulate the global warming potential of peatlands by elevating CO2:CH4 production ratios

Community and ecosystem dynamics in remnant and restored prairies

xiv, 166 p. : ill. A print copy of this title is available through the UO Libraries. Search the library catalog for the location and call number.Restoration of imperiled ecosystems has emerged as a national priority, but there is little mechanistic understanding of how to restore ecosystems so as to sustain both species diversity and ecosystem function. The main objectives of my dissertation were (i) to develop an understanding of mechanisms that structure upland and wetland prairie plant communities in Oregon's Willamette Valley, with particular focus on edaphic and competitive controls over native and exotic species, and (ii) to apply this knowledge toward more effective restoration of prairie ecosystems. I used a combination of experiments and analysis of natural gradients to examine the effects of succession, competition, and environmental heterogeneity on plant community structure and ecosystem function within a restoration framework. I conducted a large, replicated field experiment and a retroactive study of previously restored wetland prairies to assess different site preparation techniques. These techniques had variable effectiveness in suppressing the existing vegetation and seed bank, thus providing different initial successional trajectories. However, over time plant community structure converged due to a loss of early-successional species and the increasing dominance of native bunchgrasses; hence, there was a negative relationship between cover of native species and diversity. Only the more extreme treatments, such as topsoil removal, had large impacts on soil functioning. These studies underscore the importance of using a successional framework to guide restoration efforts. Given the potential importance of competition between native and exotic grasses in structuring prairie vegetation, I used a paired study of field and greenhouse experiments to determine how abiotic factors influence the competitive hierarchies between native and exotic grasses commonly found in upland and wetland prairies. Exotic grasses dominated competitive interactions with the native grasses, but this depended upon nutrient and moisture availability. Finally, I used a laboratory experiment to determine the seasonal and edaphic controls over nutrient and carbon cycling within a spatially heterogeneous upland prairie. Manipulating moisture and temperature resulted in significant changes in nitrogen, phosphorus, and carbon cycling, particularly in the winter. Under projected future climate change, these changes will likely have large effects on plant community structure. This dissertation includes my previously published and co-authored materials.Advisers: Scott D. Bridgham, Barbara "Bitty" A. Roy, Bart R. Johnso

The herbaceous landlord: integrating the effects of symbiont consortia within a single host

Plants are typically infected by a consortium of internal fungal associates, including endophytes in their leaves, as well as arbuscular mycorrhizal fungi (AMF) and dark septate endophytes (DSE) in their roots. It is logical that these organisms will interact with each other and the abiotic environment in addition to their host, but there has been little work to date examining the interactions of multiple symbionts within single plant hosts, or how the relationships among symbionts and their host change across environmental conditions. We examined the grass Agrostis capillaris in the context of a climate manipulation experiment in prairies in the Pacific Northwest, USA. Each plant was tested for presence of foliar endophytes in the genus Epichloë, and we measured percent root length colonized (PRLC) by AMF and DSE. We hypothesized that the symbionts in our system would be in competition for host resources, that the outcome of that competition could be driven by the benefit to the host, and that the host plants would be able to allocate carbon to the symbionts in such a way as to maximize fitness benefit within a particular environmental context. We found a correlation between DSE and AMF PRLC across climatic conditions; we also found a fitness cost to increasing DSE colonization, which was negated by presence of Epichloë endophytes. These results suggest that selective pressure on the host is likely to favor host/symbiont relationships that structure the community of symbionts in the most beneficial way possible for the host, not necessarily favoring the individual symbiont that is most beneficial to the host in isolation. These results highlight the need for a more integrative, systems approach to the study of host/symbiont consortia

Experimental warming decreases arbuscular mycorrhizal fungal colonization in prairie plants along a Mediterranean climate gradient

Background: Arbuscular mycorrhizal fungi (AMF) provide numerous services to their plant symbionts. Understanding climate change effects on AMF, and the resulting plant responses, is crucial for predicting ecosystem responses at regional and global scales. We investigated how the effects of climate change on AMF-plant symbioses are mediated by soil water availability, soil nutrient availability, and vegetation dynamics. Methods: We used a combination of a greenhouse experiment and a manipulative climate change experiment embedded within a Mediterranean climate gradient in the Pacific Northwest, USA to examine this question. Structural equation modeling (SEM) was used to determine the direct and indirect effects of experimental warming on AMF colonization. Results: Warming directly decreased AMF colonization across plant species and across the climate gradient of the study region. Other positive and negative indirect effects of warming, mediated by soil water availability, soil nutrient availability, and vegetation dynamics, canceled each other out. Discussion: A warming-induced decrease in AMF colonization would likely have substantial consequences for plant communities and ecosystem function. Moreover, predicted increases in more intense droughts and heavier rains for this region could shift the balance among indirect causal pathways, and either exacerbate or mitigate the negative, direct effect of increased temperature on AMF colonization

An Exploration of Hypotheses that Explain Herbivore and Pathogen Attack in Restored Plant Communities

<div><p>Many hypotheses address the associations of plant community composition with natural enemies, including: (i) plant species diversity may reduce enemy attack, (ii) attack may increase as host abundance increases, (iii) enemy spillover may lead to increased attack on one host species due to transmission from another host species, or enemy dilution may lead to reduced attack on a host that would otherwise have more attack, (iv) physical characteristics of the plant community may influence attack, and (v) plant vigor may affect attack. Restoration experiments with replicated plant communities provide an exceptional opportunity to explore these hypotheses. To explore the relative predictive strengths of these related hypotheses and to investigate the potential effect of several restoration site preparation techniques, we surveyed arthropod herbivore and fungal pathogen attack on the six most common native plant species in a restoration experiment. Multi-model inference revealed a weak but consistent negative correlation with pathogen attack and host diversity across the plant community, and no correlation between herbivory and host diversity. Our analyses also revealed host species-specific relationships between attack and abundance of the target host species, other native plant species, introduced plant species, and physical community characteristics. We found no relationship between enemy attack and plant vigor. We found minimal differences in plant community composition among several diverse site preparation techniques, and limited effects of site preparation techniques on attack. The strongest associations of community characteristics with attack varied among plant species with no community-wide patterns, suggesting that no single hypothesis successfully predicts the dominant community-wide trends in enemy attack.</p></div

Community Factors that Were Measured and Entered into AIC Analysis.

<p>Community Factors that Were Measured and Entered into AIC Analysis.</p

Interpretive Diagram: Partial Correlations of Changes in Herbivore/Pathogen Attack versus Abundance of Plant Species.

<p>Diagram is based on the results of our AIC analyses, and represents interactions that were in all or most resulting selected models. Width of arrows indicates approximate magnitude of partial correlation, ranging from 0.06 to 0.40. The arrow with a dashed border was selected in 89% of the models selected. All other relationships shown were selected in 100% of the models.</p

Average Partial Correlations from Multiple Regression Models of 14 Predictor Variables Regressed against Herbivore/Pathogen Attack.

<p>Error bars (standard error) are shown among partial correlations of each variable across all models in which that variable was selected. Numbers above each bar represent the proportion of models in which that variable was selected. Error bars represent variation in magnitude of partial correlation among selected models. R² values represent the average predictive power of the multiple selected models for each host species-natural enemy combination. Figs. <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0116650#pone.0116650.g002" target="_blank">2</a> and <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0116650#pone.0116650.g003" target="_blank">3</a> depict subsets of the data shown in this figure, arranged according to hypothesis rather than host species. Predictor variables are defined as follows; the first seven predictor variables represent relative abundance of each species denoted as percent of total plant cover, Introduced Species: relative abundance of introduced plant species as percent of total plant cover, Total: total plant cover (see manuscript for method of recording total plant cover), Simpsons: Simpson’s diversity index, Standing Thatch and Ground Thatch: percent cover of thatch, Shoot Mass: above-ground individual shoot biomass, Plant Chlorophyll: leaf chlorophyll content.</p

Partial Correlations of Variables with Herbivore and Pathogen Attack to Six Native Species.

<p>Title of each panel is the variable of interest for that panel. Panels <b>b</b>, <b>c</b>, and <b>d</b> represent relative abundance of the variable of interest. Panels <b>f</b> and <b>g</b> represent percent cover of the variable of interest. Mean partial correlations with the variable of interest and percent herbivore or pathogen attack on each of six species are represented by bars. Error bars represent variation (standard error) in magnitude of partial correlation among selected models. Numbers along x axes below each bar represent the percent of models in which that variable was selected (herbivory, pathogen).</p