Vegetation Type and Fire Severity Mediate Short-Term Post Fire Soil Microbial Responses

Background Wildfire severity mediates key dynamics, such as nutrient pulses, that regulate the recovery of ecosystem functioning. Large shifts in vegetation communities associated with plant invasions are often coupled with changes in soil communities; thus, it’s critical to understand how fire severity may interact with vegetation type and soil communities to mediate ecosystem recovery. Methods Following a 2017 wildfire in southern California, soils from areas dominated by native coastal sage scrub or exotic annual grasses that experienced a low or high severity fire event were analyzed for nutrient concentrations and two proxies for ecosystem function—microbial respiration and enzymatic activity potentials over the first-year post-fire. Aims We predicted that increasing fire severity would positively correlate with soil nutrient concentrations. Thus, higher severity burned soil would experience a greater downregulation of enzyme activity as potential microbial nutrient limitation was alleviated, a relationship that would be stronger in shrub dominated soil. Results We observed a strong soil nitrogen (N) pulse post-fire, which was greatest in shrub dominated soil; however, dominant vegetation had a variable effect on microbial responses. Enzyme activities were downregulated in CSS soil, but the grass dominated soil response was inconsistent. After 1 year, soil N remained elevated in burned soil, suggesting that basal soil N concentrations were altered. Conclusions Persistent, residual soil N concentrations are of particular concern in high fire risk ecosystems, which will likely experience increasing fire frequency associated with environmental change; thus, encouraging the regrowth of opportunistic vegetation in subsequent growing seasons will be key to minimize long-term changes to these ecosystems

Rapid Bacterial and Fungal Successional Dynamics in First Year After Chaparral Wildfire

The rise in wildfire frequency and severity across the globe has increased interest in secondary succession. However, despite the role of soil microbial communities in controlling biogeochemical cycling and their role in the regeneration of post-fire vegetation, the lack of measurements immediately post-fire and at high temporal resolution has limited understanding of microbial secondary succession. To fill this knowledge gap, we sampled soils at 17, 25, 34, 67, 95, 131, 187, 286, and 376 days after a southern California wildfire in fire-adapted chaparral shrublands. We assessed bacterial and fungal biomass with qPCR of 16S and 18S and richness and composition with Illumina MiSeq sequencing of 16S and ITS2 amplicons. Fire severely reduced bacterial biomass by 47%, bacterial richness by 46%, fungal biomass by 86%, and fungal richness by 68%. The burned bacterial and fungal communities experienced rapid succession, with 5-6 compositional turnover periods. Analogous to plants, turnover was driven by fire-loving pyrophilous microbes, many of which have been previously found in forests worldwide and changed markedly in abundance over time. Fungal secondary succession was initiated by the Basidiomycete yeast Geminibasidium, which traded off against the filamentous Ascomycetes Pyronema, Aspergillus, and Penicillium. For bacteria, the Proteobacteria Massilia dominated all year, but the Firmicute Bacillus and Proteobacteria Noviherbaspirillum increased in abundance over time. Our high-resolution temporal sampling allowed us to capture post-fire microbial secondary successional dynamics and suggest that putative tradeoffs in thermotolerance, colonization, and competition among dominant pyrophilous microbes control microbial succession with possible implications for ecosystem function

Mechanisms for Species Coexistence under Environmental Change: Insights from a California Grassland

Understanding the potential mechanisms that influence invasion resistance and coexistence in plant communities has been a central tenet of invasion ecological research during the past few decades. My dissertation used observational and experimental approaches to understand what processes influence whether a community is invaded, resists invasion, or results in species coexistence within a California grassland. Chapter 2 reviewed the impacts that alien plant species may have on communities and provided a framework for how to identify when invader impacts lead to recovery constraints for the native community and integrate these constraints into restoration efforts. Chapter 3 investigated how species effects on resource availability can result in differing invasion dynamics in native versus exotic dominated grasslands. I found that while exotic and native species differentially alter the availability of light and nitrogen in a community, nitrogen availability is key in determining invasion of an exotic into a native grassland as well as the invasion of a native into an exotic dominated community. Chapter 4 investigated how propagule pressure after an extreme disturbance can result in the invasion of intact native grasslands. I found that the recovery of native grassland stands after an extreme disturbance (fire+drought) can be stalled by an influx of exotic propagules from the surrounding matrix. Chapter 5 addressed how the strength of plant-soil feedbacks for a native and exotic may change with soil resource availability changes on soil communities and with a competitor. I found a negative effect of exotic conditioned soil on native growth and no effect of native conditioned soil on exotic growth, suggesting that plant-soil feedbacks may facilitate the establishment of the exotic as well as its dominance. Lastly, Chapter 6 investigated how seed addition and soil amendments management efforts affected native recovery after an extreme disturbance. I found that seed additions and soil N reductions were able to increase the establishment and fitness of some natives, but may not be sufficient to promote full native recovery. This work provides a tool to understand not only why native resident communities are invaded but also how to reduce the resistance of invaded communities and increase the resistance of native communities. Additionally this work allowed me to integrate the impacts that exotic species have on communities to make general predictions about the recovery of native communities after an extreme disturbance or control efforts. Overall, I observed that native communities and populations are vulnerable to invasion after a large disturbance and with nitrogen enrichment. From low to moderate nitrogen availability, native and exotic species should coexist due to niche partitioning, but not as a result of density dependent negative plant-soil feedbacks. Lastly, I found that an exotic species is able to maintain its dominance due to its strong competitive effect on native species, particularly at high nitrogen availability and its ability to culture a soil community that negatively impacts the growth of native species

Competition and soil resource environment alter plant-soil feedbacks for native and exotic grasses.

Feedbacks between plants and soil biota are increasingly identified as key determinants of species abundance patterns within plant communities. However, our understanding of how plant-soil feedbacks (PSFs) may contribute to invasions is limited by our understanding of how feedbacks may shift in the light of other ecological processes. Here we assess how the strength of PSFs may shift as soil microbial communities change along a gradient of soil nitrogen (N) availability and how these dynamics may be further altered by the presence of a competitor. We conducted a greenhouse experiment where we grew native Stipa pulchra and exotic Avena fatua, alone and in competition, in soils inoculated with conspecific and heterospecific soil microbial communities conditioned in low, ambient and high N environments. Stipa pulchra decreased in heterospecific soil and in the presence of a competitor, while the performance of the exotic A. fatua shifted with soil microbial communities from altered N environments. Moreover, competition and soil microbial communities from the high N environment eliminated the positive PSFs of Stipa. Our results highlight the importance of examining how individual PSFs may interact in a broader community context and contribute to the establishment, spread and dominance of invaders

Stem counts and seeds measurements in California grassland communities

These data were collected in the field and include initial abundance in grams and seed number, stem counts, and seed counts for each species. Data are arranged such that each row represents a plot and block combination with each species within the plot arranged in columns. The columns are arranged by species with the genus as the first part of the column header followed by the data describer (Avena=Avena fatua, Bromus=Bromus hordeaceus, Brassica=Brassica nigra, Erodium=Erodium cicutarium, Eschscholzia=Eschscholzia californica, Lasthenia=Lasthenia californica, Lotus=Lotus purshianus, Lupinus=Lupinus succulentus, Melilotus=Melilotus indicus, Vicia=Vicia villosa). For each species, the columns are arranged in the same order: "genus name initial (g)" is the initial abundance of the species in grams that were added to the plots; "genus name seeds initial" is the initial number of seeds for each species that were added to the plots; "genus name stems final" is the number of individuals of that species at the end of the growing season in the plot; "genus name seeds final" is the number of seeds of that species at the end of the growing season in the plot