The ecological impacts of global change extend through multiple scales affecting individuals, local populations and communities and species distributions. Species-specific tolerances to interacting climate variables can give rise to species’ differing abilities to track their suitable habitat. Differing tracking abilities can in turn lead to novel assemblages of species further altering community dynamics. All of these questions can be better resolved by exploring population and community processes at broader regional scales or across environmental gradients. Forests of the western United States are an excellent study system as government agencies have collected vast amounts of individual tree data spread across the country and in addition to high spatial resolution, temporal patterns are recorded in the wood as annual growth rings. Forests are also ecologically as well as economically important and are vulnerable to multiple global change factors including changes in fire regimes, pest outbreaks, and drought. Here I address three research topics regarding the response of Western forests to climate variability at multiple scales. First, I leveraged the spatial coverage of the Forest Inventory and Analysis (FIA) database to quantify how growth, survival, and reproduction responses to soil water availability are related to distributions of 59 tree species in California, Oregon, and Washington. This revealed extensive mismatches between demographic performance and species distributions that result in dramatic decreases in performance, and depend on traits related to dispersal rates and competition. Second, using tree ring data, I analyze drought resilience among three conifers and explore how factors at local scales like tree size and regional scales affect resilience. Despite being drought-adapted, populations from xeric environments still experienced detrimental effects of drought and across the range, large trees suffered from droughts through poor recovery. Last, I explore patterns in synchrony of tree ring growth across space and time to shed light on community dynamics. Synchrony among species reflects a mixture of niche partitioning and neutral dynamics at play as well as environmental stress. Over the past century, synchrony among tree growth has increased overall which has the potential to alter community dynamics and ecosystem function.

Global change is creating novel communities of organisms around the globe as species shift their geographic ranges to maintain favorable climate conditions. Species range shifts create novel plant species interactions and influence relationships between plants and soil organisms in the new range. One well documented range shift in both arctic and alpine biomes is of woody plants, primarily shrubs, moving upwards in elevation or latitude and establishing in traditionally herbaceous dominated ecosystems.

Woody plant range expansion can have significant impacts on belowground ecosystem processes through changes in litter quality and quantity, rooting depth, and interactions with soil microbial communities. In addition, both biotic and abiotic soil conditions can limit the establishment and range expansion of woody plant species. The overarching goal of this dissertation was to understand how plant-soil interactions influence and respond to the range expansion of a sagebrush shrub in the White Mountains of California. I examined multiple belowground processes including soil microbial community structure and function, biogeochemical cycling, and soil feedbacks to other native plant species. I used field and greenhouse studies, plant demographic monitoring, laboratory assays, functional trait analyses, next generation sequencing and diverse statistical modeling techniques to link ecological processes from the microbial to ecosystem scale. Overall, sagebrush expansion increased bacterial diversity, soil moisture, soil organic carbon and nitrogen pools and microbial biomass, which were all associated with increased microbial activity (substrate induced respiration). Sagebrush did not strongly influence overall fungal diversity but increased the abundance of fungal groups including the saprotrophic Agaricomycete fungi, which are important wood and litter decomposers. Soil microbial communities influenced both the performance and leaf functional traits of sagebrush seedlings through changes in microbial diversity and extracellular enzyme activity, which suggested strong plant-microbial competition for soil nutrients. Finally, sagebrush influenced the demography of two co-occurring herbaceous plant species through negative plant-soil feedbacks which overwhelmed the effects of abiotic facilitation by sagebrush. Taken together the results of this dissertation provide a more complete picture of how multiple aspects of the soil environment both respond to changes in plant community composition, and feedback to influence plant performance and plant species interactions.

While the importance of animal and plant responses to global change is generally appreciated, comparatively little is known about fungal community responses to shifting environmental conditions. Our study evaluated the role of aridity, atmospheric nitrogen deposition and annual grass invasion in driving responses of the fungal community in roots, focusing on the arbuscular mycorrhizal fungi (AMF). Previous work has found that AMF with high root colonization (rhizophilic AMF) reduce root pathogen infection and respond positively to nitrogen deposition and annual grass invasion. Other work has found that AMF with high soil colonization (edaphophilic AMF) are more important for nutrient uptake and generally respond negatively to additional nitrogen, drought and association with grasses. We ask: do guilds of AMF respond differently to multiple drivers of global change and does this impact community biomass allocation and interactions between fungi?

We sampled roots and soils from native shrubs and invasive grasses in coastal sage scrub within a long-term experiment manipulating nitrogen and precipitation. We extracted and counted spore and hyphal densities from soils, percent root length colonized by AM and non-AM fungi, and measurements of soil chemistry (Total N,C,P, NH4,NO3 and pH). Additionally, we estimated the composition of fungi in plant roots using two rRNA loci: SSU with AMF specific primers and ITS2 with universal fungal primers. We evaluated these responses of fungal composition and biomass with generalized linear models.

We found that drought reduces the relative abundance of edaphophilic AMF and overall AMF biomass. Nitrogen addition reduces the relative abundance of edaphophilic AMF, and increases AMF allocation to root colonization and spore production while reducing the extent of nutrient foraging soil hyphae. Both drought and nitrogen deposition increased the extent of root colonization by saprotrophic and pathogenic fungi, suggesting that plants become more susceptible to infection by non-mutualistic fungi as water becomes more limiting. Invasive annual grasses hosted less edaphophilic AMF than did native shrubs, while one grass species also hosted a higher abundance of ancestral AMF. Overall, this suggests that with increasing drought, nitrogen deposition and invasion of shrub communities by annual grasses, the abundance of edaphophilic AMF in coastal sage scrub will decline.