ABSTRACT

Understanding factors that allow species to migrate into new suitable habitats has become increasingly urgent because of the rapid pace of human-caused environmental change. This project combines field and lab experiments to evaluate how plant-plant and plant-microbe interactions influence the migration of creosote, one of the most common and important shrubs of the warm deserts of North America (Sonoran, Chihuahuan and Mojave), into adjacent grasslands. The expansion of woody plants at the expense of grassland, a phenomenon called shrub encroachment, is global in scale and expected to accelerate under future warmer climates and elevated atmospheric CO2. The microbes that live in and near plant roots (the plant microbiome) likely play key roles in affecting the rate of shrub encroachment but their influence on shrub establishment and migration success has not yet been examined. To predict how plant-microbe and plant-plant (in this case shrub/grass) interactions will drive or slow plant population expansion in future climates requires evaluating the spatial and temporal scales at which these interactions occur. Using both field and lab experiments and mathematical models, this project will enable forecasts of how plant-plant and plant-microbe interactions reshape the abundance and distributions of shrub and grass species at the limits of their geographic distributions. Shrubland-grassland dynamics affect the amount of carbon stored by an ecosystem, drive rangeland management decisions, influence strategies to conserve biodiversity, and extend over 330 million hectares of North America alone. This project trains students at many levels, and builds partnerships with stakeholders and managers to ensure knowledge transfer to the benefit of society. A science-art collaboration is also embedded in the research activities, including the design and creation of 3D-printed plant models and artistic interpretations of plant morphologies. A museum exhibit of these models and microbe plant interactions will be created to expand public outreach and create an aesthetic expression of the inter-relationships of microbes and plants.

The proposed research will elucidate how plant root and soil associated microbes influence plant species range movements under environmental change. Research activities include the acquisition of novel quantitative data on key parameters that determine range spread: the spatial extent and temporal speed at which plants influence their microbial environment and the relative strength of the microbe/plant feedbacks compared to plant/plant competitive interactions. Experiments on field observations will determine the underlying mechanisms of plant-microbe interactions by addressing the following questions: (1) What is the spatial extent of plant-soil feedback between foundation species at ecotones where range limits collide? This question will be addressed using spatially-explicit paired field and greenhouse experiments to quantify the spatial extent of plant/microbe feedbacks as they influence both grass and shrub performance. (2) How does elevated CO2 affect microbial feedbacks and coexistence between foundation species at these ecotones? This question will be addressed using a growth chamber experiment to contrast microbial feedbacks and competition between grass and shrubs under ambient versus elevated CO2. (3) By what mechanisms do plant species cultivate unique microbial communities through time? This will be addressed with a field experiment to evaluate mechanisms underlying soil and microbial modification by plants and the temporal scale of each mechanism. Finally, quantitative models will be developed using the data from these experiments and observations to address the question, (4) How do soil microbes contribute to range expansion, contraction, or stabilization at species range limits, and how will elevated CO2 change range limit dynamics? The project breaks new ground ni evaluating plant/soil feedbacks in a spatial and temporal context using a well-documented, ecologically important, and experimentally tractable range shift: the encroachment of C3 woody shrubs into C4 grasslands in the American Southwest. Shrub encroachment can dramatically alter biodiversity and ecosystem processes, including carbon storage

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.

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