DISSERTATION RESEARCH: Between a tree and a dry place - How competition and drought tolerance constrain tree geographic ranges

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Identifying the main constraints on species ranges and how these constraints vary across the globe has proven remarkably difficult; the degree to which the environment alone or in combination with interactions with other organisms imposes the limit is the crux of the problem. This project will combine experiments, field studies, and species distribution models to test whether plant species along a gradient from dry to wetter environments are squeezed between death from drought on one side of their range and fierce competition with species adapted to wetter conditions on the other. The results can help explain when, where, and how tree range boundaries are constrained, thereby advancing the state of the science. Such information may help guide land managers who seek to prepare for a rapidly changing climatic future by suggesting which conservation and forecasting tools are relevant in what situations, and thus enhances national welfare. It will also improve science education through activities for high school students.It has long been hypothesized that plants are constrained by a fundamental trade-off between competitive ability and environmental stress tolerance, because adaptations to tolerate stress are energetically costly and limit maximum growth rates. Using seedling experiments, dendrochronological techniques, and species distribution modeling, this project explores whether tree species inhabiting aridity gradients in the U.S. and Australia show a trade-off between fast growth (a proxy for competitive ability) and drought tolerance. It then tests whether this trade-off mirrors growth constraints on adult trees in the field, and whether the observed trade-off and/or systematic growth constraints explain species turnover across aridity gradients. For broader impacts a module based on this research will be developed and piloted in cooperation with high school teachers, and results will be communicated to the public through a university teaching greenhouse.