Does incorporating soil water dynamics improve our understanding of plant responses to climate change in the Desert Southwest?
Ecosystems in the southwestern U.S. have been identified as hotspots for climate change, and this already water-limited region is predicted to experience significant warming and drying. It has proven difficult to predict what aspects of climate may drive plant variation in plant responses to these conditions, and why some species may be more vulnerable to climate change impacts than others. In part, this is due to differences in microclimate and soils across the landscape, as well as interactions with neighboring plant species, which can both mediate plant responses to climate. Here we use long-term monitoring data collected by various researchers across the Southwestern deserts and detailed information on climate and soils to understand past responses to climate, and the mechanisms driving them. Specifically, we use a soil water model (SOILWAT) to translate changes in climate into differences in plant available water, the strongest limiting resource in the Desert Southwest. We then relate seasonal and depth patterns in soil water availability to species abundances and community composition.
Our results suggest that incorporating soil water dynamics can improve relationships between vegetation responses and climate. In particular, the abundance of perennial grass species, such as Achnatherum hymenoides, is better explained by seasonal soil water potentials than any single climate variable. However, for some species, temperature was still the dominant factor driving responses. Interspecific density within long-term monitoring plots mediated plant responses to climate, possibly by limiting the ability of neighboring plants to respond to favorable conditions. Finally, patterns for community composition suggest that extreme events, such as high temperatures or severe droughts are more important than mean conditions in driving community dynamics. Together, our research suggests that translating climate change into conditions that plants are actually experiencing in many cases requires incorporating the soils and biotic interactions that influence their abundances. By understanding the drivers of vegetation dynamics in response to past climate and soil water conditions, our research can aid in predicting and managing plant responses to current and future change.