The Influence of Seasonal Precipitation on Ponderosa Pine Carbon and Water Cycling in the Southwestern U.S.

Abstract

Water is becoming increasingly scarce in the Southwestern U.S. Projections of precipitation are uncertain, but generally suggest that arid ecosystems will become more arid. As such, understanding how ecosystems in the arid Southwest are influenced by different seasonal precipitation aids our understanding of how these ecosystems may be impacted by future conditions. This dissertation incorporates a variety of approaches using tree rings and stable isotopes to investigate how cool-season precipitation, the North American Monsoon, and less frequent precipitation from tropical cyclone remnants, impact carbon and water cycling across the region. Appendix A is focused on leveraging two different metrics of forest water-use efficiency to investigate the role of the North American Monsoon on insulating forests from heightened drought stress during the ongoing megadrought. This research illustrates that the North American Monsoon has mediated the drought response of forests across the Southwestern U.S. Appendix B of this dissertation uses high-resolution measurements of 13C from tree rings to reconstruct three periods of growing season gross primary productivity in a Madrean sky island to determine the relative influences of cool-season, North American Monsoon, and tropical cyclone remnant precipitation on season gross primary productivity. In the third and final Appendix of this dissertation, I explicitly examine how cool-season moisture influences late-season growth and carbon dynamics in Ponderosa pine forests across the region. Taken in conjunction, these chapters provide robust and thorough insights into how the three predominant precipitation sources in the Southwest U.S. influence carbon and water cycling and provide information on how they may respond to perturbations in precipitation and aridity across the region.