Investigating the Effects of Combined VPD-Soil Moisture Stress in a Semi-Arid Woodland

In arid and semi-arid ecosystems, plants are faced with a dual challenge: coping with low soil water supply while also facing high atmospheric vapor pressure deficit (VPD). A recent body of work has raised the question of which stress, VPD or soil moisture, is most limiting to transpiration and when. Here, we present an analysis of semi-arid forest function as observed over a three-year period during which we observed plant-water relations in response to well-watered conditions, months of drought, and a partial clearing experiment. We use micrometeorological, soil moisture, and water table depth measurements in conjunction with sap flow, stem water content, and leaf water potential data from three dominant semi-arid tree species Pinus remota, Quercus laceyi, and Juniperus ashei on a ranch in southwest Texas, USA to compare water use patterns among species under varying environmental conditions. Our results demonstrate an extreme form of midday stomatal closure where each species shifted transpiration patterns to early morning and late evenings to avoid the high temperature and vapor pressure deficit conditions during the middle of the day. This extreme pattern in fluxes occurs primarily when VPD and soil moisture are colimiting. Stem water content data reveals the importance of trees’ hydraulic capacitance to serve as a buffer during extended summer dry periods. Patterns in stem water content refilling and leaf water potential validate our sap flux results showing intense midday stresses. During the second year of the study, we conducted a partial clearing of the native invasive J. ashei to test Texas’s brush clearing policy for the improvement of groundwater resources. We founds that differences in elevation along the site’s topographic gradient are were tied to transpiration patterns, indicating connectivity between certain trees and the underlying shallow karst aquifer system. The majority of the benefit of juniper clearing was seen through increases in pine and oak transpiration, particularly in those located in or near local topographic lows. In the first few months after partial removal of J. ashei, water table levels were higher than previously recorded. However, natural variations in precipitation magnitude and timing cause these results to remain inconclusive. Yet, as the climate of central and west Texas continues to warm and precipitation patterns become increasingly variable, it is crucial to develop mechanistic understanding of the complex interplay between semi-arid forests, the atmosphere, and soil and groundwater availability.