Shrub strategies in a competition for water determine ecosystem state
Woody-plant encroachment over the last 100 years has shaped the way deserts in the United States look and behave today. In this work, we examine the process of woody-plant encroachment in the Chihuahuan Desert at the Jornada Experimental Range in New Mexico. Our hypothesis is that differences in shrub phenology among mesquite, tarbush, and creosotebush relative to seasonal timing of plant available water governed the observed historic shifts in shrub-dominated states and determined current dominance patterns. This work aims to test that hypothesis using data obtained from a dense instrument network along with aerial images and phenological measurements. We installed sap flow sensors on 15 shrubs, 5 each of mesquite, tarbush, and creosotebush, to measure seasonal and interspecies variations in transpiration. These sensors are located within the footprint of an eddy covariance tower used to measure ecosystem-scale evapotranspiration (ET). We use a network of soil moisture sensors as well as a cosmic-ray sensing soil moisture sensor to measure ecosystem-scale soil water and relate this to overall ET and species-level shrub transpiration. We partitioned ET into its components bare soil evaporation (E) and plant transpiration (T), using an empirical approach based on soil temperature and soil moisture, and then further partitioned T into shrub components using sapflow data. We then analyze the activity of each species throughout the year and how they compete for water by examining time series of rainfall, soil moisture, soil temperature, ET, shrub phenological data and vegetation greenness measures from aerial and ground-based platforms.
Preliminary results show that ecosystem ET is highly dependent on the mean soil moisture state in the footprint of the eddy covariance tower. A piecewise linear function to estimate ET as a function of soil moisture has been fit to the data and is capable of estimating daily ET with a mean absolute error of 0.22 mm/day. For the pre-monsoon and monsoon seasons of 2013, we found that T/ET was 0.26 and 0.70, respectively. We will further partition this into transpiration from each shrub with the use of sap flow data. We expect to find that mesquites commence transpiration in the pre-monsoon season, providing a competitive advantage due to greening prior to the monsoon storm events. Our evaluations provide a first look into how plant phenology can impact shrub competition and the implications this has on short-term soil water availability and evapotranspiration and long-term state dynamics in mixed shrublands of the Chihuahuan Desert.