Contrasting water-use strategies revealed by species-specific transpiration dynamics in the Caatinga dry forest

In forest ecosystems, transpiration patterns are important for quantifying water and carbon fluxes and are major factors in predicting ecosystem change. Seasonal changes in rainfall and soil water content can alter the sensitivity of sap flux density to daily variations in vapor pressure deficit. This sensitivity is species-specific and is thought to be related to hydraulic strategies. The aim of this work is to better understand how the sap flux density of species with low vs. high wood densities differ in their sensitivity to vapor pressure deficit and soil water content, and how potentially opposing water use strategies influence transpiration dynamics and ultimately correlations to evapotranspiration. We use hysteresis area analysis to quantify the sensitivity of species-specific sap flux density to changes in vapor pressure deficit, breakpoint-based models to determine the soil water content threshold instigating a transpiration response, and multiscalar wavelet coherency to correlate transpiration to evapotranspiration. We found that low wood density Commiphora leptophloeos had a more dynamic transpiration pattern, a greater sensitivity to vapor pressure deficit at high soil water content, required a higher soil water content threshold for this sensitivity to be apparent, and had a significant coherency correlation with evapotranspiration at daily to monthly timescales. This behavior is consistent with a drought avoidance strategy. High wood density Cenostigma pyramidale, conversely, had a more stable transpiration pattern, responded to vapor pressure deficit across a range of soil volumetric water content, tolerated a lower soil water content threshold to transpire, and had a significant coherency correlation with evapotranspiration at weekly timescales. This behavior is consistent with a drought tolerant strategy. We build on previous research to show that these species have contrasting water use strategies that should be considered in large-scale modeling efforts.