Drought Response in Three Conifer Species Detected by Sap Flow and Proximal Thermal Remote Sensing

Abstract

Dryland forests of the southwest United States face a warming and changing hydroclimate, yet our ability to monitor and predict vegetation-drought dynamics over large regions remains limited. Here, we examine drought responses and predictability of sap flow for three species (Douglas fir, ponderosa pine, and southwestern white pine) over two consecutive years. We collected tree-scale sap flow velocity and evaluated its relationship with soil water content (SWC) and vapor pressure deficit (VPD). Next, we identified the soil moisture threshold beyond which sap flow was no longer limited by SWC. We also assessed whether proximal remote sensing of canopy temperature and canopy-to-air temperature difference (ΔT) can capture ecosystem-scale drought response dynamics. Significant sap flow reductions occurred in response to drought periods, but sap flow quickly recovered following large rainfall events. When SWC was below a threshold of ∼7% (cm³/cm³), SWC and sap flow were positively correlated, indicating water-limited conditions, while above this threshold, only VPD and sap flow were positively related, indicating atmospheric demand limited conditions. Species differences were minor, but ponderosa pine sap flow responded most rapidly to soil dehydration. ΔT was significantly correlated with sap flow, but the relationship switched from positive (R = 0.73−0.94) during the pre- and post-monsoon to negative during the monsoon (R = −0.31−0.48). This shift likely reflects a transition from soil moisture supply to atmospheric demand limitation during wetter periods. These findings highlight the potential of combining tree-scale sap flow measurements with thermal remote sensing to enhance understanding of vegetation-drought dynamics in dryland forests.