Drought response in three conifer species detected by sap flow and proximal thermal remote sensing

Authors: UNI, D - University Of Arizona; Scott, Russell; JAVADIAN, M - Northern Arizona University; Biederman, Joel; DANNENBERG, M - University Of Iowa; SMITH, W - University Of Arizona

Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/18/2025
Publication Date: 8/29/2025
Citation: Uni, D., Scott, R.L., Javadian, M., Biederman, J.A., Dannenberg, M.P., Smith, W.K. 2025. Drought response in three conifer species detected by sap flow and proximal thermal remote sensing. Journal of Geophysical Research-Biogeosciences. 130(9). Article e2025JG009127. https://doi.org/10.1029/2025JG009127.
DOI: https://doi.org/10.1029/2025JG009127

Interpretive Summary: Forests of the southwest US are facing increased temperatures, reductions in rainfall, and more severe drought in the 21st Century, but our understanding of how evergreen trees respond to these conditions remains limited. Here, we evaluated how three common species of evergreen trees responded to severe drought conditions using measurements of tree water use in a mixed conifer forest near Tucson, Arizona USA. Two years were compared with contrasting snow and rainfall amounts. We also explored whether proximal remote sensing measurements can be used to capture water use changes. A significant reduction in water use was observed across all three tree species during the 2023 summer drought with all three species displaying similar seasonal dynamics and responses to soil and atmospheric dryness. Water use was only positively correlated with atmospheric water demand when soil moisture exceeded a threshold. Among the remote sensing indices tested, canopy minus air temperature difference was the most useful for understanding tree water stress confirmed by the tree water use measurements. These findings highlight the potential of combining tree measurements with remote sensing indices to enhance our understanding and monitoring of vegetation-drought dynamics across large regions of forest.

Technical 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 ('T) 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. 'T 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.