Divergent hydraulic responses of two dryland tree species to episodic summer rainfall pulses in Rajasthan

The survival of trees in dryland ecosystems depends on the balance between high atmospheric evaporative demand and episodic soil moisture availability. This study investigated the short-term hydraulic responses of two tree species to discrete summer rainfall pulses in Rajasthan, India. Continuous sap flow was monitored from April 2024 to July 2024 using the Heat Ratio Method (HRM), capturing three rainfall events per species (six events in total; 1.1–19.9 mm). Because the two species were monitored at different sites, species identity and site conditions were inherently confounded; therefore, the findings are presented as functional response patterns under contrasting site conditions rather than controlled interspecific comparisons. Statistical analyses were performed using tree × rainfall event as the independent unit of replication. Anogeissus latifolia exhibited a mean 41.7% increase in total sap flow rate after rainfall, although this trend was not statistically significant (p = 0.150), whereas morning transpiration increased significantly (p = 0.038). In contrast, Azadirachta indica showed no significant change in total sap flow rate (p = 0.117) and maintained a strong association between sap flow and atmospheric vapor pressure deficit before and after rainfall (R2 = 0.28–0.41, p < 0.0001). These findings are consistent with contrasting water-use patterns: a more pulse-responsive pattern in A. latifolia and a more atmosphere-coupled pattern in A. indica. However, given the confounded design and limited replication, these results should be regarded as preliminary and hypothesis-generating. If confirmed through broader, replicated studies, they may improve understanding of dryland forest responses to increasing climatic variability.