Asynchronous recovery of water relations and photosynthesis following natural rainfall pulses in Eucalyptus

Rainfall pulses create brief but critical opportunities for carbon uptake in seasonally dry forests; however, how tree seedlings recover photosynthetic function during the establishment phase following these short-lived rewetting events under field conditions remains poorly understood. In particular, the coordination and timing of hydraulic, stomatal and biochemical recovery processes during natural rainfall pulse-dry-down cycles are not well quantified, despite their importance for carbon-water coupling and drought resilience. Here, we investigated short-term physiological responses of establishing Eucalyptus seedlings during naturally occurring rainfall pulse-dry-down cycles. We measured leaf water potential (Ψleaf), gas exchange and photosynthetic capacity (Vcmax, Jmax) before and after rainfall to assess recovery dynamics of diffusional and biochemical processes under contrasting atmospheric demand. Across species, Ψleaf and stomatal conductance improved rapidly following rainfall, reflecting transient hydraulic relief, while net photosynthesis increased by 40-60% within 1-3 days. In contrast, biochemical capacity responded more gradually: Vcmax declined by up to ~ 15% and Jmax by 20-40% during dry-down and showed limited or partial recovery after rewetting. Limitation partitioning revealed asynchronous recovery, with stomatal limitation relaxing rapidly after rainfall under low vapour pressure deficit (VPD), whereas under high VPD, biochemical recovery preceded full stomatal reopening. The xeric-origin Eucalyptus cladocalyx sustained assimilation at more negative Ψleaf and exhibited greater biochemical stability, whereas intermediate and mesic species (E. grandis, E. urophylla, E. cloeziana) showed rapid but short-lived post-rain responses. Together, these results demonstrate that photosynthetic recovery during the seedling phase is asynchronous and strongly modulated by atmospheric demand, shaping short-term carbon-water coupling under increasingly pulsed hydroclimates.