Stem water monitoring reveals an association with rainfall and leaf flush timing in a tropical dry forest of Zambia.

Tropical dry forests (TDFs) occur in tropical regions that experience alternating dry and wet seasons, which might lead to stem water status being determined by seasonality in rainfall and leaf phenology. However, these relations may also vary between canopy and understorey trees. I tested whether relative and specific stem water content (SWC) are associated with climate factors, leaf phenology and rooting depths in 10 shrub and tree species in a TDF in Zambia. In addition, it was tested whether the pre-rain leaf flush of TDFs is sustained by stem water reserves and/or uptake of water from deep soil. At the end of the wet season, large trees contained over 65 L of water in aboveground biomass while small trees and shrubs contained <42 L, which were assumed to represent peak biomass stored water. Stem rehydration in large trees started during the hot-dry season, 2-3 months before the onset of the rainy season. In contrast, stem rehydration in small trees and shrubs only occurred after sufficient rainfall in the rainy season. Only large trees exhibited pre-rains leaf flush while small trees and shrubs did not, perhaps due to low biomass water reserves. The SWC was positively affected by cumulative rainfall prior to sampling date and leaf phenological status. High rainfall during 28 days preceding twig sample collection was associated with high SWC. The highest and lowest SWC values were recorded during the green-leaf and leafless phases, respectively. The association between SWC and taproot depth was negative. Apparently, deep-soil lateral roots of large trees play a more important role in sustaining pre-rains leaf display than taproots. These findings are useful in the assessment of soil-plant-atmosphere water transference in TDFs of East and southern Africa.