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Avoiding desiccation in tropical forests: mechanisms that maintain saplings’ plant-water status during seasonal droughts

Thursday, 27 June 2013: 08:00
La Paz - B West (Herradura San Jose)
Brett Wolfe , Biology, University of Utah
Thomas Kursar , Biology, University of Utah
Recent research has linked droughts with extensive tree die-offs and altered species composition in forests worldwide. Predicting the impacts of drought on tropical forests is hampered by a lack of understanding of the mechanisms by which trees endure drought, a period when plant-water status must remain above critical levels in order to prevent tissue damage and death. The hydraulic fuse hypothesis predicts that leaf shedding precludes tissue damage in stems by reducing tension in the soil-canopy water column. Thus, trees should shed leaves as they approach their species-specific, critical plant-water status. It is well established that trees of seasonally dry tropical forests range from deciduous to species that maintain leaves for years.  Species also vary widely in their tolerance to low plant-water status. Due to the disparity in traits among species, the validity of the hydraulic fuse hypothesis is highly uncertain. We tested this hypothesis in saplings in two seasonally dry forests in Central Panama. During the 2013 dry season, we monitored plant-water status, stomatal conductance and deciduousness in six tree species. All species restricted their stomatal conductance during the dry season, suggesting that they were water limited. The onset, rate, and duration of leaf shedding varied widely among species, indicating that deciduousness is a complex behavior that may not be adequately described by systems that classify species as “deciduous” or “evergreen.” Results supported the prediction that leaf shedding tracks species’ vulnerability low plant-water status. However, when leaf shedding occurred, the safety margin between plant-water status and critical plant-water status varied widely among species. We conclude that the hydraulic fuse hypothesis only partially explained variation in deciduousness among species.