Water-uptake depth mediates the effects of water-table depth on plant physiological performance in a tropical dune forest

Background and Aims In tropical forests both light and water influence the growth and health of woody species. In seasonally flooded dune forests, like restinga forests of the Atlantic Forest, groundwater availability may play a major role in forest functioning. This study aims to understand whether and how restinga ’s woody vegetation experience variations in physiological performance in response to water-table depth, and the mediator role of plants water-uptake depth. Methods 15 woody species were sampled along a water-table gradient, for traits related to water-use, nitrogen acquisition and photosynthetic activity. Direct, and indirect - mediated by water-uptake depth adjustments, effects of water-table depth on physiological status of restinga vegetation were tested, accounting with light and stand structure influence. Results Water-table depth strongly defined plants water-uptake depth, which mediated negative indirect effects of water-table lowering on the woody community’s photosynthetic status. Plants using deeper, possibly more abundant water resources, showed maintenance of the water status, but lower physiological performance. Besides a strong light-driven variation, water-uptake dynamics explain additional physiological variation, showing a secondary hydraulic constraint on plant photosynthetic function. Conclusion Indirectly, through water-uptake depth regulation, water-table depth can influence, at least to some degree, the physiological status of restinga plants. Despite maintained water status, greater water-uptake depth negatively influenced photosynthetic and structural leaf traits. There is a below–aboveground allocation trade-off, whereby belowground allocation possibly buffers water limitation but constrains aboveground investment, limiting leaf-level photosynthetic performance in restinga forests.