Prolonged Inundation Affects Water-Use Strategies in Bottomland Oaks

Columbia bottomland hardwoods is a network of riparian forests in the floodplain created by three major rivers in south-central Texas. Larger streams further divide into bayous, which flood to form sloughs, inundating large swaths of forests for prolonged periods. Plants in such habitats are known to cope with this hydrological heterogeneity. However, response of tree water-use to flooding early in the growing season and transitioning to water deficits in the later stages is still largely unestablished since different sequences of inundation and drought can affect plant survival and growth differently. The aim of this study was to 1) Assess how inundation affects tree water use. 2) Investigate changes in seasonal water use among upland and inundated trees as wetter conditions gradually diminish. 3) Understand how inundation affects tree water-use in response to changes in atmospheric dryness (vapor pressure deficit). We measured sap flux density continuously in 5 seasonally-inundated trees and 5 upland trees (all Quercus sp.). The upland trees had consistently higher sap flux rates (1790 kg m^-2 day^-1 tree^-1) throughout the growing season as compared to inundated trees (744 kg m^-2 day^-1 tree^-1). However, a larger proportion of this water uptake occurred in the spring (March 7-May 31, 2018). During spring, rates averaged 2114 kg m^-2 day^-1 tree^-1 in upland trees and 837 kg m^-2 day^-1 tree^-1 in inundated trees. As wet conditions diminished over the summer (May 31-July 16, 2018), sap flux rates dropped more sharply in upland trees (1086 kg m^-2 day^-1 tree^-1) as compared to inundated trees (542 kg m^-2 day^-1 tree^-1). Response of water-use was much strongly correlated to VPD during spring (R²=0.49 for upland and R²=0.63 for inundated) than summer (R²=0.14 for upland and R²=0.17 for inundated). Inundated trees show suppressed water uptake despite having higher water availability indicating flood stress possibly resulting in stomatal closure. However, much higher overall sap flow during spring inundation as compared to dry summer conditions indicates lower physiological flexibility. Thus, although the trees are wetland-adapted, the period and timing of inundation critically affects the water-use strategies of trees that are seasonally flooded.