Changing Hydroclimatic Conditions and Excessive Nitrogen Deposition Affect Ecosystem Functioning of a Bottomland Hardwood Forest

Bottomland hardwood forests (BHFs) along the Gulf coast of Texas have been experiencing a combination of disturbances in the form of hydroclimate changes and excessive anthropogenic nitrogen (N) deposition, both of which are connected through linkages between the regional hydrologic and N cycles. BHFs, which are dependent on optimally wet conditions, have received little attention amid rapidly changing climate and pollution scenarios, which are expected to become more irregular over the 21st century. This dissertation aimed to understand the integrated effects of these disturbances on BHFs by: 1) examining signals of physiological stress in response to climatic conditions in the past; 2) assessing tree water-use in flooded and non-flooded native as well as invasive species as a result of seasonal environmental changes; 3) investigating if persistent elevated deposition from pollution sources has led to N saturation and affected primary productivity, N transformations in the soil and accelerated N loss from these forests; and 4) determining the relative control of hydroclimate and N inputs on ecosystem productivity. A range of techniques such as dendrochronology, sap flow, atmospheric deposition monitoring, plant and soil isotopic analyses and biogeochemical modelling were used to address each objective. Vegetation in the drier portions of the study area experienced growth inhibition and showed signals of stress during drought periods, and was more sensitive to wetter hydroclimatic conditions. Although trees growing in the wetter patches had suppressed water-use during early-growing season flooding, their recovery indicated resistance to flooding stress. They were less sensitive to drier hydroclimate and were able to sustain their water-use during summer conditions. The younger and smaller invasive Chinese tallows had only slightly lower water-use than the mature native oaks, suggesting their potential to compete with the oaks for soil water on approaching maturity in the near future. Interestingly, despite exposure to excessive N deposition for decades, these forests did not show signs of N saturation as tree growth and N absorption continued to increase even at high deposition levels. However, despite excessive N inputs, signs of N loss were negligible, soil N concentrations were low and vegetation continued to absorb mineralized N, indicating rapid N retention and cycling through plants and soils. Persistence of these bottomland hardwood forests is contingent upon frequent flooding, optimum wetting and maintenance of hydrologic connectivity over a large spatial extent within this patchy landscape. The drying of these wetland forests, which is already underway, induces stressful conditions and reduces productivity, while rendering the vegetation to be more sensitive to precipitation inputs. As strong sinks of anthropogenic N, these forests need to be conserved as sinks and filters for air and water pollutants. This dissertation provides vital baseline ecological, hydrological and biogeochemical information about this ecosystem that can aid cohesive conservation action and inform pollutant emission standards by integrating knowledge of distinct processes within the water, carbon and nitrogen cycles, that are strongly interconnected but usually studied and applied disjointedly