Inter-relationships of vegetation, hydrology and micro-climate in a young, Douglas-fir forest

Graduation date: 2009The links between forests, streamflow, and climate are poorly understood. Despite hundreds of studies over the past 60 years, fundamental questions of forests' effects on the hydrologic cycle remain unanswered. The hydrological cycle involves mutually-dependent biological and physical processes that operate at multiple scales of time and space, and this principle is the foundation for research in ecohydrology. The objective of this research was to determine how vegetation processes (especially transpiration) affect subsurface water flow dynamics in hillslopes, and conversely how soil moisture and environmental variables affect vegetation function.\ud \ud This dissertation used multiple approaches to mechanistically assess the inter-relationships between vegetation water use, hydrology, and climate. I found that transpiration on hillslopes played an important role in diel variation in subsurface discharge in headwater catchment. However, the amount of influence transpiration had on discharge was strongly dependent upon soil moisture properties. In addition, plot-scale transpiration across a steep topographic gradient could not be predicted from measured variations in environmental variables alone. Heterogeneity in biophysical drivers, edaphic properties and whole tree conductance controlled plot scale transpiration. Last, I applied a dual isotope (¹³C and ¹⁸O) approach to infer physiological response of trees to changing environmental conditions. I found that stable isotopes of oxygen were directly related to stomatal conductance and inversely related to relative humidity; however, the relationship with relative humidity more apparent. The correlation of stable isotopes in tree rings with environmental variables can be particularly useful for assessing the impacts of environmental change on vegetation over short time series. Results demonstrated that the physiological interpretation of stable isotope in tree rings continues to be challenging in uncontrolled environments. This work represents one step forward in elucidating the linkages between vegetation processes, hydrology, and climate