Plants recycle over half of the precipitation that falls over land, yet researchers lack a basic understanding of the spatial and temporal patterns in water sources that plants transfer to the atmosphere. These patterns can be especially complex in areas with heterogeneous terrain and frequent precipitation, which can result in many potential water sources for vegetation. The aim of my dissertation research was to characterize current patterns in tree water use in order to improve empirical estimates for hydrologic process models. This work focused on three main studies: effective rooting depth for tree water uptake, temporal dynamics of water use, and characterizing and predicting the tree water isotopic landscape. Each study took place at the Susquehanna Shale Hills Critical Zone Observatory, in a first-order catchment with mixed deciduous forest in central Pennsylvania. We concluded that the majority of water uptake during the dry part of the growing season occurred at less than 50 cm soil depth throughout the catchment. While there were some trends in depth of water uptake related to slope position, tree genus, and tree size, water uptake was more uniformly shallow than we expected. Secondly, we showed tree water transport velocities between 1 and 18 m d-1, and tree water residence times of up to about 3 weeks for four hardwood tree species on a ridge top site during the peak of the growing season. There was no evidence of longer residence times in the larger trees. We found that the soil-to-leaf driving force appeared to be a strong determinant of tracer velocity across species. Lastly, we found that a simple model including tree canopy height and degree of slope as predictors of tree water δ18O compositions in the catchment, explained over half of the variance in tree water δ18O composition. Collectively, these results provide a more nuanced understanding of the relationship between tree water sources and the temporal dynamics of tree water use, in the context of catchment topography. The conclusions of these studies can inform and improve hydrologic modeling applications and have implications for the connections between trees, streamflow, and contaminant transport.
Gaines, Katie P. (2015): FOREST ECOHYDROLOGY IN A CENTRAL PENNSYLVANIA CATCHMENT: A STABLE ISOTOPE APPROACH . Doctor of Philosophy, Ecology, The Pennsylvania State University, p. 126.
This Paper/Book acknowledges NSF CZO grant support.