WATER SOURCES FOR TREES GROWING IN BEDROCK OUTCROPS

Determining the water source for vegetation is complex due to the poorly understood interactions between soil and surrounding vegetation. Analyzing the relationship between vegetation and subsurface hydrology through stable isotopes furthers understanding of the source water. We compare stable isotope (¹⁸O and ²H) values from soil and from xylem samples of trees growing on bedrock outcrops in the Gordon Gulch catchment of the Boulder Creek Critical Zone Observatory, a montane climate ecosystem. We also characterize the study site with respect to vegetation, slope, and topographic wetness index. Aerial LiDAR data provided information pertaining to these spatial characteristics. Initial isotope results indicate that trees are using groundwater sources, transient soil water, or more likely a mixture between the two. A mixture infers that water is coming from a deeper water source such as water within fractures, or a less evaporated water source that does not have exposure to the surface. Trees growing on five different rock outcrops all exhibit the same behavior. Our mixing model indicates that all tested trees obtain over 85% of their moisture from groundwater, even in weekly varying moisture conditions. Such a water source is indicative of an annual response to different moisture conditions, as the previous years’ tree water source was found to be of a potentially different source. Moreover, two sites of non-outcrop growing trees were used as a baseline for comparison of typical tree uptake behavior. Our spatial analysis indicates the trees growing on outcrops are mostly between 4 to 5 meters tall, a potential indicator of stunted growth on an exposed bedrock surface. Additionally, the site was characterized to have slopes surrounding the outcrops between 6 and 20 degrees and the topographic wetness index demonstrates less moisture in close proximity of the rock outcrop. Examining the relationship between vegetation and subsurface hydrology through stable isotopes furthers understanding of both subsurface hydrology and forest water relations.