WATER POTENTIAL IN DOMINANT TREE SPECIES ALONG A HILL SLOPE IN THE EEL RIVER CRITICAL ZONE OBSERVATORY

Tree water potential – a measurement of the effort trees expend when performing transpiration – provides critical information about tree water use and soil and rock moisture storage during annual dry seasons, which may be intensified by climate change through both increasing temperatures and droughts. Hill slope water storage also provides baseflow to sustain summer flows in the Eel River and maintain critical habitat for endangered salmonid fish.

Our group observed summer water potential in dominant tree species in the Elder Creek watershed, a tributary of the Eel River, in the Angelo Coast Range Reserve. We measured this potential by observing the pressure in megapascals (MPa) it takes to offset the tension from the vascular system of leaves. We sampled leaves from three native tree species: Madrone (hardwood), Douglas fir (softwood), and Tan oak (hardwood) at different elevation levels on the north-facing Rivendell hillslope within the Eel River Critical Zone Observatory, the ridge, and the south side. Our hypothesis was that trees lower on the north-facing hillslope have increased access to water resources due to the closer proximity of Elder Creek and the water table. Measurements were recorded throughout the day, including predawn (~6-8am) and peak sun exposure times (2-4pm) to examine diurnal variation in water potential. During predawn, Madrone and Oak trees had pressure readings of -1 MPa or higher, while Firs had -2 MPa or higher. This suggests variability between species at predawn, which has previously been interpreted as representing the baseline soil water potential. During midday, all species across the hillslope had a drop in water potential to between -2 and -1 MPa as they worked harder to extract water from the subsurface to maintain hydration during increased photosynthesis and evaporation. Position on the hillslope did not appear to play a role in a tree’s ability to access water since the tree groups had similar daily water potential patterns and value ranges. These findings indicate that there is a need for further testing to understand the dynamics of water potential within Coast Range hillslopes. Measured differences in water potential will provide insight into transpiration patterns as climate change continues to affect water availability and environmental conditions throughout the watershed.