PS 15-219 - Digging deeper: can hyperspectral retrieval of canopy water content detect edaphic influence on blue oak water stress?

Abstract: Remote detection of tree water stress could provide great insight into landscape and global patterns of climate change-induced tree stress and mortality. California has seen a deficit of below ground water storage since the beginning of the twenty first century. Soil properties like texture and structure, which vary by soil type, can influence total water storage, water holding capacity, and water movement in soil, exercising a profound influence on plant function. Studying below ground processes is difficult and labor intensive, but measures of leaf water potential (ѰL) and canopy water content (CWC) can provide insight into soil water availability. Blue oaks (Quercus douglasii), a drought tolerant California endemic species, rely heavily on below ground water storage during summer and especially during drought. Here, we investigate the potential use of remotely sensed metrics of plant water to assess edaphically varying water availability in blue oak stands. In Spring 2022, we measured blue oak ѰL and CWC across soil types at Sedgwick Reserve in Santa Ynez, CA during the NASA Surface Biology and Geology High Frequency Time Series (SHIFT) Campaign. The SHIFT campaign provided weekly hyperspectral imagery for three months at high spatial, temporal, and spectral resolution using the Airborne Visible-Infrared Imaging Spectrometer - Next Generation (AVIRIS-NG) sensor. Canopy water content retrievals were meshed with in situ measurements of plant-available water to test the potential to remotely sense edaphic controls on water stress. We found that leaf water potentials with similar elevation and aspects varied across soil types, in particular shedd silty clay loam and chamise shaly loam soils. Blue oak predawn ѰL was more negative on the shedd soil compared to chamise soil. Chamise soil has higher clay content compared to shedd which may be the cause of better water holding capacity. This soil type-driven disparity in water availability, even between closely co-located trees (< 50 m apart), persisted from the early spring well into an extreme growing-season drought. This consistent difference in hydration was at least partially detectable from remotely sensed CWC, and also detectable in other spectral features not directly related to water absorption. Soil type thus appears to influence other structural aspects of the canopy in addition to hydration, providing encouraging evidence for mapping below-ground heterogeneity with above-ground vegetation attributes.