The Effect of Elevation and Soil Moisture on Water Use Efficiency of Understory Plants

Are plants moving into higher elevations fast enough to cope with rising temperatures, or are they changing physiologically to adapt instead? With climate change and other human impacts threatening forest ecosystems it is important to investigate the physiological mechanisms that lead to population change. The Russian Wilderness in California is a botanical biodiversity hotspot with long term plots established in 1969. The area is facing two main threats: climate change and fire suppression. At high elevations, warming temperatures and decreased precipitation are likely to dry out soils, creating physiologically stressful conditions. Furthermore fire suppression leads to a buildup of biomass at lower elevations where fires have historically occurred more frequently also creating stressful conditions for understory plants. This study focuses on how elevation and soil moisture affects stress in six common understory plants in the Ericaceae family using stable isotopes as a proxy for plant stress. In summer 2024 we travelled to the Russian Wilderness in northern California and hiked in the wilderness backcountry to thirteen long term plots along an elevation and moisture gradient. We collected 114 leaf samples and additional microhabitat data including soil moisture, litter and duff depth, and canopy cover. Leaf samples were dried, pulverized, and analyzed using a mass spectrometer for two carbon isotopes, carbon-13 and carbon-12, which were used to estimate water use efficiency (WUE), a proxy for plant stress. We found that plant species that inhabit drier habitats had higher WUE than species that prefer wetter conditions, indicating that species that live in drier conditions are more stressed. We also found that soil moisture significantly decreased WUE for all species, indicating that with more available water, plants are less stressed. Finally, we found that the impact of elevation on WUE depended on the species. As elevation increased, Pyrola picta experienced a significant decline in WUE while other species had a weakly positive or no relationship between WUE and elevation. Overall our data suggest that all species are stressed by decreased soil moisture and that soil moisture is a stronger driver of plant stress than elevation in the Russian Wilderness. Physiological changes precede population shifts, and can inform why plants may or may not be shifting in elevation in response to climate change and other human impacts.