Bigcone Douglas-fir (Pseudotsuga macrocarpa) is a rare tree species, endemic to the mountains of southern California. Recent high intensity fires and climate change-induced drought conditions in Los Padres National Forest (LPNF) have reduced the health of P. macrocarpa stands across their native range. We have recently initiated a program to restore and conserve P. macrocarpa within LPNF through strategic out-planting of seedlings in degraded stands. A key piece of developing a successful restoration strategy for P. macrocarpa is investigating beneficial species associations for seedling recruitment. There is evidence for the facilitation of P. macrocarpa seedling recruitment by Canyon Live Oak (Quercus chrysolepis) but the mechanism of facilitation between these species, has yet to be investigated. One hypothesized mechanism is the hydraulic redistribution of soil water by by deep rooted Canyon Live Oak trees that reduces drought stress and seedling mortality during hot, dry summer months. To test this, field and lab-based experiments were set up to (1) observe patterns of soil moisture for evidence of hydraulic redistribution and (2) relate these patterns to seedling physiology and survival during simulated drought conditions.
Results/Conclusions
Field-based measurements pair overnight recovery of soil moisture in shallow soil (seedling root zone) with a “descending staircase” pattern of drying in deep soil, which indicates the movement of soil moisture from deep soil to shallow soil via hydraulic redistribution. The amount of diurnal rehydration in shallow soil is significantly greater than deep soil layers (P < 0.05) These patterns along with evidence of nocturnal transpiration (predawn stomatal conductance rates above zero; P < 0.05) suggest that hydraulic redistribution is prevalent in P. macrocarpa stands. Ongoing research is investigating the physiological benefits of hydraulic redistribution to P. macrocarpa seedlings and is needed to understand the importance of this phenomena for the recruitment of P. macrocarpa seedlings in the face of increasing drought and wildfire in southern California.