Under conditions supported by its archetypal fire regime, Oregon white oak woodlands are composed of widely spaced, broad-crowned oaks with a predominantly fire tolerant understory. However, when fire is excluded from these systems, structural and compositional changes occur and fire sensitive species are favored. Without fire, shade tolerant Douglas-fir (Pseudotsuga menziesii) saplings have the opportunity to establish and grow, eventually overtopping oaks, leading to growth reductions, crown dieback, and mortality. Unfortunately, these impacts are widespread, as Oregon white oak communities are declining throughout their range between central California and southern British Columbia. Restoring these landscapes to presettlement conditions requires the prioritized retention of existing oak trees coupled with the removal of conifers to release oaks from resource competition.
A number of studies have investigated oak woodland responses to conifer encroachment, but few studies have evaluated these responses at the physiological level. Although growth can be used to understand oak responses to treatment, physiological responses have the potential to detect responses much faster. The objective of our study was to understand the physiological effects of conifer encroachment on Oregon white oaks (Quercus garryana) in open, moderately encroached, and heavily encroached woodlands of northwestern California. During the 2017 growing season, we measured predawn and midday water potential (Y) using a pressure chamber and stomatal conductance (gs, positively correlated with photosynthetic rate) using a leaf porometer.
Results/Conclusions
We found that Ψ was lowest (most stressed) in open stands, identifying water availability as the most limiting factor to oak productivity in these high-light environments (Figure 1, 2). We also found that gs was highest in moderately encroached stands early in July but lowest in these stands late in the growing season, again suggesting that water availability can limit oak productivity in moderately encroached stands, particularly when winter water is exhausted (Figure 3). Lastly, we found a positive relationship between gs and Ψ (reduced gas exchange under high water stress) in open stands (Figure 4) and a negative relationship between gs and Ψ (increased gas exchange under high water stress) in heavily encroached stands (Figure 5). This last finding indicates higher stomatal regulation in open stands, likely due to xeric acclimation, and lower stomatal regulation in heavily encroached stands, likely due to mesic acclimation. Our results suggest that despite increasing competition for water, a moderate level of conifer encroachment may increase water availability to oaks by reducing evaporative losses of soil water.