Conifer encroachment increases foliar moisture content in a northwestern California oak woodland

Abstract

Background Foliar moisture content influences crown fire ignition and behaviour. Some spatial variation in foliar moisture is attributable to differences in stand conditions but evidence describing this role is lacking or contradicting. Aims To examine the role of stand conditions on tree physiology and foliar moisture content in Oregon white oak (Quercus garryana) and Douglas-fir (Pseudotsuga menziesii). Methods We monitored foliar moisture content in both species, and tree physiology (stomatal conductance and leaf water potential) in oak, across three stand conditions, including intact stands (unencroached), stands invaded by Douglas-fir (encroached), and thinned stands with Douglas-fir removed. Key results Encroached stands had higher foliar moisture content than intact or thinned stands. Higher stand density was associated with higher foliar moisture content in both species and foliage ages. Encroached stands also had higher midday leaf water potential compared with intact or thinned stands. Conclusions These findings provide strong evidence that stand conditions and thinning treatments can influence foliar moisture content in Oregon white oak ecosystems, with likely implications for other ecosystems. Implications Better understanding of the role of stand conditions on foliar moisture content may contribute to improved spatial and temporal prediction of foliar moisture content and modelling of potential crown fire behaviour.