Oak conservation maintains native grass stands in an oak woodland-annual grassland system

Abstract

Mediterranean oak woodlands serve as working landscapes and biodiversity hotspots. These landscapes have undergone dramatic land conversion, which continues to threaten their conservation. Shifting focus from traditional management practices to a balance of conservation and production goals is a key challenge on working landscapes, and evaluating potential tradeoffs and synergies among goals will be a critical first step. California’s oak woodlands have undergone marked transformations via removal of Quercus douglasii and other woody plants to enhance forage production. Within the annual grass-dominated matrix, Q. douglasii likely functions as a foundation species—providing potential habitat for native plants, such as Nassella pulchra. Via a cross-sectional survey, we examined spatial occurrence of N. pulchra relative to Q. douglasii trees across three cattle-grazed fields, which had previously undergone vegetation manipulation. We hypothesized that Q. douglasii trees provide spatial niches for N. pulchra, with bunchgrass densities declining with greater distances from Q. douglasii stems. Plots (n = 712) were located along northern/southern transects of 89 trees. N. pulchra densities and site characteristics were surveyed in 2002 and 2005. Generalized linear mixed model regression analysis revealed a significant plot position by community type interaction: N. pulchra densities significantly declined with increasing distance from target trees at grassland sites; this trend was apparent at savanna sites and no trend was observed at woodland sites, which were likely under the influence of neighboring trees. Information on native species relationships can be utilized by managers to balance agricultural production and native species conservation goals across working landscapes.