Our goal in this research is to aid oak restoration site selection by developing a quick assessment tool to predict core oak habitat.
Describing Oak Distributions
Foothill landscapes like those at Stanford will be central to many future oak woodland restoration efforts. Unfortunately, it is hard to get an accurate view of where oaks "should" be on these heterogeneous landscapes as past land use in these regions (primarily cattle ranching) has confounded the influence of local topography (slope, insolation, etc.).
We assembled a 70-year time series of aerial photographs of our study site at Stanford University to evaluate the relative importance of grazing and topography upon oak distributions. We characterized the slope, aspect, and insolation within each 30m x 30m grid across 310 ha of heavily grazed and 393 ha of lightly grazed land with a Digital Elevation Model. Oak cover, recruitment, and survivorship all changed through time. The effect of grazing was significant even after controlling for the effects of slope, aspect, and insolation. Grazing generally had the effect of restricting oak performance to a narrower range of abiotic conditions. Insolation and aspect usually interacted with grazing, suggesting grazing benefits oak performance in certain settings.
To be of greatest utility to land managers and restoration practitioners, we simplified our oak habitat model to a single topographic parameter: slope aspect. This simplified model correctly predicts oak woodland distributions outside of our initial study site 60% of the time and represents a helpful "first culling" tool for restoration planners in topographically complex regions.
