Factors Affecting Blue Oak Sapling Recruitment1

We used logistic regression to identify environmental and management history factors associated with blue oak (Quercus douglasii) sapling recruitment. Recent canopy gaps caused by natural mortality or clearing were positively associated with sapling recruitment. Plots with very high or low levels of tree canopy cover were less likely to have saplings than those with intermediate canopy cover levels. Across all locations, and within grazed locations, browsing intensity was negatively associated with sapling presence. Other factors significantly correlated with sapling recruitment include shrub presence, insolation, soil available water-holding capacity, fire, plot altitude, precipitation, and potential evapotranspiration. Different interpretations have been offered to explain the apparent flushes of blue oak recruitment that date from the 1850’s through the 1890’s (McClaran and Bartolome 1989, Mensing 1992, Vankat and Major 1978). Many interrelated ecosystem perturbations date to this period. Settlers and their successors introduced livestock, cut trees and cleared shrubs over large areas, altered fire frequency, hunted deer, and exterminated vertebrate predators. These and other actions also affected understory species composition, rodent populations, soil properties, and other factors. Although many of these factors have the potential to affect blue oak regeneration, the relative importance of these or other factors on current regeneration patterns cannot be determined from fragmentary historical data. We have shown that the frequency of small blue oak (Quercus douglasii Hook. & Arn.) saplings (>1 cm basal diameter, ≤3 cm diameter at breast height [DBH]) varies widely between different blue oak stands (Swiecki and others, these proceedings). Although such saplings tend to be scarce within a stand, their densities within a stand are highly variable (Swiecki and others 1993). In this study, we investigated whether current differences in sapling recruitment within and between blue oak stands are related to site environmental and/or history factors. Because blue oaks typically require at least 10 to 30 years to make the transition from seedling to young tree (McClaran and Bartolome 1989), our study included only locations for which we could obtain 30 years of site history.