Patterns of recruitment and mortality in mixed coast live oak/ Engelmann oak woodlands in southern California over 34 years

Background/Question/Methods

Two oak species are dominant in lower elevation woodlands of extreme southern California -- the widespread Quercus agrifolia (coast live oak) and the much rarer Q. engelmannii (Engelmann oak). Previous studies found that while growth rates are the same for both species, Q. engelmannii establishes fewer seedlings and saplings than Q. agrifolia but that it is more successful in establishing seedlings in gaps where mortality is lower and relative growth rates are higher.  In this study we compare data from 5 mixed stands of coast live and Engelmann oaks taken in 1987, 1997, and 2011. Our plots occurred in 2 elevational bands at approximately 200m and 700m.  The higher elevation band receives approximately 250mm more average annual precipitation.  We ask whether Q. agrifolia is increasing in these woodlands and if so is it doing it at the expense of Q. engelmannii.

Results/Conclusions

We found that recruitment varied by species, size and location with respect to canopies of larger oaks. Seedling density was lower at lower elevations, and is consistent with our impression that seedling recruitment is sensitive to precipitation. While the proportion of the cohort in the smallest size classes fluctuated widely, for both species we found a net increase in basal area of the largest oaks.  At the same time Q. agrifolia gained a greater number of individuals in the largest size classes. These data suggest that while Q. engelmannii populations may not be declining, Q. agrifolia may increase in dominance as these woodlands become more dense.