Long-term Growth and Persistence of Blue Oak (quercus Douglasii) Seedlings in a California Oak Savanna

We report on growth and survivorship of two cohorts of blue oaks Quercus douglasii Hook. & Arn. (Fagaceae) monitored at Hastings Reservation in Monterey County, California, the first growing in an unprotected old field and measured as seedlings in 1965 and the second planted as acorns in 1985 in several sites differing in their degree of protection from grazing. Growth of all individuals was extremely slow: among those surviving in the first cohort, mean (6 SD) height in 2006 was only 76.7 6 45.0 cm for an average growth rate of 1.8 cm yr, and only one of the original 73 oaks had grown taller than 1.5 m while one was still a seedling 28 cm in height 41 yr after being first marked. Of the second cohort, mean height 21 yr after planting was 54.3 6 31.4 cm. None of these latter individuals had grown out of the sapling stage while 25% were still seedlings , 30 cm in height. Growth of this second cohort was significantly greater when protected from grazing and when growing in the open rather than in the shade. Although growth was slow, survivorship of oaks first measured in 1965 was high, indicting that individuals can live for decades despite significant grazing pressure. Our results confirm the difficulties of inferring age from size of blue oaks, since individuals just achieving the height at which they are typically cored may be 50 or more years old. They also indicate that regeneration, although very slow, can occur in open oak savannas in California despite significant grazing pressure. Whether the observed amount of regeneration is sufficient for long-term sustainability will require continued monitoring and modeling of oak demography.