Acorn Production and California Oaks in a Changing World

Walter D. Koenig

Published May 2019 in International Oaks No. 30: 45–52

Abstract

How are oaks responding to a changing world? At Hastings Reservation in central coastal California, conditions have warmed an average of 1.08 ºC over the past 80 years, primarily due to increasing temperatures at night over much of the year. These changes appear to have had no detectable effect on acorn production by any of the five species at this site. However, variability in acorn production – masting behavior – has decreased considerably since 1980, the first year for which data are available. This decrease is matched by decreased variability in annual rainfall over the same 39-year period, although over a longer, 80-year period the trend in variability of annual rainfall has been positive rather than negative. Even more dramatic effects on oak populations have been apparently due to changes in land use. In particular, canopy cover at Hastings increased from 22.2% to 42.7% between 1979 and 2013. This was primarily due to increased density of Quercus agrifolia, the main evergreen species, at the expense of the deciduous Q. lobata, a pattern that is found elsewhere in California as well. Understanding changing oak populations and protecting them in the future will require careful consideration of multiple factors, including climate change, land-use patterns, and other ecological factors such as interspecific interactions that will potentially be even more difficult to detect and interpret.

Keywords

acorn production, climate change, land-use patterns, oak communities

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