Cues versus proximate drivers: testing the mechanism behind masting behavior

Masting, the intermittent and synchronized production of seeds, is a common and important phenomenon throughout the plant kingdom. Surprisingly, the proximate mechanisms by which populations of masting plants synchronize their seed sets have been relatively unexplored. We examined how temperature influences the acorn crop of the valley oak Quercus lobata, a masting species common in California, USA, over 33 years in order to assess whether temperature acts directly on acorn crop as a cue or whether it acts instead through intermediate steps indicative of a direct mechanistic connection to acorn production. Compared to several alternatives, the difference in temperature during the spring flowering period over the prior two years (t) was a good predictor of annual acorn crop in valley oak, as proposed recently by Kelly et al. Significantly, t correlates positively with temperatures the previous April, a likely driver of pollination success in valley oak, and negatively with the previous year's acorn crop, which is in turn negatively correlated with the current year's acorn crop, presumably due to resource limitation. Thus, the success of t is not as a cue but rather explained by its close relationship to the proximate drivers that have a direct, mechanistic relationship with acorn crop size.