Fitness consequences of seed size in the valley oak Quercus lobata Née (Fagaceae)

Abstract

We examined the functional relationship between seed size and seedling performance in the valley oak (Quercus lobata Nee) by means of a 13-year common garden experiment. Acorns were collected from five localities throughout the range of valley oak in autumn 1997, weighed and measured, and planted at Sedgwick Reserve, Santa Barbara County, California, USA. In the short term, larger acorns produced larger seedlings that had lower survival than seedlings from smaller acorns. In the longer term, large seeds correlated positively with both seedling size and survival, with path analyses indicating that the latter effect was primarily indirect via initial seedling size. The longer-term relative growth rate was only weakly related to seed size, being a combination of a slight positive direct influence of seed size on relative growth rate and a comparable negative indirect effect via larger initial seedling size. These results generally matched the predictions of the “seedling size effect hypothesis” (larger seeds yield larger seedlings with greater competitive abilities), the only one of the three hypotheses we examined that predicts an inverse relationship between seed size and initial survival and a positive relationship between seed size and longer-term relative growth rate. The factors influencing the relationships between seed size and seedling performance are complex and may involve both direct effects of seed size and indirect effects mediated through initial seedling size. Although the seedling size effect was the most important in our study, other factors may be important under different environmental conditions and/or at different growth stages.