Synchronous and highly variable patterns of seed production (‘masting’) can have profound consequences throughout ecosystems. How masting occurs is still debated, but one prominent hypothesis is the Resource Budget Model (RBM). The RBM suggests that masting plants forego reproduction and build up resources until a threshold is reached. After which, plants flower and produce seeds which accordingly depletes their stored resources. This leads to three main predictions. First, storage will be higher before years with high seed production than before years without seeds. Second, stored resources during floral induction will positively correlate with flowering. Third, higher seed production will correlate with more resource depletion during seed development. To test the RBM predictions, we collected three years of data on two syntopic oak shrubs with different patterns of acorn production: more variable huckleberry oak (Quercus vacciniifolia) and less variable tanoak (Notholithocarpus densiflorus). Because acorn production varies more for huckleberry oak and less for tanoak, we predict that the RBM will not be supported for tanoak. Furthermore, if both species show the same patterns of resource storage, with different patterns of acorn production, then we would conclude that the observed storage dynamics are tracking the environment.
Results/Conclusions
In support of prediction one, we found that non-structural carbohydrates (NSC) in huckleberry oak branches were higher at the beginning of years with acorn production and lower in a year without acorn production. However, we found that tanoak followed the same inter-annual dynamics of branch NSC, but with a different acorn production pattern, leading us to conclude that the inter-annual dynamics of branch NSC may be driven by the environment instead of by acorn production patterns. In support of prediction two, we found that branch NSC, branch nitrogen, and leaf nitrogen concentrations all had significant positive effects on flowering in huckleberry oak, but not in tanoak. In support of prediction three, we found that branch NSC concentration declined more in huckleberry oaks that produced more acorns, but not in tanoak. Overall, we found support for the RBM in huckleberry oak (more variable acorn production) but not in tanoak (less variable acorn production). This may indicate that the processes leading to variability in annual seed production are fundamentally different for masting and non-masting species.