Resource allocation and acorn production in an evergreen oak shrub (Quercus vacciniifolia)

Background/Question/Methods

Many long-lived woody plants exhibit highly variable patterns of seed production from year to year. Patterns of seed production are important not only for plant recruitment, but also for wildlife populations. For these reasons, and because the processes that determine variability in seed production are potentially sensitive to climate change, there has been much interest in gaining a mechanistic understanding of variable seed production from a physiological perspective. One hypothesis that has considerable theoretical support, but limited empirical support, is that plants rely on stored resources to produce seeds. We tested this hypothesis in an evergreen shrub, Quercus vacciniifolia, which has exhibited an alternating annual pattern of investing in growth or reproduction over the past 3 years. Specifically, our objective was to find out whether acorn production depleted stored non-structural carbohydrates (NSC) in the stem or nitrogen (N) in the leaves. To this end we measured growth, flower production, acorn production, stem NSC, and leaf N in a population of Q. vacciniifolia in the Klamath Mountains in Northern California. We compared among individuals that naturally varied in acorn production as well as those in which acorn production was experimentally suppressed through flower removal for either 2 or 3 years.

Results/Conclusions

Flower removal had no effect on acorn production or growth. There was no pattern of NSC depletion in shrubs that produced acorns. Nor was there any pattern of N depletion in acorn producing shrubs. Our results are in line with previous studies investing the role of stored carbon in variable seed production patterns, but our results contrast with such studies that have looked at stored N. Overall, our results do not support the hypothesis that stored nutrients are required for seed production in this system. Instead, evidence suggests that this population follow a pattern of switching from investment in growth to reproduction.