Growth and carbon storage trade off with acorn production in a masting shrub oak
Resource allocation trade-offs are a fundamental component of life history theory. For example, reproduction is hypothesized to come at a cost to other basic functions like growth or storage. This is expected to be prevalent in masting species of plants whose annual seed crops fluctuate greatly. However, research on carbon allocation trade-offs in masting trees has yielded mixed results. We seek to understand how a masting shrub oak species (Quercus vacciniifolia) allocates carbon to seed production at the branch level. Shrubs are more likely to be carbon limited than trees because they have smaller canopies and commonly live in the understory. We ask three questions: Are reproductive branches built for better resource acquisition (i.e. bigger and with more leaves)? Do reproductive branches pull photosynthate from neighboring branches? Does seed production depress storage? To answer these questions we used three approaches. First, we measured length, width, and number of leaves on reproductive and vegetative branches. Second, we used stable carbon isotopes to see if fruit bearing branches pulled carbon from adjacent vegetative branches. Last, we measured the change in branch carbon storage during fruit development to see if plants with larger acorn crops depleted their stores to a greater degree.
We found that reproductive branches were significantly longer and wider than vegetative branches from the same year, and had about one more leaf on average. This confirms findings for other oak species where branch growth and acorn development occur in the same growing season. In Q. vacciniifolia, most branch growth occurs in the growing season prior to acorn development. This questions the hypothesized mechanism that fruit bearing branches grow larger as a byproduct of the developing fruit drawing carbon from neighboring branches, given that growth and fruit development in our study was separated by a year. Our isotopic labelling experiment revealed that fruiting branches draw photo-assimilates from adjacent branches, suggesting that acorn production represents a significant carbon sink. Finally, we found that the change in branch carbon storage during acorn development, in terms of both concentration and mass, was significantly negatively correlated with whole-plant acorn production. This sort of relationship between reproduction and carbon storage has rarely been discovered in woody species of masting plants. Together these results suggest that reproduction in Q. vacciniifolia trades off with both growth and storage. More generally, this study highlights the need for a nuanced view of the costs of reproduction in masting plants.