Growth and Storage Trades Off with Acorn Production in Branches of Masting Shrub Oak
Resource allocation trade-offs are fundamental to life history theory. For example, reproduction is hypothesized to come at a cost to other essential functions like growth or storage. In species of masting plants whose annual seed cropes fluctuate greatly, these trade-offs are poorly understood. We seek to understand how a masting shrub oak species (Quercus vacciniifolia) allocates carbon to seed production at the branch level. 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 first 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 are significantly longer and wider than vegetative branches from the same year, and had about one more leaf on average. 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 whole-plant branch carbon storage during acorn development, in terms of both concentration and mass, was significantly negatively correlated with whole-plant acorn production. In sum, these results suggest that larger Q. vacciniifolia branches are better equipped to support acorn development, which represents a significant sink for recently assimilated carbon, and can depress allocation to carbon storage.