Endogenous resource dynamics drive sugar maple masting and maple syrup production
Plant life history determines patterns of resource gain, storage, and use in individual plants, and influences ecosystem carbon and nutrient dynamics. In masting species, resource allocation to reproduction varies dramatically from year to year. The resource budget model (RBM) of masting hypothesizes that resource dynamics drive inter-annual variation in reproductive allocation. We tested the prediction that resource dynamics should be closely linked to masting cycles by monitoring carbon resource allocation and use in a mature sugar maple (Acer saccharum Marsh.) population at the Harvard Forest in Petersham, Massachusetts, USA. Over four field seasons we monitored allocation to reproduction (flowering effort, pollination, and seed production), vegetative growth (leaf production, photosynthetic capacity, and shoot growth), and storage (sap sugar content) in 20 mature maple trees. We also used 15 years of data on seed production and maple syrup production from Vermont to explore the effects of masting on the harvest of a non-timber forest product, previously thought to be largely responsive to climate conditions.
The Harvard Forest sugar maple population exhibited clear alternate year seed production, in synchrony with seed production in Vermont, and consistent with prior studies. Sap sugar content was lower in the year following masting, while leaf production, photosynthetic capacity, and shoot growth were lower for reproductive shoots than vegetative shoots. At the landscape scale in Vermont, maple syrup production, data detrended to remove a decade-long increase in syrup production reflecting greater harvesting effort, declined in the year following a mast year, demonstrating a cost of reproduction to trees, and maple syrup producers. In addition, even though weather during the sugaring season is a strong predictor of sap flow, seed production was a stronger predictor of maple syrup production than climate alone. However, a model containing both seed production and climate best predicted syrup production. Our results confirm predictions of the RBM and demonstrate that reproduction-driven endogenous resource dynamics of trees can have impacts on forest productivity and derivative economic activity.