Reproductive costs in Acer saccharum: exploring size-dependent relations between seed production and branch extension

Life-history theory predicts that reproductive allocation should increase with age and size once plants reach reproductive maturity. This suggests that there may also be a subsequent decline in somatic growth as plants become larger or older. However, few studies have examined how the relationship between branch extension growth and reproduction varies with size or age in the longest-lived plants: trees. Using a mobile lift for canopy access, we retrospectively measured branch extension growth before, during and after two (between 2011 and 2013) Acer saccharum mast events (the synchronous production of many seeds at long intervals), quantifying seed production per internode and internode length. Branch extension was reduced by 24 and 36%, respectively, in 2011 and 2013 relative to non-mast years, consistent with the expectation that increased reproductive allocation comes at the cost of allocation to growth. Internode length decreased from 8 to 3 cm year(-1) as seed production increased from zero to 17 seeds year(-1); a similar decrease was observed at the whole-tree level using average internode extension rates and seed production per tree. Seed production alone was the most parsimonious predictor of branch extension growth, with no independent effect of tree size, suggesting that it is the increase in reproductive allocation, rather than an increase in tree size per se, that drives the decline in branch extension rates. The slope of the relationship between branch extension and reproduction did not vary with tree size, suggesting that there was no increase in the somatic cost of reproduction with tree size. We also found no evidence for lag effects of reproduction on extension growth in subsequent years. Overall, these results suggest that reproductive allocation assessed at the shoot level increases with tree size and is a major driver of the ontogenetic decline in branch extension growth.