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Harvard Forest Symposium Abstract 2017

  • Title: Resource dynamics and masting in sugar maple (Acer saccharum)
  • Primary Author: Joshua Rapp (Harvard Forest)
  • Additional Authors: Elizabeth Crone (Tufts University)
  • Abstract:

    Seed production assures the persistence of tree populations and forest cover over the long-term, and so has long interested plant demographers and foresters. Many forest tree species produce seeds synchronously and at irregular intervals across large areas, a phenomenon known as masting. Initiated in spring 2011, this study addresses the mechanisms of mast seeding in sugar maple (Acer saccharum), and its impact on pollinators, seed consumers, and forest carbon dynamics at the Harvard Forest. We monitor seed production (via counts of seeds on trees), flower production, and resource status (via sap collection) on 20 trees. Pollinator dynamics and seed predation (by weevils) are also monitored. In 2015, we added sap and seed monitoring of red maple (Acer rubrum) trees to explore the hypothesis that this non-masting species would have muted dynamics compared to its masting congener.

    We have observed clear masting patterns; abundant flowering and fruiting in 2011 were followed by a nearly complete lack of flowering in 2012, a second mast year in 2013, and low but increasing flowering from 2014-2016. Sugar maples have both male and female flowers on the same trees (and in the same flower clusters). In high flowering years, trees are approximately evenly split among those in which male flowers mature first and those in which female flowers mature first; in low flowering years, many trees either do not flower at all or produce only male flowers. Sex allocation is more male-biased in low flowering years. Total sap sugar collected is negatively correlated with seed production in the previous year, suggesting a cost of reproduction. We have also observed costs of reproduction at the twig-level; flowering branches have fewer leaves and reduced twig growth compared to branches without flowers. Leaves on reproductive twigs also have lower photosynthetic capacity (light-saturated photosynthetic rate) than those on vegetative twigs. In 2 of 3 non-mast years, flowers from which we excluded insect pollinators failed to produce seeds, while flowers open to insect pollinators produced seeds. In one mast year, and one non-mast year, wind pollination was sufficient to pollinate flowers enclosed in pollinator-exclusion bags. We have also observed a higher proportion of insect-predated seeds (presumably by weevils) during non-mast years than in a mast (2013) year.

    These preliminary data suggest sap sugar is a faithful proxy of tree resource status and is related to seed production in sugar maples. A 17-year time series of seed production and maple syrup production from Vermont provides corroborative evidence for this; syrup yield per tap was higher during mast years, and declined in the follow year. Our data also suggest a trade-off between seed production and carbon gain enforced by a meristem limitation, where buds used for flowering produce fewer leaves. The combination of fewer leaves, lower photosynthetic capacity, and a competing carbon sink in maturing seeds led to lower twig growth in these same branches. Monitoring bee populations confirmed bees (primarily in the family Andrenidae) visit maple flowers and collect pollen. Pollinator exclusion experiments provided initial evidence for bee pollination of sugar maples and indicated that sugar maples may not be pollen limited in non-mast years. Monitoring of seed, leaf, and sap production, and further experimentation will help us build an understanding of how internal resource dynamics, pollination, and climate affect mast seeding, with implications for masting on seed consumers, pollinators, and forest carbon dynamics.

  • Research Category: Physiological Ecology, Population Dynamics, and Species Interactions