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Harvard Forest Symposium Abstract 2013

  • Title: Carbon dynamics and masting in sugar maple (Acer saccharum)
  • Primary Author: Joshua Rapp (Massachusetts Audubon Society)
  • Additional Authors: James Crall (Harvard University); Elizabeth Crone (Tufts University); Dash Donnelly (Harvard Forest)
  • 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 litterfall traps and counts of seeds on trees), flower production, and resource status (via sap collection) on 20 trees. Pollinator dynamics are also monitored.



    We have observed clear masting patterns; abundant flowering and fruiting in 2011 were followed by a nearly complete lack of flowering in 2012. Sugar maples have both male and female flowers on the same trees; in 2011 male flowers matured first in 6 trees and female flowers matured first in another 6 trees, while in 2012 most trees that had flowers only produced male flowers. In 2011, male-first trees had a greater proportion of female flowers overall, and the proportion of female flowers was positively correlated with sap sugar concentration measured the following year. In 2012, sap sugar concentration was positively correlated with stem diameter (DBH) and with seed production in the previous year. Flowering branches had fewer leaves and reduced twig growth compared to branches without flowers in both years. In 2012, flowers from which we excluded insect pollinators failed to produce seeds, while flowers open to insect pollinators produced seeds.



    These preliminary data suggest sap sugar concentration is a faithful proxy of tree resource status and is related to seed production in sugar maples. A 14 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 and a competing carbon sink in maturing leaves 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: Conservation and Management
    Physiological Ecology, Population Dynamics, and Species Interactions