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Harvard Forest Symposium Abstract 2014
- Title: Carbon dynamics and masting in sugar maple (Acer saccharum)
- Primary Author: Joshua Rapp (Massachusetts Audubon Society)
- 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 2014, we expanded our sampling network by partnering with collaborators in Virginia, New Hampshire and Quebec, and began collecting sap samples to examine sap secondary chemistry. This expanded project is investigating how endogenous rhythms (masting) and environmental variability influence sap quality and maple syrup economics.
We have observed clear masting patterns; abundant flowering and fruiting in 2011 were followed by a nearly complete lack of flowering in 2012, and a second mast year in 2013. 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, and in 2013 the ratio was again more balanced (9 female first, 7 male first, and 4 male only trees). Sap sugar production mirrored masting patterns; sap sugar content was low in 2012, high in 2013, and is so far similar to 2012-levels in 2014. Sap sugar concentration is also correlated with flower sex allocation (positive correlation between sap sugar and proportion of female flowers) and tree size. 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 2012, flowers from which we excluded insect pollinators failed to produce seeds, while flowers open to insect pollinators produced seeds. This was not true in 2013, when wind pollination was sufficient to pollinate flowers enclosed in pollinator-exclusion bags. We have also observed a higher proportion of insect-predated (presumably weevils) seeds during a non-mast (2012) than a mast (2013) year.
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 15-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
- Figures: seed.sugar1.pdf
