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Harvard Forest Symposium Abstract 2010

• Title: Vegetation dynamics, coarse woody debris, and nitrogen cycling over 20 years in a virgin hemlock-hardwood forest recovering from catastrophic disturbance
• Primary Author: Maggie Wagner (Duke University)
• Additional Authors: Audrey Barker Plotkin (Harvard Forest); David Foster (Harvard University); David Orwig (Harvard Forest)
• Abstract:

  Primary forests provide a valuable opportunity to study forest processes in a natural environment, uncomplicated by anthropogenic disturbance. We surveyed two transects covering 0.57 ha of the Harvard tract of Pisgah forest in southeastern New Hampshire, one of the few parcels of old-growth forest remaining in New England. The surveys were conducted in 1989 and 2009. In each 10x10 meter plot, we measured and mapped every tree (live stems ≥ 2.5 cm diameter at breast height) and every piece of coarse woody debris (CWD; all downed wood with an average diameter ≥ 10 cm). In 2009 we also surveyed the understory vegetation and analyzed the C and N contents of soil from under CWD and soil that has incorporated recently decayed CWD. Censuses showed natural thinning of all major tree species, with Eastern hemlock (Tsuga canadensis) suffering mortality at less than half the rate of the hardwoods (Acer rubrum, Fagus grandifolia, and Betula spp.). The CWD pool decreased from 354.6 m^3/ha to 215.5 m^3/ha, but is still much larger than is typical for old-growth forests in New England. The vast majority of this CWD is the legacy of a catastrophic 1938 hurricane that destroyed the old-growth hemlock and white pine (Pinus strobus). As of 2009, white pine is almost completely absent from the forest. Between 1989 and 2009, total basal area increased from 35.5 to 41.8 m^2/ha, while density decreased from 2793 to 1768 stems/ha. Plots differed in community structure, with tree density negatively correlated with average tree size. Trends over the topographic gradient indicates that slope and aspect influence both community structure and CWD stores. Nitrogen was scarcer in soil under CWD and in soil incorporating decayed wood than in control soil (p < 0.05), indicating that CWD effects on soil nutrition add another element of spatial heterogeneity to the forest floor. Finally, we found 20 species of herbs and shrubs covering 4.42% of the study area, as well as 10 species of tree seedlings covering an additional 5.45% of the site. Our results show that despite its recent history of catastrophic blowdown, Pisgah retains many structural features typical of old-growth forests (especially huge CWD pools and microtopography related to natural disturbances) and supports more diverse vegetation than is found in New England second-growth forests. These findings are useful for foresters wishing to design a balanced management plan that maximizes forest health, and for conservation biologists aiming to restore a damaged landscape as close as possible to its original pristine condition.

• Research Category: Soil Carbon and Nitrogen Dynamics