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Harvard Forest Symposium Abstract 2007
- Title: The structure, composition, and dynamics of old-growth forests in the Berkshire Hills and Taconic Mountains, western Massachusetts
- Primary Author: Tony D'Amato (University of Massachusetts )
- Additional Authors: David Foster (Harvard University); David Orwig (Harvard Forest)
- Abstract:
We recently completed a study examining the composition, structure, dynamics, and ecosystem properties of 18 remnant, old-growth forests in the Berkshire Hills and Taconic Mountains of western Massachusetts. In this study, we quantified structural attributes, including snags and coarse woody debris (CWD), the composition of understory plant communities, ecosystem properties such as nitrogen retention and nutrient stores, and the stand and landscape-level dendroecological dynamics. In addition, these attributes were compared with adjacent second-growth stands to allow for the development of critical databases for conservation and management strategies aimed at creating old-growth structures in second-growth forests as well as old-growth conservation efforts.
Overstory composition in the old-growth forests ranges from mixed Tsuga canadensis/Picea rubens forests to northern hardwood forests containing mixtures of Acer saccharum, Fagus grandifolia, and Betula alleghaniensis. In addition, eastern hemlock (Tsuga canadensis) is a dominant overstory species in a majority (16 out of 18) of these stands. As such, this study represents a critical natural baseline for monitoring how these systems will respond if and when they become infested by the hemlock woolly adelgid.
Dendroecological analyses indicated that disturbance regimes for these systems were dominated by relatively frequent, low intensity disturbances (average of 5.0 % canopy area disturbed per decade) operating randomly on the landscape. Comparisons of dendroecological patterns with model simulations of past hurricane events and historical documents suggested that broad-scale disturbances, such as hurricanes and ice storms, resulted in common disturbance peaks in the 1790s, 1820s, and 1920s at several study areas separated by over 50 km. No stand-replacing disturbances were detected at any old-growth area during the period examined (1700-1989).
Comparisons of structural characteristics in old-growth and second-growth hemlock forests illustrated that old-growth stands exhibit a higher degree of structural complexity compared to second-growth hemlock forests. In particular, old-growth stands had larger overstory trees and a greater abundance of downed coarse woody debris (135.2 versus 33.2 m3/ha) and snags (21.2 versus 10.7 m3/ha) compared to second-growth systems. The range in variation of structural attributes within old-growth study areas was similar to those in other old-growth eastern hemlock forests located on more moderate terrain in the Upper Midwest and New York. This range in variation was related to differences in disturbance history and site productivity among old-growth stands.
Soil measurements indicated that there were no detectable differences in soil characteristics, such as total C and N, between old-growth and second-growth hemlock stands; however, inorganic N (NO3-N and NH4-N) availability was much greater in old-growth stands. In contrast, differences existed in soil characteristics and N availability between old-growth hemlock and old-growth northern hardwood forests, with hardwood dominated systems exhibiting lower forest floor C:N ratios and greater amounts of inorganic N.
Old-growth hemlock stands had higher species richness and diversity, as well as a greater abundance of tree seedlings, saplings, understory herbs, and shrubs compared to second-growth forests. In addition, several common understory plants, including Aralia nudicaulis, Dryopteris intermedia, and Viburnum alnifolia, were more abundant in old-growth stands. Nevertheless, there were no understory species found exclusively in old-growth stands.
The findings of this study illustrate the distinct nature of old-growth forests, particularly in terms of structure, and highlight the importance of conservation strategies aimed at protecting these unique areas on the landscape. Moreover, information on the relationships between forest structural development and natural disturbance patterns will be used to guide upcoming efforts aimed at restoring late successional forest structures to managed areas in western Massachusetts.
- Research Category:
