Disturbance-mediated dynamics of mid-tolerant hardwoods in an old pine-hemlock-hardwood forest, New Hampshire, USA


We studied the forest structure and dynamics of the 26 ha College Woods Natural Area (CWNA), Durham, NH. The CWNA is dominated by Tsuga canadensis, but includes a remnant cohort of ca. 300 y old Pinus strobus and abundant mid-tolerant hardwoods. Our primary objectives were to: estimate the recruitment years of four mid-tolerant hardwoods, Acer rubrum, Betula lenta, Quercus rubra, and Q. velutina; relate the temporal appearance and population structure of these species to local disturbance history; and compare our results to other old forests in central New England. We hypothesized that recruitment of mid-tolerant hardwoods at the CWNA was associated with major 20th century windstorms and was limited to within 25 y post disturbance. We used plot sampling to describe densities and size structures of all tree species in the stand. Both Tsuga canadensis, the most abundant species, and Fagus grandifolia were represented in all diameter classes, whereas the mid-tolerant hardwoods were poorly represented in both the smaller and larger size classes. We took increment cores from a sample of trees of each mid-tolerant species and estimated a recruitment year for each individual. We then related recruitment years of each species to local disturbance history. Age estimates suggest that most individuals recruited after 1900 and that recruitment was associated with disturbances, including logging between 1895 and 1919, and the hurricanes of 1938 and 1954. After 1960, there was little recruitment of mid-tolerant tree species, probably due to canopy closure and lack of additional disturbance. The dynamics of mid-tolerant species in the CWNA were similar to those in an old Tsuga–Pinus stand at the Harvard Tract, 125 km to the west, as well as in other mature stands in central New England, suggesting regionally consistent responses to disturbance. Despite the current dearth of recruitment, future regeneration of these species in College Woods is likely, if not due to severe windstorms, then to the imminent loss of the Tsuga canadensis–Fagus grandifolia canopy to introduced insects and pathogens.

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