Composition and structure of an old-growth northern hardwood forest stand in Stoddard, New Hampshire

Siriano, James H.
Rick Van de Poll
Department of Environmental Studies
The purpose of this study was to characterize the composition and structure of an old-growth northern hardwood forest community located on the east side of Lightening Hill in Stoddard, New Hampshire. Compositional and structural characteristics, including the distribution of trees among various diameter classes, tree basal area and density, coarse woody debris, and canopy gaps, were used to examine the site's in-stand diversity and to gain a better understanding of its disturbance history. Three transects were established along an elevation gradient at the site. Both variable (Bitterlcih method) and fixed circular plots were used to characterize the composition and structure of the forest stand at 5 sampling points located at equal intervals along each of the transects. Species type and dbh were recorded for all live and standing dead (i.e., snags) canopy and subcanopy stems within the variable plots. Snag height and fragmentation class were also recorded. Data collected from the variable plots were used to calculate basal area and density values for the live stems and snags, as well as total snag volume. Sapling density was calculated based on a stem count by species in a 5 meter (m) radius circular plot at each point. In addition, the volume of logs and branches within a 10 m radius circular plot was calculated by decay class. Sugar maple (Acer saccharum), northern red oak (Quercus rubra), and white ash (Fraxinus Americana) dominated the stand's canopy, representing more than 90% of the relative basal area and density. The subcanopy was also dominated by 3 species. More than 80% of the subcanopy's relative basal area and density were comprised of sugar maple, white ash, and hophornbeam (Ostrya virginiana). Total basal area (28 m² ha-1), and stem density (390 stems ha-1) for the combined canopy and subcanopy layers were within the ranges of values from eight other old-growth northern hardwood stands in central New England. Total snag basal area (4.05 m² ha-1) and density (85 stems ha-1), and the volume of logs (36.59 m² ha-1), were also within the range of values from the other sites. However, log density (321 logs ha-1) was higher than that at the other sites, in part due to the impacts from an ice storm in January 1998. Three distinct natural communities along an elevation gradient were identified within the old-growth forest stand. An analysis comparing data from the three transects highlighted several differences in composition and structure between these communities. The exposed, relatively dry and nutrient poor areas along the steep, upper elevations of the site were dominated by northern red oak. Evidence of agricultural activities, repeated fires and intense wind events indicate the importance of exogenous disturbances in this area. The fires likely have bought about the disturbances in this area. The fires likely have bought about the relatively even-aged cohort of canopy oaks along the higher elevations. Dendrochronological data from this cohort suggests a significant fire during the post civil war time period. This community also supported a more dominant hophornbeam subcanopy populations that the other communities, likely an indication of drier conditions in the higher, more exposed areas of the stand. The more mesic, nutrient rich soils of the middle elevations supported a canopy dominated by sugar maple (Acer saccharum). At the lower elevation, along a bench exposed to winds from the east and southeast, the canopy was co-dominated by sugar maple and white ash (Fraxinus Americana). Impacts from the hurricane of 1938 on this community resulted in a larger number of smaller diameter canopy tress that the other communities. This community included an ever-aged stand of white ash in the northeast portion of the stand. Aging in several of these white ash trees confirmed that the stand was initiated by the hurricane of 1938. Results of this study provide a valuable description of the stand's composition and structure, and suggest the importance of a broad-scale disturbance regime at the site. In addition, the study demonstrates that old-growth northern hardwood forest stands in central New England many be a mosaic of several different natural communities. Based on the structural characteristics of this site compared with those of other similar communities in central New England, it appears that these characteristics alone are not sufficient to identify old-growth forests in the region.

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