Migration of tree species in New England based on elevational and regional analyses

"Concerns over possible increases in global temperature have renewed interest in the detection, measurement, and analysis of tree-species migration. Previous work has consisted mostly of computer simulations of changes in species ranges. In this study, two complementary approaches were used to examine movements of tree species in New England, where there is a documented increase (at Hanover, New Hampshire) in mean annual and summer temperatures of about 2OC since 1835. We used advancing-front theory to examine age trends over distance and elevation in undisturbed stands on Haystack Mountain in New Hampshire. Then we examined changes in species occurrence over a 24-year period on USDA Forest Service survey plots throughout Maine. On Haystack Mountain, well-defined stationary fronts (no movement) were identified for red spruce (Picea rubens Sarg.) and beech (Fagus grandifolia Ehrh.); these occurred at the borders of distinct changes in site conditions, which serve as temporary obstacles to species' movements. A catastrophic front was depicted for sugar maple (Acer saccharum Marsh.): no established understory stems less than about 100 years old were evident at elevations above 720 m on a site complex composed of shallow bedrock interspersed with deeper till. A constant, very slowly moving advancing front was exhibited by hemlock (Tsuga canadensis (L.) Carr.). In Maine, white pine (Pinus strobus L.), and balsam fir (Abies balsamea (L.) (Mill.) decreased significantly in average latitude and elevation over the 24-year period; average species' elevations decreased by only 16 and 7 m, respectively, while latitudes decreased (southerly) by about 0.1 and 0.04 degrees (1 1.3 to 4.5 km). The other significant change was an increase (westerly) in longitude of sugar maple by about 0.1 degree (6.9 km). Succession--not climatic shifts-- following land use changes or cutting probably was responsible for these trends for sugar maple. Forest managers should be aware that natural succession as influenced by disturbance, land use, and site currently is the dominant influence affecting species changes in New England forests. At present, potential ranges of the major species in terms of elevation and regional position appear stable and in alignment with known site requirements."