LATE QUATERNARY HISTORY OF LOW-ELEVATION AND MID-ELEVATION VEGETATIONIN THE WHITE MOUNTAINS OF NEW-HAMPSHIRE

Pollen and plant macrofossil analyses of sediments from an altitudinal series of lakes in the White Mountains of New Hampshire, USA were use d to reconstruct the history of vegetation on the mountain slopes and to identify the factors responsible for vegetation change. Six sites, Mirror Lake (213 m), Lost Pond (625 m), Little East Pond (793 m), Lone some Lake (831 m), Carter Notch Pond (1004 m), and Lake of the Clouds (1538 m), provide paleoecological records of changes in the altitudina l limits of species, the species abundances within communities, and th e vegetation zones. These are supplemented by previously published dat a from three high-elevation sites (Spear 1989). Although past plant co mmunities were different from modern ones, differentiation of vegetati on along the slopes has always existed. At low elevations the sequence of vegetation change was: 13 700-11 500 yr BP, tundra; 11 500-9000 yr BP, transitional mixed-conifer woodlands of first spruce (Picea) and then fir (Abies balsamea), larch (Larix laricina), poplar (Populus), a nd paper birch (Betula papyrifera); 9000-7000 yr BP, forests dominated by pine (Pinus) and oak (Quercus); 7000 yr BP-present, mixed-hardwood forests. No late-glacial paleoecological records exist at mid-elevati on sites (700-1200 m). The steep slopes at these elevations stabilized by 10 000 yr BP and an early woodland of spruce was replaced by fir, larch, and paper birch. The altitudinal limits of both white pine (Pin us strobus) and hemlock (Tsuga canadensis) expanded to mid-elevations during the mid-Holecene, suggesting great warmth 6000-4000 yr BP. The modern spruce/fir forests of mid-elevations became established 2000 yr ago as spruce expanded at all elevations, suggesting cooler, moister climate similar to today. A tree line dominated by balsam fir and blac k spruce (Picea mariana) was established at its modern position 10 000 yr ago and has varied little since then, although it appears to have been slightly higher than now during the early Holocene. Changes in th e vegetation at low and mid-elevations have not been synchronous with those at tree line and result from a more complex set of environmental factors and climatic variables acting on several different species. A t low and mid-elevations disturbance by wind and frost action was impo rtant during the late-glacial. Disturbance by fire was important durin g the period of spruce woodland and later in the early Holocene pine a nd oak forests. From 7000 yr BP to the present the primary factor dist urbing New Hampshire forest was again probably wind, especially from 7 000 to 4000 yr BP when higher temperatures than present may have been associated with increased frequency and intensity of thunderstorms and tropical hurricanes. The vegetation history reveals that different la pse rates have occurred along the mountain slopes. This provides evide nce that the source of air masses reaching the White Mountains has var ied. The boundary between alpine tundra and subalpine fir forest (tree line) most likely has always been governed by temperature (summer ins olation) and wind. The montane plant communities result from individua l species response rather than community response to the numerous clim atic forces that have affected the mountains over the past 14 000 yr.