Late Quaternary history of low-and mid-elevation vegetation in the White Mountains of New Hampshire

Abstract

Pollen and plant macrofossil analyses of sediments from an altitudinal series of lakes in the White Mountains of New Hampshire, USA were used 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), Lonesome Lake (831 m), Carter Notch Pond (1004 m), and Lake of the Clouds (1538 m), provide paleoecological records of changes in the altitudinal limits of species, the species abundances within communities, and the vegetation zones. These are supplemented by previously published data from three high-elevation sites (Spear 1989). Although past plant communities were different from modern ones, differentiation of vegetation 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), and 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-elevation 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 (Pinus strobus) and hemlock (Tsuga canadensis) expanded to mid-elevations during the mid-Holocene, suggesting greater 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 black 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 the 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. At low and mid-elevations disturbance by wind and frost action was important during the late-glacial. Disturbance by fire was important during the period of spruce woodland and later in the early Holocene pine and oak forest. From 7000 yr BP to the present the primary factor disturbing New Hampshire forest was again probably wind, especially from 7000 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 lapse rates have occurred along the mountain slopes. This provides evidence that the source of air masses reaching the White Mountains has varied. The boundary between alpine tundra and subalpine fir forest (tree line) most likely has always been governed temperature (summer insolation) and wind. The montane plant communities result from individual species response rather than community response to the numerous climatic forces that have affected the mountains over the past 14 000 yr.