LONG-TERM VEGETATION DYNAMICS AND DISTURBANCE HISTORY OF A TSUGA-DOMINATED FOREST IN NEW-ENGLAND

The post-glacial history of two adjacent sites in the Harvard Forest, a 10-ha swamp (Black Gum Swamp) and a 0.006-ha hollow (Hemlock Hollow) in a Tsuga canadensis forest were investigated using pollen analysis. The sites were selected in order to contrast the regional vegetation history revealed from the swamp sediments with the local history of th e Tsuga forest reconstructed from the Hollow sediments. Specific objec tives were (1) to document the natural and anthropogenic disturbance h istory, (2) to examine the long-term vegetation dynamics of the two si tes resulting from environmental change, species migration, and distur bance, especially with respect to Tsuga, and (3) to contrast the pre- and post-settlement vegetation and environments. The Swamp and Hollow cores contain continuous sediment records covering the past 12 300 and 9500 yr, respectively. Regional vegetation changes are delimited in s ix pollen zones: I, Herb zone (12 500-11 800 yr BP); II, Picea zone (1 1 800-9350 yr BP); III, Pinus-Quercus zone (9350-8350 yr BP); IV, Tsug a-northern hardwoods zone (8350-1750 yr BP); V, Tsuga-Castanea-hardwoo ds zone (1750-200 yr BP); and, VI, Post-settlement zone (200 yr BP-pre sent). No disturbances are detected in the periods of tundra or boreal vegetation from 12 500 to 8350 yr BP. Since 8350 yr BP three distinct disturbance processes are detectable: (1) fires recorded in discrete charcoal horizons, (2) the apparent pathogenic decline of Tsuga (4700- 3500 yr BP) and the blight of Castanea (almost-equal-to 1915 A.D.), an d (3) post-settlement forest cutting, burning, land clearance, and cul tivation (1750 A.D. to present). Log-ratio diagrams facilitate the dif ferentiation of local vegetation history in the Tsuga forest from the regional upland history. Tsuga has been the dominant taxon at the Holl ow for the last 8000 yr although its abundance has fluctuated with dis turbance. Tsuga, Ulmus, Populus, Acer rubrum, A. saccharum, Betula, an d Castanea apparently are more abundant locally than in the regional v egetation, which has a larger component of Pinus, Quercus, and Carya. Post-disturbance vegetation dynamics in the Tsuga forest are controlle d by the type and intensity of disturbance and the pool of available s pecies, determined by species migration and climate change. Tsuga decr eases and subsequently recovers in 300-1200 yr following most disturba nces. Northern hardwood taxa (Acer saccharum, Fagus, Betula spp., Ulmu s, Fraxinus) generally decrease after fire and human activity and reco ver with Tsuga. For the period 8000-3000 yr BP Pinus, Quercus, and occ asionally Populus and Nyssa increase following fire and the Tsuga decl ine at 4700 yr BP. However, since its immigration at almost-equal-to 3 000 yr BP Castanea has been the major species to increase upon disturb ance of the Tsuga-northern hardwood forest. The study documents the lo ng-term dominance of a forest stand by Tsuga canadensis and northern h ardwood species despite repeated infrequent disturbances. The ability of these taxa to gradually reassume their former abundance following d isturbance is presumably a consequence of their shade tolerance and lo ngevity and the low frequency of disturbance.