Person:

Foster, David

Analyses of a sediment core from Highstead Swamp in southwestern Connecticut, USA, reveal Lateglacial and early Holocene ecological and hydrological changes. Lateglacial pollen assemblages are dominated by Picea and Pinus subg. Pinus, and the onset of the Younger Dryas (YD) cold interval is evidenced by higher abundance of Abies and Alnus viridis subsp. crispa. As climate warmed at the end of the YD, Picea and Abies declined and Pinus strobus became the dominant upland tree species. A shift from lacustrine sediment to organic peat at the YD-Holocene boundary suggests that the lake that existed in the basin during the Lateglacial interval developed into a swamp in response to reduced effective moisture. A change in wetland vegetation from Myrica gale to Alnus incana subsp. rugosa and Sphagnum is consistent with this interpretation of environmental changes at the beginning of the Holocene

The widespread and long-lasting impact of human activity on natural ecosystems indicates that land-use history must he treated as an integral aspect of ecological study and a critical component of conservation planning. The New England landscape has undergone a complete transformation as forests were converted to agriculture in the 18th and 19th centuries followed by succession to woodland as a result of widespread agricultural abandonment. Despite the prevalence of human impacts, the effect and longevity of land-use practices on modern forest conditions are poorly understood. In the present study of pitch pine-scrub oak vegetation on a sand plain in the Connecticut Valley of Massachusetts, we address the following questions: (1) what is the relative importance of human and natural disturbance and environmental factors in controlling vegetation composition, structure, and landscape patterns; (2) what are the mechanisms underlying human impacts on vegetation, and what is the duration of these impacts; and (3) what are the implications of land-use history for the interpretation and conservation of these communities? Sand plain vegetation was selected for investigation because the homogeneity of site conditions facilitates the interpretation of land-use and natural disturbance impacts, and because the uncommon vegetation and constituent species are priorities for conservation efforts. Paleoecological data suggest that pre-European fires were common on the study area, perhaps ignited by a large regional Indian population. The area was noted historically as an extensive pine plain and was used for wood products from the 18th to the mid-19th century. Eighty-two percent of the area was subsequently plowed for agriculture before being abandoned in the early 20th century. Soil analyses confirm the homogeneity of site conditions and suggest that land uses (plowing, woodlot/pasture) were determined according to ownership pattern rather than site factors. Previously cultivated parcels have distinct Ap (plow horizons) 15-33 cm deep, whereas uncultivated parcels have A horizons 3-10 cm in depth. Soil physical and chemical characteristics are similar among land uses and modern vegetation types. Aerial photographs document a dramatic transformation in plant cover over the last 50 yr. In 1939, the vegetation was grassland or shrub-heath (49%), open-canopy forest (29%), and scrub-oak shrublands (15%). In 1985, 73% of the study area was forested with pitch pine (40%), hardwood (12%), or mixed stands (21%), 9% was in open-canopy stands, and 3% was covered by grass or shrubs. Vegetation/land-use relations are striking. Pitch pine occurs almost exclusively (97%) on former plowed sites, whereas scrub oak stands occur preferentially (89%) on sites that have not been plowed. Land use explains the greatest variation in modern vegetation as well as the distribution and abundance of many taxa. Fire has been common across the study area but has influenced vegetation largely within patterns resulting from prior land use. Land-use patterns and factors controlling vegetation composition and structure are broadly paralleled at similar sites elsewhere in the Connecticut Valley. The study indicates that conservation biologists interested in preserving species, communities, and landscape patterns on sand plains in the northeastern United States need to incorporate a dynamic perspective of biological systems that includes the overriding impact of prior land use. In order to appreciate, study, and display these land-use and vegetation patterns it is essential to conserve the mosaic of assemblages and historical uses within a landscape setting.

The pervasive impact of historical land use is often underappreciated in the management and restoration of conservation areas and natural resources. We used historical and ecological approaches to determine the relative influences of past land use, fire, and site conditions on woodland vegetation patterns in Cape Cod National Seashore (CACO), the largest protected coastal landscape and area of sand-plain vegetation in New England. Coastal sand plains are the focus of intense conservation activity because they support uncommon plant and animal assemblages that are dynamic as a result of past disturbance and ongoing human impacts. CACO was predominantly wooded prior to extensive land clearance for historical agriculture. Historical maps and modern soil profiles indicate that by the mid-19th century, -44% of the area supporting sand-plain woodlands in CACO was plowed for crops or pasture, 42% was logged repeatedly but never cleared, and 14% was open and subjected to diverse uses. Relationships between modern vegetation and 19th-century land use are striking and largely independent of site conditions. Continuously wooded areas support pine-oak woodlands with abundant ericaceous shrubs, whereas previously plowed sites have less canopy oak, more pine, few ericaceous shrubs, and a distinct understory including the grass Deschampsia flexuosa and the shade-intolerant shrub Arctostaphylos uva-ursi. Current composition and historical sources suggest that past agriculture generated extensive heathland and grassland habitats, much of which has subsequently reforested. In contrast to many interpretations and management guidelines, the persistent influence of fire is principally on the canopy composition and structure of former woodlots. The results highlight a need (1) to integrate an understanding of past land use into ecological models underlying the management of biological reserves; and (2) to consider the use of management approaches that mimic past agricultural practices in order to maintain and restore important sand-plain habitats.

The pre-historical vegetation structure in temperate forest regions is much debated among European and North American ecologists and conservationists. Frans Vera’s recent hypothesis that large mammals created mosaics of forest and openland vegetation in both regions throughout the Holocene has been particularly controversial and has provoked new approaches to conservation management. Thirty years earlier, American paleoecologists Herb Wright and Margaret Davis debated whether abundant ragweed pollen at Rogers Lake, Connecticut at 9500 yr BP signified local forest openings or long-distance transport of pollen from Midwestern prairies. Using new pollen records from Harvard Forest and the North American Pollen Database, we address this question and offer insights to the openland discussion. Ragweed and other forbs exceed 3.5% at five sites in a restricted area of southern New England between 10 100 and 7700 yr BP. Strong evidence suggests this pollen originated from the landscapes surrounding these sites (supporting Davis), as ragweed pollen percentages do not increase with longitude from New England to the Midwest. Ragweed pollen percentages are also unrelated to basin size and therefore unrelated to the proportion of extraregional pollen in New England. High forbs values were associated with increases in oak, decreases in white pine, and relatively high charcoal values. Modern pollen records with similar forb and tree percentages occur along the Prairie Peninsula region of the Upper Midwest. However, the closest analogue to the southern New England early Holocene assemblages comes from Massachusetts’ Walden Pond in the early 18th century. These results and the affiliation of ragweed for open, disturbed habitats suggest that oak–pine forests with large openings persisted for over 2000 years due to dry conditions and perhaps increased fire frequency. This conclusion is corroborated by independent lake level and climate reconstructions. Because these early Holocene openlands have no detectable analogue in New England for the past 7000 years before European settlement, we suggest that all important openlands today are almost exclusively a legacy of Colonial agriculture and should be managed accordingly. Nonetheless, our results may have implications for forest dynamics accompanying projected climate change to more arid conditions in New England over the next century.

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 the Tsuga forest reconstructed from the Hollow sediments. Specific objective were (1) to document the natural and anthropogenic disturbance history, (2) to examine the long—term vegetation dynamics of the two sites resulting from environmental change, species migration, and disturbance, 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 six pollen zones: I, Herb zone (12 500—11 800 yr BP); II, Picea zone (11 800—9500 yr BP); III, Pinus—Quercus zone (9350—8350 yr BP); IV, Tsuga—northern hardwoods zone (8350—1750 yr BP); V, Tsuga—Castanea—hardwoods zone (1750—200 yr BP); and, VI Post—settlement zone (200 yr BP—present). No disturbances are detected in the periods of tundra or boreal vegetation from 12 500 to 8 350 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 (° 1915 A.D.), and (3) post—settlement forest cutting, burning, land clearance, and cultivation (1750 A.D. to present). Log—ratio diagrams facilitate the differentiation of local vegetation history in the Tsuga forest from the regional upland history. Tsuga has been the dominant taxon at the Hollow for the last 8000 yr although its abundance has fluctuated with disturbance. Tsuga, Ulmus, Populus, Acer rubrum, A. saccharum, Betula, and Castanea apparently are more abundant locally than in the regional vegetation, which has a larger component of Pinus, Quercus, and Carya. Post—disturbance vegetation dynamics in the Tsuga forest are controlled by the type and intensity of disturbance and the pool of available species, determined by species migration and climate change. Tsuga decreases and subsequently recovers in 300—1200 yr following most disturbances. Northern hardwood taxa (Acer saccharum, Fagus, Betula spp., Ulmus, Fraxinus) generally decrease after fire and human activity and recover with Tsuga. For the period 8000—3000 yr BP Pinus, Quercus, and occasionally Populus and Nyssa increase following fire and the Tsuga decline at 4700 yr BP. However, since its immigration at °3000 yr BP Castanea has been the major species to increase upon disturbance of the Tsuga—northern hardwood forest. The study documents the long—term dominance of a forest stand by Tsuga canadensis and northern hardwood species despite repeated infrequent disturbances. The ability of these taxa to gradually reassume their former abundance following disturbance is presumably a consequence of their shade tolerance and longevity and the low frequency of disturbance.

Infrequent, intense wind disturbance is an important factor in northeastern U.S. forests, yet little is known about the early stages of vegetation reorganization, or the processes that facilitate biotic regulation of ecosystem function after such storms. We designed an experiment, based on a simulated hurricane blowdown, to examine the relationship of tree damage patterns to mortality and regeneration, community dynamics, vegetation recovery, and ecosystem processes. In October 1990, selected canopy trees in a 50 X 160 m area within a 75-yr-old Quercus rubra-Acer rubrum forest in central Massachusetts were pulled over by a winch, using records from the 1938 hurricane to determine the number of trees and direction of fall. The resulting damage to 65% of trees closely approximated effects of the 1938 storm on New England forests. Damage and mortality varied by tree species and size, indicating the importance of pre-disturbance forest structure and composition in determining the range and severity of impact. Measurements of vegetation and environment in the experimental area and control indicated that, although the manipulated stand sustained dramatic damage and structural reorganization, resilience of trees and understory vegetation provided tight biotic control of ecosystem processes, including nutrient cycling. Continued leaf-out and induced sprouting by damaged trees, increased growth by saplings and understory plants, and seedling establishment on disturbed microsites stabilized the microenvironment. Our findings are in contrast to studies of disturbances in which mortality was higher when damaged trees were removed from the site. This suggests that salvage logging following wind disturbance may have serious long-term implications.