Approximately 20,000 years ago, New England was covered by a mile-thick glacier. 
By 12,000 years ago, the glacier had retreated, leaving an arctic environment very different from the New England of today.
The flora of this period was similar to that found in northern Canada. As the climate became warmer, plants, animals and humans from regions south of the glacier migrated northward.
The following was written by
Tammy Marie Rittenour
For inquiries contact Professor Julie Brigham-Grette, Depatment of Geosciences, University of Massachusetts Amherst
Tundra Vegetation (Deglaciation to 12,500 – 11,000 yr BP)
Pollen and plant macrofossil evidence from Glacial Lake Hitchcock and other non-glacial lakes indicates that tundra vegetation persisted until 12,500 years ago in southern New England and 11,000 years ago in northern New England (Davis, 1983). This late-glacial tundra consisted of species like those currently found in the arctic. Macrofossils and pollen of Dryas sp., Salix sp., and Vaccinium sp. and other tundra plants are common in lake sediments deposited during this period. Tundra pollen assemblages also contain high (10 – 40%) percentages of sedge pollen. These tundra landscapes were dominated by herb and sedge plants with some stunted spruce, balsam popular, willow and dwarf birch trees in selected microclimates (Davis, 1983). Leaf fossils from these arctic shrubs have been found in glacial varves in the Connecticut River valley. 
The presence of many past species that now occupy alpine, arctic and sub-arctic regions suggests that the year-around climate at that time was much colder than present. Frost action was probably important, producing unstable soils for plant growth. Studies of sediment accumulation in Mirror Lake, New Hampshire, and many other lakes in Atlantic Canada suggest that during tundra conditions there was permafrost (Davis and Ford, 1981; Mott and Stea, 1993). Additional evidence for permafrost conditions comes from numerous ice-wedge casts from throughout New England and the polygonal patterned ground and pingo scars found in the Connecticut River valley (Stone and Ashley, 1992).
Northward Migration of Spruce (12,300 – 11,000 BP)
Spruce was the first tree to colonize the late-glacial landscape of New England. The northward migration of this and all tree species was time transgressive. Pollen evidence for spruce was first seen in Connecticut 12,300 years ago, while in New Hampshire spruce was not present until 11,000 years ago (Davis, 1983). The high percentage of herb pollen during this period indicates open forest vegetation.
Fir, pine and larch quickly followed the spruce migration northward. 
Each new tree species that migrated northward reacted in an individualistic manner to the rapidly changing climate and availability of new habitat. Therefore, the composition of late glacial forests was constantly changing and in many cases these past forests have no modern analogues.
Younger Dryas Cold Episode (10,800 – 10,000 yr BP)
By 11,000 years ago a diverse spruce-hardwood open forest with warm-loving species such as oak and ash had developed throughout much of New England. This forest population was altered during a return to glacial climate 10,800 – 10,000 years ago. This cold episode called the Younger Dryas, named after the corresponding increase in Dryas pollen in Europe, was associated with an increase in alder (Alnus) pollen throughout New England and Atlantic Canada during this interval. This increase in alder was accompanied by a shift to a predominance of cool-climate species such as spruce and birch, and a decline of warm-climate species such as oak and ash in New England (Mayle et al., 1993). In Atlantic Canada, the Younger Dryas was associated with a change from spruce woodland back to tundra-shrub vegetation and a rejuvenation of permafrost conditions (Mott and Stea, 1993).
Development of a Hardwood Forest ( after 10,000 years ago)
About 10,000 years ago the climate in New England began to warm considerably, allowing warm-climate species to flourish. Pine and oak replaced cooler climate spruce shortly after the Younger Dryas cold interval. Additional tree species slowly and individualistically migrated into New England from the south, eventually to produce the pre-settlement forest of New England.
By 9,000 ago a closed canopy hardwood forest composed of white pine, oak, elm, ash, birch, ironwood, and sugar maple had developed in New England. This forest was different from pre-settlement forests in that it contained more poplar, birch, alder, balsam fir, and spruce (Davis, 1983). This forest more closely resembled forests currently found in the northern Great Lakes region.
Early Native American Habitation in the Connecticut River Valley
Dated archeological sites have indicated that the first migration of humans into New England occurred shortly after 11,000 years ago (Bonnichsen et al., 1985). At that time the southern portions of Glacial Lake Hitchcock had drained, although smaller lakes remained in the upper Connecticut River valley in Vermont/New Hampshire. The climate in New England was cool until 10,000 years ago when the region began to warm and a hardwood forest developed. Early Paleo-Indian inhabitants probably experienced shrub-tundra to open spruce woodland when they first arrived in New England. Bone fragments from archeological sites have suggested that some groups of Paleo-Indians hunted the herds of caribou that migrated through New England after deglaciation (Bonnichsen et al., 1985).
References:
Bonnichsen, R., Jacobson, G.L., Jr., Davis, R.B., and Borns, H.W., Jr., 1985, The
environmental setting for human
colonization of northern New England and
adjacent Canada in Late Pleistocene time: Geological Society of America,
Special
Paper 197, p. 151-159.
Davis, M.B., 1983, Holocence vegitational history of the Eastern United States: In,
Wright, H.E., Jr. ed., Late-Quaternary
environments of theUnited States, vol. 2.
The Holocene. University of Minnesota Press, Minneapolis.
Davis, M.B. and Ford, M.S., 1981, Sediment Focusing in Mirror Lake, New Hampshire:
Limnology and Oceanography,
v. 27, p. 137-150.
Mayle, F.E., Levesque, A.J., and Cwynar, L.C., 1993, Alnus as an indicator taxon of the
Younger Dryas cooling in eastern
North America: Quaternary Science Reviews,
v. 12, p. 295-305.
Mott, R.J. and Stea, R.R., 1993, Late-glacial (Allerod/Younger Dryas) buried organic
deposits, Nova Scotia, Canada:
Quaternary Science Reviews, v. 12, p. 645-657.
Stone, J.R. and Ashley, G.M., 1992, Ice-wedge casts, pingo scars, and the drainage of
Glacial Lake Hitchcock: In, Robinson,
P. and Brady, J.B., eds., Guidebook for
Field Trips in the Connecticut Valley Region of Massachusetts and Adjacent
States, v. 2 (84th Annual meeting of the New England Intercollegiate Geological
Conference), Amherst Massachusetts,
University of Massachusetts, Geology
Department, Contribution 66, p. 305-331.
