202
T.T. Veblen and P.B. Alaback
of anthropogenic disturbances such as fire and livestock (Veblen & Lorenz,
1988; Veblen & Markgraf, 1988; Veblen et al., 1992a) . In South America,
Nothofagus-dominated forests generally occur at sites that are suboptimal
for tree growth due to climatic or edaphic factors . Thus, along a west-toeast trans-Andean gradient at c. 40°S from moist to a dry climate, the
Valdivian rainforest of south-central Chile is replaced by monotypic stands
of Nothofagus, which in turn give way to xeric woodlands and Patagonian
steppe. Similarly, along north-to-south as well as along elevational gradients from warm to cooler temperatures, relatively species-rich temperate
rainforests are replaced by Nothofagus forests, then by alpine or subantarctic tundra. The occurrence of Nothofagus forests in these ecotones
between forest and nonforest vegetation makes them highly susceptible to
the effects of climate fluctuation (Markgraf, 1983, 1987).
In general for northern Patagonia, both tree-ring records and glacier
variations indicate intervals of below average temperatures for 1300 to
1380, 1500 to 1700, and around 1840 A.D. In contrast, above average
temperatures occurred in 1100 to 1300, 1850 to 1890, and 1905 to 1920
A.D. (Mercer, 1965, 1970; Rothlisberger, 1986; Villalba, 1990; Villalba,
Boninsegna, & Gobos, 1989). These climate variations are likely to
have had an influence on the occurrence of fire and on forest dynamics
generally. For example, preliminary data indicate there is a marked
concentration of Austrocedrus chilensis mortality from c. 1911 to 1919
which was a period of unusually low summer precipitation and high mean
annual temperature (Villalba & Veblen, unpublished). Similarly, the peak
of anthropogenic forest burning in northern Patagonia coincides with
drier periods in the late nineteenth and early twentieth centuries (Veblen
et aI., 1992a).
The pollen record from the Pacific coast of North America, especially
along the northern coast and interior, indicates a significant warming
trend since the last major ice advance (e.g., Brubaker, 1988; Heusser,
1960). Many tree species in Alaska are physiologically adapted to grow
much further north or west than the limit of their present day distribution
(e .g., Abies amabllis, A. lasiocarpa, Picea glauca, P. sitchensis). Picea
sitchensis continues to expand its range westward into the wet coastal
tundra areas and shrublands at the western extreme of its distribution in
Alaska (Griggs, 1934).
Subtle changes in climate in the southern part of the rainforest district
correspond with dramatic changes in northern areas including glacial
surges or rapid recessions and rapid timberline movement. For example,
during the Little Ice Age approximately 200 years ago all of Glacier Bay
was filled with ice. Presently ice only remains in the head of the Bay,
more than 95 km from the mouth of the Bay. Partly because of these
massive fluctuations in glaciers and climate, species in the northern part
of the rainforest district tend to be habitat generalists and are less likely
T.T. Veblen and P.B. Alaback
of anthropogenic disturbances such as fire and livestock (Veblen & Lorenz,
1988; Veblen & Markgraf, 1988; Veblen et al., 1992a) . In South America,
Nothofagus-dominated forests generally occur at sites that are suboptimal
for tree growth due to climatic or edaphic factors . Thus, along a west-toeast trans-Andean gradient at c. 40°S from moist to a dry climate, the
Valdivian rainforest of south-central Chile is replaced by monotypic stands
of Nothofagus, which in turn give way to xeric woodlands and Patagonian
steppe. Similarly, along north-to-south as well as along elevational gradients from warm to cooler temperatures, relatively species-rich temperate
rainforests are replaced by Nothofagus forests, then by alpine or subantarctic tundra. The occurrence of Nothofagus forests in these ecotones
between forest and nonforest vegetation makes them highly susceptible to
the effects of climate fluctuation (Markgraf, 1983, 1987).
In general for northern Patagonia, both tree-ring records and glacier
variations indicate intervals of below average temperatures for 1300 to
1380, 1500 to 1700, and around 1840 A.D. In contrast, above average
temperatures occurred in 1100 to 1300, 1850 to 1890, and 1905 to 1920
A.D. (Mercer, 1965, 1970; Rothlisberger, 1986; Villalba, 1990; Villalba,
Boninsegna, & Gobos, 1989). These climate variations are likely to
have had an influence on the occurrence of fire and on forest dynamics
generally. For example, preliminary data indicate there is a marked
concentration of Austrocedrus chilensis mortality from c. 1911 to 1919
which was a period of unusually low summer precipitation and high mean
annual temperature (Villalba & Veblen, unpublished). Similarly, the peak
of anthropogenic forest burning in northern Patagonia coincides with
drier periods in the late nineteenth and early twentieth centuries (Veblen
et aI., 1992a).
The pollen record from the Pacific coast of North America, especially
along the northern coast and interior, indicates a significant warming
trend since the last major ice advance (e.g., Brubaker, 1988; Heusser,
1960). Many tree species in Alaska are physiologically adapted to grow
much further north or west than the limit of their present day distribution
(e .g., Abies amabllis, A. lasiocarpa, Picea glauca, P. sitchensis). Picea
sitchensis continues to expand its range westward into the wet coastal
tundra areas and shrublands at the western extreme of its distribution in
Alaska (Griggs, 1934).
Subtle changes in climate in the southern part of the rainforest district
correspond with dramatic changes in northern areas including glacial
surges or rapid recessions and rapid timberline movement. For example,
during the Little Ice Age approximately 200 years ago all of Glacier Bay
was filled with ice. Presently ice only remains in the head of the Bay,
more than 95 km from the mouth of the Bay. Partly because of these
massive fluctuations in glaciers and climate, species in the northern part
of the rainforest district tend to be habitat generalists and are less likely