148
M.T.K. Arroyo et al.
especially well represented among genera of vines (50% ) and hemiparasites (53%). Although there are proportionally fewer genera of trees
(28%) and shrubs (20%) , their numbers are substantial (Table 8.1) . In
that such genera comprise such a high proportion of the woody rainforest
flora, they warrant more detailed study.
The Quaternary brought large-scale upheavals in the high-latitude biota
in each hemisphere. The Pleistocene history of southern rainforest is now
fairly well known (e.g., Ashworth, Markgraf, & Villagran, 1991; Heusser,
1983; Markgraf, 1989; Villagran, 1988, 1991), and will not be discussed in
any detail here, except to highlight a few interesting contrasts that could
bear on the patterns to be discussed later.
In South America, the lowland full glacial limit lay around 42°S, such
that a large proportion of the present seasonal rainforest zone (38°30' to
2°00' S) was not glaciated. Palynological evidence suggests that the central
west coast and much of south-central Chile supported a high precipitation
regime during the last glaciation maximum (Ashworth et al., 1991;
Heusser, 1983; Villagran 1988), whereas higher latitudes became cooler
and drier (e .g., Ashworth et al., 1991). With high precipitation favored at
the northern extreme of the rainforest zone in the Pleistocene, rainforest
species were able to descend into , or persist as, relic populations in the
nonglaciated Coast Range and northern corner of the island of Chiloe,
without being totally eliminated from the area (Villagran, 1991).
In the Pacific Northwest coast, the Pleistocene history seems to have
differed, in that climate was dry at the southern end of the ice during the
last glacial maximum (Brubaker, 1991). Here glaciers covered the Cascade
Range and Olympic Mountains from 22,000 to 29,OOOb.p. with the
lowland areas still ice-free at this stage. At 18,000 b. p. , ice advanced from
southern British Colombia into the coastal Olympic Peninsula and Puget
Sound lowland (Waitt & Thorson, 1983) (c. 48°N) . In the Pacific Northwest, according to Brubaker (1991), during the full glacial (18,000b .p .), a
cold-dry easterly airflow prevailed over the Pacific Northwest, particularly
in winter, favoring tundra and xeric treeline species, such as Engelmann
spruce south of the ice limit. According to the same author, there is no
evidence that such present dominants as Douglas fir lay immediately to
the south of the ice sheet in western Washington during late glacial times ,
as seems to have been the case for their analogs in southern South
America. Thus the nonglaciated section of the present western North
American rainforest, seems to have suffered greater climatic upheaval
and might have been recruited from smaller or more distant refugia than
its southern analog.
In summary, the South America rainforest has been drawn from a wide
array of phytogeographic elements with notable survival of those elements
that must have evolved under warmer Tertiary and Cretaceous climates.
It is difficult to estimate how many more of these elements might have
existed before the Pleistocene. What will become clear in the following
M.T.K. Arroyo et al.
especially well represented among genera of vines (50% ) and hemiparasites (53%). Although there are proportionally fewer genera of trees
(28%) and shrubs (20%) , their numbers are substantial (Table 8.1) . In
that such genera comprise such a high proportion of the woody rainforest
flora, they warrant more detailed study.
The Quaternary brought large-scale upheavals in the high-latitude biota
in each hemisphere. The Pleistocene history of southern rainforest is now
fairly well known (e.g., Ashworth, Markgraf, & Villagran, 1991; Heusser,
1983; Markgraf, 1989; Villagran, 1988, 1991), and will not be discussed in
any detail here, except to highlight a few interesting contrasts that could
bear on the patterns to be discussed later.
In South America, the lowland full glacial limit lay around 42°S, such
that a large proportion of the present seasonal rainforest zone (38°30' to
2°00' S) was not glaciated. Palynological evidence suggests that the central
west coast and much of south-central Chile supported a high precipitation
regime during the last glaciation maximum (Ashworth et al., 1991;
Heusser, 1983; Villagran 1988), whereas higher latitudes became cooler
and drier (e .g., Ashworth et al., 1991). With high precipitation favored at
the northern extreme of the rainforest zone in the Pleistocene, rainforest
species were able to descend into , or persist as, relic populations in the
nonglaciated Coast Range and northern corner of the island of Chiloe,
without being totally eliminated from the area (Villagran, 1991).
In the Pacific Northwest coast, the Pleistocene history seems to have
differed, in that climate was dry at the southern end of the ice during the
last glacial maximum (Brubaker, 1991). Here glaciers covered the Cascade
Range and Olympic Mountains from 22,000 to 29,OOOb.p. with the
lowland areas still ice-free at this stage. At 18,000 b. p. , ice advanced from
southern British Colombia into the coastal Olympic Peninsula and Puget
Sound lowland (Waitt & Thorson, 1983) (c. 48°N) . In the Pacific Northwest, according to Brubaker (1991), during the full glacial (18,000b .p .), a
cold-dry easterly airflow prevailed over the Pacific Northwest, particularly
in winter, favoring tundra and xeric treeline species, such as Engelmann
spruce south of the ice limit. According to the same author, there is no
evidence that such present dominants as Douglas fir lay immediately to
the south of the ice sheet in western Washington during late glacial times ,
as seems to have been the case for their analogs in southern South
America. Thus the nonglaciated section of the present western North
American rainforest, seems to have suffered greater climatic upheaval
and might have been recruited from smaller or more distant refugia than
its southern analog.
In summary, the South America rainforest has been drawn from a wide
array of phytogeographic elements with notable survival of those elements
that must have evolved under warmer Tertiary and Cretaceous climates.
It is difficult to estimate how many more of these elements might have
existed before the Pleistocene. What will become clear in the following