Latest Touch - A Natural History of the New World - page 289

solar variations, and archeology. When this is done, a remarkably detailed
and consistent history emerges for New World vegetation since 2.6 Ma.
REGIONAL HISTORIES
I have been emphasizing the origin, sequence of appearance, and causal
factors involved in the development of ecosystems. By the Quaternary, the
current ecosystems of the New World had long been established, so the
discussions below are intended to (1) give some selective examples of recent
modifi cations in their range and composition; (2) demonstrate how vari-
ous forcing mechanisms for ecosystem change are being revealed; (3) show
how these mechanisms, originally demonstrated mostly for the high polar
regions, are now known to extend into the tropical and southern latitudes;
and (4) explain how methods and interpretations have changed over the
past few decades. Some selectivity is necessary because compared to the
fewer than twenty Oligocene fl oras studied relatively recently and in detail
for the New World, there are a thousand or more for the Quaternary (I,
chap. 8; II, chaps. 4-7).
North America (North of Mexico)
In northeastern and upper midwestern North America, the biotic record
within the glacial boundary, along the southern margin of the ice sheet,
and in interlobate areas of the region begins around 12,000-15,000 years
ago. This is when the ice retreated and peat and lake sediments began fi lling
basins created by the glaciers, and when vegetation moved onto the barren
soil (fi gs. 2.3, 8.5; Jackson et al. 2000). A composite sequence is shown in
table 8.1, starting with sediments at the bottom containing a prominent
representation of pollen from herbaceous plants that suggests a treeless
vegetation or tundra. Macrofossils from this “T” zone are important be-
cause the zone usually includes some tree pollen blown in from beyond the
glacial boundary (e.g., Abies , Picea ). As climates warmed, soils matured,
and succession progressed, a boreal forest developed at the site (zone A),
followed by a pine maximum (zone B) refl ecting the hypsithermal warm
interval. Temperatures then cooled and various combinations of deciduous
trees moved in (zone C), persisting until early agriculturalists arrived sev-
eral hundred years ago (zone C-3). The last zone is marked by an increase
in charcoal, weeds typically associated with cultivation (e.g., Ambrosia , rag-
weed; Rumex , buckwheat), and crop plants (e.g., Zea mays , corn or maize).