Agriculture Reference
In-Depth Information
Fig. 10.8
Islands of xeric chaparral (arrowed) within a matrix of more mesic woodlands in
northern California as a model of Miocene chaparral distribution. (Photo by Jon Keeley.)
profiles showed a decline in species diversity associated with decreasing precipita-
tion and an increasingly severe dry season (Frederiksen
1991
). Despite this lower
community diversity, one might expect the landscape mosaic of mesic and arid
assemblages would have contributed to higher beta diversity. The Eocene-
Oligocene (35 Ma) transition resulted in continental-scale cooling and drying
and set the stage for further expansion of sclerophylls on most continents in the
northern and southern hemispheres (Raven &Axelrod
1972
). By the mid-Oligocene
drier and colder climates had spread sufficiently to eliminate many tropical lineages
from temperate latitudes (Axelrod
et al.
1991
), although little consideration has
been given to the interaction between climate and fire in driving these changes.
In North America, sclerophyllous chaparral vegetation came into its own in
the Oligocene (Wing
1987
), although unlike contemporary landscapes domin-
ated by pure chaparral, this early chaparral was restricted to rocky well-drained
slopes, rain shadows or coarse-grained low-fertility substrates, and formed a
mosaic with more mesic woodlands (e.g.
Fig. 10.8
). The Creede flora (27.2 Ma)
in southern Colorado is one example (Wolfe & Schorn
1989
). The climate was
dominated by winter rain or snow and summer rainfall was low. This chaparral
occupied the drier slopes around a mixed woodland and comprised largely two
species of
Cercocarpus
, plus a few other chaparral taxa such as
Mahonia
and
Ribes
.WolfeandSchorn(
1989
) disagreed with Axelrod's (
1987
) contention that
these chaparral elements were understory species in woodlands and posited that
they likely were present on more arid ridgelines and other surrounding sites. In
western North America and as far south as southeastern Mexico,
Cercocarpus
