Agriculture Reference
In-Depth Information
drought-stressed sites occupied a much smaller proportion of the landscape and
thus were subject to a less predictable fire regime.
The model that species track proximal soil moisture conditions more than
average annual climatic patterns is perhaps not surprising when one considers
that the life span of most species is between 1 and 30 Ma, and thus due to
Milankovitch and other climatic cycles they are almost guaranteed to experience
multiple climates (Bennett
1990
). It is commonly believed that species simply
migrate to stay matched with the precise climate under which they evolved.
Alternatively though, species that have persisted through multiple climates may
have adapted to features of their environment that are common across multiple
climates. In light of this, Bennett (
2004
) has questioned the validity of trying to
distinguish adaptations from exaptations (see
Chapter 9
). Over time as climates
change there will always be reshuffling and ecological sorting of species into new
assemblages. Despite this sorting process species may still be within their potential
climatic niche and thus one need not necessarily view adaptive evolution and
ecological sorting as mutually exclusive options.
The demonstration that one cannot adequately predict contemporary global
vegetation patterns without including fire (Bond
et al.
2003
,
2005
) might very well
be applied to paleohistorical vegetation patterns as well. Most MTV taxa have
traits that are reasonably interpreted as being derived in response to fire. In
addition, plant traits appear to have the potential for significant effects on
flammability and fire spread (Bond & Midgley
1995
; Schwilk
2003
; Bond &
Keeley
2005
). These considerations suggest that plants can control fire in ways
that determine the ultimate assembly of MTC plant communities and thus climate
may be only a subtle indirect effect.
Lastly, perhaps drought has been overemphasized as a selective factor in MTC
taxa. What about the selective role of winter rains under mild temperatures?
A substantial proportion of taxa have their life histories closely linked to the winter
growing season, from germination to flowering. In this respect comparison of
species in common between the California MTC chaparral and the Arizona
bimodal rainfall chaparral is instructive. These regions share many species of
shrubs and postfire herbs that are winter active (Fotheringham
2009
). For example,
flowering phenology shows little difference in two of the dominant shrub species
that presently occur in both California and Arizona chaparral as they track winter
rains and not summer rains (J. Vankat unpublished in Keeley &Keeley
1988
). If we
assume the Arizona chaparral represents an earlier climatic condition under which
the MTC chaparral evolved (e.g. Axelrod
1989
) then it is apparent that the select-
ively important features of the MTC appeared before a true MTC developed.
Considerations in Evaluating MTV Origins
The dominant species in MTV are sclerophyllous-leaved shrubs and trees but the
communities in MTC regions comprise a diversity of other growth forms. Tracing
their evolutionary history requires recognition of several factors:
