Agriculture Reference
In-Depth Information
on the interpretation of how climates developed within each of the regions
considered. Seasonality and summer aridity apparently developed gradually over
a period of 10-20 million years and the contemporary MTC is an extreme
expression of a climate that has been a long time in the making.
Not only are there big unknowns in the timing of the MTC origin, the climatic
progression is unclear. Was the predecessor a bimodal rainfall regime with pro-
gressive loss of summer rains or a winter drought/summer rainfall regime that
switched the timing of both drought and rainfall. A potential model in south-
western North America is that the bimodal rainfall regime of Arizona chaparral
was the early Tertiary climate and this gave rise to the winter-drought vegetation
in northeastern Mexico and the summer-drought vegetation of California. If true
this raises serious questions about the interpretation of phenology comparisons
made between winter-drought and winter-rain regions (Verdu´
et al.
2002
).
However, complicating the role of climate is the observation that on some
landscapes substrate appears to have played a strong role in selecting for sclero-
phylly (Loveless
1961
). This makes it problematical to categorize the selective
environment responsible for the evolution of sclerophyllous MTV taxa. We are
rather limited in our ability to draw firm conclusions about the exact selective
environment driving plant traits. For example, the conclusion that sclerophylly in
southwest Australian heathlands was strictly an adaptation to nutrient stress and
neither drought nor fire played a role (Hopper
2003
) is unwarranted. Despite the
early origins of sclerophylly on poor soils, under a regional climate capable of
supporting mesic forest this would not rule out a role for soil drought. Indeed,
these coarse-textured soils are capable of generating significantly greater drought
stress than other soils (Sperry & Hacke
2002
). Annual droughts typical of MTC
regions are not required to provide a selective advantage to sclerophyllous leaves
as contemporary sclerophyll vegetation exists under aseasonal environments with
longer-interval droughts such as in southeastern Australia. Also, although this
latter region lacks a predictable annual fire threat, longer-interval cyclical fires are
commonplace and there is little reason to assume this was not the case during the
early evolution of sclerophyllous heathlands. Indeed, it has been argued that the
absence of forests on nutrient-stressed sites cannot be accounted for without
consideration of fire (Bond
2010
), and thus fire has potentially been a factor since
the origins of heathlands in South Africa and Australia. In short, interactions
between climate, fire and geology need to be considered in understanding the
selective environment of plant traits.
Selective Environment for MTV
Drawing conclusions about adaptations vs. exaptations in contemporary MTV
requires consideration of the scale at which proximal selective factors operate. The
niche relationships of the MTC paleofloras typically have been interpreted within
a narrow niche space that includes mean temperature, total precipitation and
its seasonal distribution (Raven & Axelrod
1978
; Ackerly
2004a
) or substrate
