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became longer, dominance in the chaparral vegetation shifted from evergreen to
deciduous species. They concluded that so long as the dry period was not too
prolonged, the deeply rooted, sclerophyllous evergreens with their relatively low
photosynthetic rates were more productive over the year than the shallow-rooted,
mesophyllic deciduous species. Conversely, when the dry period was not long, the
high photosynthetic rates typical of mesophyllic leaves conferred an advantage on
the deciduous species that were better able to exploit the cool, wet winter season
and to avoid water loss by being leafless in the hot, dry summer. In a related cost-
benefit analysis of leaves as photosynthetic organs, Orians and Solbrig (1977) were
the first to offer a functional explanation at the leaf level for sclerophylly and the
evergreen habit. They postulated that plants adapted to hydric conditions should
have drought-deciduous, mesophyllic leaves, photosynthesize rapidly when water
was readily available, and cease photosynthetic activity quickly as conditions
became drier (Fig.
1.1
). On the other hand, they expected plants adapted to xeric
conditions to have evergreen leaves persistent through drought periods, with
relatively low photosynthetic rates even when water was readily available, but able
to withstand drought through conservative stomatal regulation and low cuticular
water loss associated with sclerophylly. In this their ideas followed Mooney and
Dunn (1970a,b), but they also specifically suggested that the association of sclero-
phylly with the evergreen habit arose in the time required to recover leaf construc-
tion costs. Given the low photosynthetic capacity of sclerophyllous leaves, only an
evergreen habit allowing amortization over more than a single year could recover
Fig. 1.1
The Orians and Solbrig (1977) expectations for photosynthetic activity in response to
water availability as a function of different plant strategies. Soil water availability is on the
abscissa
, from wet to dry; photosynthetic activity is on the
ordinate
. Four hypothetical species are
illustrated:
darker shading
shows the portion of the moisture gradient where each species is
respectively at an advantage in terms of potential productivity.
X
indicates a species with xero-
morphic (sclerophyllous) leaves;
m
indicates a species with mesomorphic leaves
