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nitrogen increased as the favorable period length shortened across the sampled
microhabitats (Kudo 1996). Consistent with Kikuzawa's cost-benefit analysis of
foliar carbon economy (Kikuzawa 1991, 1995a,b, 1996), there clearly is interplay
between leaf longevity and foliar habit that should figure in analyses of the
geography of foliar habit as well.
Modeling Foliar Habit in Relationship to Climate
The greatest current concern in predicting the distribution of foliar habit at a global
scale is in models for shifts in vegetation in response to climate change. These
dynamic global vegetation models (DGVMs) draw on the distinction between ever-
green and deciduous foliar habit to characterize future vegetation zones but are cast
at the scale of global zonation in broadly defined plant functional types (Woodward
et al. 2004; Sitch et al. 2008). The scale and the definition of DGVMs unfortunately
do not allow detailed attention to the relationships between leaf longevity and foliar
habit. One place where climate models of foliar habit, however, have explicitly
considered the role of leaf longevity is in analyses of the possible origin of the
deciduous habit in polar forests during warmer periods in earth history (Brentnall
et al. 2005). In an adaptation of the University of Sheffield's conifer forest growth
model (Osborne and Beerling 2002), Brentnall and his colleagues (2005) use leaf
longevity as a key driver in analyses of variation in foliar habit along simulated
mid-Cretaceous climate gradients. Using wood from extant conifers, they calibrate
their model with a method relating the fine structure of wood anatomy to leaf lon-
gevity (Falcon-Lang 2000a,b; Falcon-Lang and Cantrill 2001) and then test their
predictions against a broad sampling of fossil wood deposits. Their analyses dem-
onstrate the advantage of the deciduous habit in high-latitude conifers during the
mid-Cretaceous when the earth was warmer and polar regions were forested
(Fig.
9.6
), a finding substantiated by experimental studies of extant conifers with
evergreen versus deciduous habits (Royer et al. 2005).
0.5
Fig. 9.6
Fractional cover of
deciduous conifers (
open
circles
) versus evergreen
conifers (
closed circles
) as a
function of latitude in simu-
lations that consider the role
of leaf longevity in affecting
survival, reproduction, and
competitive ability during the
mid-Cretaceous. (From
Brentnall et al. 2005)
0.4
0
0.2
0.1
0
60
70
80
90
Latitude
