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have long been evergreen broadleaf forests in tropical regions, the extensive
needle-leaved boreal forests that influenced the perspectives of nineteenth-century
phytogeographers did not exist until recently (Taggart and Cross 2009). During
most of the more than 400 million years since terrestrial plants first evolved in
the Silurian, the planet has been in a “greenhouse” mode characterized by
relatively warm climates worldwide that were only occasionally interrupted by
episodes of cooling and glaciation (Tabor and Poulsen 2008; DiMichele et al.
2009). During this time the deciduous habit arose, most likely as an adaptation to
seasonally dark and fire-prone polar environments (Brentnall et al. 2005; Royer
et al. 2003, 2005). By the Eocene, when the flora had close affinity with that of
today, the distribution of evergreen and deciduous vegetation differed substan-
tially from the present day. Broadleaf evergreen forests extended from equatorial
regions to latitudes as high as 60°, and deciduous conifer forests were found in
polar regions (Brentnall et al. 2005; Utescher and Mosbrugger 2007). This long
period of “greenhouse” conditions is in contrast to the “icehouse” conditions that
began and have persisted since the transition from the Eocene to Oligocene about
34 million years ago (MYA) when the planet became cooler and more subject to
cyclic glaciation than it had been during the late Paleozoic and earlier Cenozoic
(Coxall and Pearson 2007; Zachos et al. 2001). Although the contemporary phy-
togeography of vegetation types (Melillo et al. 1993) has arisen in these “ice-
house” conditions, on uniformitarian principles a well-grounded theory of foliar
habit should predict both contemporary and paleo-distributions of evergreen and
deciduous habits.
Contemporary Distribution of Deciduous
and Evergreen Habits
Chabot and Hicks (1982) posed the question: What can account for the bimodal
distribution of evergreen forests along latitudinal gradients (Fig.
9.2
), ecosystems
dominated by broadleaf evergreen species at low latitudes and needle-leaf ever-
green species at high latitudes (Melillo et al. 1993)? This query may, in fact, not be
the best question around which to develop a theory of foliar habit. The potential
problem is that despite the conceptual frameworks imposed by those interested in
classifying, modeling, and mapping the broad patterns of global vegetation, plant
communities only rarely, if at all, are composed entirely of evergreen or deciduous
species. The norm is co-occurrence of species with these contrasting foliar habits
within and among plant growth forms and vegetation strata, albeit in varying
proportions. For example, the low-growing woody species of the tundra are mostly
evergreen, but there are also many deciduous species of
Salix
. Boreal forests are
dominated by evergreen conifers, but deciduous species of
Populus
and
Betula
are
frequent on successional sites,
Salix
species widespread, and
Fraxinus
,
Alnus
, and
Ulmus
not uncommon trees in rich, moist sites. There is no shortage of deciduous
herbs and shrubs in the boreal forest understory. The transition from boreal forests
