Foliar Habit and Leaf Longevity - Ecology of Leaf Longevity - page 4

habit appears to be more a variant of the deciduous habit rather than a true
evergreen habit. Even a north temperate forest tree such as Carpinus caroliniana
that is commonly perceived as unambiguously deciduous in response to harsh
winter conditions is brevideciduous at the southern limits of its native geographic
range (Borchert et al. 2005). The widespread tropical tree, Shorea robusta , which is
usually considered evergreen, in fact shows similarly plastic rangewide responses
in the timing of leaf turnover (Singh and Kushwaha 2005). There is clearly a degree
of plasticity and ambiguity in what at first seems a straightforward dichotomy
between the deciduous and evergreen habits. Similarly, the simple association
between the deciduous habit and strongly seasonal climates is belied by its
occurrence in aseasonal tropical forests as well (Hatta and Darnaedi 2005).
In these same tropical forests, some of the evergreen species maintained relatively
constant leaf numbers through either steady or episodic turnover of leaves through-
out the year, while others were evergreen but allowed their leaf numbers to drop to
only 30-60% of full canopy at some point in the year (Hatta and Darnaedi 2005).
Although the evergreen-deciduous dichotomy has been recognized since ancient
times, it is only in the 20th century that appreciation for the diversity in leaf demog-
raphy that underlies observations at the scale of whole trees and forests has emerged
to make sense of these variations within the basic dichotomy.
The pioneering phytogeographic studies of Alexander von Humboldt and Aimé
Bonpland (1807) were the first to stimulate scientific interest in the contrast
between evergreen versus deciduous trees and forests. Western botanists already
were familiar with the broad-leaved deciduous forests of central Europe and
needle-leaved conifer forests of northern Europe, but Humboldt and Bonpland
called attention to the somewhat surprising existence of tropical forests dominated
by broad-leaved evergreen species of flowering plants. Since then, the distinction
between the evergreen and deciduous habit has figured in the classification of
vegetation types by phytogeographers and ecologists (Grisebach 1838, 1884;
Warming 1909; Whittaker 1962; Walter et al. 2002; Woodward et al. 2004). By the
late nineteenth century a complementary stream of inquiry had arisen that sought
to explain the environmental basis for predominance of the evergreen habit and the
frequently allied condition of small, tough, long-lived leaves referred to as sclero-
phylly (Beadle 1954, 1966; Loveless 1961; Monk 1966; Mooney and Dunn
1970a,b). Schimper's (1903) classic topic entitled Plant-Geography Upon a
Physiological Basis consolidated the earliest work in this field and raised questions
that continue to be investigated to the present day. It is these attempts to discover
the adaptive value of evergreenness and sclerophylly that eventually led to the study
of leaf longevity in its own right.
Recognizing that the evergreen habit and sclerophylly were associated with dry
and infertile sites, most of the work following Schimper (1903) focused on the
evergreen and deciduous habits as alternative strategies for managing water and
nutrient resources. Mooney and Dunn (1970a,b), for example, adopted a whole-
plant perspective on adaptation to explain the occurrence of evergreen and deciduous
species along gradients of moisture availability in the Mediterranean climates of
Chile and southern California. They observed that as the summer dry period