Agriculture Reference
In-Depth Information
alternative evolutionary outcomes in the organization of foliar phenology that
involve distinct differences in leaf longevity. For some groups of plants sufficient
data have been compiled (cf. Wright et al. 2004) to detect broad differences in leaf
longevity, but other groups are too little studied to identify any characteristic leaf
longevity. Here we briefly review what we know about patterns of leaf longevity
among and within diverse groups of plants, illustrating our points with selected
examples.
Box 5.1
Adaptive Radiation
The diversity of species at any time in Earth's history arises in the balance
between rates of speciation and extinction. There are background rates of
speciation and extinction, but occasionally events trigger a rapid increase in
the rate of speciation. Such bursts of speciation are referred to as an adaptive
radiation. Adaptive radiations are often associated with colonization of species-
poor environments such as an isolated oceanic island that allows colonizing
species to diversify and exploit a wider variety of resources and habitats with-
out facing strong competitive interactions from other species. A well-known
example of adaptive radiation is the finches on the Galapagos Islands, which
now include many species derived from a single ancestor that have diversified
to use different habitats and food resources within and among the islands in
this archipelago far off the coast of South America.
Leaf Longevity of Ferns
The extant ferns trace their ancestry to the early Paleozoic but their current diver-
sity to an adaptive radiation in the early Tertiary (Schneider et al. 2004). Most
species are herbaceous, but there are some woody ferns that are tropical and
evergreen, with leaf longevity generally a year or longer. Leaf longevities were
328 days for
Cyathea furfuraca
, 525 days for
C. pubescens
, and 730 days for
C. woodwardioides
(Tanner 1983). Mean leaf longevity averaged 1.1-1.6 years
for
Cyathea hornei
(Ash 1987) and 2-2.5 years for
Leptopteris wilkesiana
(Ash
1986). The herbaceous ferns are more diverse in both their climatic affinities and
their leaf longevities. Sato and Sakai (1980) classified 67 herbaceous ferns in
northern Japan into four groups in terms of foliar habit: evergreen, semiever-
green, summergreen, and wintergreen. Evergreen species such as
Lepisorus
ussuriensis
and
Pyrrosia tricuspis
produce new leaves in June and July that are
shed from April to August 2 years later. Other evergreen species such as
Asplenium incisum
,
Blechnum niponicum
, and
Phyllitis scolopendrium
also produce
leaves early in the growing season but shed them after only about 1 year.
Semievergreen species such as
Dryopteris crassirhizoma
and
Polystichum
tripteron
produce leaves in late May and early July that begin to senesce by
