Agriculture Reference
In-Depth Information
Box 7.1
Self-Shading and Leaf Emergence
There is a dichotomy between plants that produce essentially all their leaves
each year in a single burst (
simultaneous-type
leaf emergence) and those that
produce leaves in a steady progression throughout all or part of the year
(
successive-leafing
type). As all potential leaves appear at once at the start of
a growing season in the simultaneous type, all the leaves of this type can carry
out photosynthesis throughout the growing season. However, if many leaves
are attached on a shoot, leaves in lower positions will be shaded by those in
upper positions (
self-shading
), a disadvantage that can be reduced by the ori-
entation of shoots and leaves (Kikuzawa et al. 1996). By this means, all the
leaves on a shoot can receive sunlight evenly and the photosynthetic perfor-
mance of the shoot will increase, although inclining the shoot will also reduce
the height growth of the plant and increase biomechanical support costs. In
contrast, successive leafing essentially is an alternative method to avoid self-
shading within the plant canopy. The first leaf produced on a growing shoot
will enjoy full sunlight until the shoot extends and the second leaf emerges
and begins to shade the first leaf, and so on as successive leaves emerge.
Consequently, there are some linkages among leaf phenology (leaf emer-
gence pattern), self-shading, and shoot architecture (Kikuzawa et al. 1996;
Kikuzawa2003) in deciduous broad-leaved species. Simultaneous leafing
species (
Fagus crenata
,
Quercus crispula, Tilia japonica
) have strongly
inclined shoots and avoid self-shading whereas successive leafing species
(
Alnus hirsuta
,
A. sieboldiana
,
Betula platyphylla
) have upright shoots
(Kikuzawa et al. 1996). Similar linkages between leaf phenology and archi-
tecture exist in herbaceous species as well (Kikuzawa 2003).
Canopy Architecture and Leaf Longevity
Intrinsic controls on the development of canopy architecture determine the degree
of mutual shading among different branches and leaves within a canopy and hence
influence the longevity of leaves throughout the canopy. If shoot elongation is
rapid and leaf turnover on the elongating shoot high, the inner canopy of the tree
tends to become leafless as the outer canopy expands. The inner canopy of
Alnus
sieboldiana
, a species that elongates upright apical shoots with short leaf longevity,
illustrates this canopy-hollowing phenomenon (Shirakawa and Kikuzawa 2009).
Crown hollowing incurs an increasing cost in maintaining interior branches to
support the leafy shoots in the expanding outer canopy, perhaps explaining why
crown hollowing occurs mostly in species that never attain heights sufficient to
occupy the upper strata of mature forests. In some early successional trees canopy
hollowing is diminished by production of dimorphic shoots, long shoots that expand
