Agriculture Reference
In-Depth Information
the canopy periphery and short shoots that produce leaves along interior branches
without elongating internodes. Long shoots function in both space acquisition and
leaf display, but short shoots only play a role of leaf display. Short shoots can persist
over many years along interior branches, producing only a few relatively long-
lived leaves and thus reducing canopy hollowing in species of
Betula
and
Populus
(Critchfield 1960; Pollard 1970; Isebrands and Nelson 1982) and some
Acer
spe-
cies as well (Critchfield 1971; Sakai 1987). Such differentiation of leaf display and
space acquisition through variation in shoot structure and leaf longevity is a general
phenomenon, with the short shoot-long shoot dichotomy only a particular case of
a broader range of structural variation in shoots (Takenaka 1997; Yagi and
Kikuzawa 1999). For example, shifts in the relationships between bud dormancy,
needle longevity, and total needle area per unit shoot length in some evergreen
trees alter the balance between leaf display and space acquisition in canopy devel-
opment and reduce canopy hollowing (Takenaka 1997). In some evergreen broad-
leaved tree species such as
Cleyera japonica
, leaves at the inner canopy have
prolonged longevity, or burst bud only after some years of dormancy, thus avoiding
canopy hollowing (Suzuki 2002).
The balance between leaf display and space acquisition in canopy development
is inextricably linked to leaf longevity through the feedback to leaf lifetime carbon
gain. Maximizing the capture of light energy is not simply a question of growing
taller to shade competing neighbors, but also a question of how effectively a plant
captures light from the part of the overall plant canopy surface that it occupies.
There is a trade-off between growing taller to shade neighbors and spreading
laterally to claim more surface area in the upper canopy of the plant stand. For
example, a tree maximizing only height growth could simply extend its apical
shoots straight and upright, but many canopy tree species in mature temperate
deciduous forests such as
Fagus
,
Quercus
, or
Acer
in fact have determinate shoot
growth and apical shoots declined toward the horizontal. These trees avoid self-
shading among leaves within the canopy by branch and shoot angles that allow
light penetration to deeper layers of the canopy (Posada et al. 2009). On the
other hand, successional tree species with indeterminate shoot growth such as
Alnus
or
Betula
elongate their apical shoots strongly upward, growing tall more
quickly but with a higher degree of self-shading in their canopy (Kikuzawa et al.
1996). Such successive leafers can attain higher photosynthetic rates by receiving
full sunlight at the time of first leaf appearance. When the first leaf's photosyn-
thetic rate declines with aging, a second leaf appears and again receives full
sunlight at the shoot apex but also shades the preceding leaf on the shoot and so
forth. Thus, successive leafing, high but early decline of photosynthetic rate, and
short leaf longevity are functionally linked with one another. In contrast, leaves
appearing simultaneously on a determinate shoot mutually shade one another
from the initial stage of leaf appearance, and thus plants avoid self-shading by
more horizontal placement of shoots, branching angles, leaf angles, and the like.
Simultaneous leafing, lower but persistent photosynthetic rates, relatively long
leaf longevity, and a more horizontally oriented canopy structure are also parts of
