Agriculture Reference
In-Depth Information
a functional syndrome (Kikuzawa 1995a, b). Similarly contrasting morphological
and phenological characteristics related to light interception are found in the
essentially horizontal leaves of herbaceous forb species (Kikuzawa 2003),
although not in unbranched graminoid species that typically orient their leaves
near vertical in turf or cespitose clumps.
Box 7.2
Impact of Deep Versus Partial Shading
The way that individual leaves react to shading depends on the light regime in
which the entire plant exists. If the entire plant is subjected to low insolation,
as in forest understory species, then leaf longevity is relatively long and leaves
lower in the canopy do not translocate resources to less-shaded leaves higher
in the canopy. Conversely, leaves on trees in the forest canopy exist in a broad
range of insolation regimes from well lighted in the upper canopy to progres-
sively more and more partially shaded deeper in the canopy. In this case, leaf
longevity is shortened in proportion to shading, and resources are translocated
to the upper, brighter portion of the canopy. These responses reflect a balance
between optimization of resource gain and loss at the leaf level versus the
whole-plant level.
Canopy Heterogeneity and Leaf Longevity
The insolation regimes of leaves set by intrinsic controls on canopy architecture in
a uniform and stable light regime can be disrupted by external influences that create
asymmetry such as adjacent objects, forest edges, or gaps. In such instances, variation
in leaf longevity within individual plants does not appear to follow the general
pattern seen between individuals and species but is actually reversed: leaf longevity
on shaded shoots is shortened compared to sunlit shoots. For example, Miyaji and
Tagawa (1973) reported that shaded leaves in the lower canopy of a
Tilia japonica
