Agriculture Reference
In-Depth Information
The origins of the study of leaf longevity lie in the distinction between evergreen
and deciduous plant species, which is not as simple as it first seems. The ever-
green habit basically is defined by the retention of functional leaves in the plant canopy
throughout the year, as opposed to the deciduous habit in which a plant is leafless
for some part of the annual cycle. This simple evergreen-deciduous dichotomy
most often is applied to woody trees, shrubs, and vines. Herbaceous perennials that
retain leaves through winter are sometimes referred to as evergreen, or more often
as wintergreen, in contrast to summergreen (Sydes 1984; Ohno 1990; Tessier
2008), but the evergreen-deciduous dichotomy has had less attention in herbaceous
species than in woody plants.
Box 1.1 Plant Canopy
The plant canopy can be thought of as a three-dimensional array of leaves for
the capture of solar energy. The term applies at two spatial scales, but in all
cases it refers to an array of leaves in space. At the level of individual plants,
the structure of the canopy is determined by the way that leaves are arrayed
along herbaceous stems or woody branches. The canopy of individual trees is
also referred to as the tree crown, the array of branches above the trunk. At the
level of a plant community, canopy structure depends on the canopy architec-
ture of neighboring plants and the way that individuals adjust their canopy
architecture in response to neighbors. In grasslands, the low-growing canopy
is often a heterogeneous mix of vertically oriented grasses and laterally
branching, broad-leaved herbs. In forests the canopy can be multilayered,
with taller trees forming the forest canopy but other, less tall, trees forming a
distinct subcanopy.
Box 1.2 Foliar Habit
Foliar habit refers to the common distinction between evergreen and deciduous
plants, which in fact is not as straightforward as most people think. Foliar habit
is a characteristic of the plant canopy as a whole, not of individual leaves. A
plant is commonly referred to as evergreen if it retains at least some leaves
throughout the year, in contrast to deciduous plants, which are bare of leaves for
some part of the annual cycle of the seasons. Depending on the timing of emer-
gence and fall of individual leaves and the number of leaves retained in the plant
canopy, some subdivisions of the evergreen and deciduous habits are possible.
Variations on the evergreen habit
Leaf exchanger : Leaves are exchanged within a year; thus, leaf longevity is
shorter than 1 year but there are always viable leaves in the plant
canopy.
(continued)
 
Search WWH ::




Custom Search