Agriculture Reference
In-Depth Information
Box 3.1
(continued)
Heterophylly refers to conspicuous differences in shape, size, or function
among the leaves on a plant. For example, the leaves that appear on a shoot of
Cercidiphyllum japonicum
early in the season are round and heart shaped at
the base, but those appearing later in the season are flat at the base and more
triangular in shape. Such early and late leaves often differ not only in shape
but also in longevity and physiological function.
Estimating Leaf Longevity from Leaf Turnover on Shoots
The shoot is the modular unit of leaf production, and hence the natural focus for
sampling coherent sets of observations to derive estimates of leaf longevity.
Monitoring the emergence and fall of leaves on a particular shoot at frequent
intervals over an extended time period is the definitive method for estimating leaf
longevity. Counts of leaves are usually recorded at the midpoint of census intervals,
so the more frequent the observations, the more precise is the estimate of leaf
longevity. Frequent counts of all the leaves on a shoot are tedious, but the accumu-
lated data are highly informative. The method gives a complete record of temporal
variation in leaf production and leaf longevity, which can be especially important
for species with indeterminate shoot growth (Fig.
3.1
). Since the date of emergence
and the date of fall are known for each individual leaf, both the mean and the variance
in leaf longevity can be calculated. These demographic data can be reworked to
describe the probability of leaffall as a function of leaf age (Dungan et al. 2003).
Seasonal or interannual differences in leaf longevity or differences between early
and late leaves in heterophyllous species can also be analyzed by partitioning the
data accordingly. In principle, a census can be carried out over many years, but in
practice this approach often is restricted to observations within a growing season.
Data most commonly are summarized initially in a leaf survival curve (Fig.
3.2
),
which can illustrate in detail the differences in leaf demography that underlie the
calculation of leaf longevity. The total number of leaf-days (the area under the
curve showing the number of living leaves) divided by the total number of leaves
produced is the mean leaf longevity over the period of observation, which typically
would be one complete growing season (Kikuzawa 1983).
There are various alternative calculations for estimating the mean leaf longevity
from a census of the numbers of leaves emerging and falling over a time interval.
A graphical framework introduced by Navas et al. (2003) helps us to understand the
ways that the relative timing of leaf emergence and leaffall can affect estimates of
mean leaf longevity. If for simplicity the increase (leaf emergence) and decrease
(leaffall) in numbers of leaves are approximated by straight lines over time, then
leaf longevity (
L
) can be considered in a graphical framework (Fig.
3.3
) linked to
the following equation (Navas et al. 2003):
( )
/
Lt
=+ +
p L
2
t
t
(3.1)
