Agriculture Reference
In-Depth Information
time in the period between intervals, so the common practice of referencing the
data to the day midway between two sequential observations can introduce
considerable error as the observation interval increases beyond a week. Up to
about a week the uncertainty in the timing of leaffall or emergence is of the order
of the few days potential error associated with the intrinsic ambiguity in obser-
vations of the phenophases themselves. In a study of leaf emergence and fall on
the shoots of trees observed at intervals as long as a month, Dungan et al. (2003)
introduced an approach to minimizing the error associated with longer intervals
between observations. They observed shoots at weekly or biweekly intervals
early in the seasons, so that they could fit their observations on leaf production
and mortality to sigmoid growth functions. These functions can be combined to
estimate the number of living leaves at any time, including times between actual
observations. Fitting their leaf survivorship data to a gamma function, Dungan
et al. (2003) then used failure-time analysis to estimate the probability that a leaf
would survive to any given day after budburst and report leaf longevity as the
age at which the probability of a leaf dying reaches 50%, the leaf half-life.
Strictly speaking the leaf half-life and mean leaf longevity may not be perfectly
identical because of seasonal changes in half-life, but when leaf longevity is
longer than about 80 days it appears that half-life can provide a convenient
surrogate for mean leaf longevity (Diemer 1998a; Dungan et al. 2003). A reanalysis
of the Navas (2003) data confirmed the utility of this method and showed it to
be more accurate than estimates based on the midpoint between consecutive
observations (Dungan et al. 2008).
Box 3.2
Leaf Cohort
Some plants produce leaves sequentially through the growing season, others
all at once in a single episode early in the growing season. Any leaves emerging
together at some time form an even-aged cohort: these may be all the leaves
that will be produced in a year or just those produced at one time by a sequen-
tial leafing species. Following the death of individual leaves in a cohort over
time provides a survivorship curve, which often yields insights into foliar
function and canopy architecture. In successive leafing species, multiple
cohorts of leaves coexist on the plant at any time in the season, each cohort
following its own survivorship curve. A cohort produced early in the growing
season has older leaves than a cohort produced in midseason, and more leaves
in the older cohort may have senesced and fallen by the time the midseason
cohort leaves emerge. In other words, in successive leafing species the leaves
on a plant are multiaged, and the total number of leaves at any time in the
season is the difference between all the leaves that have emerged across all
cohorts and those that have fallen.
