Agriculture Reference
In-Depth Information
Fig. 8.2
Leaf survivorship
curves for
Rhododendron
maximum
in different light
regimes:
O
canopy gap,
D
deciduous broad-leaved forest
floor,
E
evergreen forest floor.
(From Nilsen 1986)
100
Rhododendron
maximum
(1983)
90
80
70
60
50
40
O
D
30
E
20
10
0
1
2345 6789
YEARS
10
Aridity and Leaf Longevity
On the assumption that leaf longevity is governed by the time required to pay back
the costs of leaf construction, we can generally expect sublethal levels of water
shortage to be associated with longer-lived leaves. There is a variety of experimen-
tal and observational evidence supporting this point of view at the level of indi-
vidual species, but interspecific comparisons of the relationships between water
availability and leaf longevity are not straightforward.
The contrast between the deciduous and the evergreen habit illustrates the ambi-
guities of the relationship between water availability and leaf longevity. The vegeta-
tion of regions prone to water shortage can include both drought-deciduous species
that drop their leaves at the onset of a dry season and evergreen species that retain
leaves through the dry season. Drought-deciduous plants usually have higher maxi-
mum photosynthetic rates than evergreen plants (Comstock and Ehleringer 1986;
Ackerly 2004), which is consistent with the general relationship between leaf
longevity and photosynthetic rate. On the other hand, the co-occurrence of species
with different foliar habits indicates that leaf longevity is only part of a larger syn-
drome of adaptive alternatives to water shortage. A study comparing species with
short-lived versus long-lived leaves in the understory of a seasonal tropical forest
in Panama illustrates this point (Tobin et al. 1999). Species with long-lived leaves
had deeper root systems than species with short-lived leaves and thus could avoid
drought conditions during the dry season. We cannot expect a simple pattern of leaf
longevity in relationship to water stress across species, but if the cost-benefit
perspective on leaf longevity (Chabot and Hicks 1982; Kikuzawa 1991) is valid,
then it must apply within species.
