Agriculture Reference
In-Depth Information
If we regress nutrient use efficiency of various forests against nutrient absorption
rate, we obtain a decreasing relationship between the two (Vitousek 1982). This
finding suggests that in nutrient-rich forests, nutrient use efficiency (NUE) is low
whereas in nutrient-poor forests the trees use nutrients efficiently. Fertilization
decreased the nutrient use efficiency, indicating this relationship is not merely the
outcome of autocorrelation. It is also suggested that if there is a lowest limit of
nutrient amount to achieve positive net production, the relationship is not a simple
decreasing function, but should be an optimum curve (Bridgham et al. 1995).
When nutrient is resorbed from senescing leaves, total CO
2
assimilation of the
canopy can be improved by the shedding of older leaves only when the increase
in photosynthesis from resorbed nitrogen (
N
) exceeds the photosynthesis of the
leaves lost. This condition is satisfied if the ratio (100 × PNUE in the old leaves/
PNUE in the young leaves) is less than the percentage of
N
recovered from
senescing leaves before abscission. In other words, the
N
of old leaf × efficiency of
old leaf should be less than the retranslocation ratio ×
N
of old leaf × efficiency
of new leaf:
<
(10.2)
PNUE / PNUE
old
new
Otherwise, retention of the old leaves would result in a higher total CO
2
assimila-
tion for the whole-leaf biomass. Accordingly, under
N
limitation, maximum leaf
longevity must be constrained by both the rate of decline in PNUE with leaf age
and the efficiency of
N
resorption; the balance between these factors will determine
a minimum relative PNUE for leaf retention. In agreement with the foregoing
expectations, instantaneous PNUE of the leaf cohorts in nine Mediterranean tree
species was usually above the predicted minimum PNUE for a leaf to be retained
(Escudero and Mediavilla 2003).
Defense of Leaves and Leaf Longevity
Consistent with a carbon-focused cost-benefit analysis of leaf longevity, PNUE
will also be adversely affected and leaf longevity altered if the photosynthetic
capacity of leaves is impaired during their lifetime. This impairment can occur not
simply because of aging of tissues but also because of either damage through
herbivore and pathogen attack or damage associated with abiotic factors such as
wind or falling branches. The characteristics of the leaf can modulate these risks to
at least some degree, and at some cost, the biotic risks through constitutive and
facultative defenses and the abiotic risks through investments in stronger foliar tis-
sue. Although Chabot and Hicks (1982) raised these points, the associated costs and
benefits have not been incorporated into a theory for leaf longevity. Nor will it be
easy to do so (see Agrawal (2006) for an entry into the tangled history of research
on plant defense), but there are at least two possibilities among existing theories of
defense for establishing links to theory for leaf longevity. Both, in turn, have links
to aspects of ecosystem ecology.
