Agriculture Reference
In-Depth Information
the realized growth; note that the herbivore damage is not given by the percentage
but by the absolute difference of the two terms.
The resource availability theory predicts a close correlation between leaf longevity
and defense. Plant species in resource-rich, sunny environments should invest more
for growth rather than defense, replacing leaves quickly to avoid declines in aging
leaves and attain vigorous growth at the whole-plant level. Nitrogen-rich leaves
have this high growth potential and short longevity, on one hand, but also are attrac-
tive resources for herbivores on the other (Mooney and Gulmon 1979). Leaves in a
resource-rich environment may incur more herbivore damage (Coley 1988; Coley
and Barone 1996) but can tolerate losses because of the high return on investment
possible in the resource-rich environment. On the other hand, plant species in
resource-poor environments will have lower photosynthetic potential and hence
longer-lived leaves. This situation places a premium on investments in defense
over growth. To summarize, the resource availability theory predicts: (a) a negative
correlation between growth and defense, and a positive correlation between growth
and amount of herbivory, and (b) a positive correlation between leaf longevity and
defense and a negative correlation between leaf longevity and growth. The theory
does not address interspecific variation in leaf longevity among co-occurring
species, but at least it has the potential to link theory for leaf longevity to environ-
mental gradients in resource availability that affect ecosystem productivity.
Timing of Leaf Emergence, Leaf Longevity,
and Leaf Defense
One of the earliest theories for plant defense, the apparency theory (Feeny 1970,
1976), also has bearing on theory for leaf longevity. Apparency theory made a
qualitative argument that if plants were more easily found by herbivores or patho-
gens because of their abundance, stature, persistence, or some similar factor, then
they would be subject to more frequent and diverse attacks and should have a
quantitative defense founded on reducing foliage quality for the attacker by heavy
investments in tannins, fiber, and other constitutive defenses. Conversely, a less
apparent plant would escape generalist herbivores or opportunistic pathogens and
need only mount a less costly, qualitative defense against potential attackers spe-
cially adapted to finding the plant despite its lack of apparency. The ideas are
simple but in some ways compelling and not without support (Agrawal 2006).
This apparency perspective on defense is interesting for theory of leaf longevity
because it might provide a link between leaf phenology and leaf longevity, and
leaf phenology in turn is being altered by global warming (Parmesan 2006).
Collating coherent data on the frequency and intensity of losses to herbivory and
disease at the community and ecosystem levels may allow probabilistic estimates
of leaf vulnerability that could be incorporated into an improved theory of leaf
longevity.
