Induced Plant Responses to Herbivory - Terrestrial Systems Ecology
Induced Plant Responses to Herbivory - Terrestrial Systems Ecology
Induced Plant Responses to Herbivory - Terrestrial Systems Ecology
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INDUCED RESPONSES 335<br />
Annu. Rev. Ecol. Syst. 1989.20:331-348. Downloaded from www.annualreviews.org<br />
by ETH- Eidgenossische Technische Hochschule Zurich - BIBLIOTHEK on 03/29/11. For personal use only.<br />
following defoliation may result from a passive rearrangement of resources<br />
within the plant. The distinction is that active responses involve de novo<br />
synthesis or energetically costly enzymatic processes, whereas passive responses<br />
involve only the consequences of tissue removal. Passive responses<br />
have been described as nutrient stress by Tuomi and coworkers (98, 99), as<br />
carbon-nutrient imbalance by Bryant and his associates (13, 14), and as<br />
passive deterioration by Myers & Williams (80). According <strong>to</strong> this hypothesis,<br />
a tree growing in an area with abundant soil nutrients (a fast growing<br />
tree) loses proportionately more nitrogen and other nutrients and less carbon<br />
during defoliation because it had proportionately more nitrogen in its leaves.<br />
Subsequently, carbon may be replaced in the leaves at a faster rate than<br />
nitrogen, and the surplus allocated <strong>to</strong> carbon-based allelochemicals (terpenes,<br />
resins, tannins, and other phenolics) and fiber. These foliar changes are<br />
expected <strong>to</strong> reduce the preference and performance of herbivores on trees that<br />
were previously defoliated. On the other hand, trees growing in nutrient-poor<br />
conditions or which s<strong>to</strong>re proportionately more carbon in their leaves (evergreens)<br />
may respond in the opposite way; defoliation may reduce the concentrations<br />
of carbon-based chemicals and increase the palatability of leaves<br />
of these slow-growing trees in the next growing season (14, 15, 24, 98).<br />
This model leads <strong>to</strong> several testable predictions (see also 98). (a) Nitrogen<br />
fertilization of defoliated trees should negate the nutrient imbalance and<br />
cancel the induced response; (b) carbon stress should result in a collapse of<br />
carbon-based resistance; (c) if herbivory and plant crowding reduce the same<br />
nutrients, then the effects of these two stresses should be qualitatively similar<br />
(57). Experimental N fertilization of birch trees increased foliar nitrogen and<br />
reduced phenolics, while root damage, which reduced nutrient uptake, reduced<br />
foliar nitrogen and increased phenolics (97). Larsson et al (64) found<br />
similar patterns between carbon availability (light) and carbon-based phenolics.<br />
Shading (reduced C) increased the palatability of willows <strong>to</strong> snowshoe<br />
hares, presumably because of reduced carbon-based defenses (12). Clipped<br />
and shaded willows produced regrowth shoots with lower concentrations of<br />
carbon-based secondary compounds, that were more preferred than clipped<br />
and unshaded trees.<br />
The resource rearrangement model does not explain all observations,<br />
however. Nitrogen fertilization of artificially defoliated birch trees did not<br />
negate the induced resistance as assayed by autumnal moth caterpillars (39).<br />
Crowding cot<strong>to</strong>n plants reduced their suitability <strong>to</strong> spider mites; however,<br />
crowding and herbivore damage did not act additively <strong>to</strong> reduce foliage<br />
quality for mites or verticillium fungus (57). On the contrary, induced resistance<br />
was only apparent when plants were not crowded, suggesting that<br />
resources are required for the induced response <strong>to</strong> occur.<br />
These tests of the passive model are not easy <strong>to</strong> interpret. For example,