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Spatial variation in anti-herbivore defenses of Pinus.
Ayres, Matthew1, Lombardero, Maria2, Ylioja, Tiina1, Lorio, Peter3, Klepzig, Kier3, 1 2 3
ABSTRACT- Experimental manipulations of water and nutrients consistently reveal strong phenotypic effects on tree defenses. Through the same physiological effects, spatial variation in site quality could affect tree defenses, yielding regional patterns in herbivory and biotic disturbance within forests. Alternatively, trees could undergo phenotypic adjustments to compensate for differences among sites. We compared the oleoresin defenses of red pine and loblolly pine across sites that permitted different growth rates. Resin flow from standardized wounds was highly variable across sites (10-40 km) and regions (400 km), indicating spatial patterns in suitability for herbivores such as bark beetles, but patterns differed between spatial scales. Among sites, there was no relationship between tree defenses and site quality. However, red pines in western Minnesota, near the prairie-forest ecotone where tree growth is very slow, had twice the resin flow as in Wisconsin, where tree growth is very rapid. This pattern matches studies showing that experimental increases in water or nutrient supply tends to increase pine growth and decrease oleoresin flow (apparently because constitutive secondary metabolism is increased when growth is limited by water or nutrients). Results were contrary to the plant stress hypothesis, which predicts that tree defenses should be least in marginal sites. Tree species such as red pine are less likely to have their realized distribution truncated through the effects of herbivores than those that behave as predicted by the plant stress hypothesis.
KEY WORDS: plant defense, secondary metabolism, growth-differentiation balance, plant stress