The site on which a stand of trees grows has extremely important effects on the type of vegetation, and the health of the plants, growing upon it. In general, sites with poor water-holding capacity, that have low levels of essential nutrients, or that receive high levels of solar radiation are generally less suitable for plant growth and more suitable for insect survival and reproduction. The overall effects of site on plant and insect populations are summarized in the slide. Remember that the feedback involving insect defoliation, nutrients on the site and tree health and vigor is a stabilizing negative feedback loop. Also remember that site conditions have a very strong influence on gradient outbreaks but a much lesser effect on eruptive outbreaks.

EXAMPLE

Douglas-fir tussock moth. Site factors have a decisive effect on the occurrence of outbreaks of the Douglas-fir tussock moth. The moth exhibits cyclical gradient outbreaks with cycles of very high amplitude (outbreak cycles) occurring on dry or nutrient impoverished sites (Slide). Simulation experiments on models of tussock moth dynamics show how the amplitude of cycles can be influenced by the physical conditions of the site (Slide).

Mountain pine beetle. Hazard maps define regions where outbreaks have consistently occurred, particularly where droughts are common (slide).

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