Modifying Lodgepole Pine Stands to Change Susceptibility to Mountain Pine Beetle Attack

The premise that mature lodgepole pine forests are susceptible to mountain pine beetle attack when physiologically stressed was supported experimentally by manipulating the canopy density and availability of nitrogen in a 120—yr—old forest exposed to a high population of beetles. Where canopy density was reduced, ether by us or by the insects, surviving trees significantly increased their resistance to attack over a 3—yr period. Increased resistance was reflected by changes in wood production per unit of leaf area (tree growth efficiency). Improved nitrogen nutrition hastened tree recovery but did not prevent attacks by beetles until growth efficiencies exceeded 100 g of wood production per square metre of foliage. Growth efficiency, as here defined, is an index of vigor that may reflect the relative ability of susceptible trees to produce defensive compounds following attack.