. Hazard-rating systems in forest insect pest management : symposium proceedings, Athens, Georgia, July 31-August 1, 1980. Forest insects Biological control United States Congresses; Forest insects United States Analysis; Forest insects United States Management. 20 40 60 80 100 120 .09 t. 20 40 60 80 100 120 TREE AGE Figure relationship between ob- served (solid line), calculated annu- al increment, mean annual increment, and age for lodgepole pine at Riske Creek, cm , a greater number of brood is produced than the number of beetles that attack; and therefore an increase
. Hazard-rating systems in forest insect pest management : symposium proceedings, Athens, Georgia, July 31-August 1, 1980. Forest insects Biological control United States Congresses; Forest insects United States Analysis; Forest insects United States Management. 20 40 60 80 100 120 .09 t. 20 40 60 80 100 120 TREE AGE Figure relationship between ob- served (solid line), calculated annu- al increment, mean annual increment, and age for lodgepole pine at Riske Creek, cm , a greater number of brood is produced than the number of beetles that attack; and therefore an increase in population occurs (Safranyik et al. 1975). At the time of cutting, nine of the 12 trees were cm in diameter or grea- ter. One tree has been greater than cm for 38 years, another for 28 years, and four additional trees have exceeded cm since 1971, which was the begin- ning of the outbreak. The outbreak has therefore been sustained from the time when about half of the stems began to ex- ceed the diameter when a yearly increase in the MPB population is possible. Average diameters of the two 50-tree samples— and cm—would suggest that this con- clusion can be applied to the parkland in general. Periodic Growth Ratio and Hazard to MPB Periodic growth ratio (PGR) has been suggested as a method of assessing hazard of lodgepole pine stands from MPB. The method was evaluated for 21 stands with considerable success (Mahoney 1978). A ratio less than was used to define hazard. On a theoretical basis, however, the use of this ratio would not appear sound. Growth in radial increment is at a maximum when the tree is about 20 to 30 years old and afterward tends to decline (Duff and Nolan 1953). On average, there- fore, PGR should tend to decline after about 30 years. A ratio greater than 1 would be the result of release, or periods of above-average rainfall. But most of the time, after the trees reach ca. age 30, the ratio should be less than Periodic gr
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