Ten-Year Risk-Rating Systems for California Red Fir and White Fir: development and use

Logistic regression equations predicting the probability that a tree will die from natural causes-insects, diseases, intertree competition-within 10 years have been developed for California red fir (Abies magnifica) and white fir (A. concolor). The equations, like those with a 5-year prediction period already developed for these species, are based on analysis of crown and bole characteristics of trees, and are readily programmed into pocket calculators for field use. For user convenience, the equations are formulated into Award-Penalty Point Systems, in which a tree is awarded or penalized points on the basis of ratings of its crown characteristics. The resulting Risk Point Total is related to the percentage of a hypothetical population of identical trees expected to die within 10 years. Three risk classes (low, medium, and high) are defined and depicted for rapid visual estimation of risk in applications where a more precise estimate is not required. Previous systems for these firs applied only to trees at least 10 inches (25.4 cm) in d.b.h. growing in mature stands. These new systems are applicable to firs down to 4 inches (10 cm) in d.b.h., growing in stands with a wide variety of size and age structure, in northern California. Outside this range, in central and southern California, the systems may be used tentatively, pending testing in these regions.