Competition thresholds for the survival and growth of ponderosa pine seedlings associated with woody and herbaceous vegetation

Abstract

Patterns of survival and stem-volume growth for planted seedlings of ponderosa pine (Pinus ponderosa Dougl. ex Laws.) competing with various levels of woody and herbaceous vegetation were derived from three previous studies—one in Oregon and two in Montana. Negative hyperbolic curves of opposite concavity describe the relation between the abundance of woody or herbaceous vegetation and (1) the survival and (2) the stem volume of the pine seedlings. From these curves, two types of competition thresholds for managing forest vegetation are identified: (1) maximum-response threshold—a level of vegetation abundance where additional control measures will not yield an appreciable increase in tree performance; and (2) minimum-response threshold—a level of vegetation abundance that must be reached before additional control measures will yield an appreciable increase in tree performance. The maximum- and minimum-response thresholds for pine stem volume occurred at lower levels of vegetation abundance than those for pine survival. Thus, forest managers may need to consider ponderosa pine survival and stemvolume growth as separate objectives when managing woody and herbaceous vegetation in young plantations. Knowledge of maximum- and minimum-response thresholds also can be used to improve herbicide prescriptions.