Effects of Interference from Calamagrostis rubescens on Size Distribution in Stands of Pinus ponderosa

SUMMARY (1) Pinus ponderosa Dougl. was grown as widely-spaced trees in monoculture and in mixture with the grass Calamagrostis rubescens Buckl. for four growing seasons on a forest site in north-western Montana, U.S.A. Interference from C. rubescens reduced biomass production of P. ponderosa. Weights of foliage, of above-ground woody tissue, and of roots were 430%, 450%, and 460% greater in the monoculture than in the mixture. (2) The frequency distribution of stem volumes in the monoculture became positively skewed after two growing seasons. Therefore, dominance and suppression associated with above-ground competition between trees are not necessary for the development of skewed size distributions in stands of P. ponderosa. (3) The size distribution became positively skewed more quickly, and remained more highly skewed, in the mixture than in the monoculture. (4) Interference from C. rubescens decreased the mean growth rate of stem volume in P. ponderosa, regardless of initial size class, but simultaneously increased the relative variation in growth rate. The greater variation in growth rate more than offset the tendency of slower growing stands to have more symmetrical size distributions.