Effects of interference from Calamagrostis rubescens on size distributions in stands of Pinus ponderosa.
P. ponderosa was grown as widely-spaced trees in a hexazinone-treated monoculture and in mixture with the grass Calamagrostis rubescens for 4 growing seasons on a forest site in NW Montana, USA. Interference from C. rubescens reduced biomass production of P. ponderosa. Weights of foliage, of above-ground woody tissue, and of roots were respectively 430%, 450%, and 460% greater in the monoculture than in the mixture. The frequency distribution of stem volumes in the monculture became positively skewed after 2 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. The size distribution became positively skewed more quickly, and remained more highly skewed, in the mixture than in the monoculture. 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.