Root distribution of 12-year-old forests at rocky sites in southwestern Oregon: effects of rock physical properties

Abstract

Distribution of root length density (in terms of centimetres of fine and large roots per cubic centimetre of soil and bedrock) was analyzed at harsh forest sites supporting 12-year-old stands of pure whiteleaf manzanita (Arctostaphylosviscida Parry), pure ponderosa pine (Pinusponderosa Dougl. ex Laws.), or mixtures of Pacific madrone (Arbutusmenziesii Pursh) and Douglas-fir (Pseudotsugamenziesii (Mirb.) Franco) in southwest Oregon. On such sites, soil (usually <50 cm deep) dries below the wilting point of plants before the middle of the growing season. The bedrock contains enough water to support a dense stand of woody plants. The study shows that about one quarter to one third of the total root length is located in the rock layer. Stepwise procedures were applied to find the best fitting multiple-regression model for relating root length density to depth, bulk density of the soil and rock, and the space penetrable by roots (SPR). Linear expressions of SPR and depth were found to be the only significant explanatory variables. Bulk density had very limited explanatory power, reflecting the high general density of the bedrock containing the water but lower density of the fine material in minuscule fissures in which roots were found.