SOIL DRAINAGE CLASS EFFECTS ON EARLY RESPONSE OF BALSAM FIR TO PRECOMMERCIAL THINNING

Precommercial thinning (PCT), a silvicultural treatment commonly used to increase individual tree growth rate and reduce the time required to attain merchantability for young fir (Abies spp.)-spruce (Picea spp.) stands, is both biologically effective and expensive ($300-500/ha in 1993). Forest managers desire to allocate silvicultural capital to maximize return on investment. This study was initiated to identify criteria that could be utilized to rank sites according to response of balsam fir [Abies balsamea (L.) Mill.] to PCT. Sixty-two plots were established in young fir-spruce stands in Maine that had been precommercially thinned between 1977 and 1987. Volume increments 3 yr prior to (VI3PRE) and 3 yr after (VI3POST) PCT were reconstructed from detailed stem analyses of 427 balsam fir trees. Increased radial increment following PCT was visually apparent and significant for each sample tree. The magnitude of the response was strongly affected by soil drainage class. Regression with dummy variables showed that VI3POST, expressed as a function of VI3PRE, increased as soil drainage class improved from poorly (mottling within 10 cm of mineral soil surface) to well (mottling >60 cm below mineral soil surface) and somewhat-excessively drained (loam textured cap overlying deep sand), then decreased for trees growing on excessively drained (sandy-textured) soils. Soil drainage class can be used effectively to rank sites according to expected response to PCT on large land bases.