Low densities in white pine stands reduce risk of drought-incited decline

Abstract

White pine (Pinus strobus) is commonly found in naturally regenerated even-aged stands on former agricultural lands throughout its range. These stands can be managed for rapid production of high-quality sawtimber, and are thus a valuable timber resource. Yet, periodic growth decline and mortality events have occurred, such as that observed in the late 1990s in southern Maine, USA. The present study uses increment measurements from white pine tree cores collected in the early 2000s to compare increment chronologies in high- and low-mortality stands in southern Maine. Periods and severity of decline were quantified, revealing a growth decline and mortality event that began in 1996 following a late-summer drought in 1995. Further, the sites on which mortality was most pronounced were observed to have soil restrictions resulting in shallow rooting depth, ranging from 19 cm to 32 cm deep. Restrictions included bedrock, lithological discontinuity (loamy cap overlaying sand), and plow pans. Stand stocking was also a predisposing factor; on average, stands that experienced greater mortality during the decline event had higher stocking than those that experienced less mortality. As a consequence, stand densities in declining stands were reduced through mortality to levels more common in nearby non-decline stands; those densities correspond to recommendations for low-density management of white pine (i.e., 330–540 trees ha−1 and 17–25 m2 ha−1 of basal area at 20–30 cm DBH). The smaller diameters of the affected trees indicate the need to give priority to retention of large-diameter trees when thinning pole- and small sawtimber-sized (20–30 cm DBH) white pine stands. This and other studies demonstrate the need for low-density management of pole-size white pine stands to not only maximize growth and value but also reduce the risk of drought-incited decline and mortality on sites with rooting restrictions.