Predicting forest dieback in Maine, USA: a simple model based on soil frost and drought

Tree roots of northern hardwoods are shallow rooted, winter active, and minimally frost hardened; dieback is a winter freezing injury to roots incited by frost penetration in the absence of adequate snow cover and exacerbated by drought in summer. High soil water content greatly increases conductivity of frost. We develop a model based on the sum of z-scores of soil frost (December-February) and drought in summer (May-September) that accurately predicts timing and severity of dieback on sugar maple (Acer saccharum Marsh.), Betula spp., Fraxinus spp., and red spruce (Picea rubens Sarg.) in Maine, USA, during 1920-1995. Discrepancies between predicted and actual dieback relate largely to tree population dynamics. Extraordinary levels of dieback on Betula during 1935-1950 occurred under low stress but in mature populations; under high climate stress during 1951-1975, younger surviving Betula proved resistant and showed little dieback. Severe dieback in Acer stands during 1975-1995 may relate to extensive partial cutting, leaving stands open to frost and high water tables. Widespread use of forwarder feller/buncher and wheeled skidders after 1960 commonly left hummocky, irregular surfaces subject to pooling and accelerated frost penetration. Silvicultural applications include modifying harvesting practices to minimize frost complications and use of the model to track and map climatic risk to avoid conditions and locales subject to dieback.