Effects of insufficient chilling on budburst and growth of six temperate forest tree species in Ontario

Abstract

Insufficient winter chilling related to rising temperatures has raised questions about its potential effects on budburst timing (growth initiation) and subsequent tree growth. We quantified budburst timing and growth of seedlings of six Ontario temperate forest tree species after exposure to different levels of winter chilling using controlled forcing experiments. Based on chilling–forcing relationships, chilling requirements for the species tested ranged from 1000 to 1500 weighted chilling hours. Our findings showed that the species’ chilling needs ranked from highest to lowest were yellow birch (Betula alleghaniensis), eastern larch (Larix laricina), eastern white pine (Pinus strobus), red pine (Pinus resinosa), red spruce (Picea rubens), and eastern white cedar (Thuja occidentalis). Species that needed more chilling to release dormancy generally required less cumulative forcing for budburst. The variations in chilling–forcing relationships among species suggest that they would respond differently to temperature changes, resulting in year to year variation in their relative phenology. Insufficient chilling increased cumulative forcing needed for budburst but only extreme chilling deficiency (< 700 weighted chilling hours) caused abnormal budburst and growth, including reduced leaf and shoot expansion, early and erratic budburst in lower branches and stems, and even lack of budburst after 7 months in optimal growing conditions. Overall, our results suggest that based on climate warming projected for this century, insufficient winter chilling is not likely to occur in Ontario temperate forests in the foreseeable future.