A case of severe frost damage prior to budbreak in young conifers in Northeastern Ontario: Consequence of climate change?

In spring 2007, young planted and natural conifers suffered extensive needle and bud injury near Hearst and Kapuskasing in northeastern Ontario. Damage was observed on all species of conifers up to 20 years old and 8 m in height. Taller trees, especially those in the overstory, and young understory trees protected by a closed canopy had less damage. The damage was caused by earlier than normal loss of cold hardiness followed by late spring frosts according to damage observations, weather station data, and calculated thermal time requirements for budbreak and cold hardiness of conifers. During May 19 to 21, 2007, minimum temperatures were recorded between -8°C and -9°C, and before this period, temperatures were warm enough to induce early dehardening and loss of trees' cold hardiness. According to the historical weather data for Kapuskasing, estimated budbreak time has become earlier since 1918 and freezing temperatures during budbreak and shoot elongation (between growing degree days 100 and June 15) have...