Effects of thermal model and base temperature on estimates of thermal time to bud break in white spruce seedlings

To improve the predictability of bud burst and growth of boreal trees under varying climate, the thermal time for bud break in white spruce (Picea glauca (Moench) Voss) seedlings was evaluated under a range of temperature conditions in controlled environment chambers. Thermal time requirements were calculated as the sum of growing degree days or growing degree hours above base temperatures ranging from -1 to 5 degrees C. The results indicated that the common modeling approach, which uses a high base temperature of 5 degrees C and growing degree days, may not be appropriate for future climatic conditions. Estimates of thermal time requirements using a base temperature of 5 degrees C varied considerably among temperature treatments and thus would reduce the predictability of bud burst under changing climate. In contrast, estimates of thermal time requirements with lower temperatures closer to 1 degrees C were relatively consistent among treatments. Growing degree hour models were less sensitive to base temperature than degree day models. These results should help in the selection of appropriate base temperatures and thermal time models in quantification of thermal time for bud burst modeling in other boreal trees.