A 2ND-ORDER DYNAMIC-MODEL FOR THE FROST HARDINESS OF TREES

The development of frost hardiness in forest trees is described by a d ynamic model in which the input variables are the prevailing environme ntal conditions and the developmental stage of trees. The assumption o f the model is that for each temperature and photoperiod there is a di screte stationary level of frost hardiness, which is attained if these environmental factors remain constant. The dependence of the stationa ry level on temperature and photoperiod is assumed to be piece-wise li near and additive. The rate of acclimation, i.e. frost hardening or de hardening, is described as a second-order dynamic process with two tim e constants, the second of which changes depending on the stage of the annual development of the trees. The frost hardiness model was calibr ated and tested using experimental data from Douglas fir [Pseudotsuga menziesii var. glauca (Beissn.) France] seedlings. The results suggest that the second-order model describes the changes in frost hardiness better than the first-order model with only one time constant. (C) 199 5 Annals of Botany Company