Predicting spring phenology and frost damage risk of Betula spp. under climatic warming: a comparison of two models

Timing of bud burst and frost damage risk for leaves of Betula spp. in resp onse to climatic warming in Finland was examined with two models. In the fi rst model, ontogenetic development in spring was triggered by an accumulati on of chilling temperatures. The second model assumed an additional signal from the light climate. The two models gave radically different estimates o f frost damage risk in response to climate warming. The chilling-triggered model forecast a significant and increasing risk with increased warming, wh ereas the light-climate-triggered model predicted little or no risk. The chilling-triggered model is widely applied in phenological research; ho wever, there is increasing experimental evidence that light conditions play a role in the timing of spring phenology. Although it is not clear if the light response mechanisms are appropriately represented in our model, the r esults imply that reliance on a light signal for spring development would a fford a degree of protection against possible frost damage under climate wa rming that would not be present if chilling were the sole determinant. Furt her experimental tests are required to ascertain the light-related mechanis ms controlling phenological timing, so that credible model extrapolations c an be undertaken.