Many studies in ecology and phenology implicitly assume that plants behave in an optimal manner in relation to their environment. This may not be the case for boreal trees in Scandinavia, as their current growing sites were released from ice-cover less than 10 000 years ago. It is unclear if evo-lutive processes of the trees have been rapid enough for full adaptation to the prevailing growing conditions. The aim of this study was to assess if the leaf bud burst timing of two common boreal tree species is optimal. With phenological modelling I estimated the type of timing of bud burst in spring that would maximise the photosynthetic production, and compared the results to those ob-served in Betula spp. and Populus tremula in Finland.

In the study I utilised 93 years of meteorological data, with 3 daily observations (morning, midday and afternoon) of temperature and cloud coverage together with daily minimum temperature. From these data, I calculated the daily potential photosynthetic production as a function of irradiation, day length and temperature.

I used two phenological models to predict the leaf unfolding dates with varying model parameter values. The first was a modified thermal time model in which the bud development starts from a fixed day in spring and proceeds with warm air temperatures. The other, sequential or Sarvas model assumes that the start of spring development is connected to the release of winter dormancy. Both models have shown consistent agreement with the observation data. I then calculated potential photosynthetic production after the predicted leaf unfolding, corrected by a penalty function in the case of air temperatures below a frost damage threshold after the bud burst occurred. The figure shows the potential photosynthetic production (in grams of carbon per m2) as a function of model parameters. The two dots show the parameter values that fir observed data best.

The results indicate that bud burst occurs on average 17 days for Betula spp. and 30 days for Populus tremula later than it might do. Thus the boreal trees lose up to 15% of potential photosynthetic production during the growing season as they are not optimally adapted to prevailing climatic conditions. The incomplete adaptation and seemingly slow adaptation rate of boreal trees bring interesting new aspects to modelling the outcome of climatic warming.