Growth of mountain birch seedlings in early-successional patches: A year-round perspective

Seedlings of mountain birch (Betula pubescens ssp. czerepanovii), a subarct ic tree, mainly survive and establish in early-successional patches with lo w vegetation cover. In particular, during the first years after seed germin ation, a rapid seedling growth rate is important for winter survival. Seedl ing growth rate is controlled by plant nitrogen (N) concentration. On a yea r-round perspective, the N concentration is influenced by N uptake rate dur ing both summer and winter and by N loss during autumn. The aim of the pres ent study was to evaluate the effects of autumn N loss and winter N uptake for seedling growth during summer. The study used young seedlings growing i n situ in northern Sweden. Since the growth rate of whole plants cannot be measured in situ, it was estimated using a simple, empirical seedling growt h model. The model was based on data from controlled experiments and valida ted using growth data from a field study. The field study included sequenti al seedling harvests which were carried out at two sites differing in altit ude, from autumn 1994 until autumn 1996. The seedling growth model was used to simulate the effects on growth rate of autumn N losses and winter N upt ake. It was found that a decrease in the amount of N lost in autumn and an increase in the amount of N taken up during winter could enhance the growth rate of mountain birch seedlings by the same order of magnitude as an incr ease in growing season soil temperature by 1 to 2 K.