BUD DEMOGRAPHY OF THE MOUNTAIN BIRCH BETULA-PUBESCENS SSP TORTUOSA NEAR TREE LINE

The aim of this study was (1) to evaluate the importance of dormant buds for the bud demography of the mountain birch Betula pubescens ssp. tortuosa near the tree line and (2) to study whether sexual reproduction leads to costs for bud production rates. A bud population census was taken in two consecutive years for six branches of each of 90 mountain birch trees. The trees were growing in a common garden and belonged to 10 different progenies originating from different parts of Finnish Lapland. The data were analyzed with matrix population models. The most important transformations of the bud populations were between vegetative short and long shoots. However, if most apparently dead buds are actually latent dormants, they make an even more important contribution to the bud population growth rate than vegetative long and short shoots. Dormant buds may have considerable importance especially after events such as herbivore outbreaks, in which short and long shoots are damaged. Generative long shoots (with male catkins) and short shoots (with female catkins) had approximately the same bud production rate as the corresponding vegetative shoots, i.e., bud populations did not show any major costs due to sexual reproduction. Meristem costs, i.e., a decrease in the number of buds due to sexual reproduction, may be relatively low in mountain birch, because new axillary buds develop and compensate for lost shoot apices. This compensation capacity may be especially well developed under suboptimal conditions, where canopy expansion is limited by the harsh environment rather than by the availability of meristems. The resource cost of reproduction (e.g., in terms of carbon or mineral nutrients) may also be partly compensated especially when flowering intensity is sufficiently low.