Biomass allocation and equations for silver birch (Betula pendula) and downy birch (Betula pubescens) in Estonia

The values of the fractional distribution of aboveground (AGB) and belowground biomass (BGB), root/shoot ratios (R/S) and stemwood density were estimated for silver (Betula pendula) and downy birch (Betula pubescens) along site index and age gradients for Estonia. Biomass models were used, in which the diameter at breast height (D-1.3, measured above root collar), tree height, age and site index served as independent variables. The models for AGB and BGB were elaborated based on the data of 117 and 30 model trees, respectively. Strong allometric relationships were revealed between D-1.3 and total AGB (with foliage), stem mass and BGB. For trees with D-1.3 < 15 cm, individual models predicted similar biomass patterns for both species, for larger trees, species-specific models are suggested. The relative share of stem mass in AGB was 80-90% for all site index and age classes, being significantly higher for silver birch. The average relative share of stembark, branches, leaves and the R/S, were significantly higher for downy birch. Mean oven-dry mass stemwood density for silver and downy birch was 0.573 g cm(-3), there was no statistically significant difference between the species. The mean R/S ratio was 0.29 and 0.43 for silver and downy birch, respectively.