The tree age-diameter relationship and large old trees in natural spruce forest landscapes of boreal Europe

Abstract The age and size of trees are key characteristics in forest ecosystems, and large old trees have significant roles in them. Tree age and size of trees are often decoupled in naturally developed forests, but their relationship is still known only vaguely, for example regarding the strength of their correlation or the co-occurrence of large size and old age. Given their significance, understanding the occurrence of large and old trees is relevant to conservation value assessments in forests. To study the tree age-size relationship in old-growth forests, we collected data from 25 small plots (0.1 ha) in two forest landscapes in northern boreal Europe. The plots were dominated by spruce ( Picea abies ), with birch ( Betula pubescens ) as a secondary tree species. The sites had not been impacted by major disturbances for at least several centuries. We inventoried all trees in the plots with diameter ≥ 10 cm ( n = 1,156) and successfully obtained age estimates from tree ring samples for 991 trees. Diameter explained 31% and 28% of variation in age for spruce and birch, respectively. The variation explained by the models increased to 48% and 46% when the spatial structure of the data (i.e., study landscape and plot) was accounted for. Age varied slightly more in large than in small trees, and outlier trees in terms of the age-size-ratio were always anomalously old in relation to their size. The median age of trees (dbh ≥ 10 cm) was 127 and 156 years in the two study landscapes. Large or old trees (largest or oldest 25th percentile) were present throughout the study landscapes, but their plot-level frequency varied widely (0–24 per plot). There was only partial overlap between large size and old age in the tree populations: about 55% of trees that were either large or old were also the other. The study landscapes consisted mostly of relatively young trees, but old trees (> 200 years) were ubiquitous. Therefore, we propose that the presence of large old trees may be a more viable metric for forest conservation value assessments than forest age, as the latter is, in many cases, infeasible to quantify.