Increasing stem dimensions of European temperate tree species between 1990 and 2015

Increasing forest CO₂ absorption is ensured by enhanced gross primary production (GPP) which exhibited increasing trends as documented by CO₂ flux measurements or by global vegetation models. Considering the simultaneous increase in ecosystem respiration, it is, however, uncertain how the growing GPP imprints in tree stem biomass increase. There is still a certain discrepancy between estimates of forest biomass trends derived from standardized tree-ring series, information acquired from repeated re-measurements of stem biomass at permanent plots, and information derived from vegetation models or flux-tower measurements.  Standardization procedures of tree-ring series related to age/size trend removal make this data source unique for the assessment of climate-growth relationships and for climatic reconstruction, however it also increases uncertainty of this data source for biomass trends assessment. Here, we present an approach mimicking repeated data collection at permanent plots based on an extensive data set of tree-ring sites.  In this way, we connected two strong benefits of above-mentioned data – reliable age-independent estimates of stem biomass by repeated measurements at permanent plots and a dense network of highly replicated data covering wide environmental gradients provided by tree-ring time series. Our tree-ring network captures core parts of distribution ranges of five main European temperate tree species. Density of tree-ring network is roughly 1 site per 25 km² of forested area in Central Europe namely Czech Republic (area of 78 000 km²) making this tree-ring network probably densest in the world.  We first manipulated original tree-ring data sets by their truncation in 1990 (data set mimicking sample collection in 1990) and then adapt the original data set so that it has similar age structure as the 1990 data set mainly by excluding old age classes (data set 2015) assuring age independency of our data. For both data sets and all sites included, we calculated mean stem diameter at breast height (DBH) of average 100-year old tree based on basal area increments. We then tested for differences in DBH between 1990 and 2015. We found that all species except Pinus sylvestris showed a significant increase in stem dimension as indicated by DBH between 1990 and 2015. The highest DBH increase exhibited Abies alba (+13.5%), followed by Fagus sylvatica (+5.5%), Quercus sp.(+5.2%) and Picea abies (+4.7%).  Differences in DBH between 2015 and 1990 were relatively homogenous across environmental gradients suggesting prevailing influence of large-scale factors independent on local conditions.  Picea abies and Fagus sylvatica exhibited lesser increase in stem dimensions in colder areas. Furthermore, Picea abies and Quercus sp. showed a significant enhancement of growth at productive sites with fast growing individuals. Quercus also significantly enlarged DBH at locations with more positive trends in SPEI, i.e. those experiencing a trend towards wetter climate. Our results corroborate the pervasive growth acceleration in core region of European temperate forests leading to presence of larger canopy-level trees in current forests than in the past. Increasing stem size makes trees more sensitive to disturbances and potentially leads to their shorter life spans as reported in other studies.