The annual increment rings found in the trunks and stems of trees tell numerous stories. Some of these stories are about climate while others are about the life of the tree, but the two are often difficult to disentangle. As a result, previous studies found conflicting results between a strong climate and carbon dioxide response of mid- and high-latitude vegetation in ecosystem models and a weaker or non-existent one in many tree ring chronologies.
Trees grow faster or slower both due to changes in surrounding species from natural processes of aging and thinning and to increases in favorable climate or nutrients. Further, sampling is typically limited to live and dominant trees at any time, further complicating analysis who entire forest growth.
Hember et al.  systematically evaluate sources of biases in estimating tree growth from these rings and develop an approach that attempts to minimize those. By combining forest growth models with a large database of tree ring chronologies, the authors were able to demonstrate that environmental change contributed to significant growth in Canadian black spruce trees.
Citation: Hember, R. A., Kurz, W. A., & Girardin, M. P. . Tree ring reconstructions of stemwood biomass indicate increases in the growth rate of black spruce trees across boreal forests of Canada. Journal of Geophysical Research: Biogeosciences, 124. https://doi.org/10.1029/2018JG004573
—Ankur Rashmikant Desai, Editor, JGR: Biogeosciences