Growth-ring studies show no growth enrichment in Canadian boreal forests despite 50 years of CO2 enhancement

It has been argued that rising atmospheric CO₂ levels might benefit plants by providing them more substrate for photosynthetic carbon-fixation. However, numerous studies have indicated that other factors interfere with a so-called CO₂-fertilization benefit. Girardin et al. explore recent tree growth by analysing 873 tree-ring chronologies obtains from Canada’s National Forest Inventory. Although they observed species-related and regional differences, there was no strong overall change in growth over the past 60 years. The authors conclude that increased temperatures and effects on soil moisture have overridden any positive CO₂-enrichment effects. They further observe that their data, which measures tree growth directly, shows some significant differences from observations obtained by remote sensing (NDVI: normalized difference vegetation index), supporting the need for continuing ground-based observations. Proc. Natl. Acad. Sci. USA 10.1073/pnas.1610156113