Tree rings provide no evidence of a CO2 fertilization effect in old-growth subalpine forests of western Canada

Abstract

Atmospheric CO₂ concentrations are now 1.7 times higher than the pre-industrial values. Although photosynthetic rates are hypothesized to increase in response to rising atmospheric CO₂ concentrations, results from in situ experiments are inconsistent in supporting a CO₂ fertilization effect of tree growth. Tree ring data provide a historical record of tree-level productivity that can be used to evaluate long-term responses of tree growth. We use tree-ring data from old-growth, subalpine forests of western Canada that have not had a stand-replacing disturbance for hundreds of years to determine if growth has increased over 19^th and 20^th centuries. Our sample consisted of 5,858 trees belonging to five species distributed over two sites in the coastal zone and two in the continental climate of the interior. We calculated annual increments in tree basal area, adjusted these increments for tree size and age, and tested whether there was a detectable temporal trend in tree growth over the 19^th and 20^th centuries. We found a similar pattern in 20^th century growth trends among all species at all sites. Growth during the 19^th century was mostly stable or increasing, with the exception of one of the coastal sites, where tree growth was slightly decreasing; whereas growth during the 20^th century consistently decreased. The unexpected decrease in growth during the 20^th century indicates that there was no CO₂ fertilization effect on photosynthesis. We compared the growth trends from our four sites to the trends simulated by seven Earth System Models, and saw that most of the models did not predict these growth declines. Overall, our results indicate that these old-growth forests are unlikely to increase their carbon storage capacity in response to rising atmospheric CO₂, and thus are unlikely to contribute substantially to offsetting future carbon emissions.