Testing for a CO 2 fertilization effect on growth of Canadian boreal forests

The CO 2 fertilization hypothesis stipulates that rising atmospheric CO 2 has a direct positive effect on net primary productivity (NPP), with experimental evidence suggesting a 23% growth enhancement with a doubling of CO 2 . Here, we test this hypothesis by comparing a bioclimatic model simulation of NPP over the twentieth century against tree growth increment (TGI) data of 192 Pinus banksiana trees from the Duck Mountain Provincial Forest in Manitoba, Canada. We postulate that, if a CO 2 fertilization effect has occurred, climatically driven simulations of NPP and TGI will diverge with increasing CO 2 . We use a two‐level scaling approach to simulate NPP. A leaf‐level model is first used to simulate high‐frequency responses to climate variability. A canopy‐level model of NPP is then adjusted to the aggregated leaf‐level results and used to simulate yearly plot‐level NPP. Neither model accounts for CO 2 fertilization. The climatically driven simulations of NPP for 1912–2000 are effective for tracking the measured year‐to‐year variations in TGI, with 47.2% of the variance in TGI reproduced by the simulation. In addition, the simulation reproduces without divergence the positive linear trend detected in TGI over the same period. Our results therefore do not support the attribution of a portion of the historical linear trend in TGI to CO 2 fertilization at the level suggested by current experimental evidence. A sensitivity analysis done by adding an expected CO 2 fertilization effect to simulations suggests that the detection limit of the study is for a 14% growth increment with a doubling of atmospheric CO 2 concentration.