Effect of natural atmospheric CO2 fertilization suggested by open-grown white spruce in a dry environment

Evidence of an atmospheric CO2-fertilization effect on radial growth rates was uncovered for open-grown white spruce in a mixed-grass prairie of southwestern Manitoba, Canada. Consistent upward trends of the residuals from dendroclimatic models indicated a decreased ability for climatic parameters to predict radial growth. Despite that a similar amount (61%) of the total variation in radial growth index was explained by climate for both young and old trees, residuals from young trees for the period of 1955-1999 demonstrated a stronger upward trend (R-2=0.551, P < 0.0001) than old trees for the period of 1900-1996 (R-2=0.020, P=0.167). Similar to young trees, the residuals from the early growth period (1900-1929) of old trees also demonstrated a stronger upward trend (R-2=0.480, P < 0.0001) than the period of 1900-1996. Likewise, a comparable period (1970-1999) of young trees also demonstrated a stronger upward trend (R-2=0.619, P < 0.0001) than the early growth period (1900-1929) of old trees. In addition, postdrought growth response was much stronger for young trees (1970-1999) compared with old trees at the same development stage (1900-1929) (P=0.011) or within the same time period (1970-1999) (P=0.014). There was no difference (P=0.221) in drought recovery between the early (1900-1929) period and the late (1970-1999) period within old trees. Together, our results suggest that (1) open-grown white spruce trees improved their growth with time at the early developmental stage, and (2) at the same developmental stage, a greater growth response occurred in the late period when atmospheric CO2 concentration, and the rate of atmospheric CO2 increase were both relatively high. While it is impossible to rule out other factors, these results are consistent with expectations for CO2-fertilization effects.