In theory, rising CO2 levels could cause plant growth to increase. Higher levels of CO2 can stimulate growth, as can the associated warmer temperatures. This would provide a welcome negative feedback – some of the manmade CO2 increase would be counteracted by higher rates of photosynthesis.
This month, scientists reported unexpected widespread declines in tree growth in Canadian forests. One of the causes of these declines is heat stress and water stress is another concern. However, not all the declines reported in this study could be attributed to heat or water stress. The study suggests that nutrients may be increasingly limited. Nutrient limitation is known to be a constraint for photosynthesis, and some of the declines in US forests have been recently attributed to progressive nutrient limitation.
Silva, LCR; Anand, M; Leithead, MD (2010) Recent Widespread Tree Growth Decline Despite Increasing Atmospheric CO2 PLOS ONE 5 (7) Number: e11543