North American temperate forests are a critical component of the global carbon cycle. The response of these forests, which are in different stages of development, to extreme weather events such as drought and heat stresses, and climate variability is not fully understood. In this study, eddy covariance flux measurements in an age sequence (76-, 41-, and 13-years old as of 2015) of pine forests in southern Ontario, Canada are examined to determine the impact of heat and drought stresses and climate variability over 13 years (from 2003 to 2015).
The mean net ecosystem productivity (NEP) values were 199, 506 and 115 g C m-2 year-1 in 76-, 41- and 13-year-old forests, respectively over the study period. The youngest forest became a net carbon sink in the fifth year of its growth. A large decrease in annual NEP was observed during years experiencing heat waves such as 2005 and 2012. Removal of seasonal trends by data normalization indicated that air temperature was a dominant control on summer NEP anomalies, largely because heat stress reduced GEP much more as compared to Re. Summer droughts were a limiting factor for GEP and Re, but had little impact on NEP. Overall, drought stress had the strongest impact on the middle age forest which had the largest carbon sink and water demand. In contrast, young forest was more sensitive to heat stress. The simultaneous occurrence of heat and drought stress in the early growing season such as in 2012 had a drastic negative impact on carbon balance in these forests, suggesting that the combined effect of heat and drought could be enhanced due to plant-soil-atmosphere feedbacks. Further research about forest response to climate stresses should consider the timing of the extreme events, the interaction among heat and drought effects and the stage of forest development.