Long-term c i /c a response of trees in western North America to atmospheric CO2 concentration derived from carbon isotope chronologies.

To evaluate how the land carbon reservoir has been responding to the rising CO2 concentration of the atmosphere, it is important to study how plants in natural forests adjust physiologically to the changing atmospheric conditions. Many experimental studies have addressed this issue, but it has been difficult to scale short-term experimental observations to long-term ecosystem-level responses. This paper derives carbon-isotope-related variables for the past 100-200 years from measurements on trees from natural forests. Calculations show that the c i/c a ratios [c i/c a is the ratio of the CO2 concentration (μmol mol-1) in the intercellular space of leaves to that in the atmosphere] of the trees were constant or increased slightly before the 20th century, but changed more rapidly in the 20th century; some increased, some decreased, and some stayed constant. In contrast, the CO2 concentration inside plant leaves increased monotonically for all trees.