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 i n natural forests adjust physiologically to the changing atmospheric condit ions. Many experimental studies have addressed this issue, but it has been difficult to scale short-term experimental observations to long-term ecosys tem-level responses. This paper derives carbon-isotope-related variables fo r the past 100-200 years from measurements on trees from natural forests. C alculations show that the c(i)/c(a) ratios [c(i)/c(a) is the ratio of the C O2 concentration (mu mol mol(-1)) in the intercellular space of leaves to t hat in the atmosphere] of the trees were constant or increased slightly bef ore the 20th century, but changed more rapidly in the 20th century; some in creased, some decreased, and some stayed constant. In contrast, the CO2 con centration inside plant leaves increased monotonically for all trees.