Here, we use annually resolved δ13C tree rings measurements across a NE-USA forest network to derive intercellurar CO2 (ci) response caused by increasing atmospheric CO2 (ca) trends and climate change over the past two decades, concurrent with direct long-term measurements of ecosystem carbon and water exchange. We find a substantial increase in ci suggesting that for each one 1 ppm increase in ca, ci increased proportionally or at the same rate. This response corresponds to static or moderate increase in water use efficiency-the ratio of carbon gain to water loss, respectively.
The observed ci trends are consistent with a regional increase in precipitation magnitude and intensity. These results suggest a weak stomatal regulation in response to continuously increasing ca, and a higher photosynthetic assimilation (uptake) of CO2. The trends are associated with increasing ecosystem level carbon uptake and forest productivity. Our findings challenge the hypothesized CO2 fertilization effect documented at the ecosystem level.