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.
