Nonlinear increases in water-use efficiency near maximum height: their influence on inferred CO2 response

Tree-ring carbon isotope data show evidence of increasing intrinsic water-use efficiency over the past 150 years. Some of that increase is almost certainly due to rising CO₂ levels in the atmosphere, but the CO₂ signal is often confounded with hydraulic limitations, which also increase water-use efficiency as trees grow taller. Although trees do reach and maintain a maximum height, many require a century or more to do it—and they almost never grow shorter. The hydraulic consequences of this long period of height increase have seldom been accounted for in the rush to attribute isotopic change to CO₂. We have, in our previous work, developed linear corrections for the height increase. However, we now present evidence that the height effect can be exponential, with higher sensitivity near maximum height. Because this occurs in tall, old trees, it is quite different from the "juvenile effect" traditionally removed from the inner rings in tree-ring studies. Maintenance of turgor near maximum height could easily generate such nonlinear effects. We present data from northern Italy, southern Germany, and the western USA that has been collected so as to disentangle the height and CO₂ effects. We present height-corrected inferences of CO₂ effects and call for a renewed focus on the physiology of tall trees.