Cold‐blooded forests in a warming world

Leaf temperature influences almost all physiological processes in leaves, and consequently, is one of the most important drivers of carbon, water, and energy fluxes through ecosystems. Plants have many ways of regulating temperature, both actively and passively, to maximize carbon uptake and minimize water loss. Photosynthesis is particularly sensitive to temperature, as it increases with warming to a threshold, and then declines (Berry & Bj€orkman, 1980). Because measuring leaf temperatures can be challenging, many vegetation process models use air temperature to drive plant physiological function (Blum, et al., 2013). Recent studies have argued, however, that air temperature (Tair) is not an appropriate proxy for leaf temperature (Tleaf) (Michaletz et al., 2016; Dong et al., 2017; Fauset, et al., 2018). At the same time, recent analyses of oxygen isotopes (O) in wood cellulose have revealed that photosynthesis is most often occurring at temperatures of c. 21°C, suggesting a remarkably consistent signal of leaf thermoregulation across the globe (Helliker & Richter, 2008; Song et al., 2011). In this issue of New Phytologist, Drake et al. (2020; pp. 1511–1523) link these two distinct fields of study for the first time: the biophysical and the isotopic aspects of leaf thermodynamics. This study provides two important contributions: (1) a case study showing a surprising lack of leaf thermoregulation in whole tree crowns; and (2) the first direct, independent validation of the tree ring O method for reconstructing past leaf temperatures at the whole-tree level. Drake et al. leveraged a unique whole-tree chamber experiment to investigate carbon flux and canopy thermodynamics. Twelve chambers enclosing individual Eucalyptus parramattensis trees were instrumented for continuous measurements of Tair, Tleaf, and tree-level carbon dioxide (CO2) uptake for nine months. A +3°C warming treatment, a shorter-term heat wave treatment, and seasonal variation allowed for a wide range of air temperatures to investigate ( 1 to 45°C). The whole-tree chambers provided the high level of control that glassshouse and growth chamber studies have, while also allowing for continuous carbon and water flux data for full-size trees grown in situ, a scenario that is notoriously difficult to achieve.