High-Resolution Measurements of the Temperature Response of Leaf Respiration.

At each field site, replicate branches of sunlit leaves were used to generate high-resolution R–T curves (see Supporting Information for details). In brief, whole replicate leaves from these branches, or shoot segments for conifers and small-leaved species, were placed in a T-controlled, well-mixed cuvette, and allowed to adapt to darkness for 30 min. Leaf cuvettes were T controlled via a thermostatically controlled circulating water bath as in O’Sullivan et al. (27) and Heskel et al. (34), or via a Peltier system (3010-GWK1 Gas-Exchange Chamber; Walz, Heinz Walz GmbH). After the 30-min dark adaption period, the cuvette chamber was cooled to 10 °C. Thereafter, the cuvette chamber was heated continuously at a rate of 1 °C min⁻¹ until a maximum rate of respiration was reached (generally leaf T between 55 and 70 °C), although only data up to T = 45 °C was used in our model. The net release of CO₂ from leaves was recorded at 30-s intervals. Postmeasurement, each replicate leaf was removed from the cuvette, placed in a drying oven at ∼60 °C for a minimum of 2 d, and weighed afterward, so that rates could be expressed on a dry-mass basis (nmol CO₂ g⁻¹ s⁻¹).