3.11. Leaf dark respiration

Characterising leaf dark respiration (Rleaf) in a fashion that enables comparison among species and especially among sites and times of year is challenging, given the sensitivity and acclimation of Rleaf to temperature. However Rleaf under typical field conditions is valuable because it is both a measure of basal metabolism and a rough correlate of average realised night-time respiratory C flux. Rleaf scales with other metabolic, structural, chemical and longevity aspects of the leaf economic spectrum and, along with those other variables, enables scaling to canopy processes of whole ecosystems.

What, when and how to measure?

Sample young to medium-aged fully expanded leaves (see Section 3.1) to ensure negligible respiration associated with biosynthesis (‘growth respiration’). Do not make measurements during or soon after atypical conditions (such as e.g. heat or cold stress, water stress), unless that is the focus of the research. Sample foliage from parts of the canopy sunlit during daytime, unless one is specifically focussed on the shaded understorey taxa. If possible, measure intact leaves at night. In any case, leaves must have been in the dark for ~30 min to minimise variation resulting from very recently fixed photosynthate or transient light-induced respiratory CO2 losses.

Any reliable leaf gas-exchange system that can control leaf temperature can be used. If possible, it is best to measure intact foliage. Detached leaves should be kept moist, cool (to minimise C and water loss), and in the dark until measurement. If possible, tests of intact v. detached foliage should be made for a subsample, to ensure that similar rates are observed. See under Section 3.10 for any subsequent leaf handling.

Leaf dark respiration (Rleaf) can be measured while measuring photosynthetic rate (see Section 3.10), merely by turning off, or shielding, the chamber completely from incident light. However, flux rates for Rleaf are roughly an order of magnitude lower than those for Amax and, therefore, the signal to noise ratio of the typical portable photosynthesis system may be suboptimal for taxa with lower flux rates. One can reduce flow rates and/or increase the amount of foliage into the chamber, to alleviate this problem; however, this is not always sufficient to obtain reliable measurements. In such cases, using specialised chambers (which may hold more foliage) and/or choosing a standardised temperature that is at the high rather than low end of the candidate range (next paragraph) can help ameliorate this problem by increasing the flux rate.

For comparative Rleaf measurements, one typically chooses a standardised temperature appropriate for the site conditions (e.g. 25°C in the tropics, 20°C in the temperate zone, 10°C or 15°C in cold boreal or summer tundra conditions). However, because cross-study comparisons are often made of taxa grown in and/or measured under different temperatures, instantaneous temperature response functions can help in calibrating respiration across temperature regimes. Where possible, measure (subsets of) leaves at appropriate contrasting measurement temperatures, with 10°C intervals, or ideally at least four different temperatures over a 15–35°C range.

References on theory, significance and large datasets: Reich et al. (1998, 2008); Tjoelker et al. (2001); Wright et al. (2004); Atkin et al. (2005); Rodriguez-Calcerrada et al. (2010).