The light-saturated photosynthetic rate (Amax) under typical field conditions, usually expressed in µmol m–2 s–1 or nmol g–1 s–1, or both, is a valuable metric as a measure (or at least as an index) of metabolic capacity and a factor determining average realised photosynthetic rate (for upper-canopy foliage). Amax scales with other structural, chemical and longevity aspects of the leaf economic spectrum and, along with those other variables, enables scaling to canopy processes of whole ecosystems. Simultaneous measures of leaf water-vapour conductance are typically made in concert with the photosynthetic measurements.
What, when and how to measure?
Sample young, fully expanded leaves (see Section 3.1). These should be from sunlit parts of the canopy, unless specifically focusing on the shaded taxa of understorey. Measure leaves only if they have been in sufficiently high light just before measurement (e.g. direct sun for 5–10 min) to minimise concerns about leaf induction status or stomatal closure as a result of shading (see Section 3.1. for discussion).
Because realised photosynthesis is less than maximal because of a host of factors, including low or high temperatures, limited soil moisture or air humidity, negative leaf water potential, and source–sink inhibition, among others, care must be taken in choosing the time of year, time of day, and general conditions under which measurements can be made. Some knowledge of gas-exchange responses of the taxa under study will be essential. Do not make measurements during or just following (days to weeks) periods of severe water deficit, or unusual temperatures. Do make measurements on days when soil moisture, plant water status, air humidity, irradiance and temperatures are near optimal for the taxa in question. Measurements in most ecosystems should be made at mid- to late morning (e.g. from 0800 hours to 1100 hours local time) under non-limiting vapour-pressure deficits or temperatures. This minimises the risk of sampling during midday and afternoon declines in gas-exchange rate as a result of stomatal closure, source–sink inhibition or other causes. If a given morning, or mornings in general, are cold relative to photosynthetic temperature optima, measurement can be made later in the day. Because most published measurements have been made under ambient CO2 concentrations, that would be recommended. If rates can also be measured under saturating CO2 concentrations, that is also useful.
Any reliable leaf gas-exchange system can be used, and conditions in the chamber can be either set at levels considered optimal or left to track the in situ conditions (which need to be near optimal). If possible, measure intact foliage or else leaves on branches cut and then re-cut underwater. In the latter case, check whether given individuals fail to stay hydrated. Conduct some test comparisons of gas exchange on intact and ‘re-cut’ branches, to ensure the technique works for your taxa and system. Measurements can also be made on detached foliage; however, this requires even greater attention. Leaves should be measured within seconds, to a few minutes at most, after detachment, and tests of intact v. detached foliage should be made for a subsample, to ensure similar rates are observed.
If possible, the leaf material inside the chamber should be collected (see Section 3.1), measured for LDMC and SLA, and stored for any subsequent chemical analyses.
References on theory, significance and large datasets: Reich et al. (1992, 1997, 1999); Wright et al. (2004).
More on methods: Wong et al. (1979, 1985a, 1985b, 1985c); Reich et al. (1991a, 1991b); Ellsworth and Reich (1992).