Text by Lauri Laanisto
A big part research done nowadays in various plant (eco)physiology labs all over the world is about calibrating different methods and measurement approaches. There are innumerable ways of how to measure some specific function in some specific species or group of species. These have developed during time, vary by countries, labs, persist because of whatever traditions, available infrastructure, price etc.
But how well these standards, or more precisely, the results measured with different standards agree with each other? Even if the method in use is exactly the same, results can differ. And even if the things that are measured are exactly the same, and methods of measurements as well – there can still be such variations in results that opposing hypothesis seem to be supported. (For example the curious case of Samuel Morton´s skull collection.)
Martin G. De Kauwe with 18 colleagues from 17 different institutions, including Ülo Niinemets, tested the one-point method of measuring the maximum carboxylation rate on “564 species from 46 field sites, covering a range of plant functional types” and got some really nice prediction values (r square of 0.98). Thus, there is hope in unifying the standards, unlike the cartoon predicts below…
Citation: De Kauwe, M.G., Lin, Y.S., Wright, I.J., Medlyn, B.E., Crous, K.Y., Ellsworth, D.S., Maire, V., Prentice, I.C., Atkin, O.K., Rogers, A. and Niinemets, Ü., 2015. A test of the ‘one‐point method’for estimating maximum carboxylation capacity from field‐measured, light‐saturated photosynthesis. New Phytologist, doi: 10.1111/nph.13815 (link to full paper)
- Simulations of photosynthesis by terrestrial biosphere models typically need a specification of the maximum carboxylation rate (Vcmax). Estimating this parameter using A–Ci curves (net photosynthesis, A, vs intercellular CO2 concentration, Ci) is laborious, which limits availability of Vcmax data. However, many multispecies field datasets include net photosynthetic rate at saturating irradiance and at ambient atmospheric CO2 concentration (Asat) measurements, from which Vcmax can be extracted using a ‘one-point method’.
- We used a global dataset of A–Ci curves (564 species from 46 field sites, covering a range of plant functional types) to test the validity of an alternative approach to estimate Vcmax from Asat via this ‘one-point method’.
- If leaf respiration during the day (Rday) is known exactly, Vcmax can be estimated with an r2 value of 0.98 and a root-mean-squared error (RMSE) of 8.19 μmol m−2 s−1. However, Rday typically must be estimated. Estimating Rday as 1.5% of Vcmax, we found that Vcmax could be estimated with an r2 of 0.95 and an RMSE of 17.1 μmol m−2 s−1.
- The one-point method provides a robust means to expand current databases of field-measured Vcmax, giving new potential to improve vegetation models and quantify the environmental drivers of Vcmax variation.
De Kauwe MG, Lin YS, Wright IJ, Medlyn BE, Crous KY, Ellsworth DS, Maire V, Prentice IC, Atkin OK, Rogers A, Niinemets Ü, Serbin SP, Meir P, Uddling J, Togashi HF, Tarvainen L, Weerasinghe LK, Evans BJ, Ishida FY, & Domingues TF (2015). A test of the ‘one-point method’ for estimating maximum carboxylation capacity from field-measured, light-saturated photosynthesis. The New phytologist PMID: 26719951