calculates GPP_ref at a reference (usually saturating) PPFD and ecosystem quantum yield (alpha) using a rectangular light response curve.
light.response(data, NEE = "NEE", Reco = "Reco", PPFD = "PPFD",
PPFD_ref = 2000, ...)Data.frame or matrix containing all required columns
Net ecosystem exchange (umol CO2 m-2 s-1)
Ecosystem respiration (umol CO2 m-2 s-1)
Photosynthetic photon flux density (umol m-2 s-1)
Reference PPFD (umol m-2 s-1) for which GPP_ref is estimated. Default is 2000 umol m-2 s-1.
Additional arguments to nls
A nls model object containing estimates (+/- SE) for alpha and GPP_ref.
A rectangular light response curve is fitted to NEE data. The curve takes the form as described in Falge et al. 2001:
$$-NEE = \alpha PPFD / (1 - (PPFD / PPFD_ref) + \alpha PPFD / GPP_ref) - Reco$$
where \(\alpha\) is the ecosystem quantum yield (umol CO2 m-2 s-1) (umol quanta m-2 s-1)-1, and GPP_ref is the GPP at the reference PPFD (usually at saturating light). \(\alpha\) represents the slope of the light response curve, and is a measure for the light use efficiency of the canopy.
The advantage of this equation over the standard rectangular light response
curve is that GPP_ref at PPFD_ref is more readily interpretable
as it constitutes a value observed in the ecosystem, in contrast to
GPP_ref (mostly named 'beta') in the standard model that occurs at infinite light.
PPFD_ref defaults to 2000 umol m-2 s-1, but other values can be used. For
further details refer to Falge et al. 2001.
Falge E., et al. 2001: Gap filling strategies for defensible annual sums of net ecosystem exchange. Agricultural and Forest Meteorology 107, 43-69.
Gilmanov T.G., et al. 2003: Gross primary production and light response parameters of four Southern Plains ecosystems estimated using long-term CO2-flux tower measurements. Global Biogeochemical Cycles 17, 1071.
Reichstein M., Stoy P.C., Desai A.R., Lasslop G., Richardson A. 2012: Partitioning of net fluxes. In: Eddy Covariance. A practical guide to measurement and data analysis. Aubinet M., Vesala T., Papale D. (Eds.). Springer.