light.response {bigleaf}

Ecosystem Light Response

Description

calculates GPP_ref at a reference (usually saturating) PPFD and ecosystem quantum yield (alpha) using a rectangular light response curve.

Usage

light.response(
  data,
  NEE = "NEE",
  Reco = "Reco",
  PPFD = "PPFD",
  PPFD_ref = 2000,
  ...
)

Arguments

data

Data.frame or matrix containing all required columns

NEE

Net ecosystem exchange (umol CO2 m-2 s-1)

Reco

Ecosystem respiration (umol CO2 m-2 s-1)

PPFD

Photosynthetic photon flux density (umol m-2 s-1)

PPFD_ref

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

Details

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.

Value

A nls model object containing estimates (+/- SE) for alpha and GPP_ref.

Note

Note the sign convention. Negative NEE indicates that carbon is taken up by the ecosystem. Reco has to be 0 or larger.

References

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.