Photosynthetically active radiation (Q)-use efficiency (F) is an important parameter for deriving carbon fluxes between forest canopies and the atmosphere from meteorological ground and remote sensing data. A common approach is to assume gross primary production (P,) and net primary production (P-n) are proportional to Q absorbed by vegetation (Q(abs)) by defining the proportionality constants epsilon(Pg) and epsilon(Pn) (for P-g and P-n respectively). Although remote sensing and climate monitoring provide Q(abs) and other meteorological data at the global scale, information on c is particularly scarce in remote tropical areas. We used a 16-month continuous CO2 flux and meteorological dataset from a mountainous tropical rain forest in central Sulawesi, Indonesia to derive values of epsilon(Pg). and to investigate the relationship between P-g and Q(abs). Absorption was estimated with a 1D SVAT model from measured canopy structure and short wave radiation. The half-hourly P, data showed a saturation response to Q(abs). The amount Of Q(abs) required to saturate P-g was reduced when water vapor saturation deficit (D) was high. Light saturation of P-g was still evident when shifting from half-hourly to daily and monthly time scales. Thus, for a majority of observations, P-g was insensitive to changes in Q(abs). A large proportion of the observed seasonal variability in P-g could not be attributed to changes in Q(abs) or D. Values of epsilon(Pg) varied little around the long-term mean of 0.0179 mol CO2 (mol photon)(-1) or 0.99 g C MJ(-1) (the standard deviations were +/- 0.006 and +/- 0.0018 mol CO2 (mol photon)(-1) for daily and monthly means, respectively). In both cases, c p. values were more sensitive to Q(abs) than to daytime D. These findings show that the current epsilon-approaches fail to predict P-g at our tropical rain forest site for two reasons: (1) they neglect saturation of P-g when Q(abs), is high; and (2) they do not include factors, other than Q(abs) and D, that determine seasonality and annual sums of P-g.
Tree Physiology, 2008, Vol 28, Issue 4, p. 499-508