PAR extinction in shortgrass ecosystems: effects of clumping, sky conditions and soil albedo

The amount of photosynthetically active radiation (PAR) absorbed by a canopy (APAR) is an important driving variable for vegetation processes such as photosynthesis. PAR extinction in clumped canopies of shortgrass ecosystems is the focus of this paper. Directional gap fractions estimated at peak biomass on several Mexican shortgrass ecosystems with a hemispherical radiation sensor (Li-Cor, LAI-2000) were higher than those predicted by a Poisson model assuming a random leaf dispersion (RLD). LAI-2000-estimated gap fractions, together with independent estimations of plant area index (PAI), and leaf and stem angle distribution (LSAD) were used for estimating the angular course of a leaf dispersion parameter λ(θ). Radiation extinction coefficients simulated for all solar zenith angles using Markov chain processes and estimated λ(θ) were subsequently incorporated in a simple radiative transfer model for estimating the efficiencies of instantaneous and daily integrated PAR interception and absorption, and for studying the effects of clumping, sky conditions and soil albedo on PAR absorption. For clear sky condition, instantaneous PAR absorption showed marked directional effects, therefore indicating that using a constant extinction coefficient in canopy photosynthesis models working at hourly time step would be inaccurate. The effects of clumping, sky conditions and soil albedo were all found to be significant for low PAI, and decreased with higher PAI. As shortgrass ecosystems are characterized by low PAI, neglecting these effects would give inaccurate estimations of PAR absorption. Daily PAR absorption was found to be significantly higher than PAR interception for low PAI, especially when soil albedo was high, and lower than PAR interception for high PAI. These results indicate that in canopy photosynthesis models where APAR is estimated from simple exponential-like relationships calibrated using PAR interception measurements, the PAR available for photosynthesis might be significantly underestimated in the first stages of the growth, and may be overestimated in the later stages of the growing season.