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

The amount of photosynthetically active radiation (PAR) absorbed by a canop y (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 bi omass on several Mexican shortgrass ecosystems with a hemispherical radiati on sensor (Li-Cor, LAI-2000) were higher than those predicted by a Poisson model assuming a random leaf dispersion (RLD), LAI-2000-estimated gap fract ions, together with independent estimations of plant area index (PAI), and leaf and stem angle distribution (LSAD) were used for estimating the angula r course of a leaf dispersion parameter lambda(theta). Radiation extinction coefficients simulated for all solar zenith angles using Markov chain proc esses and estimated lambda(theta) were subsequently incorporated in a simpl e 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 directiona l effects, therefore indicating that using a constant extinction coefficien t in canopy photosynthesis models working at hourly time step would be inac curate. The effects of clumping, sky conditions and soil albedo were all fo und to be significant for low PAI, and decreased with higher PAI. As shortg rass ecosystems are characterized by low PAI, neglecting these effects woul d give inaccurate estimations of PAR absorption. Daily PAR absorption was f ound to be significantly higher than PAR interception for low PAI, especial ly 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 i nterception measurements, the PAR available for photosynthesis might be sig nificantly underestimated in the first stages of the growth, and may be ove restimated in the later stages of the growing season. (C) 2000 Elsevier Sci ence B.V. All rights reserved.