Submitted to: Agricultural and Forest Meteorology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 1, 2000
Publication Date: N/A
Interpretive Summary: The amount of light absorbed by plants is an important variable for vegetation processes such as photosynthesis. Reliable estimations of light absorption are therefore required in models simulating photosynthesis and plant growth. Some of the methods for estimating light absorption are based on simple relationships between light absorption and foliage density. For some ecosystems, some elements of these relationships are still poorly documented, as well as their variation with environmental variables such as soil or sky conditions. In this paper, this information is estimated for shortgrass ecosystems, and their variations with soil and sky conditions are evaluated. These results are important for improving simulations of plant growth models, which are increasingly used as a tool by ranchers, government agencies, and consultants for rangeland management.
The amount of 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 were higher than those predicted by a Poisson model assuming a random leaf dispersion. Measured gap fractions, together with independent estimations of plant area index (PAI)were used for estimating the angular course of a leaf dispersion parameter. Radiation extinction coefficients simulated for all solar zenith angles using Markov chain processes and the estimated leaf dispersion parameter 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 higher than PAR interception for low PAI, especially when soil albedo was high, and lower than PAR interception for high PAI.