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

Abstract

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. 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 8(2). Radiation extinction coefficients simulated for all solar zenith angles using Markov chain processes and estimated 8(2) 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 3 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.

Identification of a general light use efficiency model for gross primary production

“…The calibration of p shows that temperature has indeed a high influence on , especially in cooler ecosystems, as shown by numerous studies (Runyon et al, 1994;Chen et al, 1999;Nouvellon et al, 2000;Turner et al, 2003;Schwalm et al, 2006). SWC as modulating variable had the highest impact at summerdry sites and grasslands what is not surprising considering the short rooting depth and the low depth at which the SWC-measurements were made.…”

“…Predictions with this model approach obviously require sufficiently long measurement time series covering optimal periods for vegetation growth. The minimum required measurement period of three years presupposed in this study can be critical in this sense (Nouvellon et al, 2000). But this restriction will become less important when FLUXNET measurement time series get longer and are made available for such calibration studies.…”

A conceptual dynamic vegetation-soil model for arid and semiarid zones

“…Several models for specific vegetation physiological processes have been developed at plant or vegetation plot scale. Processes like interception (Calder, 1990;Eltahir and Bras, 1993), transpiration (Mackay et al, 2003;Guswa et al, 2004) and the sensibility to water availability (Nouvellon et al, 2000a;Gracia et al, 2003;Mackay et al, 2003;Rosati and Dejong, 2003) are modelled, but with high parameter requirements.…”

How does the global Moderate Resolution Imaging Spectroradiometer (MODIS) Fraction of Photosynthetically Active Radiation (FPAR) product relate to regionally developed land cover and vegetation products in a semi-arid Australian savanna?

“…Yuri Knyazikhin, 18.11.2008): MODIS FPAR C4 provides FPAR retrievals under diffuse illumination conditions [61]. Diffuse illumination conditions enhance PAR absorption at the leaf and canopy level more than under angular illumination conditions in areas where plants with erectophile leaves dominate (such as grasses, eucalypts or acacias) and with lower plant area index [10,35]. These environmental conditions (erectophile leaves and low PAI) occur in more than 70% of our study area.…”

“…Hence is it problematic to assume little or no contribution from NPV components to absorbed PAR by a canopy or landscape. Increased knowledge and improved radiative transfer modelling (RTM) of PAR absorption at leaf, canopy, pixel, and landscape level in different environments [19,34,35] has led to the development of a metric termed the fraction of absorbed photosynthetically active radiation (fAPAR) [36,37]. The fAPAR is of particular importance, because it provides a direct connection between ecosystem structure and function, including nitrogen use, CO2 assimilation, and water loss [19,38].…”