Coupled carbon-water exchange of the Amazon rain forest, I. Model description, parameterization and sensitivity analysis

Abstract

Abstract. Detailed one-dimensional multilayer biosphereatmosphere models, also referred to as CANVEG models, are used for more than a decade to describe coupled watercarbon exchange between the terrestrial vegetation and the lower atmosphere. Within the present study, a modified CANVEG scheme is described. A generic parameterization and characterization of biophysical properties of Amazon rain forest canopies is inferred using available field measurements of canopy structure, in-canopy profiles of horizontal wind speed and radiation, canopy albedo, soil heat flux and soil respiration, photosynthetic capacity and leaf nitrogen as well as leaf level enclosure measurements made on sunlit and shaded branches of several Amazonian tree species during the wet and dry season. The sensitivity of calculated canopy energy and CO 2 fluxes to the uncertainty of individual parameter values is assessed. In the companion paper, the predicted seasonal exchange of energy, CO 2 , ozone and isoprene is compared to observations. A bi-modal distribution of leaf area density with a total leaf area index of 6 is inferred from several observations in Amazonia. Predicted light attenuation within the canopy agrees reasonably well with observations made at different field sites. A comparison of predicted and observed canopy albedo shows a high model sensitivity to the leaf optical parameters for near-infrared short-wave radiation (NIR). The predictions agree much better with observations when the leaf reflectance and transmission coefficients for NIR are reduced by 25-40%. Available vertical distributions of photosynthetic capacity and leaf nitrogen concentration suggest a low but significant light acclimation of the rain forest canopy that scales nearly linearly with accumulated leaf area.Evaluation of the biochemical leaf model, using the enclosure measurements, showed that recommended parameter values describing the photosynthetic light response, haveCorrespondence to: E. Simon (simon@mpch-mainz.mpg.de) to be optimized. Otherwise, predicted net assimilation is overestimated by 30-50%. Two stomatal models have been tested, which apply a well established semi-empirical relationship between stomatal conductance and net assimilation. Both models differ in the way they describe the influence of humidity on stomatal response. However, they show a very similar performance within the range of observed environmental conditions. The agreement between predicted and observed stomatal conductance rates is reasonable. In general, the leaf level data suggests seasonal physiological changes, which can be reproduced reasonably well by assuming increased stomatal conductance rates during the wet season, and decreased assimilation rates during the dry season.The sensitivity of the predicted canopy fluxes of energy and CO 2 to the parameterization of canopy structure, the leaf optical parameters, and the scaling of photosynthetic parameters is relatively low (1-12%), with respect to parameter uncertainty. In contrast, modifying leaf model parameters within...

Optimum leaf size predicted by a novel leaf energy balance model incorporating dependencies of photosynthesis on light and temperature

“…a 1 and D 0 are both empirical parameters, and a 1 = 10 and D 0 = 1.5 have been used most frequently in the literature (Leuning 1995;Simon et al 2005). Results obtained with different a 1 are shown in Figs.…”

An interannual assessment of the relationship between the stable carbon isotopic composition of ecosystem respiration and climate in a high-elevation subalpine forest

“…One approach to extrapolating site‐specific studies to regional and global scales involves “process modeling.” General ecosystem and global models of C cycling have been developed [e.g., Running and Gower , 1991; Amthor , 1994; Simon et al , 2005; Atkin et al , 2008] using knowledge obtained from observations at the scale of leaves or soil plots, combined with hierarchical scaling schemes that transfer process knowledge across multiple spatiotemporal scales [ Jarvis , 1995]. An assumption that is inherent in this approach is that physiological and biochemical processes that control CO 2 and H 2 O exchange at small scales (e.g., leaf or stand scale) transfer directly as controls over ecosystem exchange at larger scales.…”

Sensitivity and predictive uncertainty of the ACASA model at a spruce forest site

“…The overall number of the external parameters used within this study is 16, and a few external parameters were held constant, such as the soil type and the measured normalized LAI profile (Fig. 1b, fitted following Simon et al, 2005). Even though the normalized LAI profile was kept the same for all model runs, the absolute LAI value was allowed to vary between 0.5 and 9 m 2 m −2 (see Table 2).…”