Scaling CO2-photosynthesis relationships from the leaf to the canopy

Abstract

Responses of individual leaves to short-term changes in CO2 partial pressure have been relatively well studied. Whole-plant and plant community responses to elevated CO2 are less well understood and scaling up from leaves to canopies will be complicated if feedbacks at the small scale differ from feedbacks at the large scale. Mathematical models of leaf, canopy, and ecosystem processes are important tools in the study of effects on plants and ecosystems of global environmental change, and in particular increasing atmospheric CO2, and might be used to scale from leaves to canopies. Models are also important in assessing effects of the biosphere on the atmosphere. Presently, multilayer and big leaf models of canopy photosynthesis and energy exchange exist. Big leaf models - which are advocated here as being applicable to the evaluation of impacts of 'global change' on the biosphere - simplify much of the underlying leaf-level physics, physiology, and biochemistry, yet can retain the important features of plant-environment interactions with respect to leaf CO2 exchange processes and are able to make useful, quantitative predictions of canopy and community responses to environmental change. The basis of some big leaf models of photosynthesis, including a new model described herein, is that photosynthetic capacity and activity are scaled vertically within a canopy (by plants themselves) to match approximately the vertical profile of PPFD. The new big leaf model combines physically based models of leaf and canopy level transport processes with a biochemically based model of CO2 assimilation. Predictions made by the model are consistent with canopy CO2 exchange measurements, although a need exists for further testing of this and other canopy physiology models with independent measurements of canopy mass and energy exchange at the time scale of 1 h or less.

Environmental controls on the photosynthesis and respiration of a boreal lichen woodland: a growing season of whole-ecosystem exchange measurements by eddy correlation

“…We parameterized the carbon assimilation module of the Amthor model for the Schefferville site using field observations from a nearby stand, and compared model results with the eddy flux measurements. Our modeling goals in this study were to (1) test the model of canopy C02 assimilation (Amthor 1994) at the Schefferville site, (2) determine the physiological and environmental factors regulating C02 exchange, and (3) assess the sensitivity of gross ecosystem productivity to cloudiness and temperature.…”

Testing a Mechanistic Model of Forest-Canopy Mass and Energy Exchange Using Eddy Correlation: Carbon Dioxide and Ozone Uptake by a Mixed Oak-Maple Stand

“…Global shortwave irradiance is then partitioned between direct beam and diffuse components according to Erbs et al (1982) and direct beam and diffuse radiation are each divided into photosynthetically active radiation (PAR) and near-infrared radiation (NIR) wave bands based on solar elevation (after Szeicz 1974). (2) A simple multilayer model of NIR and PAR absorption by the canopy that accounts for individual leaf and forest floor reflectance and absorptance of NIR and PAR (Amthor 1994a Sinclair et al 1976) and the soil by a horizontally uniform slab. Environmental and biological parameters that govern big-leaf physiology are listed and defined in Table 1, variables predicted by the big-leaf portion of the model are defined in Table 2, and the structure of the model is shown in Table 3.…”

“…Except for the substitution of a humidity term for the leaf water potential term, gs is related to Ps as before (Amthor 1994a) as follows where 6 is a signal (0-1) from roots pertaining to soil water status that partially controls stomatal conductance (see, e.g., Gollan et al 1986); gS(,in) is the minimum stomatal conductance, i.e. the value with closed stomata (mol H20 m-2 leaf s-I); kstoma is an empirical coefficient; Ci is the intercellular CO2 partial pressure (Pa); and exp (x) means 8 .…”

Energy, water, and carbon fluxes in a loblolly pine stand: Results from uniform and gappy canopy models with comparisons to eddy flux data

“…However, the computational need for ecosystem models with multilayer representation of vegetation canopies and the challenge to provide model parameters for each layer make such models practically impossible for operational use over a heterogeneous landscape. Although it is possible in principle to test multilayer models with field data, this is difficult in practice [Amthor, 1994].…”