Predicting Effects of C02 Enrichment with Simulation Models and a Digital Computer

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A simulation model modified from one described by Duncan, et al was used to evaluate the variable factors that affect rates of photosynthesis in the field with particular emphasis on the carbon dioxide profile existing within the plant canopy. The photosynthetic assimilation rates were taken from data by Hesketh. The paper also deals with computing the carbon dioxide profile that might be expected under several conditions of air movement and photosynthetic assimilation.

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Humans have been struggling with the plant environment since they started sowing seeds and cultivating vegetation for food. Early attempts were primarily to keep the plants alive. As time passed, trial and error showed that certain conditions favored growth and production. With this knowledge came cultural practices for each species, which were often mixtures of folklore, superstition and fact. Many of these early growing methods have been passed down to recent times.
A model to simulate the effect of increased carbon dioxide levels on the gas exchange by a crop canopy was developed. The distribution of carbon dioxide in the canopy, released from a ground level area source, was calculated by a method reported elsewhere. In this submodel, the wind speed in the crop surface boundary layer, the CO2 concentration at the upwind edge of the release area, the CO2 release rate, and the aerodynamic canopy parameters (crop height, zero-plane displacement and roughness length) were the inputs. The radiant energy distribution inside the canopy was computed by a modified Duncan-Stewart submodel, using leaf architecture and optics as inputs. Using the outputs of the submodels and the air temperature, dewpoint and effective soil water potential as inputs, a leaf gas and energy exchange submodel calculated CO2 and water vapor fluxes for each leaf layer, each characterized by physiological parameters that were assumed constant with height.
Plants have a remarkable ability to cope with diverse environments. However, the whims of man can lead to plants being so misplaced that environmental modification may be a useful enterprise for improving productivity. Unfortunately, the potential for modifying canopy environments on large scales under drought conditions is limited. The canopy radiation balance can be modified with misting, crop covers or albedo changes, and canopy temperatures modified by area fogging to cool the air or intermittent sprinkling to cool the leaves. Perhaps the most common environmental modification derives from windbreaks, which reduce air movement. The last promising method for large areas is CO2 enrichment.Comprehensive plant-environment models can be valuable tools for assessing the impact of various environmental modification techniques because of the complexity of plant-environmental relations. Two examples of model applications are: (a) CO2 enrichment; and (b) retarding spider mite development with irrigation. The results suggest that field CO2 enrichment is not practical. Retarding spider mite development by 3 to 6 days may be possible.
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