Water vapor plant community

Of the 19 LAII projects 3 are part of the International Tundra Experiment (ITEX), which looks at the response of plant communities to climate change. Three others are concerned with atmosphere processes, including weather patterns affecting snowmelt, Arctic-wide temperature trends, and water vapor over the Arctic and its relationship with atmospheric circulation and surface conditions. Another project deals with the response of birds to climate and sea level change at river deltas, and yet another studies the balance and recent volume changes of McCall Glacier in the Brooks Range. 

Our next task is to discuss concentrations and fluxes within a plant community. When we analyze water vapor and CO2 fluxes from the soil up to the top of plants, we are confronted by the great structural diversity among different types of vegetation. Each plant community has its own unique spatial patterns for water vapor and CO2 concentration. The possibility of many layers of leaves and the constantly changing illumination also greatly complicate the analysis. Even approximate descriptions of the gas fluxes within carefully selected plant communities involve complex calculations based on models incorporating numerous simplifying assumptions. We will consider a cornfield as a specific example. 

Generally, 70 to 75% of the water vaporized on land is transpired by plants. This water comes from the soil (soil also affects the C02 fluxes for vegetation). Therefore, after we consider gas fluxes within a plant community, we will examine some of the hydraulic properties of soil. For instance, water in the soil is removed from larger pores before from smaller ones. This removal decreases the soil conductivity for subsequent water movement, and a greater drop in water potential from the bulk soil up to a root is therefore necessary for a particular water flux density. 

During the daytime, a transpiring and photosynthesizing plant community as a whole can have a net vertical flux density of CO2 (/coz) downward toward it and a net vertical flux density of water vapor (71W) upward away from it into the turbulent air above the canopy. These flux densities are expressed per unit area of the ground or, equivalently, per unit area of the (horizontal) plant canopy. Each of the flux densities depends on the appropriate gradient. The vertical flux density of water vapor, for example, depends on the rate of change of water vapor concentration in the turbulent air, c, with respect to distance, z, above the vegetation ... 

In contrast to the concentration of water vapor, which continuously decreases with increasing distance above the ground, on a sunny day the CO2 concentration generally achieves a minimum somewhere within a plant community (Fig. 9-5). This occurs because both the turbulent air above the canopy and also the soil can serve as sources of C02. During the day, C02 diffuses toward lower concentrations from the soil upward into the vegetation and from the overlying turbulent air downward into the plant community. 

Water has reached equilibrium with its vapor in this ecosphere. Equilibrium also has been reached by the food and waste products of the inhabitants—a carefully balanced community of plants, shrimp, and a hundred or so kinds of microorganisms. 

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