Canopy uptake

In concentrations approximating present air quality standards (Table III), O3 or SO2 in combination with NO2 could measurably suppress CO2 uptake rates of sensitive plants if exposed under favorable growing conditions. In the controlled environmental chamber studies, 1-hr exposures to 10 pphm O3 (which is slightly above the primary and secondary standards — i.e., 8 pphm for 1 hr) for example, depressed alfalfa CO2 absorption rates by approximately four percent. Exposures to 15 pphm hr SO2 in combination with an equal amount of NO2 reduced uptake rates by 7 percent. Alfalfa, barley or oat canopies exposed to these pollutants singly required higher concentrations (i.e., 1- to 2-hr treatments with more than 20 pphm SO2 or 40 pphm NO2) to measurably reduce canopy uptake rates. 

Figure 12 Interactions of soil emissions of NO with O3 in plant canopies and NO, uptake by vegetation in determining the net exchange of NO between soil-plant and the atmosphere.

HilF reported pollutant uptake values for a number of gaseous pollutants, including ozone and PAN, with alfalfa as his test organism (Table 11-26). These values were obtained with a dynamic, but closed, exposure facility. Uptake was determined by the amount of pollutant needed to maintain a constant chamber concentration over an alfalfa bed. Uptake values, expressed on the basis of leaf area, reflect the effect of the plant canopy on the exchange of gases within the canopy and do not... 

At least six major phytotoxic air pollutants have been shown to reversibly inhibit apparent photosynthetic rates in plants (1 - ). Studies indicate that these phytotoxicants ranked in the following order according to the relative amount of inhibition effected after several hours of exposure to equal pollutant concentrations HF>Cl2-03>S02>N02>N0. A summary of the experimental results which compares measured depressions in CO2 uptake rates of barley and oat canopies after 2-hr pollutant exposures in environmental chambers appears in Figure Typical inhibition and recovery rate curves for exposures that reduced CO2 absorption rates by 20 percent at the end of the 2-hr fumigations are also shown. Similar data have been obtained for alfalfa, another important crop species which was cultured and exposed under identical conditions In contrast, equivalent... 

The possible effects of carbon monoxide on alfalfa canopies have also been tested but CO did not measurably depress CO2 uptake rates when present in concentrations ranging up to 80 ppm in the fumigant (3). 

Although information concerning the inhibition of photosynthesis by air pollution is limited, we may gain perspective into the potential problem through appraising available data on the extent that CO2 uptake by oats, barley, and alfalfa canopies can be suppressed by short-term (a few hours) exposure to the major air pollutants and simple combinations investigated. 

Figure 2.21 schematically depicts the dry deposition of a pollutant to a typical surface in the form of resistances (Lovett, 1994 Wesely and Hicks, 1999). In this case, the surface resistance rsurf has been broken down even further into a combination of parallel and series resistances (rs, rm, rct, rsoil, rwa(cl, etc.). Since leaves may absorb pollutants either through stomata or through the cuticles, the absorption into the leaf is represented by two parallel resistances, rcl for the cuticular resistance and rs for the stomatal resistance, which is in series with a mesophyllic resistance rm. Also shown are resistances for uptake into the lower part of the plant canopy and into water, soil, or other surfaces. 

Figure 5. A photograph of the fast response ln-sltu CO2 sensor monitoring carbon uptake by a soybean canopy. The open, folded optical absorption cell allows for fast sample exchange. The surrounding instruments measure air flow and humidity variations.

We will use representative values of Jcoz to calculate the decreases in the concentration of C02 that can occur over a certain vertical distance in the turbulent atmosphere above vegetation. When a net C02 uptake is occurring, the flux density of C02 is directed from the turbulent air downward into the canopy. JCOz above the vegetation is then negative by our sign convention, which means that c1 increases as we go vertically upward (Jcoi = - coz coa/ Eq. 9.3). This is as we would expect if C02 is to be... 

E. The minimum in occurs at the level corresponding to the cumulative uptake of all the CO2 coming down into the canopy (15.4 pmol m-2 s-1) net uptake of... 

The first 2-m layer thus takes up 10.7 (imol m-2 s 1 and the second layer one-half of this, or 5.3 pmol m-2 s-1. Thus, at 4 m from the top of the canopy, the total uptake is 16.0 pmol m 2 s 1, which is slightly more than the flux of C02 from the air above the canopy. Therefore, will be minimal at slightly above 12 m from the ground. 

Piedade, M. T. F, S. P. Long, and W. J. Junk. 1994. Leaf and canopy CO2 uptake of a stand of Echinochloa polystachya on the Central Amazon floodplain. Oecologia. 97 159-174. 

A more detailed model is shown in this Fig. 5b. The throughfall Fa of Fig. 5a is replaced by the partial fluxes stem flow (Fu) and canopy drip (Fu). There are also included additional fluxes as Utter fall F e, dry deposition to soil Fse and internal fluxes of the system as uptake by roots and flow to the crown Fee, leaching of leaves Fse and flow from roots to soil F9e. 

The focus of research in terrestrial canopies has been on the vertical exchange between a canopy and the overlying atmosphere, i.e. at the scale of the canopy height, h. In the study of aquatic systems, however, there is interest in processes occurring at several scales both smaller and greater than h - from the stem-scale up to the marsh-scale. For example, the details of flow around an individual stem or leaf sets the length-scale of the diffusive boundary layer that, in many instances, controls the uptake of nutrients... 

The iron is especially important. In freshwater ecosystems, fluxes of hydrogen sulfide are also relatively small owing to the lack of sufficient sulfate as a substrate for dissimilatory reduction and to the relatively greater incorporation of the available sulfur into biomass. However, the release of hydrogen sulfide is significant from wetlands. In addition, H2S emission from plant canopy occurs when S plant uptake is in excess of biosynthetic demands. The latter process may account for as much as 40% of total natural S emission. 

Chamber experiments on uptake by plants provide useful information for the determination of deposition velocities, however, the deposition velocities over forest canopies will be different to those calculated in the chamber experiments. Deposition velocities take into account not only the resistance to mass transfer due to the plant itself, but also the resistance due to the forest canopy stmcture, and the turbulence over the canopy. 

Figures 7c and 7d show the result of the forward calculation, in which the canopy profiles of q (the CO2 concentration) and (for "C) were inferred from

Biological exchange processes in the forest canopy are absorption or leaching, e.g., irreversible stomata uptake or leaching of elements originating from root uptake. 

The amount of sulfur found in stand precipitation is significantly greater than in bulk precipitation, because forests collect atmospheric sulfur much more efficiently than do bulk collectors. In contrary to sulfur, nitrogen fluxes to bulk collectors may exceed the fluxes in stand precipitation. This is due to the uptake of nitrogen by the canopy. 

Throughfall fluxes of N tend to be lower than atmospheric inputs because of uptake of atmospherically deposited N in the canopy. The inorganic nitrogen flux in throughfall will be underestimated in the order of 0.2-0.5 g N m a . Total deposition of nitrate and also ammonium can be estimated via throughfall monitoring only during winter conditions, when the trees are less active. 

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