WIND FLOW WITHIN OPEN-TOP GROWTH CHAMBERS AND THE GAS-EXCHANGE IMPLICATIONS

Field studies of atmospheric pollutant effects on plants commonly use an open-top plant growth chamber (OTC). Altered wind flow characteristics within an OTC may create aerodynamic and boundary layer transport resistances (r(a) and r(b)) which differ from field values, complicating the extension of OTC results to field conditions. An OTC was placed in an enclosed area and the wind velocity was measured at five heights at nine locations using a triple hot-film sensor. Measurements were made in an empty OTC and at two different surface roughnesses. The flow was turbulent, highly variable with location, and had well-defined vertical and horizontal circulation patterns. Average wind speed in empty OTC ranged from 0.28 to 1.33 m s-1. Increased roughness decreased the mean flow speed but had little effect on the turbulence level or the circulation pattern. The flow within the OTC was used to estimate the r(a) and r(b) for O3 uptake in a hypothetical soybean [Glycine max (L.) Merr.] plant canopy, a representative low stomatal resistance species. Results showed that the variable flow in the OTC results in widely varying r(b) values within the OTC itself, depending on actual physical location within the chamber. In addition, the OTC r(a) and r(b) would differ from field values under typical meteorological conditions. For plants with low stomatal resistance this could create significant differences in gaseous uptake for individual plants within the OTC as well as for the average plant in the OTC vs. that in the field.