Measuring water use efficiency of Eucalypt trees with chambers and micrometeorological techniques

Enclosure appears to be the only feasible way to examine the gas exchange of small groups of trees or to answer questions about the effects of increased atmospheric CO₂ on the assimilation, evaporation and water use efficiency of forests. To be effective, enclosures must necessarily change the microclimate, but few studies have been made of the consequences. In this paper, the assimilation, evaporation and water use efficiency of a community of Eucalyptus trees inside a ventilated chamber are compared with the same attributes for the surrounding forest. Assimilation and evaporation for the chamber were measured by the depletion in CO₂ and the enrichment in water vapour of air passing through the chamber. For the forest, assimilation and evaporation were determined by micrometeorological techniques based on the energy balance, and for CO₂, additional chamber measurements of the soil effiux. Water use efficiencies were calculated as the ratio of mol CO₂ assimilated to mol water evaporated. There are some important microclimatic differences between chamber and forest: net radiation is reduced by about 30% in the chamber, the vapour pressure deficit of the chamber air is lower, and the light climate there tends to be diffuse rather than direct. Despite these differences, evaporation rates for both chamber and forest were generally similar, perhaps due to compensating effects in the chamber from higher boundary layer conductances (because of greater ventilation rates) and higher stomatal conductances (because of increased humidity). However, assimilation rates and water use efficiencies were markedly different for the two communities in clear sky conditions, with higher values of both being recorded in the chamber for most of the daylight hours. Only on cloudy days, when the light climate was diffuse in both chamber and forest, were similar assimilation rates and water use efficiencies observed. This behaviour seems to be attributable in part to the light climate in the chamber being predominantly diffuse and that in the forest predominantly direct. Diffuse light enhances the photosynthesis of lower leaves in the canopy. This contention is supported by model calculations of canopy assimilation under diffuse and direct radiation which produced qualitatively the same light response functions as observed for chamber and forest. The study suggests that the use of chambers for exploring questions of forest productivity and water use efficiency must be circumspect. The act of enclosure, by itself, can change the daily water use efficiency of the tree community by as much as 50%.