Importance of Aerodynamic Resistance to Water Use Efficiency in Three Conifers under Field Conditions

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Abstract
The quantitative importance of aerodynamic resistance to H2O vapor and CO2 exchange was determined for shoots from saplings of 3 conifers (Abies lasiocarpa [Hook] Nutt., Pinus contorta Dougl., Juniperus communis L.) under natural conditions in the field. A combination of relatively low stomatal resistances (< 300 s/cm) and low wind speeds (< 30 cm/s led to substantial contributions of the aerodynamic resistance .**GRAPHIC**. to water use efficiency (WUE = photosynthesis/transpiration) for all 3 spp. For A. lasiocarpa, transpiration was calculated to be 44% less and photosynthesis 17% less due to the presence of .**GRAPHIC**. which led to a predicted increase in WUE of 57% compared to the calculated WUE when .**GRAPHIC**. was assumed negligible. Similar increases in WUE were computed for P. contorta (48%) with somewhat smaller values for J. communis (34%). These results are discussed in terms of the estimated importance of .**GRAPHIC**. on water and photosynthetic relations of plants that have relatively low stomatal resistances and grow in microhabitats with low winds.