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

The quantitative importance of aerodynamic resistance to H(2)O vapor and CO(2) exchange was determined for shoots from saplings of three 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 seconds per centimeter) and low wind speeds (<30 centimeters per second) led to substantial contributions of the aerodynamic resistance (R(wv) (a)) to water use efficiency (WUE = photosynthesis/transpiration) for all three species. For A. lasiocarpa, transpiration was calculated to be 44% less and photosynthesis 17% less due to the presence of R(wv) (a), which led to a predicted increase in WUE of 57% compared to the calculated WUE when R(wv) (a) 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 R(wv) (a) on water and photosynthetic relations of plants that have relatively low stomatal resistances and grow in microhabitats with low winds.