Woody plants generally exhibit more conservative water-use characteristics than herbaceous perennials and annuals, including lower stomatal conductance, net photosynthesis, and transpiration, but higher photosynthetic water-use efficiency. Under soil moisture stress, stomatal conductance usually decreases more than net photosynthesis, resulting in lower ratios of internal to atmospheric CO₂ concentrations and higher photosynthetic water-use efficiency. However, we found that shrubs of a semiarid grassland employed opportunistic water-use strategies, which likely evolved due to the short duration of soil moisture availability. We measured photosynthetic gas exchange, leaf reflectance, and foliar stable carbon isotope composition in Artemisia cana, Prunus virginiana, Rhus trilobata, and Ribes aureum in southern Alberta. Under moist conditions, net photosynthesis was very high in A. cana, R. trilobata, and R. aureum, but less so in P. virginiana. Declining volumetric soil moisture caused stomatal conductance to decrease in proportion with net photosynthesis in 3 of 4 species (P. virginiana, R. aureum, and R. trilobata). Consequently, ratios of internal to atmospheric CO₂ were unaffected by soil moisture depletion, and photosynthetic water-use efficiency declined. At the leaf level, the normalized difference vegetation index did not vary with season in these species, while the chlorophyll index decreased only slightly and the photochemical reflectance index showed a small mid-season peak. A. cana had the lowest stable carbon isotope ratio, highest stomatal conductance, highest late-season net photosynthesis, and highest leaf nitrogen (% dry weight). As soil moisture decreased, A. cana exhibited a decline in ratios of internal to atmospheric CO₂ concentrations but no change in photosynthetic water-use efficiency. This study shows that even long-lived, woody species of a semiarid grassland have evolved traits facilitating rapid water-use to maximize carbon gain during brief periods of soil moisture availability.

Nomenclature: Kartesz, 1994.