Gas Exchange and Growth Responses of the Desert Shrubs Artemisia tridentata and Chrysothamnus nauseosus to Shallow- Versus Deep-Soil Water in a Glasshouse Experiment

The aridland shrub species, Artemisia tridentata (big sagebrush) and Chrysothamnus nauseosus (rubber rabbitbrush), are distributed widely in the Intermountain region of western North America. Earlier research indicated that A. tridentata can utilize upper soil water from transient summer rain events while C. nauseosus apparently cannot, although both species have similar rooting depths. Thus, we hypothesized that C. nauseosus relies more on deep water than A. tridentata, while A. tridentata can take advantage of soil moisture in upper soil layers. We examined this hypothesis by growing A. tridentata and C. nauseosus in two-layer pots in which soil water content in the upper and lower layers was controlled independently. After plants were well established, they were subjected to one of the three water treatments: water applied both to upper and lower layers, water applied only to the upper layer, or water applied only to the lower layer. We measured above- and belowground biomass, leaf gas exchange and leaf carbon isotope composition (δ13C). Close to the end of the experiment, a deuterium- and 15N-labeled solution was applied in a localized patch in either the upper or lower soil layer to quantify resource-pulse utilization. In general, our hypothesis was supported; δ13C and biomass indicated that A. tridentata performed better when water was available in the upper soil layer and gas exchange, and δ13C indicated that C. nauseosus performed better when water was available in the lower layer. There was, however, no significant variation among treatments for many of the variables examined. Roots of both species were involved in hydraulic redistribution of soil water between layers, which may have reduced the effectiveness of the water distribution treatments to some extent