Shifts in depth of water extraction and photosynthetic capacity inferred from stable isotope proxies across an ecotone of Juniperus osteosperma (Utah juniper) and Artemisia tridentata (big sagebrush)

Summary1 In western North America, juniper trees ( Juniperus spp.) are apparently encroaching into numerous communities including sagebrush‐dominated ( Artemisia tridentata ) valleys, where, as density of juniper increases, the density and condition of sagebrush decline but juniper condition appears unaffected. 2 We examined stable isotope proxies of plant gas exchange and relative depth of soil water extraction of Juniperus osteosperma and Artemisia tridentata as their relative densities changed across a transition zone in northern Utah, USA. Measurement of 13 C and 18 O of foliage allowed separation of the contributions of stomatal and biochemical factors to differences in mean intercellular CO 2 concentration, while deuterium composition of stem water served as an indicator of the relative depth of water extraction. 3 Leaf δ 13 C of juniper and, to a lesser extent, of sagebrush increased with decreasing density of juniper. Foliage 18 O did not vary significantly with juniper density, indicating that this was primarily caused by an increase in photosynthetic capacity rather than decreased supply of CO 2 . Increased foliage nitrogen concentration of both species with decreasing juniper density is also consistent with increased photosynthetic capacity. 4 The much greater mean age of juniper trees in the high juniper density plots may explain their lower photosynthetic capacity. 5 Averaged across densities, juniper always extracted water deeper in the profile than did sagebrush, but in June and August, only the high juniper density plots showed an effect. Sagebrush is either restricted from using water at greater depth in the presence of juniper at high density or was in such poor condition that deep‐water sources were not necessary to maintain their reduced function. 6 Ecotones can serve as useful, spatially constrained stages for examining resource partitioning and relationships among plant species. We observed spatial and temporal variation in resource use and partitioning of limited resources that will enable better design of manipulative experiments to explicitly examine competition.