| Carbon and nitrogen stable isotope data from herbarium specimens of Artemisia
tridentata and Bromus tectorum across southern Idaho were used to determine potential changes
in water use efficiency (WUE) over the last century as CO₂ in the atmosphere has increased, and
climate has varied. Multiple regression analysis revealed a small but significant effect of CO₂ on
WUE when other factors were held constant. A significant decrease in nitrogen availability over
time, indicated by decreased nitrogen percentage and δ¹⁵N, was also found, which as nitrogen is
also correlated with WUE, countered the effects of CO₂. This suggests that the sagebrush steppe
is generally becoming more nitrogen limited over the past ca. 100 years, and that the terrestrial
carbon sink, for this ecosystem, could be reaching a plateau.
Keywords: Artemisia tridentata, Bromus tectorum, water use efficiency, nitrogen limitation |