Sagebrush carbon allocation patterns and grasshopper nutrition: the influence of CO2 enrichment and soil mineral limitation.

Artemisia tridentata seedlings were grown under carbon dioxide concentrations of 350 and 650 μl l-1 and two levels of soil nutrition. In the high nutrient treatment, increasing CO2 led to a doubling of shoot mass, whereas nutrient limitation completely constrained the response to elevated CO2. Root biomass was unaffected by any treatment. Plant root/shoot ratios declined under carbon dioxide enrichment but increased under low nutrient availability, thus the ratio was apparently controlled by changes in carbon allocation to shoot mass alone. Growth under CO2 enrichment increased the starch concentrations of leaves grown under both nutrient regimes, while increased CO2 and low nutrient availability acted in concert to reduce leaf nitrogen concentration and water content. Carbon dioxide enrichment and soil nutrient limitation both acted to increase the balance of leaf storage carbohydrate versus nitrogen (C/N). The two treatment effects were significantly interactive in that nutrient limitation slightly reduced the C/N balance among the high-CO2 plants. Leaf volatile terpene concentration increased only in the nutrient limited plants and did not follow the overall increase in leaf C/N ratio. Grasshopper consumption was significantly greater on host leaves grown under CO2 enrichment but was reduced on leaves grown under low nutrient availability. An overall negative relationship of consumption versus leaf volatile concentration suggests that terpenes may have been one of several important leaf characteristics limiting consumption of the low nutrient hosts. Digestibility of host leaves grown under the high CO2 treatment was significantly increased and was related to high leaf starch content. Grasshopper growth efficiency (ECI) was significantly reduced by the nutrient limitation treatment but co-varied with leaf water content.