Fig. 2.

Responses of NPP (g of DM·m⁻²·yr⁻¹) (A), N uptake (g of N·m⁻² ·yr⁻¹) (B), and NUE (g of DM·g of N⁻¹) (C) to forest growth under present-day and elevated CO₂ at the Rhinelander (RHI), Duke, ORNL, and POP-EUROFACE (EURO) sites. The error bar corresponds to one standard error of the mean. Significant differences between CO₂ treatments within a site are given by *, P < 0.05; **, P < 0.01; and ***, P < 0.001.