Fig. 2.

Growth response to nitrogen addition. Responses in the N fertilizer experiment (dashed lines) are compared with responses in the FACE experiment (solid lines). Elevated CO₂ (solid circles) caused a significant increase in wood increment in the first year after treatment initiation (1998), but the response diminished in subsequent years and in later years was not statistically different from FACE controls (open circles). N fertilization (shaded squares) caused an immediate and sustained increase in wood increment compared with unfertilized plots (open squares) (P < 0.001).