Elevated CO 2 and elevated temperature have no effect on Douglas‐fir fine‐root dynamics in nitrogen‐poor soil

Summary•  Here, we investigate fine‐root production, mortality and standing crop of Douglas‐fir ( Pseudotsuga menziesii ) seedlings exposed to elevated atmospheric CO 2 and elevated air temperature. We hypothesized that these treatments would increase fine‐root production, but that mortality would be greater under elevated temperature, leading to a smaller increase in standing crop. •  Seedlings were grown in outdoor, sun‐lit controlled‐environment chambers containing native soil. They were exposed in a factorial design to two levels of atmospheric CO 2 and two levels of air temperature. Minirhizotron methods were used to measure fine‐root length production, mortality and standing crop every 4 wk for 36 months. •  Neither elevated atmospheric CO 2 nor elevated air temperature affected fine‐root production, mortality, or standing crop. Fine roots appeared to root deeper in the soil profile under elevated CO 2 and elevated temperature. •  Low soil nitrogen (N) levels apparently limited root responses to the treatments. This suggests that forests on nutrient‐poor soils may exhibit limited fine‐root responses to elevated atmospheric CO 2 and elevated air temperature.