What was done
Douglas-fir -- Pseudotsuga menziessi (Mirb.) Franco -- seedlings were grown for four years in ambient or CO2-enriched air (ambient + 180 ppm CO2) at ambient or elevated air temperature (ambient + 3.5°C) in outdoor sunlit chambers in lysimeters filled with soil collected from the perimeter of an old-growth Douglas-fir stand located at 1200 m elevation in the Oregon Cascade Mountains.  This soil had a total nitrogen content that was only one-half of what is deemed to be optimum for highly productive Douglas-fir stands in this region.  It was kept at approximately 100% of field capacity during the winter wet seasons, with a gradual dry-down to about 25% of field capacity during the dry summers and then brought back rapidly to winter conditions in the fall.

What was learned
In the words of the authors, "elevated CO2 produced no statistically significant effects on biomass of individual Douglas-fir organs: leaves, buds, branches, stems, fine foots, coarse roots, or main root."  Hence, as they continue, "elevated CO2 had no effect on either total shoot, root, or whole-plant biomass."  They also report that "elevated temperatures did not significantly affect the biomass of specific organs," so that warming also "had no significant effects on total shoot, root, or whole-plant biomass."

What it means
The authors say their results suggest that "for Douglas-fir seedlings growing under naturally limiting soil moisture and nutrition conditions, elevated CO2 and temperature may have little impact on biomass."  As to why this is so, they mention the obvious limitations to enhanced growth provided by the limiting moisture and soil nutrients of their study.  In addition, they note that Douglas-fir "may be inherently less responsive to elevated CO2 than other tree species," and this is surely a possibility as well.

Still another possibility is growth-restricting root confinement.  Fourteen tree seedlings growing for four years in each 1- x 2-meter surface-area soil lysimeter of 1-meter depth may not have had enough "growing room" to display their true ability to fully respond to the extra CO2 to which they were exposed.  As Idso (1999) discovered in analyzing 176 CO2 enrichment experiments where woody plants were grown in containers with finite rooting volumes, all initial positive growth responses typically declined with the passage of time at a rate that suggested they were unlikely to persist beyond five years.

Reference
Idso, S.B.  1999.  The long-term response of trees to atmospheric CO2 enrichment.  Global Change Biology 5: 493-495.