Shoot growth was stimulated 44% during 1996 and total biomass stimulation increased subsequently to about 60% (Dijkstra et al., 2002) and has leveled off at 65% since 1999. However, during recent years, there have been very large inter-annual variations in the amount of carbon added annually. Most of the additional growth each year (70%) is added by the largest shoots (> 2 cm diameter), which constitute a small fraction (<10%) of the total number of shoots. Leaf area was stimulated by elevated CO2 40-60% during the summer growing season but only 20-30% during the fall and winter (Hymus et al. 2002). Leaf fall occurs throughout the year but most strongly during March and April. Litter measurements of leaf fall are consistent with the LAI measurements made using optical methods in showing the seasonally variable effect of elevated CO2 on canopy leaf area.

Growth of fine roots, studied with periodic measurements using a Bartz camera lowered into minirhizotron tubes (Day et al. 1996), showed an initial stimulation during the first two years of exposure (Dilustro et al. 2002) but growth of fine roots at normal ambient CO2 has now equaled that of growth of fine roots in elevated CO2 (unpublished data). Soil cores taken in April 2002 showed no stimulation of coarse and fine root growth by elevated CO2. This is not surprising given that the bulk of the root mass was present prior to the commencement of the elevated CO2 treatment in 1996. The largest roots have still not been adequately sampled and ground penetrating radar is being proposed as a technique that may fill that gap. Fine root mortality and turnover were greater under elevated CO2 early in the study (Dilustro et al. 2002) but this treatment effect also was absent after several years.

Research Questions