Are plants growing close to the floors of tropical forests exposed to markedly elevated concentrations of carbon dioxide?

The study tested the frequently expressed perception that the concentration of CO2 in the vicinity of establishing seedlings growing close to tropical forest floors is generally high. CO2 concentration was monitored 10 cm from the forest floor over several days during wet and dry seasons at three Panamanian lowland and montane tropical forest sites. Air was sampled at a low flow rate with a peristaltic pump to minimise contamination by air from other strata. The average CO2 concentrations observed during the dry and wet seasons were 387 and 423 muL CO2 L-1 air, respectively, a relatively small enrichment compared with the above-canopy CO2 concentrations. The highest CO2 concentration recorded at 10 cm was 494 muL L-1. The generally modest levels of enrichment-far below concentrations required to saturate photosynthesis-were nonetheless sufficient to significantly increase the rates of CO2 uptake relative to above-canopy CO2 concentrations by shade-grown seedlings of Piper cordulatum C. DC., an understorey shrub and Virola surinamensis (Rol.) Warb., a late successional tree species.