Influence of elevated CO2 on canopy development and red:far-red ratios in two-storied stands ofRicinus communis.

Vertical structure of plant stands and canopies may change under conditions of elevated CO2 due to differential responses of overstory and understory plants or plant parts. In the long term, seedling recruitment, competition, and thus population or community structure may be affected. Aside from the possible differential direct effects of elevated CO2 on photosynthesis and growth, both the quantity and quality of the light below the overstory canopy could be indirectly affected by CO2-induced changes in overstory leaf area index (LAI) and/or changes in overstory leaf quality. In order to explore such possible interactions, we compared canopy leaf area development, canopy light extinction and the quality of light beneath overstory leaves of two-storied monospecific stands ofRicinus communis exposed to ambient (340 μl l-1) and elevated (610 μl l-1) CO2. Plants in each stand were grown in a common soil as closed "artificial ecosystems" with a ground area of 6.7 m2. LAI of overstory plants in all ecosystems more than doubled during the experiment but was not different between CO2 treatments at the end. As a consequence, extinction of photosynthetically active radiation (PAR) was also not altered. However, under elevated CO2 the red to far-red ratio (R:FR) measured beneath overstory leaves was 10% lower than in ecosystems treated with ambient CO2. This reduction was associated with increased thickness of palisade layers of overstory leaves and appears to be a plausible explanation for the specific enhancement of stem elongation of understory plants (without a corresponding biomass response) under elevated CO2. CO2 enrichment led to increased biomass of overstory plants (mainly stem biomass) but had no effect on understory biomass. The results of this study raise the possibility of an important indirect effect of elevated CO2 at the stand-level. We suggest that, under elevated CO2, reductions in the R:FR ratio beneath overstory canopies may affect understory plant development independently of the effects of PAR extinction.