Response of plant communities to elevated CO_2

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Elevated CO_2 may increase dry mass production of canopies directly through increasing net assimilation rate of leaves and also indirectly through increasing leaf area index (LAI). We studied the effects of CO_2 elevation on canopy productivity and development in mono-specific and mixed(1 : 1) stands of two co-occurring C_3 annual species, Abutilon theophrasti and Ambrosia artemisiifolia. The stands were established in the glasshouse with two CO_2 levels (360 and 700ml/l)under natural light conditions. The planting density was 100 per m2 and LAI increased up to 2.6 in 53-day growth. Root competition was excluded growing each plant in an individual pot. However, interference was apparent in the amount of photons absorbed by the plants and in photon absorption per unit leaf area. Greater photon absorption by Abutilon in the mixed stand was due to different canopy structures : Abutilon distributed leaves in the upper layrs in the canopy while Ambrosia distributed leaves more to the lower layrs. CO_2 elevation did not affect the relative performance and light interception of the two species in mixed stands. Total aboveground dry mass was significantly increased with CO_2 elevation, while no significant effects on leaf area development were observed. CO_2 elevation increased dry mass production by 30-50%, which was mediated by 35-38% increase in the net assimilation rate (NAR) and 37-60% increase in the nitrogen use efficiency (NUE,=net assimilation rate per unit leaf nitrogen). Since there was a strong overall correlation between LAI and aboveground nitrogen and no significant difference was found in the regression of LAI against aboveground nitrogen between the two CO_2 levels, we hypothesized that leaf area development was controlled by the amount of nitrogen taken up from the soil. This hypothesis suggests that the increased LAI with CO_2 elevation observed by several authors might be due to increased uptake of nitrogen with increased root growth.