Long term effects of naturally elevated CO₂ on mediterranean grassland and forest trees

Abstract

We investigated the carbon supply status in species-rich mediterranean plant communities growing in a bowl-shaped 1-ha “CO₂ spring” area near Sienna, Italy. A geothermic “lime-kiln” has provided these communities, for as long as historical records are available, with pure CO₂ that mixes with ambient air at canopy level to daytime means of 500–1000 ppm CO₂. Immediately outside the spring area similar plant communities are growing on similar substrate, and in the same climate, but under ca. 355 ppm CO₂. We found no evidence that plants in the CO₂ spring area grow faster, flower earlier or become larger. However, we found very large differences in tissue quality among the 40 species studied inside and outside the spring area. Depending on weather conditions, the mean concentration of total non-structural carbohydrates (TNC, sugars and starch) in leaves of herbaceous plants was 38–47% higher in the spring area. Fast growing ruderals growing on garden soil inside and outside the spring area show the same response. Among trees, leaves of the deciduousQuercus pubscens contain twice as much TNC inside as outside the vent area, whereas evergreenQ. ilex leaves show no significant difference. TNC levels in branch wood paralleled leaf values. TNC in shade leaves was also higher. Elevated CO₂ had no effect on the sugar fraction, therefore differences in TNC are due to starch accumulation. Leaf nitrogen concentration decreases under elevated CO₂. These observations suggest that the commonly reported TNC accumulation and N depletion in leaves growing under elevated CO₂ are not restricted to the artificial conditions of short-term CO₂ enrichment experiments but persist over very long periods. Such an alteration of tissue composition can be expected to occur in other plant communities also if atmospheric CO₂ levels continue to rise. Effects on food webs and nutrient cycling are likely.