Biodiversity and CO₂: Global Change is Under Way

In this article our current understanding of CO₂ effects on complex vegetation is outlined and first results of the CO₂ enrichment experiments conducted as part of the Biodiversity Module of the Swiss Priority Programme Environment are discussed.

It is known for two centuries that plants need CO₂ for photosynthesis. Elevated concentrations of CO₂ were found to stimulate growth, and thus CO₂ fertilization became a standard tool in greenhouse horticulture.

Meanwhile it became clear that this stimulating effect of CO₂ on plant growth depends on the availability of other resources like water, nitrogen and phosphate, which are usually very abundant in agro-systems but limited in natural ecosystems. However, even when resources are short some plants (the “winners”) may become more efficient competitors for these resources than others (the “losers”), causing global CO₂ enrichment to become a biodiversity issue.

While 1 per cent of the global biomass is growing under fertile agricultural conditions, more than 95 per cent of the global vegetation produces biomass without any artificial addition of resources. Hence, understanding CO₂ effects on biodiversity requires experiments conducted in diverse, low-fertility systems, a model for which are calcareous grasslands. First results indicate increased carbon uptake by plants during the day but no change in biomass.

In addition, tissue quality changes and species-specific responses were observed which were not predicted by any current theory.

These results indicate that predictions of the future development of complex plant assemblages can not be made from data obtained in simplified fertile systems but need to consider the multitude of interactions between organisms as they occur in the field.

With more than 70 plant species per 100 m² this precious grassland system can serve as a model for other highly diverse natural ecosystems.