Alpine ecosystems and elevated CO₂

Plant photosynthesis depends on CO₂. The lesson on carbon relations has made it clear that there is no evidence that alpine plants are short of carbon. In order to test the C-limitation hypothesis, alpine grassland at 2500 m in the Swiss Alps was exposed to twice the current CO₂ concentration for 4 seasons.

The results confirmed the C-saturation hypothesis. Elevated CO₂ caused:

• no change in growth or biomass production per unit land area
• an increase in the carbon to nitrogen ratio in plant tissue (diminished tissue quality; more carbohydrates, less protein)
• compensatory feeding (increased intake) by the major natural herbivore (grasshopper)
• differential responses of plant species, which could, in the very long run, change community structure and ecosystem functioning.

It is concluded that the enrichment of the atmosphere with CO₂ will exert little direct influence on late successional alpine biota. There is a possibility that small species specific reponse differences and plant-animal interactions will translate into longer term biodiversity effects. However, other environmental changes are likely to mask such CO₂ effects. No data are available for alpine pioneer vegetation, which may be more responsive given their better access to nutrients. The indirect effects of atmospheric CO₂-enrichment via enhanced greenhouse forcing will be discussed later.