Plant growth enhanced by increased carbon dioxide, but food webs give rise to significant variations

Carbon dioxide emissions from fossil fuels such as coal, petroleum and other sources have resulted in the atmospheric CO2 concentration almost doubling over the past few decades, from 280 to approx. 400 ppm. Researchers agree that a higher CO₂ concentration will stimulate photosynthesis and growth of wetland plants. However, there is debate as to whether wetland conservation might therefore be an effective strategy to absorb excess CO₂ and mitigate future climate change. Various experiments are being conducted around the globe to investigate these specific effects.

The longest running of these experiments was initiated in 1987 by Professor Bert Drake in brackish wetlands on the Atlantic coast of the US state of Maryland. The transitional zone between salt and fresh water was chosen as these highly productive marshland areas provide many essential ecosystem services, and are threatened in many regions by human land development. The Smithsonian Environmental Research Center is examining plant communities on a marsh that is dominated by American bulrush (Scirpus olneyi, a C3 sedge) and saltmeadow cordgrass (Spartina patens, a C4 grass). Two different types of plants, therefore, are being evaluated. Photosynthesis of C3 plants, which include over 95 per cent of plant species on earth (e.g. trees), are thought to be more sensitive to changes in atmospheric CO₂ than C4 plants which include crops such as sugar cane and corn. An experiment was conducted in several test chambers to examine the influences of elevated CO₂ and on growth of C3 and C4 plants. At 365 ppm, the CO₂ concentration in some chambers was the same as that found in the atmosphere today. The CO₂ concentration was almost doubled to 705 ppm in other chambers, to simulate a future scenario.

The team evaluated the biomass and growth of the plants, not only to elevated CO₂, but also experimental manipulation of detritivores such as isopods and their predators such as Pardosa littoralis spiders. These invertebrate food chain treatments were chosen based on a six-year field study where invertebrate populations were monitored. "The field study showed strong variations in detritivore and spider populations, whereby the top-level predators were the most sensitive to climate variation. " commented Dr Jes Hines from the iDiv, whose study was able to demonstrate a new mechanism which helps explain the inter-annual variations in plant growth when CO₂ concentrations are elevated.

In the case of American bulrush, biomass production was enhanced by the presence of detritivores especially in elevated atmospheric CO₂ concentrations. These findings represent an important step in illustrating the connections between climate, the food chain and biomass production in the wetland ecosystem. "The study emphasized the importance of viewing the ecosystem in its entirety rather than looking at sub-divided compartments," stressed Professor Nico Eisenhauer from the iDiv and Leipzig University. Carbon dioxide is just one of many variables. Besides CO₂, the sensitive wetland ecosystem is also affected by nitrogen deposition, salinity, and precipitation. There are therefore many more variables and much still to be done for experimental ecology. Tilo Arnhold