Seventeen-Year Response of Wetland Sedge Community
to Elevated CO2.

D. P. Rasse1 and B. G. Drake2
1 French Agricultural Research Institute - INRA - Paris, France
2 Smithsonian Environmental Research Center - SERC - Edgewater, Maryland
Question:
Will the response of natural ecosystems to elevated CO2 be sustained beyond the often-reported positive initial response?

 
Measured Parameters:
Reponses to elevated CO2 of : shoot numbers (species response) and net ecosystem exchange of CO2 (NEE, community response).

Experiment:
Location: Chesapeake Bay wetland : the Kirkpatrick salt marsh (38°53'N, 76°33'W), Maryland
Plant community for this study: Scirpus olneyi
Treatment: open top chambers with either ambient or elevated CO2 levels (ambient + 340 ppm). Five replicated chambers per treatment.
Duration: 17 years : 1987 to 2003 (ongoing study)
Measurements: - shoot census in early August of each year, - Net ecosystem exchange of C02 measured by differential CO2 flux in and out of the chambers prior to the shoot census.

 
Main Results:

Table 1. Probability values for a repeated measure analysis of the CO2 effect and the CO2 interaction with time on shoot numbers and NEE for the 17-year period.



 

1.Shoot densities were significantly increased by elevated CO2. (table 1. Fig.1)

2.The stimulation of shoot densities by elevated CO2 increased with time (Table 1)

3.The Net ecosystem exchange was significantly increased by elevated CO2 (Table 1, Fig 3)

4.The stimulation of net ecosystem exchange decreased from an initial high of 80 percent to about 20 percent after nine years, it then stabilized and further re-increased to exceed 50 percent after 17 years.
Fig.1
Fig.2
Fig.3
Fig.4
Conclusions:

In the the longest in situ elevated CO2 experiment in the world, we observe that elevated CO2 stimulated both the number of shoots and the net ecosystem exchange (NEE) in the Scirpus olneyi community. While the ecosystem response (NEE stimulation) became fairly stable after a decade, the plant specific response (shoot numbers) kept increasing over time.