Journal of Conference Abstracts

Impacts of Increased Temperature and Carbon Dioxide
on a Forested Catchment in Norway:
The CLIMEX Climate Experiment

Nico Van Breemen¹ (Nico.vanbreemen@bodeco.ben.wau.nl), Alan Jenkins² (jinx@ua.nwl.ac.uk),
R. F. Wright³ (richard.wright@niva.no), David J. Beerling⁴ (d.j.beerling@sheffield.ac.uk),
Wim Arp⁵ (wim.arp@staf.ton.wau.nl), David Fowler⁶ (d.fowler@ite.ac.uk),
Paul Verburg¹ (paul.verburg@bodeco.beng.wau.nl), Frank Berendse⁵ (frank.berendse@staf.ton.wau.nl),
Rob Collins² (r.collins@uk.ac.nerc-wallingford.unixa) & Anke Lükewille³ (anke.lukewille@niva.no)

¹ WAU, POB 37 6700 AA Wageningen. Netherlands.

² IH, Wallingford, Oxon OX1 8BB, U.K.

³ NIVA, Box 173 Kjelsås, N-0411 Oslo, Norway.

⁴ Univ.Sheffield S10 2TN, U.K.

⁵ WAU, Bornse Steeg 69, 6708 PD Wageningen, Netherlands.

⁶ ITE, Penicuik, Midlothian EH26 0QB, U.K.

Boreal ecosystems may be particularly sensitive to climate change. In the CLIMEX (Climate Change) experiment researchers from six European research groups manipulate the climate of a whole boreal forest headwater catchment as predicted by GCM's for the middle of next century, and study the response of the ecosystem to those perturbations. CLIMEX began in 1992 using the former RAIN (Reverse Acidification in Norway) project facilities at Risdalsheia near Grimstad, Norway. From April 1994, CO₂ and air temperature were increased in a greenhouse enclosing the KIM catchment, which receives unpolluted precipitation.

Measurements include gas exchange at catchment and leaf scale, plant growth and nutrition, litter decomposition, mineralisation of soil N, soil water chemistry, soil water dynamics and catchment output fluxes of water and chemicals to characterize whole-ecosystem response. Experimental design incorporates monitoring of soil and vegetation plots and of individual plants, comparison of pre- against post-treatment data at KIM, and three non-enclosed reference catchments. Carbon dioxide and temperature treatments started in April 1994. To date this is the world's only catchment-scale experiments with climate change.

After two years of exposure to increased CO₂ and temperature, all of the dominant tree and ground shrub species increased leaf water-use efficiency (WUE). Periodic instantaneous gas exchange measurements indicated that this response was sustained by continued increased photosynthetic rates and reduced stomatal conductance. Despite increased WUE of individual leaves, no major changes in catchment runoff were observed between pre-and post treatment years, indicating that higher WUE was compensated for by higher soil evapo(trans)piration and/or increased plant growth. In the first three years of treatment, soil N mineralisation increased significantly from 210 to 430 mmol m^-2 yr^-1 under Calluna, and from 295 to 390 mmol m^-2 yr^-1 under Vaccinium. Increased availability of N helped to sustain increased growth caused by elevated CO₂, higher temperatures and longer growing season. Runoff export of N increased as a result of treatment from 1 to 6 mmol m^-2 yr^-1 for NO₃^- and from 1 to 4 mmol m^-2 yr^-1 for NH₄⁺. Gaseous loss of N, as NO_x, increased too, as did whole ecosystem respiration of CO₂. These responses indicate that climate change may have profound effects on ecosystem biogeochemistry.