Long-term responses of boreal vegetation to global change: an experimentaland modelling investigation

The response of boreal ecosystems to future global change is an uncertain b ut potentially critical component of the feedback between the terrestrial b iosphere and the atmosphere. To reduce some of the uncertainties in predict ing the responses of this key ecosystem, the climate change experiment (CLI MEX) exposed an entire undisturbed catchment of boreal vegetation to CO2 en richment (560 ppmv) and climate change (+ 5 degrees C in winter, + 3 degree s C in summer) for three years (1994-96). This paper describes the leaf met abolic responses of the vegetation to the experimental treatment and model simulations of possible future changes in the hydrological and carbon balan ce of the site. Randomized intervention analysis of the leaf gas exchange m easurements for the dominant species indicated Pinus sylvestris had signifi cantly (P < 0.01) higher photosynthetic rates and Betula pubescens and Vacc inium myrtillus had significantly (P < 0.01) lower stomatal conductances af ter three years treatment compared to the controls. These responses led to sustained increases in leaf water-use efficiency of all species of trees an d ground shrubs, as determined from carbon isotope analyses. Photosynthesis (A) vs. intercellular CO2 (c(i)) response curves (A/c(i) responses), RuBis Co analysis and leaf nitrogen data together suggested none of the species i nvestigated exhibited down-regulation in photosynthetic capacity. At the wh ole ecosystem level, the improved water economy of the plants did not trans late into increased catchment runoff. Modelling simulations for the site in dicate this was most likely brought about by a compensatory increase in eva potranspiration. In terms of the carbon budget of the site, the ecosystem m odel indicates that increased CO2 and temperature would lead to boreal ecos ystems of the type used in CLIMEX, and typical of much of southern Norway, acting as moderate net sinks for CO2.