Possible impacts of global warming on tundra and boreal forest ecosystems: Comparison of some biogeochemical models

Global warming affects the magnitude of carbon, water and nitrogen fluxes between biosphere and atmosphere as well as the distribution of vegetation types. Biogeochemical models, global as well as patch models, can be used to estimate the differences between the mean values of annual net primary production (NPP) for present and for future climate scenarios. Both approaches rely on the prescribed pattern of vegetation types. Structural, rule-based models can predict such patterns, provided that vegetation and climate are in equilibrium. The coupling of biogeochemical and structural models gives the opportunity of testing the sensitivity of biogeochemical processes not only to climatic change but also to biome shifts. Whether the annual mean NPP of a vegetation type increases or decreases depends strongly on the assumptions about a CO2 fertilization effect and nitrogen cycling. Results from our coupled model show that, given that direct CO2 effects are uncertain, (i) average NPP of these northern biomes might decrease under global warming, but (ii) total NPP of the region would increase, due to the northward shift of the taiga biome.