Modelling stimulation of plants and ecosystem response to present levels of excess atmospheric CO₂

Abstract

Rising atmospheric CO₂ concentrations (16-26% above the preindustrial value) along with other anthropogenically released nutrients (especially nitrogen as well as phosphate compounds) may already have induced a fertilization effect of the biosphere. A great number of greenhouse and phytotron experiments along with only a few field observations suggest a stimulation of the net primary production which may be expressed by an experimental stimulation factor β between 0.25 and 0.50 on the average. Using these data, a global CO₂ fertilization effect can be calculated, which is supported at present by one open field observation at high altitude pines where an increase in tree ring width is correlated with the atmospheric CO₂ increase. The ecological response to increased biomass production in different ecosystems is still debated; global estimates for long-term storage of carbon range from zero to the order of several Gt C. A 3-compartment model for living biota, litter and humus is discussed for a global 1-biome as well as a global 4-biome aggregation of the land vegetation. Different kinetic models for NPP have been studied, of which the most realistic appeared to be dependent both on atmospheric CO₂ concentration and on standing biomass level. For a global 4-biome model, it was estimated that the annual carbon sequestering in the biota pool corresponds to a net ecosystem production of NEP_b, = 0.7 ± 0.3 Gt C/a, in the litter pool to NEP₁=0.07?0.03 Gt C/a and in the humus pool to NEPh=0.60 ± 0.35 Gt C/a, adding up to a present total carbon storage of NEP = 1.4 β 0.7 Gt C/a, assuming a β value of 0.25 to 0.50.