Comparison of atmospheric carbon dioxide concentration and metabolic activity in Boreal Forest ecosystems

Abstract

The increasing seasonal amplitude of atmospheric CO₂ concentrations may reflect biotic influences such as increased photosynthesis due to higher CO₂ concentrations or increased winter respiration due to warmer temperatures. Boreal forests have a large rôle in the seasonal dynamics of atmospheric CO₂ in the Northern Hemisphere, and if terrestrial ecosystems contribute to the greater amplitude, such a signal should appear in the metabolic activity of boreal forests. Analyses in which a mechanistic model of photosynthesis, respiration, and decomposition was driven with the observed annual increase in atmospheric CO₂ concentration and observed daily air temperature and precipitation show that from 1974 to 1982 the seasonal amplitude in metabolic activity in boreal forests near Fairbanks, Alaska, increased by 0.52 ± 0.03% year^-1. The increasing amplitude was due to increased tree photosynthesis as CO₂ concentration increased. However, the increase in annual aboveground biomass production was so small (3 g m^-2 year^-1 year^-1) that it is undetectable given field sampling errors. The small biomass growth increase indicates that a larger seasonal amplitude of atmospheric CO₂ can occur without large changes in net primary production, nutrient availability, or nutrient use efficiency. The increase in CO₂ concentration that produced the higher biomass production was also small (1.1 ppm year^-1), suggesting more attention should be paid to the ecological effects of small changes in CO₂ concentrations.