Parameters for global ecosystem models

Abstract

Tian et al. reply — Our model-based analysis of the effects of interannual climate variability and increasing atmospheric CO₂concentration on carbon storage in Amazonian ecosystems focused on CO₂exchanges between the atmosphere and undisturbed forests and other upland eco-systems of the region¹. Crutzen et al. urge us to add the emissions of isoprene and other volatile organic compounds (VOCs) to our analysis. They argue that ignoring these emissions could lead to an overestimation of annual net carbon storage (net ecosystem production) in the Amazon Basin.

Main

We did not include VOCs in our Terrestrial Ecosystem Model because not enough is known about their production, such as controls on rates, and tree species involved². The parameterization of CO₂uptake (gross primary production) in the model is based on an estimate of the sum of net primary production and plant respiration, and does not include allocation of carbon to support the production of VOCs. Because we make no allowances in the model for CO₂uptake by plants to support the production of VOCs, we make no allowances for emissions associated with VOCs. The estimates of net ecosystem production in our current version of the model are therefore independent of VOC emissions, and should not be corrected downwards for them.

However, the future development of the Terrestrial Ecosystem Model will certainly include the addition of VOCs because of their importance in tropospheric chemistry. Because the model is subject to mass balance constraints, we expect that our estimates of both gross primary production and net primary production will increase to accommodate the addition of VOC fluxes.

Schulman et al. suggest that we consider carbon fluxes between the atmosphere and peatlands in our calculations of net ecosystem production for the Amazon Basin. They state that these ecosystems cover a large area, and they assume that tropical peatlands are likely to be at least as responsive to climate changes as their boreal counterparts. From our review of the literature on the areal extent of peatlands in Amazonia³,⁴, we conclude that Schulman et al.'s estimate of peatland area in the basin, 150,000 km², is reasonable. If we combine the total peatland area of Amazonia with the boreal peatland flux rates cited by Schulman et al., the resulting basin-wide fluxes are small. A carbon storage rate (positive net ecosystem production) of 119 g C m⁻² yr⁻¹ translates to an annual basin-wide storage of about 0.02 Pg C, and a carbon loss rate (negative net ecosystem production) of 83 g C m⁻² yr⁻¹ translates to an annual basin-wide release of about 0.01 Pg C. For interannual climate variability to have a significant effect on the net ecosystem production of Amazonia through peatlands, these ecosystems would have to be much more sensitive than boreal peatlands to climate shifts.

There is evidence that this is not the case. The literature on peatlands in warm climates indicates that, because of the poor quality of their organic matter, decomposition rates in these ecosystems are low under both aerobic and anaerobic conditions⁵. This is not true for boreal peatlands, where the low temperatures that prevail for most of the year slow the decay of plant litter. Slow decay leads to a build-up of relatively high-quality organic matter that decomposes rapidly under warmer and drier conditions. Because tropical peatlands may be less sensitive than boreal peatlands to interannual climate variability, and because the area of peatlands is relatively small in Amazonia, we conclude that the net ecosystem production of the Amazon Basin is little influenced by the effects of year-to-year variability on carbon storage in its peatlands.

See also Crutzen et al. and Schulman et al.