Carbon inputs and outputs may vary with forest age

Law et al. (2001) compared patterns of carbon storage and flux in a young (clear cut 22 years previously) and an old forest (not previously logged, trees from 50 to 250 years old) of ponderosa pine (Pinus ponderosa) in Oregon, USA. Their results are summarized in Figure 18.5.

Total ecosystem carbon content (vegetation, detritus and soil) of the old forest was about twice that of its young counterpart. There were notable differences in percentage carbon stored in living biomass (61% in old, 15% in young) and in dead wood on the forest floor (6% in old, 26% in young). These differences reflect the influence of soil organic matter and woody debris in the young forest derived from the prelogged period of its history. As far as living biomass is concerned, the old forest contained more than 10 times as much as the young forest, with the biggest difference in the wood component of tree biomass.

Below-ground primary productivity differed little between the two forests but because of a much lower above-ground net primary productivity (ANPP) in the young forest, total net primary productivity (NPP) was 25% higher in the old forest. Shrubs accounted for 27% of ANPP in the young forest, but only 10% in the old forest. Heterotrophic respiration (decomposers, detritivores and other animals) was somewhat lower in the old forest than NPP, indicating that this forest is a net sink for carbon. In the young forest, however, heterotrophic respiration exceeded NPP making this site a net source of CO2 to the atmosphere. In nutrients can be lost...

... and to groundwater and streams an old forest is a net sink for carbon (input greater than output)...

Figure 18.5 Annual carbon budgets for an old and a young ponderosa pine forest. Carbon storage figures are in g C m-2 while net primary productivity (NPP) and heterotrophic respiration (Rh) are in g C m-2 year-1 (arrows). The numbers above ground represent carbon storage in tree foliage, in the remainder of forest biomass, in understory plants, and in dead wood on the forest floor. The numbers just below the ground surface are for tree roots and litter. The lowest numbers are for soil carbon. (After Law et al., 2001.)

both forests, respiration from the soil community accounted for 77% of total heterotrophic respiration.

These results provide a good illustration of the pathways, stores and fluxes of carbon in forest communities. They also serve to emphasize that nutrient inputs and outputs are by no means always in balance in ecosystems.

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