Ecosystem and Biome Productivity
The sum of assimilatory and respiratory processes in carbon balance is called net ecosystem productivity (NEP).
where Ra is the respiration of autotrophic plants and Rh the respiration of heterotrophic organisms (Fig. 3.3,9; Schulze and Heimann 1998; Schulze et al. 2000).
The NEP gives no information where the accumulated C is deposited. This could take place with low Ra in the biomass (increase of stems and heartwood in a forest) and with limited Rh in the soil as humus. Thus, NEP should not be equated with formation of humus only. Whether NEP is accumulated temporarily at the level of the stand, or as undecomposed carbon in soils waiting to be transferred into the chain of heterotrophic organisms at a later date (for trees >100 years), can only be established after observation over longer periods or at a regional scale.
Figure 3.3.10 (Rebmann 2001) shows net ecosystem carbon fluxes as measured by a micro-
meteorological approach (eddy covariance method, see Chap. 2.2, Box 2.2.6), as an example. Photosynthetic activity during summer is clearly detectable. It balances and exceeds respiration during the night from mid-March until September. Also in winter there are times when net C02 assimilation is greater than respiration of spruce. Respiration of the ecosystem is significantly higher in summer than in winter. Under certain conditions (low irradiation) the forests may also be at the compensation point in summer. This stand assimilated, over the whole year, a total of 13.8 t C ha"1 year"1 (GPP). Of this, 13 t C ha"1 year"1 was lost through respiration of biomass and soil. In the end NEP was only 0.8 t C ha"1 year"1, which was deposited mainly in wood, as a consequence of growth. The balance may also become negative, i.e. losses exceed the gain from assimilation. As the wood is harvested in managed forests the amount of C is only transiently available in the ecosystem and thus the balance of this stand in the ecosystem is negative.
In addition to heterotrophic degradation, natural and anthropogenic exports of C occur mainly by harvesting (of agricultural crops or wood or animals) and fire. Including non-respi-
NBP (net biome productivity)
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