Variability in Processes

The Kyoto protocol allows the compensation of fossil fuel emission by biological sinks without defining its components. In contrast to plant physiologists who are mainly concerned with photosynthesis, land managers are mainly interested in growth of products (timber, grain), but the atmosphere integrates carbon assimilation and respiration which includes the soil. In addition, carbon is released from ecosystems not only by respira-

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FIGURE 4 The European Transect: Latitudinal changes of needle and leaf nitrogen concentration, 5''C-isotope ratio and S' ^N-isotope ratio, net primary productivity (NPP) and leaf area index (LAI) for conifers {Picea abies) and deciduous trees (Rigus sylvuticu). In Fig. 4a minimum (min) and maximum (max) values show the absolute range of data.

tion but also by harvest and fire. In the case of harvest, respiration may take place elsewhere on the globe due to the globalization of trade. Schulze and Heimann (1998) illustrated the different views of productivity and quantified the consequences thereof in a flux scheme (Fig. 5), where the input by photosynthesis represents 100% and this quantity is generally termed gross primary productivity (GPP). Since the plant needs energy for its own growth and maintenance metabolism, 50% of GPP is used by the plant itself. The resultant quantity is termed net primary production (NPP) which includes growth of all components, especially leaves, stems, fine roots, and fruits. The har-vestable fraction may only be a small quantity of NPP and the resultant biomass. The so-called harvest index (crop/biomass) is 50% in high yielding crop varieties, but generally averages 30%. Also, in trees, timber production is generally less than 20% of NPP. The largest quantity of NPP is not retained by the plant but shed as litter. This may take place in a seasonal rhythm or continuously, but it is a rejuvenescence process and compensates for aging of organelles and organs. The litter of roots and foliage reaches the ground and is decomposed by heterotrophic organisms which use the litter as the sole carbon source. Thus, the largest fraction of the litter returns to the atmosphere as C02 and some undigestable carbon remains as humus. The balance between assimilation and ecosystem respiration is termed net ecosystem productivity (NEE). However, also this fraction may be remobilized and converted to C02 by disturbance, or by fire. The remaining carbon, mainly in the form of recalcitrant humus and charcoal, contributes to the net biome productivity (NBP Schulze ct al., 2000). The definitions are based on the assumption, that the observations are made on an increasing area, namely, it moves from the leaf level (GPP) to the plant cover (NPP) and to the stand level (NEP), and finally reaches the landscape level (NBP).

The terrestrial surface looks quite different, depending on which quantity we chose (Schulze and Heimann, 1998). Photosynthesis is related to leaf structure and available nitrogen, and reaches highest rates in the temperate climate and in regions with intensive agriculture (Eastern US, Europe, India, East Asia). In contrast, NPP, which depends on the length of the growing season and on leaf biomass, reaches highest rates in the humid tropics and monsoon climates. Predictions on NEP are problematic, because according to ecological theory respiration should balance assimilation in the long term. However, a disequilibrium exists between assimilation and decomposition due to a continuous increase in atmospheric C02. Based on this effect, NEP would reach a maximum in subtropical and temperate regions, not in the boreal climate.

Schulze et al. (1999) compared a European Picea forest with a Siberian Pine forest. The European forest has a high NPP (15 m' stem growth per year) but also high respiration, while the Siberian forest has a low NPP (1 m3 stem growth per year) but also low respiration. Integrated NEP over the growing season of both sites was surprisingly similar. Both sites assimilated about 15 mol m~2 during the summer. However, it would not be appropriate to generalize from this observation, because a high variability exists on a landscape basis in Siberia (Fig. 6), ranging from plots that are carbon neutral or carbon sources after logging to very effective carbon sinks, such as old growth-unmanaged forests. In fact, sphagnum bogs, representing a totally different plant cover than forest, reach rates of net carbon sequestration similar to those of a forest.

The main natural factor that disturbs the Siberian forest are fires which either occur as repeated ground fires (fire frequency about 50 years) or burn the whole forest (crown fires, every 200-300 years). The study of ground fires shows, that the forest ecosystem

FIGURE 5 Schematic explanation and estimates of productivity at the leaf (GPP), the whole plant (NPP), the ecosystem (NEP) and the biome (NBP) level (Schulze and Heimann, 1998).

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