Hectare Scale Impacts of Past Actions

I now return to the patch of shrub and grassland mentioned in the introduction to assess how the landowner's decision to convert to high forest 180 years ago affects the carbon balance today. I consider four alternative scenarios:

1. Maintain the land under the grazing regime and thus maintain the shrub-grass landscape (not plotted in Figure 16.3);

2. Plant beech and manage it in a 150-year rotation with four thinnings per rotation;

3. Plant Norway spruce in an 80-year rotation with four thinnings;

4. Abandon the land (i.e., stop grazing), allow natural regeneration, and do no further managing.

If we assume the shrub-grassland system with grazing was established in early Medieval times, then it was in equilibrium in the 1820s. If this system had continued until the year 2000, it would have a baseline carbon balance around 0 MgC ha-1 y-1.

1 beech

- Norway spruce

■ stop grazing & allow natural regeneration

Figure 16.3. Comparison of the temporal development of the annual flux (positive = sink) in the total system of forest biomass, soils, and products for beech, Norway spruce, and the natural regeneration system. Peaks in sources are due to logging events. Results were obtained with the CO2FIX modeling framework (Masera et al. 2003).

1 beech

- Norway spruce

■ stop grazing & allow natural regeneration jS? &

Figure 16.3. Comparison of the temporal development of the annual flux (positive = sink) in the total system of forest biomass, soils, and products for beech, Norway spruce, and the natural regeneration system. Peaks in sources are due to logging events. Results were obtained with the CO2FIX modeling framework (Masera et al. 2003).

The 80-year rotation of Norway spruce yields a rapid increase of the net sink after establishment, peaking at 7.5 MgC ha-1 y-1 after 21 years, in 1841 (Figure 16.3). Then, because of increasing density in the stand, the net sink starts to decrease. This decrease is counteracted by the thinnings at 25, 45, 55, and 70 years. These interventions produce peaks of carbon release sources at the level of the whole system. These peaks are the result of slash decomposing on the ground and wood products decaying. Immediately after the final felling in 1900 the system lost a lot of carbon, with a peak of the source of about 38 MgC ha-1 y-1. This cycle is repeated in the second rotation and leads in our reference year 2000 to a net sink of 5.75 MgC ha-1 y-1. The cumulative sum of all sources and sinks from 1820 until 2000 amounts to 132 MgC ha-1 (mean net biome production, or NBP, is 0.73 MgC ha-1 y-1 over the whole period).

The pattern is similar for the beech system except that the accumulation is slower in the beginning but continues longer. The net sink also peaks at 7.5 MgC ha-1 y-1, but in 1850 instead of 1841. In 2000 the beech system was a net sink of 5.0 MgC ha-1 y-1. The overall net storage, however, is much larger than in the Norway spruce system, at 218 MgC ha-1 (mean NBP is 1.2 MgC ha-1 y-1 over the whole period).

The third alternative of "stopping grazing + natural regeneration" shows very different dynamics. The growth is much slower in the beginning but continues much longer. In a managed forest, growth is stimulated by keeping the stand young, but in this case growth is more continuous and levels off from natural mortality. The overall net storage is much larger, with a final stocking of around 440 MgC ha-1 (mean NBP is 2.4 MgC ha-1 y-1 over the whole period). In the reference year 2000, however, the remaining net sink is only 0.7 MgC ha-1 y-1.

Clearly the decision of a landowner in 1820 still has an impact on the carbon budget in 2000. This analysis, however, assumes "model" forest growth dynamics based on long-term measurements in permanent plots (the yield table information). It does not account for

• variability due to weather;

• N fertilization;

• CO2 fertilization;

• other site amelioration from, for example, recovery from past grazing; or

• risks from natural disturbances.

As a consequence, this analysis probably overstates the impact of the landowner's decision in 1820. But the magnitude of the overstatement remains uncertain. We know that, at the local level, interannual variability can be very large (Kirschbaum 1999) and that CO2 fertilization experiments have a short-term impact on NPP (King et al. 1997) but that longer-term impacts are not so easily detectable (Nadelhoffer et al. 1999; Caspersen et al. 2000). Furthermore we know that recovery from past land use change is apparent in Europe (Spiecker et al. 1996). From other analyses, it appears that the impact of current management on the carbon budget far outweighs the effects of CO2 fertilization and N deposition (Houghton 2002).

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