This section describes issues regarding processes, time scales, and carbon accounting rules that are specific to ARD activities. Where carbon accounting is different between definitional scenarios, we identify these differences.
As described in Chapter 1, carbon accumulates in living biomass when forest stands are established. The accumulation rate depends on climatic conditions (especially temperature and precipitation), site conditions (slope, exposure, soil texture, fertility, etc.), tree species/genetic stock, and human activities (e.g., tree cutting, burning, thinning, litter removal). Dead organic matter (i.e., litter, coarse woody debris) and soil carbon may start to accumulate once living biomass produces litter as substrate for soil organic matter formation. The rate of soil carbon accumulation varies as a function of the type and amount of living biomass present and the site's land-use history. If the site was previously tilled, afforestation and reforestation are likely to lead to a significant accumulation of soil carbon.
A distinction between afforestation and reforestation under Article 3.3 is relevant only for assumptions concerning changes in soil and dead organic matter stocks. In general, the land-use history (tillage, type of cropping system, etc.) and site conditions are more relevant than the afforestation/reforestation labels for the determination of assumptions and methods to calculate stock changes on land converted from non-forest to forest.
Reforestation does not always lead to immediate increases in ecosystem carbon stocks. When the definitional scenario creates ARD land where a stand is cleared and a new forest crop is established (e.g., in the FAO definitional scenario), the loss of carbon from decaying slash, stumps, dead root systems, and soil organic matter may exceed the removal of carbon by young trees for several years. The choice of definitions and accounting rules for the implementation of Article 3.3 will determine whether such negative changes in carbon stocks will be reported (see numerical examples in Section 3.5.2).
Afforestation/Reforestation and Deforestation Differ in Their Dynamic Effects
The magnitude and rate of change in carbon stocks differ greatly between afforestation/reforestation activities and deforestation activities, as well as among carbon pools within a single stand. Afforestation and reforestation generally cause small annual changes in carbon stocks over long periods of time, whereas deforestation can cause large changes in carbon stocks over a short period of time. Deforestation causes an immediate reduction in aboveground biomass carbon stocks, followed by several years of decreases in other carbon stocks. The differing pace of response among different activities "since 1990" creates outcomes that are not intuitively obvious. Even if the total forest area and carbon stocks in a country remain constant ("normal forest management")-for example, if afforestation or reforestation in one area are matched by deforestation in another area-a decline in carbon stocks may be reported under Article 3.3. This imbalance occurs because most emissions from deforestation take place rapidly (except where a high percentage of the wood enters long-lived products), whereas carbon removal from afforestation and reforestation activities is a slow process. Afforestation and reforestation take decades to centuries to come to completion; under Article 3.3, however, only the portion of the uptake that originates from stands created "since 1990" is considered. In subsequent commitment periods, if the pattern of normal forest management is maintained, the Article 3.3 imbalance will diminish: A larger and larger number of parcels reforested or afforested since 1990 will make their cumulative carbon gain contributions in comparison to the constant number of parcels being deforested and having their carbon loss. This Article 3.3 imbalance is more pronounced for regions with low growth rates and long rotation periods, such as the boreal zone. In summary, ARD activities can result in a reported carbon stock change that may not reflect the actual changes in carbon stocks (see also Section 3.5.1). Avoiding this effect does not seem possible with the language of Articles 3.3 and 3.7.
An afforestation and reforestation program with a fixed amount of land afforested or reforested each year will yield large positive stock changes after an initial period of slow carbon accumulation. In the first few years, afforestation and reforestation tree growth is slow, and not all land in the program will have been afforested or reforested. Over time, more and more land will become part of the program, and benefits will continue to increase for decades (Nilsson and Schopfhauser, 1995). Because the carbon gain is low at the beginning of afforestation and reforestation programs, in most parts of the world such programs will yield only small carbon gains during the first commitment period.
Avoidance of deforestation under Article 3.3, however, will immediately produce a large amount of avoided emissions. The magnitude of carbon benefits from avoidance of deforestation is high initially, whereas the optimal level in afforestation and reforestation that is initiated at the same time is reached decades later.
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