Sustainable Land Management for Mitigating Climate Change

A. Natural Regrowth and Forest Succession

32. Restoration of tropical forests is important to improving the environment while adapting to and mitigating CC. The strategy is to let the forest regrowth happen on degraded soils and agriculturally marginal land through natural succession. Yet, sustainable forestry in the tropics is not a panacea (Pearce et al., 2003), and must be objectively and critically assessed.

33. Reforestation has a large potential to fix atmospheric CO₂ through photosynthesis into biomass (Yokoyama, 1997), and humification of a part of the biomass (Lal, 2005a; b). Potential land area for reforestation may be 293 Mha in South America and Africa, and 47 Mha in Asia (Yokoyama, 1997). Globally, land area available for afforestation was estimated at 865 Mha by Houghton (1990), 952 Mha by Depert et al. (1992) and 600-1200 Mha by Winjum et al. (1992). In tropical dry forests of Jamaica, McLaren and McDonald (2003) evaluated several ecological factors (e.g., soil moisture and shade) that affect ecosystem C pool including seed germination and seedling survival. Also in Jamaica, McDonald and Healey (2000) concluded that soil fertility and elemental cycling (C, N) are fully restored during 20 years of secondary succession.

34. The impacts of forest ecosystems on C sequestration have also been studied at a national scale such as in Spain (Bravo et al., 2008). A regional study conducted across 10 countries in Africa by Lewis et al. (2009) indicated that between 1968 and 2007, above ground C storage in live trees increased by 0.63 t C/ha/yr. Lewis and colleagues estimated the C sequestration potential of African tropical forest to be 0.34 Gt C/yr, and also synthesized the available data across tropical Asia, Africa and Latin America. They observed the mean rate of C sequestration in the above ground biomass at the rate of 0.49 t C/ha/yr which indicates a C sink capacity across all tropical forests of 1.3 Gt C/yr (Table 5). The data in Table 5 from Costa Rica show that forest regeneration is extremely effective in restoring the SOC pool. Lal (2005a;b) assessed the technical potential of forest ecosystems on C sequestration. While various strategies are useful in enhancing SOC, Figure 10 shows that tropical plantations are extremely effective in enhancing the biomass and SOC pool. However, forest ecosystems can also enhance emisson of methane (Sanhuwza, 2006).

Table 4. Estimated annual increase in tropical forest carbon pool (Lewis et al., 2009).
Continent	Area		Rate of C sequestration (Gt C/yr)
			Mean		Range
Central and South America	786.8		0.62		0.39-0.73
Africa	632.3		0.44		0.26-0.53
Asia	358.3		0.25		0.15-0.30
Total	1777.3		1.31		0.79-1.56

Table 5. SOC pool (t C/ha) to 60-cm depth 4 years after planting (calculated from Montagnini and Porras, 1998).
Site 1		Site 2		Site 3
Species	SOC Pool	Species	SOC Pool	Species	SOC Pool
Jacaranda copaia	90.4	Albizia guachapele	75.3	Genipa Americana	61.1
Calophyllum brasiliense	73.7	Dipteryx Panamensis	73.1	Hieroyma alchorneoides	64.4
Styrphnodendron microstachyum	74.8	Terminalia Amazonia	70.6	Balizia elegans	85.9
Vochysia guatemalensis	76.1	Virola koschnyi	71.9	Vochysia ferruginea	66.9
Mixed	74.9	Mixed	73.2	Mixed	68.5
Regeneration	85.1	Regeneration	95.4	Regeneration	76.8
Assumptions: Organic matter comprised 58% C, and soil bulk density equals 0.6 T/m^3 (Fisher, 1995) for all depths and under all species.