In terrestrial ecosystems the carbon cycle exhibits natural cyclic behaviour on a range of time scales. Most ecosystems, for example, have a diurnal and seasonal cycle. Often this means that the ecosystem functions as a source of C in the winter and a sink for C in the summer, and this shows up in fluctuations at the global scale, as shown by the annual oscillations in the global atmospheric CO₂ concentration. Large-scale fluctuations occur at other temporal scales as well, ranging from decades (Braswell et al., 1997; Turner et al., 1997; Karjalainen et al., 1998; Kurz and Apps, 1999; Bhatti et al., 2001) to several centuries (Campbell et al., 2000) and longer (Harden et al., 1992).

The net balance of C flows between the atmosphere and the terrestrial biosphere also undergoes management-induced cycles that occur over long time scales (decades to millennia), and that can cause the transition of terrestrial systems from sink to source and back (Harden et al., 1992). Of relevance for C mitigation are the human-induced changes that occur on an annual to centennial time scale. This would include the harvest cycle of managed, production forests.

The intent of any mitigation option is to reduce atmospheric CO₂ relative to that which would occur without implementation of that option. Biological approaches to curb the increase of atmospheric CO₂ can occur by one of three strategies (IPCC, 1996):

• conservation: conserving an existing C pool, thereby preventing emissions to the atmosphere;
• sequestration: increasing the size of existing carbon pools, thereby extracting CO₂ from the atmosphere; and
• substitution: substituting biological products for fossil fuels or energy-intensive products, thereby reducing CO₂ emissions.

The benefits of these strategies show contrasting temporal patterns. Conservation offers immediate benefits via prevented emissions. Sequestration impacts often follow an S-curve: accrual rates are often highest after an initial lag phase and then decline towards zero as C stocks approach a maximum (e.g., Figure 4.3). Substitution benefits often occur after an initial period of net emission, but these benefits can continue almost indefinitely into the future (Figure 4.6).

This section deals primarily with carbon conservation and sequestration in the terrestrial biosphere, but acknowledges the complementarity and trade-offs among the three strategies. Carbon sequestration in forest products is included here and the substitution benefits of forest products are treated briefly. The role of energy cropping is treated in greater depth in Chapter 3 (Section 3.6.4.3) and in the IPCC Special Report on LULUCF (IPCC, 2000a). Here the discussion is restricted to the secondary use of biomass products for energy (e.g., waste products) and non-commercial uses (e.g., domestic heating, cooking, etc.).

The general goal of sequestration activities is to maintain ecosystems in the sink phase. However, if the system is disturbed (a forest burns or is harvested, or land is cultivated), a large fraction of previously accumulated C may be released into the atmosphere through combustion or decomposition (Figure 4.2). When the system recovers from the disturbance, it re-enters a phase of active carbon accumulation. Thus, the disturbance history of terrestrial ecosystems involves in large C losses in the past (Houghton et al., 1999; Kurz and Apps, 1999), but opportunities for C sequestration in the present.

An upper bound for the technical potential for global C mitigation in the terrestrial biosphere, a physical upper limit, can be estimated for conservation, sequestration, and substitution measures. The technical potential for conservation measures would equal the current existing C stock of the world’s ecosystems. This assumes that all ecosystems are threatened, but all could be conserved by implementing protection measures. The technical potential for sequestration would roughly equal the carbon stocks lost in deforestation, desertification, and other human-induced changes in land cover and land use over centuries and millennia. The theoretical upper limit would thus correspond to the full recovery of lost biomass in ecosystems, and to a steady state at the natural carrying capacity for biomass on earth. The technical potential for substitution is related to the sustainable production of harvestable biomass and its substitution for fossil fuels and energy-intensive products. Clearly, each of these upper limits violates in practice the ideals of development, equity, and sustainability. And yet, they help to appreciate that there are bounds on the role that managing the biosphere might play in carbon mitigation.

Table 4.3: Mitigation options, mitigation potential, and investment cost per tonne of carbon (US$/tC) abated in selected countries (Sathaye and Ravindranath, 1998)
Mitigation option	Mitigation potential (tC/ha)	Investment cost1 (US$/tC)	Mitigation option	Mitigation potential (tC/ha)	Investment cost1 (US$/tC)
ASIA
China				India
North & North West
Assisted natural regeneration2	13.0	1.3		Natural regeneration2	62.0	1.5
Plantation	55.0	1.3		Enhanced natural regeneration2	87.5	2.5
Agroforestry	15.0	16.3		Agroforestry	25.4	1.6
South, South West & North East				Community woodlot	75.8	5.6
Assisted natural regeneration	13.9	3.5		Softwood forestry	80.1	7.3
Plantation3	71.0	5.0		Timber forestry	120.6	3.3
Agroforestry	6.0	9.8
Indonesia				South Korea
Timber estate	165.0	1.9		Improved management of natural forest	99.4	6.0
Social forestry	94.0	1.1		Urban forestry	299.0	9.2
Reforestation4	214.0	0.9		Enhanced regeneration of L. leptolepsis	123.0	13.8
Private forests	99.0	2.1		P. koraiensis	85.0	21.0
Afforestation	106.0	0.6
Mongolia				Pakistan
Private forests	99.2	0.8		Intensified forest management
Natural regeneration	67.5	0.6		- Conifer forest – protection	41.6	0.1
Agroforestry	9.8	0.8		- Conifer forest – natural regeneration	33.8	8.8
				(enhanced)
Bioenergy	80	-		Reforestation4	39.1	19.3
Shelter belt	101.7	0.9		Riverain forest plantation	32.9	40.6
				Commercial forest plantation	54.6	40.6
				Watershed management	26.7	34.8
				Agroforestry	29.7	1.6
				Plantation on agricultural land2	7.5	0.7
				Rangeland management.	20.0	17.4
Philippines				Thailand
Forest protection plus sustainable	215.0	1.3		Short rotation in:
management				- Managed forests	185.5	2.5
Forest protection – total log ban	215.0	0.5		- Non protected areas	158.9	2.9
Long rotation forestry	236.0	2.1		Long rotation in community managed forests	169.0	3.2
Urban forestry	90.0	5.3		Medium rotation in non protected areas	112.5	4.3
				Forest protection and rotation forestry for conservation in
				- Protected area	38.6	7.5
				- Community managed forests	38.1	10.7
Vietnam				Myanmar
Forest protection	106.9	0.1		Natural regeneration	33.0	0.1
Degraded forest protection	64.3	0.2		Reforestation long4	155.0	0.8
Natural regeneration (enhanced)	57.1	0.8		Forest protection	47.0	1.6
Scattered trees	64.0	0.9		Reforestation short4	55.0	3.8
Reforestation short4	43.0	2.2		Bio electricity	78.0	21.4
Reforestation long	68.2	1.7
AFRIKA
Ghana				Cameroon
Evergreen forest				Evergreen forest
- agroforestry	13-88	1-6		- agroforestry	16-58	1-5
- slowing deforestation	35-140	1-2		- slowing deforestation.	40-160	1-2
Deciduous forest				- forestation5	73-195	1-19
-slowing deforestation	35-140	1-2		Deciduous forest
- forestation5	31-154	1-27		- forestation5	27-169	21-19
Savannah				Savannah
- agroforestry	29-61	4-12		- forestation5	36-170	1-31
1 Investment costs (US$/tC): This largely includes forest or plantation establishment costs incurred during the initial 2-3 years; discounted for only the initial 2-3 year period. For forest protection, the costs include expenditure on erecting barriers for protection, training, and other organizational costs incurred during the initial 2-3 year period. Mitigation potential is in pertuity, assuming one full cycle; rotation length for mitigation option subject to harvesting (such as short and long rotation) and for others 40 years. 2 Natural regeneration of forest is increasing the biomass density to that of closed forests on partially degraded open forest areas; assisted or enhanced natural regeneration would involve planting a (few) trees and/soil and water conservation activity to assist or enhance natural regeneration. 3 Plantations involve planting of one or more species at high densities. 4 Reforestation in a short rotation has a 5 to 15 year harvest cycle, reforestation in a long rotation has a 30-100 year harvest cycle. 5 Forestation includes both afforestation and reforestation.

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