Land Use, Land-Use Change and Forestry

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4.4. Activities Categorized and Described

In this section, we define activities broadly, primarily to keep the complexity and length of the presentation manageable. Some of the activities listed here, such as agroforestry and restoration of degraded lands, cut across several of the "land cover" boxes of the matrix in Figure 4-6. Key practices are discussed in some detail in several of the Fact Sheets.

4.4.1. Overview of Rates and Duration of Potential Carbon Sequestration

Potential rates of carbon sequestration in response to improved management vary widely as a function of land use, climate, soil, and many other factors (refer to Tables 4-5, 4-6, 4-7, 4.8, 4-9, 4-10, 4-11 and 4-12). Although research to date does not allow definitive evaluation of potential rates of carbon gain for all regions and management options, Table 4-4 presents rough estimates for broad activity and eco-zone groupings. For each grouping, Table 4-4 shows the carbon-conserving practices that are most likely to achieve substantial rates of carbon gain. The rates of carbon gain alongside each practice reflect the best combination of these practices for that activity and eco-zone. For example, estimated rates of potential carbon gain for cropland management might reflect the effects of reduced tillage, better fertilization, and improved crop rotation applied together on the same unit of land.

Table 4-4: Summary of potential rates of carbon gain and associated impacts for various activities.

Activity Ecozonea Keyb

(t C ha-1 yr-1)

Confidenced Duration

Cropland management Boreal Ley/perennial forage crops, organic amendments 0.3-0.6 (0.4) M 40 +N2O Increased food production, improved soil quality
Temperate - dry Reduced tillage, reduced bare fallow, irrigation 0.1-0.3 (0.2) H 30 +N2O Increased food production, improved soil quality, reduced erosion, possibly higher pesticide use
Temperate - wet Reduced tillage, fertilization, cover crops 0.2-0.6 (0.4) H 25 +N2O Increased food production, improved soil quality, reduced erosion, possibly higher pesticide use
Tropical - dry Reduced tillage, residue retention 0.1-0.3 (0.2) L 20 +N2O Increased food production, improved soil quality, reduced erosion, possibly higher pesticide use
Tropical - wet Reduced tillage, improved fallow management, fertilization 0.2-0.8 (0.5) M 15 +N2O Increased food production, improved soil quality, reduced erosion, fertilizers often unavailable, possibly higher pesticide use
Tropical - wet (rice) Residue management, fertilization, drainage management 0.2-0.8 (0.5) L 25 ++CH4, +N2O Increased food production

Agroforest management Tropical Improved management 0.5-1.8 (1.0) M 25 +N2O  

Grassland management Temperate - dry Grazing management, fertilization, irrigation 0-0.3 (0.1) M 50 ±CH4, +N2O Increased energy use, salinity, higher productivity
Temperate - wet Grazing management, species introduction, fertilization 0.4-2.0 (1.0) M 50 ±CH4, ++N2O Higher productivity, acidification, erosion, reduced biodiversity
Tropical - dry Grazing management, species introduction, fire management 0.1-1.5 (0.9) L 40 -CH4, ++N2O Reduced soil degradation, higher productivity, woody encroachment (reduced productivity)
Tropical - wet Species introduction, fertilization, grazing management 0.2-3.9 (1.2) L 40 -CH4, ++N2O Increased productivity, reduced biodiversity, acidification

Forestland management Boreal and
Temperate - dry
Forest regeneration, fertilization, plant density, improved species, increased rotation length 0.1-0.8 (0.4) L 80 +N2O, +NOX Leakage (rotation length), high cost efficiency
Temperate - wet Forest regeneration, fertilization, species change 0.1-3.0 (1.0) L 50 +N2O, +NOX Leakage (rotation length), reduced biodiversity
Tropical - dry Forest conservation, reduced degradation (1.75) L 40 Ecological improvement, high cost efficiency
Tropical - wet Reduced degradation 3.1-4.6 (3.4) L 40 Environmental improvement

Wetland management All Restoration 0.1-1 (0.5) L 100 ++CH4, ±N2O Increase in water quality, decrease in flooding, increased biodiversity

Restoration of degraded land All Restoration of eroded lands, saline soil reclamation 0.1-7 (0.25) M 30 +N2O Increased productivity, may be expensive

Urban land management All Tree planting (0.3) M 50 Increased biodiversity

Conversion to agroforestry Tropics Conversion from cropland or grassland at forest margins 1-5 (3) L 25 Improved biodiversity, CH4 sinks, poverty alleviation, food security

Conversion (cropland to grassland) Temperate - dry Marginal cropland re-seeded to grassland 0.3-0.8 (0.5) H 50 -N2O; -CH4 Enhanced biodiversity, reduced erosion
Temperate - wet Surplus cropland seeded to grassland 0.5-1.0 (0.8) M 50 --N2O; - CH4 Enhanced biodiversity, reduced erosion

a "Wet" vs. "dry" based on potential evapotranspiration:precipitation ratio. "Wet" <1 and "Dry" >1; "Tropical" = latitude <30º.
b List of practices that may yield largest gains in carbon stocks, roughly in order of importance.
c Range of carbon increase rates that might reasonably be expected to occur in response to adoption of best-possible complement of key practices. Actual rate will depend on previous management practices (e.g., rates of gain may be higher in a carbon-depleted system), climate, ecosystem properties (e.g., soil carbon gain may be favored by higher clay content), and many other factors. Value in parentheses is default estimate. Rates for tropical forest management to recover carbon stocks on degraded forests apply only to the present area of degraded forest (as of 1990) as reported by FAO (1996)-that is, closed-canopy forest having full biomass stocks are excluded.
d Relative reliability of rate estimates. Generally, confidence increases with the number of studies conducted in the activity-ecozone grouping. L = low, M = medium, and H = high.
e An estimate of the time required for the system to approach a new steady state after the adoption of the new practices.
f Relative magnitude of potential effects on emission of N2O, CH4, and other GHGs. "+" denotes increased emission; "-" denotes reduced emission; number of "+" or "-" denotes relative magnitude of possible effects.

The rates of carbon gain in Table 4-4 refer to the average accumulation rate from the time a practice is started until carbon storage again reaches a new equilibrium (see Box 4-1). Almost invariably, the rates in Table 4-4 are lower than those from published studies, which are usually measured for time intervals shorter than that needed to reach saturation. As shown, some of the rate values have greater uncertainty than others, largely because of a paucity of studies in many regions. The net effect of the practices on climate forcing is also affected by impacts on other GHGs (Table 4-4). For example, where the proposed practice increases N2O or CH4 emission, the rate of carbon sequestration alone overestimates the net benefit of the practice to the atmosphere.

Box 4-1. The Rate of Carbon Gain

The rate of carbon gain following application of a given practice that stores carbon will decrease over time (see Chapter 1 and Section 4.2). Figure 4-8a shows an approximation of this relationship, which normally is used to describe changes in soil carbon following application of a particular practice that stores carbon. Therefore, rates of carbon change observed during the initial stages of an activity are usually higher than the average rates (Figure 4-8a). Idealized saturating curves such as those in Figure 4-8a require two constraining parameters: the rate of inputs and the time required to reach "saturation." Data for carbon stock increases reported in the literature sometimes do not cover the entire duration; hence, the rates reported are higher than the average rate. We have corrected for this in determining the average rates in Table 4-4. Many management practices are applied over a period of years before saturation can occur. In such cases, the average rate of carbon storage underestimates stock changes that will occur using the approach outlined in Figure 4-8a. In this case, we have estimated the average change in stock once the management has come to an approximate steady-state for carbon (Figure 4-8b).

Figure 4-8. Rate of carbon gain.

The magnitude of change in carbon stocks for a given practice depends on three factors: the average rate of carbon stock change per unit area after the practice has been applied, the time required for saturation to occur, and the total area over which the activity is applied. Figure 4-8a shows how we have defined these terms for calculation of magnitudes.

Adoption of a proposed practice seldom depends solely on perceived effects on the atmosphere; indeed, other benefits or disadvantages of the proposed activities will usually outweigh any effect on atmospheric CO2 in affecting land-use decisions. Table 4-4 shows a few of the key associated impacts for each of the proposed activities.

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