Fact Sheet 4.12. Forest Regeneration

Forest regeneration is the act of renewing tree cover by establishing young trees naturally or artificially-generally, promptly after the previous stand or forest has been removed. The method, species, and density are chosen to meet the goal of the landowner. Forest regeneration includes practices such as changes in tree plant density through human-assisted natural regeneration, enrichment planting, reduced grazing of forested savannas, and changes in tree provenances/genetics or tree species. "Human-assisted natural regeneration" means establishment of a forest age class from natural seeding or sprouting after harvesting through selection cutting, shelter (or seed-tree) harvest, soil preparation, or restricting the size of a clear-cut stand to secure natural regeneration from surrounding trees. "Enrichment planting" means increasing the planting density (i.e., the numbers of plants per hectare) in an already growing forest stand.

Use and Potential
This activity influences carbon storage through changes in the growth of aboveground and below-ground tree biomass and changes in wood end use. The impacts on the litter layer and soil vary with many factors (see other chapters in this Special Report). Generally, over the rotation period, annual growth in carbon storage in tree biomass in most cases is much higher than soil/litter carbon storage. Regarding carbon sequestration, the connection between forest regeneration and the end use of wood is important. For example, higher planting density is not generally preferable for carbon sequestration. High densities can lead to rapid crown closure and early growth, but such stands reach maximum increment early and may suffer onset of mortality and rapid growth decline-thereby potentially becoming sources of carbon considerably faster than stands that are managed at lower densities. Additionally, trees that are grown in less dense conditions generally reach a suitable size for solid wood products earlier; as a result harvest and conversion to long-lived wood products occurs sooner, adding to the stock of sequestered carbon and substituting for non-wood products that may use more fossil fuel in their production.

All of these activities are used today, with varying intensities, in many countries without consideration of carbon sequestration, largely on the basis of decisions about present costs and expected future benefits from timber and other forest values. If carbon management is introduced, these activities could be effective in sequestering considerably more carbon than is occurring today (e.g., Lunnan et al., 1991; Hoen and Solberg, 1994; Xu, 1995; Row, 1996; Nabuurs et al., 1999; Ravindranath et al., 1999).

No published estimate of the global carbon sequestration potential of these practices is available.

Methods and Uncertainty
At least two methods can be used to quantify changes in carbon stocks from these practices: existing forest growth yield tables and forest inventories that measure standing and incremental aboveground stem volume. The second method can be done as accurately as one wants, though with increasing costs. The first method is less accurate but would be good enough in some cases, at least in the initial phases, and could later be checked by more accurate inventories to secure adequate precision for verification.

Tree biomass growth (and correspondingly carbon accumulation) processes are well known. Soil carbon accumulation generally is less certain.

Mortality caused by wind, fire, pest, rot, or insect damages can lead to a loss of carbon pools for all of these activities. Most yield tables include estimates of natural plus mortality rates (for example, mortality is estimated to be 0.4 percent of living trees per year in Norway). Regarding accidental mortality, fewer estimates exist. Thorsen and Helles (1998) estimate the probability of total damage caused by strong winds for a Picea abies stand in Denmark to be about 1.5-3.0 percent per year if the stand were thinned no more than a year previously (the probability declines strongly with time after thinning). Climate changes may increase the risks of tree loss-for example, by more frequent winds or increased insect attacks.

The accuracy of national forest inventories varies considerably. Hobbelstad (1999) reports that the present national inventory of Norway gives estimates of total standing volume and annual yield for the country as a whole at an accuracy of 1.6 percent as standard deviation. This level of accuracy is based on 8000 permanent sample plots, of which 20 percent are measured each year. At a regional level, the standard deviation is 3.2 percent (the country is divided into four regions). Countries such as Sweden and Finland have the same accuracy in their forest inventories.

In addition to national inventories, Norway conducts a county inventory, which covers one-third of the counties every 5 years. This inventory provides a county-level accuracy that corresponds to a standard deviation of 3-4 percent (Norway is composed of 20 counties). The costs for the national inventory and county inventory are about US$0.17 ha^-1 yr^-1, covering a total productive forest area of about 7.5 Mha.

Time Scale and Monitoring
The accumulation time for aboveground and below-ground biomass ranges from 5 years (for the shortest rotation times in tropical plantations) to 150 years or more on low-potential sites in boreal forests. The tree biomass carbon accumulation process is not difficult to quantify and predict, particularly where well-developed forest growth and yield models exist. Allometric studies provide factors that can be used to estimate total biomass (aboveground and below-ground) from the timber yield tables (Marklund, 1988; Birdsey, 1996). Soil carbon accumulation processes are generally less confidently predicted, but increasingly there are research results to guide these estimates.

The duration of the carbon biomass stored in forests or forest products depends on factors such as the following:

• Forest rotation length (or harvest intensity over time for selection felling systems)
• Thinning intervals and intensity
• Decaying time of timber not used (roots, branches, stumps, logging residues)
• Average lifetime of end use of wood and decay time of end-use product after its use.

These times vary widely; the best estimates for 1992 come from Norway (Hoen and Solberg, 1994), as tabulated below.

End-Use Category	Anthropogenic Time (years from felling until decay starts)	Decay Time (years until all fiber has decayed)
Bark in land fillings	0	8
Bark for burning	0	1
Needles	0	7-11
Branches, stumps, stems in forest	0	12
Root system after felling	0	100
Construction material	80	80
Furniture and interiors	20	50
Impregnated lumber	40	70
Pallets	2	23
Losses	0	1
Composites, plywood	17	33
Sawdust	1	2
Pulp/paper	1	2
Fuelwood	0	1

Verifiability
In principle, all of these activities can be verified, at varying accuracy and costs. The capacity varies between countries, and combinations of methods might be applied. To estimate the carbon impact from enrichment planting, for example, one would measure a control plot and take the difference as the estimated impact of the activity. Where several activities are combined, land-based measures will probably be required. These estimates can be made from yield models (if available), historical inventory data for similar stands, or a combination of these methods.

Transparency
The assumptions and methodologies associated with this activity can be explained clearly to facilitate replication and assessment of carbon impacts. The scientific and technical methods are open to review and are replicable over time.

Permanence
Carbon will be stored in a forest as long as the forest is not harvested or damaged by natural events. Where the harvested timber is used for bioenergy or forest industry production, for example, the degree of permanency will depend on the end use of the timber extracted and the carbon substitution impact of these products.

Associated Impacts
Improved natural regeneration would result in most cases in increased biodiversity and recreational/landscape improvements. These effects could also result from increased mixed-species stands and higher tree density in savanna woodlands. Regarding environmental damage, tree planting and change of tree species could result in decreased biodiversity and reduced recreational benefits, particularly if monoculture stands are emphasized. All activities will produce more jobs and income in the establishment phase, as well as at harvesting and end-use activities, especially in rural areas. The potential is probably highest in tropical countries; as such, developing countries may benefit more than developed countries. The costs and benefits of associated impacts are difficult to quantify. The economic benefit from increased timber production, by comparison, is easy to estimate by using market prices. Leakages through market dislocations may occur. For example, increased investment in forest management for increased carbon sequestration may increase the long-term timber supply, implying lower future timber prices and thereby reducing total forest management investments. These leakages, however, are probably not higher than for those occurring for other GHG mitigation options in other sectors of the economy (see Chapters 2 and 5 for more discussion of leakage).

Relationship to IPCC Guidelines
All forest management practices that affect the rate of biomass increment and biomass losses through harvesting or other removals are implicitly included in the Reference Manual under the calculations for "Changes in Forest and Other Woody Biomass Stocks." Changes in soil carbon, litter, and below-ground biomass stocks as affected by forest management practices are not included in the Workbook.

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