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
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
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
- 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
These times vary widely; the best estimates for 1992 come from Norway (Hoen
and Solberg, 1994), as tabulated below.
(years from felling until decay starts)
(years until all fiber has decayed)
|Bark in land fillings
|Bark for burning
|Branches, stumps, stems in forest
|Root system after felling
|Furniture and interiors
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
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.
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
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.