Forêts

3. How can forests affect climate change?

The source document for this Digest states:

Growing stock

Growing stock has formed part of global forest resources assessments since the first report. In addition to providing information on existing wood resources, growing stock estimates constitute the basis for estimation of biomass and carbon stocks for most countries.

Country information on total growing stock and forest area was used to estimate growing stock per hectare as an indicator of how well or poorly stocked the forests are. FRA 2005 has also collected country information on commercial growing stock. Chapter 5 (Productive functions of forest resources) presents results for this indicator, as well as a more detailed discussion of total growing stock.

Information availability

Of the 229 countries and territories covered by FRA 2005, 150 countries, representing 88 percent of the world’s forest area, reported on growing stock for 2005. Oceania was the only region for which information was available for only a small portion of forest area (15 percent), given that Australia did not provide information on this variable. With a few exceptions, reporting countries gave information for all three reporting years (see Figure 5.6 in Chapter 5).

Although many countries provided information on growing stock, the quality of the information is variable. A few countries with repeated national forest assessments have very reliable information, but many countries do not have good inventory data to support growing stock estimates and changes in growing stock over time.

Status

In order to obtain consistent global, regional and subregional estimates of total growing stock, growing stock per hectare was estimated for each region/subregion for those countries providing information. These estimates were then multiplied by the total forest area of each region and subregion. Table 5.7 in Chapter 5 shows the status of growing stock in 2005 and its distribution by region and subregion.

Total growing stock is estimated at 434 billion m³, of which some 30 percent is found in South America.

The five countries with the greatest total growing stock account for almost 261 billion m³, which corresponds to 60 percent of the global total. Of these, Brazil has the largest growing stock, with 81 billion m³ or 19 percent of the total.

The global average for growing stock per hectare is 110 m³/ha. The countries with the highest growing stock per hectare are found in central Europe and in some tropical countries.

Trends

Based on data from the 147 countries that reported growing stock figures for all three reporting years, total growing stock shows a slight decreasing tendency at the global level (see Table 5.9 in Chapter 5). There are some regional tendencies: Africa, Asia and South America show a slight decrease, while Europe and North and Central America show a slight increase.

As regards growing stock per hectare, changes at the global level are not significant. At regional and subregional levels, however, there are more significant changes. For example, Europe, excluding the Russian Federation, shows a net increase of 0.3 percent (or 1.2 m³ per hectare) annually for the last 15-year period, while South and Southeast Asia show a net decrease of 1.0 percent (or 1.0 m³ per hectare) annually, mainly due to a decrease in growing stock per hectare in Indonesia.

Changes in total growing stock reflect the combined effects of changes in forest area and in growing stock per hectare. However, for many countries, changes in growing stock reflect only the changes in forest area, because their estimates of growing stock are based on a single figure per hectare determined at one point in time (see Chapter 5). Thus the actual trends may be more pronounced than those in this analysis.

Biomass and Carbon

At a casual glance, the amounts of biomass and carbon seem simply to reflect the extent of forests and their growing stock. A more meaningful understanding emerges in the context of the global carbon cycle, climate change and related international agreements such as the United Nations Framework Convention on Climate Change (UNFCCC). Since half the dry weight of biomass is carbon (IPCC, 2003), the following analysis addresses biomass implicitly. For data related to biomass stock in forest and other wooded land, refer to Table 13 in Annex 3.

Forests, like other ecosystems, are affected by climate change, be it a sea-level rise that threatens coastal forests or changes in temperature and rainfall patterns. In some places, impacts may be negative, while in others they may be positive. However, forests also influence climate and the climate change process. They absorb carbon in wood, leaves and soil and release it into the atmosphere when burned, for example during forest fires or the clearing of forest land.

The Kyoto Protocol entered into force in the same year that this assessment was carried out. The protocol and the UNFCCC oblige all member countries to regularly assess and report national greenhouse gas emissions, including emissions and removals of carbon reflected as stock changes in forests. To that end, IPCC has created guidelines, methods and default values for all parameters needed to assess carbon stocks and their changes in forests (IPCC, 2003). It has thus furnished all countries with the means of estimating and reporting carbon stocks, greenhouse gas emissions and removals, irrespective of the availability of country-specific data. Striving for synergies and for streamlined country reporting to international organizations, FAO incorporated the IPCC guidelines on assessment of carbon stocks in forests into its guidelines for country reporting for FRA 2005.

Reporting on carbon stocks in forests under the UNFCCC, the Kyoto Protocol and to FAO may overlap, but are not necessarily identical. For FRA 2005, countries reported carbon stocks for the years 1990, 2000 and 2005. The UNFCCC mandates reporting carbon stock changes. However, in one of its methods, IPCC estimates net emissions of carbon as the difference between periodic carbon stocks.

In a further difference, UNFCCC members report on ‘managed forests’ only. The convention does not define ‘forest’ or managed forest. However, IPCC considers managed forests as “all forests under direct human influence” or “forests subject to the process of planning and implementing practices for stewardship and use aimed at fulfilling relevant ecological, economic and social functions” (IPCC, 2003). Given this broad definition, many countries may classify all their forests as managed forests. However, only by assuming both a steady state of biomass in ‘unmanaged forests’ and identical definitions of ‘forest’ will carbon stock changes be the same under the two reporting systems. Even in this case, estimates of the total carbon stock may still differ, depending on whether all forests are included or not in reporting under the UNFCCC.

Quantifying the substantial roles of forests as carbon stores, as sources of carbon emissions and as carbon sinks has become one of the keys to understanding and modifying the global carbon cycle. Global forest resources assessments have the potential to contribute to or substantiate estimates of the magnitude of stocks and flows by scientific bodies such as IPCC. Simultaneously, they complement and facilitate international reporting by countries on greenhouse gas emissions and removals under the UNFCCC.

Information availability

By integrating IPCC guidance into the guidelines for country reporting for FRA 2005, FAO sought to facilitate complete reporting on biomass and carbon pools by all countries. Yet many of the 229 countries and territories had difficulty in providing complete information for all pools of carbon, i.e. above- and below-ground biomass, dead wood, litter and soil carbon to a depth of 30 cm.

With few exceptions, countries that reported growing stock also successfully transformed this data into above- and below-ground biomass and then to carbon stock in forest biomass (Figure 2.11). Many countries based the conversion from growing stock to biomass on the IPCC good practice guidance factors (IPCC, 2003), reflecting a lack of country-specific biomass expansion factors.

Of the 151 countries that reported on forest biomass:

• 87 have used the IPCC good practice guidance biomass expansion factors exclusively;
• 41 have used the IPCC factors in combination with factors from other sources;
• 13 have used national data – either direct estimates or national expansion factors;
• 5 have used factors/models from FAO and FAO/UNECE publications;
• 5 are based on expert estimates.

Response rates for carbon pools other than forest biomass decreased steeply, to merely 20 percent of the countries, representing 51 percent of the total forest area for soil carbon.

It is clear that many countries do not possess country-specific information on the parameters necessary for calculating all carbon pools. However, perhaps blanks in the reporting tables also reflect political concerns, institutional and human capacity for reporting, or difficulties with the IPCC guidelines. Response rates for carbon in biomass were high from developing countries in all subregions except the Caribbean, while some large industrialized countries in North America and Oceania did not report biomass and carbon data at all or only incompletely, because they are currently in the process of finalizing their overall carbon inventories.

Overall, this report assesses carbon in all pools based on a fairly representative fraction of over half the global forest area for all components and more than 80 percent of total forest area for carbon in forest biomass.

Although countries were asked to provide information on carbon in forest soils in the top 30 cm, some countries used other threshold values. In these cases, the figures were adjusted to the common threshold of 30 cm.

Status

Carbon stock per hectare. Table 2.8 provides forest-area-weighted average carbon stocks per hectare for biomass, dead wood, litter and soils by region for the year 2005. Biomass and dead wood account for 44 and 6 percent of total forest ecosystem carbon respectively, while soils to a depth of 30 cm and litter contribute approximately 46 and 4 percent respectively.

Carbon stocks in forest biomass reach the highest values per hectare in Central and South America and Western and Central Africa, while East Asia, Northern Africa and Western and Central Asia report the lowest values.

IPCC (2000) estimated an average carbon stock of 86 tonnes per hectare in the vegetation of the world’s forests for the mid-1990s. The corresponding carbon in biomass and dead wood in forests reported here amounts to 82 tonnes per hectare for the year 1990 and to 81 tonnes per hectare for the year 2005.

Each cubic metre of growing stock equals different amounts of biomass and carbon in biomass in the regions. Table 2.9 provides average conversion factors compiled from country submissions. Globally, each cubic metre of growing stock equals, on average, 1 tonne of above-ground biomass, 1.3 tonnes of total biomass and 0.7 tonnes of carbon in biomass.

Total carbon stock. As a consequence of missing data, it is not possible to sum country data to obtain complete regional or global totals for carbon in any pool. Yet, in the context of climate change, these totals and their changes over the years are beyond mere academic interest. Figure 2.12 shows estimated total carbon stock for all pools by region. The figures were obtained by expanding reported data through the use of subregional estimates of carbon per hectare of forest, multiplied by the total forest area for each subregion.

The country reports indicate that global forest vegetation stores 283 Gt of carbon in its biomass, and an additional 38 Gt in dead wood, for a total of 321 Gt. A prior estimate by IPCC (2000) assumed 359 Gt of carbon in these pools. An assumed amount in FRA 2005 of only 10 tonnes per hectare of carbon in dead wood, on average, probably represents an underestimate and might be one reason for the discrepancy between the IPCC and country reports. Another may be exclusion of the biomass of undergrowth by some countries.

Soils (down to 30 cm) and litter contain 317 Gt of carbon according to country estimates in this assessment. There are large data gaps for major boreal forests with typically large amounts of soil carbon; thus the figures are likely underestimates.

The total carbon content of forest ecosystems for the year 2005 is, therefore, 638 Gt of carbon, which is more than the amount of carbon in the entire atmosphere. Roughly half of total carbon is found in forest biomass and dead wood combined, and half in soils and litter combined.

Trends

From 1990 to 2005, carbon in biomass decreased in Africa, Asia and South America, remained approximately constant in Oceania and increased in Europe and in North and Central America. Not all subregions followed this trend. Thus total biomass carbon stocks increased in East Asia and in Western and Central Asia, and decreased in Central America (Table 2.10). The decrease in overall biomass carbon stocks since 1990 was driven by South and Southeast Asia (33 percent decrease), Western and Central Africa (7 percent) and South America (6 percent).

If an average change of total biomass carbon stocks of at least 0.5 percent per year is defined as significant, then of a total of 146 countries and territories, 42 reported decreases, 55 increases and 49 reported no significant change in total carbon stocks within forest biomass.

In interpreting the reliability and meaning of these results, it is helpful to examine carbon stocks per hectare concurrently. Based on the same significance level, 99 countries reported no substantial change of carbon stock per hectare for the 1990–2005 period, 11 countries reported a decrease and 36 countries an increase.

Of the 42 countries communicating significant declines in total carbon stocks in forest biomass, only 17 percent also described lower levels of carbon stocks per hectare. In contrast, 78 percent – overwhelmingly developing countries – presumed virtually identical carbon stocks per hectare at the beginning and end of the 15-year period. In these countries, therefore, a reduction in total carbon stock in forest biomass reflects a net loss of forest area. Of the 20 countries reporting the highest absolute reduction in carbon stock, 15 did not report decreases in carbon stock per hectare. Essentially all the carbon stock reduction, therefore, is due to a net loss of forest area. Of the two countries with the highest decrease in carbon stocks, Brazil and Indonesia, only Indonesia recorded a significantly lower level of carbon per hectare in 2005, indicating that not only the forest area but also the biomass and carbon stock per hectare had decreased.

In contrast, of all countries reporting significant total carbon stock increases (mainly Chile, China, many European countries, India, Japan and the United States), 67 percent also documented substantially higher levels of carbon stock per hectare, indicating a higher likelihood that stocks were actually assessed more than once. For 25 percent of these countries, carbon stocks per hectare remained essentially the same, pointing to an increase in forest area as the main reason for increased total stocks.

Source & ©: FAO  Global Forest Resources Assessment 2005, Progress towards sustainable forest management, Chapter 2: Extent of forest resources, p.30-36