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Denmark's Greenhouse Gas Projections until 2012, an update including a preliminary projection until 2017

6 Land-use Change & Forestry

The total area of Denmark is 43000 km². About 10% or 4170 km² (417,000 ha) of the area was forest in 1990, this includes 60 km² not covered at the moment of counting [30]. The forest area is defined as the area covered with trees. This means that open woodland and open areas within the forests are not included.

At the moment there is a discussion in IPCC whether sequestration in forest can be included in the Danish totals. In this report it is assumed that CO₂ sequestration in new forests planted after 1. January 1990 can be included. The sequestration in forests existing prior to 1990 is not included in the totals in the projections in this report.

Table 22 shows the conversion factors used for the calculations. In the Danish literature an expansion factor of 2.0 is used both for broadleaves and conifers [33]. This value also includes roots and some carbon in the undergrowth and soil (IPCC default expansion factors [36] are 1.75 for undisturbed forests and 1.90 for logged forests). In order to estimate the stored amount of CO₂, the stem wood volume is first multiplied with the expansion factor to include the additional biomass apart from commercial stem wood: branches, leaves/needles, stumps and roots. This is finally multiplied with the carbon content (as CO₂ equivalents) per cubic meter.

In the default IPCC methodology the last multiplication is separated into three steps: first the roundwood volumes are converted to tons of dry biomass by multiplying with 0.65 for broadleaf trees and 0.45 for conifers, next converted to tons of carbon (by multiplying with 0.5) and finally converted to CO₂ (by multiplying with 44/12). Combining these three factors gives 1.19 t CO₂/m³ and 0.83 t CO₂/m³. Comparison with Table 22 shows that the Danish calculations use a lower carbon content than default IPCC values for both broadleaves and conifers. This is due to lower conversion factors for dry mass per volume of wood.

Table 22.
Conversion factors

6.1 CO₂ Sequestration in Existing Forests

Two hundred years ago, the forests of Denmark were exploited to such a degree that only about 2-3% of the land was covered by forest [31]. One of the consequences was a serious threat of sand drift. The trend was successfully changed and the forest area has increased until the present 10% forest cover. During the last decade, the stock of existing forests has also been increasing on a per hectare basis. The reasons for this are a distorted age distribution because of wind-throw and a diminished harvest due to current low prices.

The CO₂ uptake by existing forests has not been calculated with a model. It is based on the forest statistics. According to the Forest Act of 1989, a forest census must be produced every 10 years. The latest census [59] of the total standing volume over bark (stem wood) in 2000 showed a much higher CO₂ sequestration than for the former 10-year period. For the period 1980 to 1989 0.916 Mt CO₂ per year was sequestered in Danish forests. According to the new count it is now estimated that the average annual uptake in the period 1990-99 was four times higher, or about 3.9 Mt CO₂/year. This large increase is partly caused by the large areas regenerating from the massive fall of conifers in the storm in 1981. However, the sequestration in forest existing before 1990 is not included in the projections in the present report.

6.2 CO₂ Sequestration in New Forests

Since 1987 it has been a strategy of the Danish Government to double the forested area within the next 80-100 years, that is within about a forest generation. Using the forest model mentioned in the following it was calculated that the permanent storage in mature Danish forest is between 500 and 850 tCO₂/ha [33]. The current prognosis of afforestation is that about 200,000 ha will be afforested within the period. This would result in about 125 million tons CO₂ being stored in the first 125 years or about 1 million tons CO₂/year as an average annual sequestration over the period (see Table 24)

The CO₂ sequestration in new forests has been calculated using a model developed in Denmark [33,35,37]. As opposed to the IPCC guidelines [36], the model calculates the annual changes in the carbon stored over the whole tree generation. It does not only take into account the growth rates of the forests. It also attempts to bring into the calculation the "fate" of the stored carbon, thus recognising the "delay" in the release of carbon stored in (1) commercial wood products: fuel wood, paper and wood products with short and long lifetimes and (2) roots, leaves/needles and branches/stumps. The model uses constant five-year decomposition rates in the calculations. However, in our projections we assume that the carbon stored in the commercial wood products just replace similar wood products on the marked, which are burned after the replacement, thus resulting in no net increase of carbon stored in wood products.

The model consists of an EXCEL file with three parts: The model calculates the carbon stored annually in the 100 year period 1990-2089.

The first input to the main module of the forest model is the annual area afforested according to the Danish Afforestation Plan (see Table 23). The model operates with the two major species used for afforestation, oak (Quercus robur) representing broadleaves and Norway spruce (Picea abies) representing conifers.

Table 23.
Annual area afforested.

The total afforestation area for the period 1990-1999 is based on table 1 in the recent evaluation of the afforestation in the period 1989-1998 [56]. Distribution between broadleaves and conifers are based on the estimate, that 2/3 is broadleaves and 1/3 is conifers [34]. However, based on the new forest census [59] the historic distribution for 1990-1999 has been changed to 48% broadleaves and 52% conifers. Also the total afforested area has been reduced compared to the former projection [58], since it has been recognised that a large part of the afforested area has been planted with Normanns fir. These stands are more representative of short-rotation forestry, as trees are cut about 10 years for Christmas trees and greenery. Furthermore, the land-use change is not permanent, as changes in the market for Christmas trees may force land owners to revert the land use to agriculture after a few years.

The present prognosis for the years 1999-2003 do not satisfy the parliamentary agreement on the aquatic environment (Vandmiljøplan II), that 20000 ha shall be afforested during the period 1998-2003 (6 years). Only about 16000 ha is expected to be afforested. The projection for the areas afforested in the period 2004-2012 is based on information in the report from "Wilhelmudvalget" [38]. After 2012 it is assumed that the rate of afforestation is going to be equal to the level before the parliamentary decision, i.e. in average a total of 2000 hectares [38]. However, this means that we will only reach an afforestation of about 200,000 ha in 2089, e.g. a 50% increase in the forest area. In order to reach the 100% increase of the forest areas additional measures have to be implemented.

The next input to the main module of the forest model is how fast one hectare of new broad-leaved or coniferous forest sequesters CO₂ over its lifetime. This information is calculated in the two other modules "broadleaves" and "conifers". "Broadleaves" represents the broad-leaved forest and contains empirically based yield tables for annual increment (5-year steps), thinning harvests every 10 years, and the final harvest after a rotation time of 140 years (in m³ roundwood). "Conifers" represents the conifers and contains similar yield tables for a rotation time of 70 years. These key values are based on the experience of Danish foresters documented in [39]. The "Broadleaves" module contains yield tables for 4 different site qualities ("boniteter") and "Conifers" includes only for 3 site qualities, since afforestation sites for conifers are assumed to be among the three best site quality classes. So far, the model only uses the values for site quality 2, since this site quality is assumed to be average.

The annual increment and thinning harvests in m³ in "Broadleaves" and "Conifers" are then converted to CO₂ stored in the trees and the products by using the conversion factors in Table 22 and assumptions about the percentage composition of the wood product categories.

Based on these inputs the main module of the forest model separately calculates the CO₂ stored in the forest area planted each year over the next 100 years. Finally, for each of the next 100 years, the respective amounts of CO₂ stored in plantations of different age are added. The final result (without products) for the two tree types is shown in Table 24. In the first commitment period 2008-12 under the Kyoto Protocol the total absorption in new forest is 1416 kt CO₂ or equal to an average annual absorption in these five years of 283.2 kt CO₂. Table 24 also shows that the average annual sequestration until 2090 is about 0.8 Mt CO₂.

Table 24.
CO₂ sequestration in new forest.

6.3  CO₂ Sequestration in forest soils

According to IPCC [36] carbon stock estimates should include the total organic carbon content to a depth of 30 cm in the mineral soils as well as the carbon content of the organic layer accumulated on top of the mineral soil. A recent Danish study quantified the amounts of carbon in well-drained Danish forest soils and found an average of 125 t C/ha in the forest floor and to 1 meter depth for 140 forest sites [61]. For the forest floor and to 30 cm in the mineral soil, the carbon storage was 93 t C/ha.

The development in carbon storage over time since afforestation is currently being studied in detail, but the knowledge of differences in carbon content between agricultural soils and forest soils is still scarce for Danish conditions. However, a current Danish study aims [62] aims at quantifying the carbon pools in former agricultural soils following afforestation with oak and Norway spruce. This study could provide important input to the model calculations of carbon storage due to afforestation activities. There are a few results available for nutrient rich soils as yet, but for the organic layer alone, about 8-10 ton C/ha may be sequestered in Norway spruce stands and about 2 ton C/ha may be sequestered in oak stands after 30 years in the upper 5 centimetre of the soil [60]. There was no evidence of increased carbon content in the mineral soil 30 years after afforestation, but a similar study on nutrient-poor soil showed an increase in the upper 25 cm of the mineral soil of 23 t C/ha over 40 years [34].

However, this CO₂ sequestration in forest soil is not included in the values in Table 24, because it is still not possible to generalize for the whole country. It is assumed that the rather large expansion factor in Table 22 includes some forest soil carbon.

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