Energy forests with Salix as a Carbon Dioxide sink

The time span of 50 years can to some extent be justified with human memory. If the present or the previous generation does not remember the particular site as a forest, then it will be afforested. And if they do remember that there has been forest on that site, then the site will be reforested.

For the sake of clarity, reforestation must be separated from forest regeneration which is a normal activity in sustained forestry. Regeneration establishes a new tree generation on a harvested forest land. Regeneration does not cause any land use change: the forest stays as a forest. Regeneration has neither any affect on the carbon dioxide stock of that particular site, in the long run at least, whereas reforestation builds up a new carbon dioxide stock.

Afforestation is typically artificial establishment by planting or seeding of forest on an area of agricultural land. In Finland, other Nordic countries or Baltic countries afforestation may well be an establishment of long rotation forest of silver birch, Betula pendula Roth on an agricultural land. For saw log production a long rotation of 60 years is needed. Afforestation can also establish a medium rotation forest of hybrid aspen, Populus x wettsteinii Hämet-Ahti. With it, a medium rotation of 30 years for pulpwood, may be sufficient. A more recent approach is to afforest the agricultural field with a short rotation Salix species, like osier, S. viminalis L. For bioenergy purposes a short rotation of 3-5 to years, or up to 10 years can be applied.

For reforestation the best example from Finland may be the cut-away areas of peat lands which have been utilized by peat industry enterprises for fuel and growing peat. Large scale peat fuel industry was initiated in Finland in 1971. Considerable areas for peat harvesting were cleared already by 31 December 1989 (FCCC reforestation time line divider). Towards the end of the 1990s the area of active peat fuel production had stabilized to about 50,000 ha. As the time span of peat fuel excavation with modern machinery reaches over 20-25 years, the first cut-away areas have already been vacated. Already towards the end of 1990s their total area was about 10,000 ha and in 2000s they will be vacated at a rate of 2,000-3,000 per annum (Pohjonen 1998).

Before the peat harvesting started, the Finnish peat bogs were growing mostly peat land forest, and only partly they were open. The start of peat industry effected a land use change (deforestation) in the area. Reforestation returns these open areas back into forests. Thus these areas fulfill the conditions of FCCC; the older areas most certainly for the first commitment period (until end of 2012), and the newer areas most probably for the second commitment period (beyond 2012).

The total area of technically and economically suitable peat production bogs in Finland covers 622,000 ha (Lappalainen and Hänninen 1993), out of which the Finnish peat industries have reserved 123,000 ha. Wettest bottom parts of the of the peat cutaway areas, maybe some 15-20 per cent of the total, will be reserved for regeneration of marshes and small lakes. The potential area for reforestation can therefore counted to be of some 100,000 ha, which is of the same order than the vacated agricultural fields for afforestation

Reforestation of cutaway peat lands has been studied in Finland over 50 years (Mikola, P. and Mikola, I. 1958, Kaunisto and Aro 1996). If the peat has been harvested to a remainder thickness of 30 cm of the bottom peat, the recommendable species for long rotation reforestation is silver birch. It benefits if the roots can penetrate through the remaining bottom peat into the bottom mineral soil. If the remainder peat thickness is over 30 cm a more recommendable species is downy birch, Betula pubescens Ehrh. Of the short rotation willows the best results has given tea leaved willow, Salix phylicifolia L. (Hytönen et al. 1995). Exotic willows to Finland, like Salix viminalis, Salix burjatica Nasarov and Salix x dasyclados Wimmer have suffered badly from the harsh microclimate of the peat land sites, especially from spring, summer and autumn frosts.

In the calculations of excess carbon dioxide movements between the atmosphere and the biosphere it is important to separate the concepts of carbon dioxide stocks and carbon dioxide flows. The excess carbon dioxide flow from the biosphere to the atmosphere is expressed as carbon dioxide emissions. The emissions are caused by man’s activities like burning of fossil fuels. The carbon dioxide emissions are known from the national energy statistics.

In Finland the carbon dioxide emissions in 2000s induced by fossil fuels have been on average 60 million tons per annum (Energiakatsaus 2007). Since 2005 the carbon dioxide has been valued in the stock market as emission allowances (www.europeanclimateexchange.com). In mid February 2008 the price of one ton of carbon dioxide emission allowances was 21.5 Euros. Calculating from these data the total value of annual Finnish carbon dioxide emissions was 1.3 billion EUR. It can be interpreted as annual Finnish national environment cost to the global climate.

The flow of excess carbon dioxide from the atmosphere to the forest sinks, also called carbon dioxide sequestration, is expressed as carbon dioxide capture from the atmosphere. The CO2 capture can be thought as a negative CO2 emission. In theory at least, if not yet in practice in Finland such carbon dioxide capture should be possible to deduct from the carbon dioxide emissions when calculating the national greenhouse gas balances. To some extent such principle, however, is globally already in practice in the Green Development Mechanism (CDM) under the Kyoto Protocol (http://cdm.unfccc.int/index.html).

The downwards flow of carbon dioxide from the atmosphere cannot be measured directly. Instead, it is measurable indirectly by determining the changes of carbon dioxide stock in the forest sink over a commitment period, which can be one year, 5 years, 10 years, or a calculation period from 1990 to present. The measurement is a modification of a standard forest inventory. First, the forest sink is measured by cubic meters (m3/ha), or rather by its dry matter biomass (tn/ha). The same is repeated in the next year. Bearing in mind that roughly 50 per cent of dry woody biomass is in elementary carbon, the change in carbon stock (tn C per ha per annum) is calculated. This change has been caused by the capture of carbon dioxide from the atmosphere. The capture as CO2 is calculated by multiplying the change in carbon stock by the atomic weights ratio 44/12 and is finally expressed in tons of CO2.

Which kind of change in the carbon dioxide stock is verifiable, as required by the Kyoto Protocol? An immediate possibility is to measure only the above ground woody biomass, using standard non destructive forest mensuration methods. It is known, at least academically, that an important part, some additional 25 – 50 per cent or even more, of the forest sink is in the roots of the trees and in the soil humus. However, as the underground forest sink cannot currently be measured in a non destructive verifiable manner, it was ruled out from the Kyoto Protocol.

The determination the effect of forest sinks in the sense of the Kyoto Protocol requirements can be simplified to the following three steps. First, starting from the beginning of the commitment period, from year 1990, the number of hectares which have been afforested or reforested, are determined year by year. Second, the above ground woody biomass stock over the total afforested and reforested lands is determined either year by year or at least for the beginning year 1990 and for the ending year (present). Third, the annual changes in carbon stock are calculated and converted into captured carbon dioxide from atmosphere to the forest sinks, again starting from 1990 and ending in present (or in fixed year in the future).

This paper examines by simulation means the value of short rotation energy forests with Salix as carbon dioxide sinks. Two special cases are considered: Salix viminalis in southern Finland on agricultural land with 7 years rotation and Salix phylicifolia in northern Finland on peat cutaway land with 10 years rotation. The commitment period in both cases is 30 years starting from 2008 and ending in 2037.

The rotation of 7 years rotation for Salix viminalis is 2-3 years longer than in the standard Swedish energy forest approach (www.agrobransle.se). The aim is to raise the woody biomass and carbon dioxide stock to somewhat higher level which is beneficial in the forest sink calculations. The growth of the energy forest plantation continues still after the 4th or 5th year, not at highest but still at moderate level, as shown in the growth and yield studies by Hytönen (1995). He let Salix burjatica plantation grow until 7 years of coppice age. Similarly, the rotation of 10 years for Salix phylicifolia is longer than normally anticipated. This rotation time is based on experience and long term measurements from Piipsanneva peat cutaway trial area in Finland (Hytönen et al. 1995, Hytönen 1998).

The mean carbon dioxide captured, tons CO2/ha, is a concept analogical to mean volume, m3/ha, in conventional forest management.

Fig. 2. Mean CO2 in S. viminalis

The mean carbon dioxide is calculated by dividing the captured carbon dioxide in the forest by the area that has already been planted.

Building a carbon dioxide sink with any forest is essentially an aim at raising the mean carbon dioxide of the forest as quickly and as high as possible, and keeping it there. In the energy forest of Salix viminalis the mean carbon dioxide raises within 7 years to 50 tn CO2/ha and is kept in the long run at level of about 55 tn CO2/ha (Figure 2).

The total amount of carbon dioxide captured into 150,000 ha and over 30 years, is 8.81 million tons. Applying the carbon dioxide stock price 21.5 EUR per ton CO2 this accrues a total saving of 189 million EUR in the carbon dioxide emission allowances that have been saved during the commitment period 2008-2037. Calculated over the whole energy forest area of 150,000 ha this makes at the end of the 30 years commitment period a sink value of Salix viminalis energy forest at 1,263 EUR per hectare. The development of the sink value (in EUR/ha) of the Salix viminalis energy forest as a function of time is shown in Figure 3.

Case 2: Salix phylicifolia on cutaway peat land