Energy forests with Salix as a Carbon Dioxide sink

Introduction

The United Nations Kyoto Convention on Climate Change (Kyoto Protocol … 1997) accepted in 1997 the principle of forest sinks as a means to combat the threatening climate change. According to that principle the increasing emissions to atmosphere of carbon dioxide can be counterbalanced by capturing the excess carbon dioxide from the atmosphere down to the biosphere. The growing forests were understood to be the most realistic sink onto which man can have an immediate and sufficient effect.
The acceptable forest sinks were defined with rather strict characteristics. First of all, the forest sinks must be a result of direct, human induced activity, limited to afforestation and reforestation. The natural forests were ruled out, because they are acting as sinks also without direct human activity.  Secondly, only the afforested or reforested age classes since the beginning 1990, of the commitment period, were to be counted. The end of the initial commitment period was defined as five-year period of 2008 to 2012 over which the necessary mean values are calculated. And thirdly, the changes in stocks of the dry matter or in the captured carbon dioxide of the afforested or reforested areas had to be measurable in a verifiable manner.
Afforestation and reforestation refer to land use change where the land has been treeless for a long time. For the difference between afforested and reforested sites the Intergovernmental Panel for Climate Change (IPCC … 1996) defined the time span of 50 years: ”They (plantations) are either on lands that previously have not supported forests for more than 50 years (afforestation), or lands that have supported forests within the last 50 years and where the original crop has been replaced with a different one (reforestation)”.
Later the United Nations Framework Convention of Climate Change (FCCC 2001) redefined the reforestation as ”the direct human-induced conversion of non-forested land to forested land through planting, seeding and/or the human-induced promotion of natural seed sources, on land that was forested but that has been converted to non-forested land. For the first commitment period, reforestation activities will be limited to reforestation occurring on those lands that did not contain forest on 31 December 1989”.
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).

Simulation of carbon dioxide capture

Case 1: Salix viminalis afforestation on agricultural land

 
For the simulation standard (virtual) plantations of Salix viminalis, using Swedish energy forestry approach and technology are established on agricultural soils in Southern Finland. The planting starts in the year 2008 and ends in 2037. The plantation establishment rate is 5,000 ha per annum, so in 30 years a total area of 150,000 ha of energy forest carbon sink will be established. After the first growing year the stems are cut down to allow for sufficient coppicing. Then the trees are let grow for seven years and cut again. The first harvest therefore takes place at age of 8 years. The consecutive coppice stands are always harvested after 7 years.
The yield target of 60 tn/ha is set for the Salix viminalis at harvest. For the first rotation of 8 years this equals to mean annual increment (MAI) of 7.5 tn/ha/a. For the coppice rotation of 7 years the MAI is 8.6 tn/ha/a. The provisional growth and yield tables are presented in Table 1. The carbon dioxide stock target at the end of all rotations (time of harvesting) is 110 tn CO2/ha.
Table 1. Provisional growth and yield table for Salix viminalis energy forest growing on an agricultural land in Southern Finland. First (seedling) rotation. Cumul Y: cumulative standing woody biomass dry tonnes per hectare, CAI: Current Annual Increment, MAI: Mean Annual Increment. CO2: cumulative carbon dioxide capture to the stand, tn/ha. Carbon content of the dry matter is 50 per cent, atomic weight ration CO2/C is 44/12 or 3.6667.
 Year Cumul. Y,tn/ha  CAI tn/ha/a  MAI tn/ha/a  CO2 tn/ha
    1        0      0.0      0.0      0.0
    2        5      5.0      2.5      9.2
    3      14      9.0      4.7    25.7
    4      26    12.0      6.5    47.7
    5      36    10.0      7.2    66.0
    6      45     9.0      7.5    82.5
    7      53     8.0      7.6    97.2
    8      60     7.0      7.5  110.0
Table 2. Provisional growth and yield table for Salix viminalis energy forest growing on an agricultural land in Southern Finland. Coppice rotation. Cumul Y: cumulative standing woody biomass dry tonnes per hectare, CAI: Current Annual Increment, MAI: Mean Annual Increment. CO2: cumulative carbon dioxide capture to the stand, tn/ha. Carbon content of the dry matter is 50 per cent, atomic weight ration CO2/C is 44/12 or 3.6667.
 Year Cumul. Y,tn/ha  CAI tn/ha/a  MAI tn/ha/a  CO2 tn/ha
    1        5      5.0      5.0      9.2
    2      14      9.0      7.0    25.7
    3      26    12.0      8.7    47.7
    4      36    10.0      9.0    66.0
    5      45      9.0      9.0    82.5
    6      53      8.0      8.8    97.2
    7      60      7.0      8.6  110.0
Each year the planting of 5,000 ha makes an age class or block, which starts to grow according to the growth and yield table.

Fig. 1. Total CO2 in S. viminalis

The first age block (2008) is harvested in autumn 2015, the coppice continues the growth according to the coppice growth and yield table and it is again harvested in autumn 2022, 2029 and 2036.

Each year the carbon stock is summed over the already established and grown age blocks (Figure 1.). The carbon dioxide capture starts in 2009, after the cut back of the first year shoots. The later harvestings are seen in the carbon dioxide accumulation curve as slight bends.
After the 30 year commitment period the total amount of carbon dioxide captured into 150,000 ha is 8.81 mill. tons.
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.
Fig. 3. Sink value of S. viminalis
The sink value can be thought to be an additional value of the energy forest, besides its main function to produce the biomass which can be used to substitute the fossil fuels. In practical terms the per hectare sink value can be thought as a partial payment for the afforestation cost. The sink value is of course strongly dependant on the level of the carbon dioxide price (the stock value for carbon dioxide emission allowances).

Case 2: Salix phylicifolia on cutaway peat land

Rather substantial research work into growing energy forests on cutaway peat lands was carried out in Finland between 1979 and 1995, as has been summarized by Hytönen (1996). Common to the experiments has been the early reliance on the exotic willows Salix burjatica and Salix x dasyclados Wimmer. The results with the exotics have been poor. A more recent interest has been shifting to the possibilities of the frost hardy indigenous willows, for which there is a range of 21 species for selection (Pohjonen 1991).
Little is known on the growth of indigenous willows of Finland on cutaway peat lands. Some early results were given by Lumme and Törmälä (1988), but without biomass growth data. The only trial of 1980s with large enough plots for woody biomass measurement was established in 1984 in Piipsanneva peat cutaway area in Haapavesi, 100 km south of the city of Oulu. The trial was measured twice, at the age of 6 years (Hytönen et al. 1995) and at the age of 11 years (Hytönen 1998). Of the tested species Salix phylicifolia produced best. At the age of 6 years it had a biomass stock of 38 tn/ha. Between the 6-11 years the growth was even higher. The woody biomass stock was raised to 96 tn/ha (Table 3). It equals to mean annual increment of 8.7 tn/ha/a.
Table 3. Woody biomass dry tn/ha of various deciduous trees in Piipsanneva peat cutaway area in Finland at ages of 6 and 11 years. Species: Betula pendula, Betula pubescens, Alnus incana, Salix phylicifolia, Salix triandra, Salix x dasyclados. The original planting density for Salix sp. was 40,000 cuttings per hectare, for the others 20,000 seedlings per hectare (Hytonen et al. 1995, Hytonen 1998).
Species 6 yrs 11 yrs
 BetPen  21 81
 BetPub  25  72
 AlnInc  24  52
 SalPhy  38  96
 SalTri  31  62
 SalDas  16  0
It is notable that the (cumulated) woody biomass production of the exotic willow Salix x dasyclados was poor (16 tn/ha) over the first six years. Besides, it died away between 6 and 11 years. The other indigenous, fully winter hardy species, Salix triandra L. performed moderately. It is natural and more suitable on mineral soils, whereas Salix phylicifolia typically grows on peat lands and other moist soils in Central and Northern Finland. The rather high production of longer rotation birches and grey alder, Alnus incana (L.) Moench can partly be explained at exceptionally high planting density, 20,000 seedlings per hectare.
For the simulation, and based on the experience from Piipsanneva peat cutaway area, standard (virtual) plantations of Salix phylicifolia, again using Swedish energy forestry approach and technology, are established on peat cutaway soils. The climatic conditions refer to Northern and Middle Finland where the peat soils dominate and large areas of peat industries are situated. The planting starts in the year 2008 and ends in 2037. The plantation establishment rate is 2,000 ha per annum, so in 30 years a total area of 60,000 ha of energy forest carbon sink will be established. After the first growing year the willow stems are cut down to allow for coppicing. Then the willows are let grow for 10 years and cut again. The first harvest is therefore taken place at age of 11 years. The consecutive coppice stands are always harvested after 10 years.
The yield target of 70 tn/ha is set for the Salix phylicifolia at harvest. For the first rotation of 11 years this equals to mean annual increment of 6.4 tn/ha/a. For the coppice rotation of 10 years the MAI is 7.0 tn/ha/a. The provisional growth and yield tables are presented in Tables 4 and 5. The carbon dioxide stock target at the end of all rotations (time of harvesting) is 128 tn CO2/ha.
Table 4. Provisional growth and yield table for Salix phylicifolia energy forest growing on a peat cutaway land in Northern and Middle Finland. First (seedling) rotation. Cumul Y: cumulative standing woody biomass dry tonnes per hectare, CAI: Current Annual Increment, MAI: Mean Annual Increment. CO2: cumulative carbon dioxide capture to the stand, tn/ha. Carbon content of the dry matter is 50 per cent, atomic weight ration CO2/C is 44/12 or 3.6667.
 Year Cumul. Y,tn/ha  CAI tn/ha/a  MAI tn/ha/a  CO2 tn/ha
    1        0      0.0      0.0      0.0
    2        3      3.0      1.5      5.5
    3        8      5.0      2.7    14.7
    4      15      7.0      3.8    27.5
    5      24      9.0      4.8    44.0
    6      34    10.0      5.7    62.3
    7      44    10.0      6.3    80.7
    8      52      8.0      6.5    95.3
    9      59      7.0      6.6  108.2
  10      65      6.0      6.5  119.2
  11      70      5.0      6.4  128.3
Table 5. Provisional growth and yield table for Salix phylicifolia energy forest growing on a peat cutaway land in Northern and Middle Finland. Coppice rotation. Cumul Y: cumulative standing woody biomass dry tonnes per hectare, CAI: Current Annual Increment, MAI: Mean Annual Increment. CO2: cumulative carbon dioxide capture to the stand, tn/ha. Carbon content of the dry matter is 50 per cent, atomic weight ration CO2/C is 44/12 or 3.6667.
 Year Cumul. Y,tn/ha  CAI tn/ha/a  MAI tn/ha/a  CO2 tn/ha
    1        4      4.0      4.0      7.3
    2      10      6.0      5.0    18.3
    3      19      9.0      6.3    34.8
    4      30    11.0      7.5    55.0
    5      39      9.0      7.8    71.5
    6      47      8.0      7.8    86.2
    7      54      7.0      7.7    99.0
    8      60      6.0      7.5  110.0
    9      65      5.0      7.2  119.2
  10      70      5.0      7.0  128.3
Each year the planting of 2,000 ha makes again an age class or block. It starts to grow according to the Salix phylicifolia growth and yield table.

Fig. 4. Total CO2 in S. phylicifolia

The first age block (2008) is harvested in autumn 2018, the coppice continues the growth according to the coppice growth and yield table and it is again harvested in autumn 2028. Each year the carbon dioxide stock is summed over the already established and grown age blocks. The carbon dioxide capture starts in 2009, after the cut back of the first year shoots. The later harvesting are seen in the carbon accumulation as slight bends. After the 30 year commitment period the total amount of carbon dioxide capture into 60,000 ha is 4.07 mill. tons (Figure 4).

In the energy forest of Salix phylicifolia the mean carbon dioxide rises within 10 years to 60 tn CO2/ha and is kept in the long run at that level (Figure 5). The first drop is caused by the first harvesting of the matured age blocks. The later drops are slighter.

Fig.5. Mean CO2 in S. phylicifolia

The mean carbon dioxide with longer rotation Salix phylicifolia is slightly higher than with shorter rotation Salix viminalis (55-60 tn CO2/ha).

The total amount of carbon dioxide captured into 60,000 ha and over 30 years, is 4.07 million tons. Applying the carbon dioxide stock price 21.5 EUR per ton CO2 this accrues a total saving of 87 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 60,000 ha this makes at the end of the 30 years commitment period a sink value of Salix phylicifolia energy forest of 1,457 EUR per hectare. The development of the sink value (in EUR/ha) as a function of time is shown in Figure 6.
Fig. 6. Sink value in S. phylicifolia

Conclusions

The Kyoto protocol and the concept of afforestation and reforestation as a means to establish carbon dioxide sinks, gives additional value to energy forestry with Salix. On rotations from 7-10 years a mean carbon dioxide density of 50-60 tons CO2 per hectare can be built. Of all the bioenergy crops on agricultural, or otherwise arable, this gives an advantage to perennial Salix. Annual crops like reed canary grass (Phalaris arundinacea), rape seed (Brassica sp.) for biodiesel or Miscanthus sp. do not build an above-ground carbon dioxide sink in the sense the Kyoto protocol definition. The carbon dioxide density of the short rotation energy forest will always remain lower than the carbon density in the long rotation forests (typically about 150 – 200 tn CO2/ha in Finland), but on the other hand the carbon sink can be built faster with Salix.
The carbon dioxide sinks between Salix viminalis on agricultural lands and Salix phylicifolia on cutaway fuel peat areas do not differ much. Rather than selection of species the building of carbon dioxide sinks depends on selection of sites. Agricultural lands all over Europe are most obvious land resource for afforestation. Reforestation possibilities are smaller, like finding cutaway peat lands in Finland. Also some other exploited land areas may become available for reforestation, like old quarries, although their land area might be small.
The Kyoto convention also accepted the principle of Joint Implementation for establishment of forest sinks in various parts of the world. For building the carbon dioxide sinks the joint implementation opened up a possibility for joint afforestation or reforestation so that the forests are planted in second country. Later the first country can count for the carbon dioxide sink benefits in her national energy statistics, if the mutual country agreement so defines. The simulation approach described above could be applied in such joint implementation short rotation projects, with Salix or other fast growing species.
Another possibility to use the simulation approach is to use it in advance studies of Clean Development Mechanism (CDM) reforestation projects in developing countries. For instance in a country like Ethiopia farmers have planted after 31 December 1989 substantial amounts of fast growing eucalypts onto previously deforested lands. Similarly to Salix, plantations with Eucalyptus start quickly capture carbon dioxide from the atmosphere. Similar coppice silviculture is used with both tree species, and similarly they build rather constant, but considerable mean carbon dioxide stock per hectare. Clean Development Mechanism provides a method to quantify the positive reforestation effect into the climate change and channeling for instance the European carbon dioxide emission funds as a new-type development assistance to developing countries and to their farmers.

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