An evaluation of land suitability for forest fertilization with biofuel ash on organic soils in Sweden

Forest Ecology and Management 209 (2005) 43–55 www.elsevier.com/locate/foreco

An evaluation of land suitability for forest fertilization with biofuel ash on organic soils in Sweden Bjo¨rn Ha˚nella,*, Tord Magnussonb,1 a

Department of Silviculture, Swedish University of Agricultural Sciences, SE-901 83 Umea˚, Sweden Department of Forest Ecology, Swedish University of Agricultural Sciences, SE-901 83 Umea˚, Sweden

b

Abstract The nutrients removed from the forest when branches and tree tops are harvested for fuel can be returned to the site by bringing back the wood-ash from the burning. In Sweden, this compensation measure is not carried out to any appreciable extent, mostly because there is no economic incentive to the landowner. The ash contains all the elements required for tree growth except for nitrogen (N), the most important element limiting growth on mineral soils. Since ash, so far, is brought back only to mineral soils, increased forest growth cannot be expected. In contrast, for organic soils, N is often abundant whereas the amounts of other mineral nutrients in peat are small. This means that the peatland forests provide an opportunity for ash amendment in order to increase forest production. Old fertilization trials using wood-ash show that the growth increase can be very large. The aims of this study were (i) to calculate the area of peat-covered land that, with respect to stand growth responses, could be regarded as most suitable for biofuel ash (wood-ash and peat-ash) fertilization and (ii) to assess the amount of biofuel ash needed for fertilizing this area. Most of the area calculations were based on data from the Swedish National Forest Inventory (SNFI) 1997–2001. Sites were selected using existing knowledge about ash fertilization effects on tree growth and with the aid of registrations made in SNFI regarding peat thickness, site productivity, drainage, condition of drains, dominating field vegetation, and stage of stand development. Additional calculations were made to estimate the area of abandoned peat harvesting fields ready for after-use by afforestation. According to the selection criteria used, the most suitable sites for biofuel ash fertilization are drained, mid-rotation or old peatland forests where the field vegetation is dominated by shrubs or low sedge plants indicative of medium-low site productivity. The selected area comprises 190,000 ha. Most are located in North and North Central Sweden (90,000 ha), whereas South Central and South Sweden accounted for 30,000 and 70,000 ha, respectively (Figs. 1 and 2). To these areas, 2000–3000 ha of abandoned peat fields ready for afforestation should be available within a period of about 5 years. The present annual production of biofuel ash in Sweden, of acceptable quality for forest fertilization, is 250,000– 300,000 tonnes. If it were desired to fertilize all the sites (190,000 ha) identified in this study with 5000 kg per ha, it would require 3–4 years of the current annual production of bio-ash. # 2005 Elsevier B.V. All rights reserved. Keywords: Wood-ash; Peat-ash; Recycling; Growth response; Peatland forests

* Corresponding author. Tel.: +46 90 786 8297; fax: +46 90 786 8414. E-mail addresses: [email protected] (B. Ha˚nell), [email protected] (T. Magnusson). 1 Tel.: +46 90 786 8344; fax +46 90 786 8163. 0378-1127/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2005.01.002

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1. Introduction About one third of the 1,000,000 tonnes of ash annually produced in Sweden originates from biofuels, which include the branches and tree tops from tree harvesting, residues from saw mills, and fiber sludge from pulp and paper industries (Bjurstro¨ m et al., 2003). At present, most of the biofuel ash is dumped in landfills. The Swedish National Board of Forestry recommends forest amendment with biofuel ash to compensate for the reduction in base cations (calcium, magnesium, and potassium) that could result from whole-tree harvesting (Anon, 2001a). There are concerns that the base cation reduction jeopardizes the long term site productivity, and the aim is thus to maintain productivity by compensation fertilization. The need for such fertilization is different for mineral and organic soils. Mineral soils account for 80% of the Swedish forest land area. The most important limiting element for tree growth in these soils is nitrogen (N). Another mineral soil characteristic is that the reduction of base cations is counteracted by weathering of rock, although the speed of this process in relation to losses is uncertain. Recycling of ash constituents in Sweden has so far been practised exclusively on mineral soils and only to a small extent compared to the estimated compensation need (Anon, 2002a). One reason for this could be that the short-term production losses from biofuel harvesting on mineral soils have not been conclusively proven (Jacobson et al., 2000). Also, bringing back mineral nutrients as a compensation measure on these soils is believed to enhance sustainability of site productivity, but results to date have not shown significant shortterm effects (Sikstro¨ m, 1992; Egnell et al., 1998; Jacobsson, 2001). Hence, a reason for the low level of ash recycling activity could be the lack of economic incentive for the landowner. One other desirable effect expected from the compensatory ash fertlilization is its counteraction of anthropogenic acidification of forest soils. This motive is particularly important in southwestern Sweden, because of the concomitant impoverishment of the flora in this region (Ho¨ gbom et al., 2001; Anon, 2001b). One-fifth of the forest land in Sweden is covered by a thin or a thick peat layer (organic soils). Here, no weathering of minerals is occurring, and tree growth is limited by the amount of mineral elements present in

the peat layer, especially phosphorus (P) and potassium (K). Wood-ash contains all the nutrient species a tree needs except for nitrogen. Thus, woodash amendments to organic soils most often result in significantly increased forest growth (Malmstro¨ m, 1952; Silfverberg and Hotanen, 1989; Silfverberg and Huiukari, 1985; Hallenbarter et al., 2002; Moilanen et al., 2002). The growth effects are related to the properties of the soil substrate, and are apparent for most common tree species, e.g., pine, spruce and birch. Obviously, ash may be used as a forest fertilizer on organic soils. Indeed, the measure may be highly profitable from an economic point of view. Lauhanen et al. (1997) found very favourable internal rates of return between 3.7 and 9.3%. This option suggests utilizing land already claimed for forest production through drainage. A selection of suitable sites from the 1.5 milllion ha of forest drainage projects carried out (Ha˚ nell, 1990) would therefore be desirable. Calculations on the areas of drained and undrained peatcovered land have been done earlier but these reports (e.g., Ha˚ nell, 1989) only allow for a small part of the selections aimed for in the present analysis. In the search for the most suitable sites for forest fertilization with ash, the highest priority should be given to drained and wooded peatlands where tree growth is significant but reduced due to lack of those nutrients originating from minerals (category 1). Land that has been drained for forestry for a long time but still is non-productive, most often due to lack of both available nitrogen and other nutrients, should be given lower priority (category 2). Drainage done to utilize the peat substrate for fuel or horticultural purposes is also significant in this context. Machine harvesting of peat has been carried out to various extents in Sweden for at least 100 years. Peat harvesting for household requirements probably has a history of several hundred years. So far about 15,000 ha have been used for large-scale harvesting. According to current Swedish law, some kind of restoration should be started soon after the harvesting is completed. One of several treatment alternatives is afforestation (Svensson et al., 1998). Thus, peat fields (category 3) where the harvesting is finished and afforestation is chosen, as after-use treatment should be regarded as potential areas for ash amendments (Ha˚ nell et al., 1996; Magnusson and Ha˚ nell, 1996, 2000, 2001; Nilsson, 2001).

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On all of the three mentioned categories (1–3) relatively large doses of biofuel ash (wood-ash, peatash) can be added and a rapid and significant growth response can be expected in most cases. As for afforestation on depleted peat fields (category 3 sites), the stand establishment phase is promoted by a moderate ash dose (Ha˚ nell, 1997). Larger doses should not be given until a stand is established. Large ash doses increase the risk of nutrient leaching (Silfverberg, 1998). Since the ash also contains undesirable elements, for example, heavy metals, the ash doses should be restricted and kept within environmentally sound limits (Egnell et al., 1998; Perkio¨ ma¨ ki, 2004). The present study has been restricted to the possibilities for using biofuel ash as forest fertilizer on organic soils in Sweden. The aims of the study are: (i) to calculate the areas of peat-covered land that with respect to tree growth responses are the most suitable for forest fertilization with biofuel ash and (ii) to assess the amounts of ash that would be required to fertilize this land. Fig. 1. The four main regions of Sweden described in this study.

2. Materials and methods 2.1. Data selection The best available basis for calculations of peatcovered land in Sweden consists of data collected by the National Forest Inventory (SNFI). This is an allencompassing annual inventory of Swedish forests. Data are collected from temporary and permanent circular sample plots with a 7 and 10 m radius, respectively. The plots are laid out in a grid pattern, which is systematically placed over the country, a total land area of about 410,000 km2 in four regions and 21 counties (Fig. 1; Anon, 2000). The results reported in the inventory are not true values but rather estimates which are subject to random and systematic deviations. The former, commonly expressed as standard error, decreases with increasing sample size. In order to obtain acceptable accuracy at the county level, results are presented as the means for periods of at least 5 years, commonly the most recent 5-year period. Estimates of areas with shallow and deep peat layers (wet mineral soils < 30 cm, and peatlands  30 cm) have been reported earlier (Ha˚ nell, 1989, 1990). These

give the size of the peat-covered wetland resource but do not distinguish between drained wet mineral soils and drained peatlands. In the latter, the detailed examination is restricted to four main regions and not to counties. Thus, the earlier analyses of SNFI data have only limited value as a basis for assessing suitable areas for forest fertilization with biofuel ash. As for the assessment of the area of completed peat harvesting fields, SNFI data cannot be used at all since the sampling is too sparse. Data in the 1997–2001 SNFI material were selected with the aid of definitions documented in the SNFI Field Work Manual (Anon, 2002b) regarding peat thickness, site productivity, drainage, condition of drains, dominating plant species in the field layer, and stand maturity class (Table 1). Peat harvesting for soil amelioration and fuel purposes in Sweden is presently taking place on over 100 peatland areas totalling about 15,000 ha. Owing to a significant variation in peat thickness within most peat fields, the harvesting will be finished earlier in some parts of a field than in others. Only rarely, where the peat cover is more or less even, the harvesting will

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Table 1 SNFI definitions relevant to this work Peat thickness Peat-covered land

Sites with thin peat, less than 30 cm, are classified as wet mineral soils whereas peatlands have thicker peat Areas with surface peat, irrespective of thickness

Site productivity

Separates productive forest land (stand growth  1 m3 ha mires (<1 m3 ha 1 per year)

1

Drainage

A site is registered as drained when a ditch is found within 25 m from sample plot centre

per year) from non-productive

Drain condition

Each drain is registered as functioning or not functioning with respect to ability to draw off water

Dominating field vegetation

Sites were classified by nutrient and productivity status into four groups and named according to occurrence of indicator plants in the field layer Sites characterized by tall and/or low herbs: Circium heterophyllum, Tall Dryopteris species, Crepis paludosa, Cirsium palustre, Paris quadrifolia, Aconitum septentrionale, Angelica silvestris, Filipendula ulmaria, Rumex acetosa, and Potentilla erecta, Broad-leaved grass species, Gymnocarpium dryopteris, Maianthemum bifolium, Oxalis acetocella, Geum rivale, Equisetum palustre, Orchis species, Viola species, respectively Sites dominated by Vaccinium myrtillus, Vaccinium vitis idaea, Equisetum silvaticum, and tall Carex species Sites dominated by Eriophorum vaginatum, Carex limosa, Carex pausiflora, and Scirpus caespitosus Sites dominated by Empetrum nigrum, Calluna vulgaris, Vaccinium uliginosum, Ledum palustre, Andromeda polyfolia, and Vaccinium oxycoccus

Herbs

Rich shrubsa Low sedge Poor shrubsa Stand maturity class (for productive forest land) Bare land Seedling and sapling stands Unthinned stands Thinned stands

Old stands a

In this study, stands were grouped into five classes Forest land where the number of main stems is below the critical limit for stand (A1) Includes sapling stands with height <1.3 m (B1), young stands (diameter at breast height <10 cm) between 1.3 and 3 m (B2) and 3 m (B3) Most of dominant and co-dominant trees thinner than 20 cm at breast height (C1) Dominant and co-dominant trees either thinner or thicker than 20 cm at breast height (C2 and C3, respectively). Also, old stands that should be thinned once more (C4) are included here, and Forests registered for final felling, including both stands which have reached and not reached lowest recommended age for final cut (D1 and D2, respectively)

The division of the low-shrub communities into ‘‘rich’’ and ‘‘poor’’ is merely a reflection of their relative indicative value.

be finished simultaneously in all parts of the field. A database with area information on after-use treatments in peat fields has been established at the Swedish Peat Research Foundation. In our study, the field area potential for afforestation and ash-fertilization was estimated on the basis of this information and information from interviews with those persons responsible for the peat harvesting. Throughout this article, we use the following terminology related to ash. We have used the term biofuel defined as fuel originating from non-fossil biomass. The fuel may have undergone some chemical or biological process or change, or been subject to intermediate utilization. This includes tree-based fuels collected at forest harvest, by-products from the woodworking industry, and fiber sludge produced in

the pulp and paper industry. It also includes fast growing tree species, e.g., Salix, and agricultural crops grown for energy production. Whether or not to regard peat as a fossil fuel is debated, but in the context of this paper peat is considered a biofuel. Thus, peat-ash is included in the term ‘‘biofuel ash.’’ The ashes considered as available for forest fertilization, in this paper, are only pure biofuel ashes, i.e., excluding the vast amounts that are mixed with other types of ashes (e.g., from coal, oil, household waste, etc.). The annual production of pure biofuel ashes in Sweden is estimated to 250,000–300,000 tonnes (Bjurstro¨ m et al., 2003). The composition of these ashes will normally fall within recommended limit values for recyclable ashes set up by the National Board of Forestry (Table 2).

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Table 2 Recommended minimum or maximum contents of elements in ashes intended for spreading on forest land

Table 3 Drained forest land, categorized according to soil type, dominant field vegetation and regions, in ’000s ha

Element/substance

Soil type

Macronutrients (g kg 1) Ca Mg K P Trace elements (mg kg 1) B Cu Zn As Pb Cd Cr Hg Ni V Toxic organics (mg kg 1) Sum PAH

Lower limit

Upper limit

Dominating field vegetation Herbs

125 20 30 10

1000

500 400 7000 30 300 30 100 3 70 70 2

Figures for total polyaromatic carbons (PAH) are preliminary. Source: Swedish National Board of Forestry, 2001.

Peatland North North Central South Central South

3. Results—the areas of suitable sites for biofuel ash fertilisation 3.1. Drained peatland and wet mineral soil forest land The total area of drained peat-covered forest land encompasses over one million hectares and is fairly evenly distributed between thin and thick peat (wet mineral soils and peatlands, Table 3). Most of this area

Low sedges

Poor shrubs

Total

36 42 52 129

71 37 42 85

10 3 4 20

12 2 7 5

131 84 104 239

Whole country

259

238

37

26

559

Wet mineral soil North North Central South Central South

53 61 64 44

76 39 54 44

14 4 5 3

20 3 6 0

162 107 128 91

Whole country

244

212

26

29

488

89 104 116 173

147 76 95 129

25 7 9 23

31 6 13 5

293 192 232 330

482

447

63

54

1047

Total North North Central South Central South Whole country

The major part of available biofuel ashes originates, directly or indirectly, from tree residues, while peat-ashes constitute a small part. The composition of ashes may vary considerably, irrespective of the source of the fuel. However, some systematic differences between peat and wood-ashes, which need to be considered in the fertilization context, should be stressed: the potassium content is normally much lower in peat-ash – often only one-tenth compared to wood-ash (Eriksson, 1993); the phosphorus content is also lower in most peat-ashes – on average 60% of the content in wood-ash, according to data from Eriksson (1993).

Rich shrubs

(80%) was drained with ditches that, at the date of the inventory, were categorized as functioning (data not shown). The majority of drained forest land, about 90%, was mostly characterized by herbs and rich shrubs, and the remainder was dominated by low sedge plants and poor shrubs (Table 3). Of the 559,000 ha of drained peatlands, almost half are located in the South region (Table 3). For the wet mineral soils the areas (488,000 ha) are more evenly distributed over the regions, largest in North region and smallest in South region. All in all, the area of peatland and wet mineral soil land claimed for forestry through drainage is somewhat larger in the southern part of the country; 54% of all drained forest land is found in the South and South Central regions. Nearly all drained peatland and wet mineral soil forest land with non-functioning drains (180,000 ha) were dominated by herbs and richer shrubs (data not shown). About 120,000 ha of this area were peatlands and about 60,000 ha of those were found in South Sweden. Approximately, 5% (55,000 ha) of the one million hectares of drained forest land are bare land, including

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Table 4 Drained forest land categorized according to soil type, maturity class and region, in ’000 ha Soil type

Stand maturity class Bare land

Peatland North North Central South Central South

Seedling and sapling stands

Unthinned stands

Thinned stands

Older stands

Total

12 8 6 15

18 13 16 34

49 19 21 63

26 16 22 73

25 28 39 54

130 84 104 239

Whole country

41

81

152

136

147

559

Wet mineral soil North North Central South Central South

2 3 6 3

48 33 38 16

48 26 27 13

22 21 17 25

42 24 39 34

162 107 127 91

Whole country

14

135

115

139

489

14 11 12 18

66 46 54 50

97 45 48 76

48 37 38 98

67 52 78 88

292 191 230 330

55

216

266

221

285

1048

Total North North Central South Central South Whole country

clearcuts (Table 3). The large remainder is fairly evenly distributed among seedling, sapling, and young stands, unthinned stands, thinned stands and older stands. Land, which was drained with functioning ditches, covered about 200,000 ha in each of these four groups (data not shown). Most of the area with non-functioning ditches was peatlands (data not shown). About 60% of thinned stands and older stands were located in the South and South Central regions (Table 4). There is a significant north–south gradient regarding the older stands on peatlands. These have the least area in region North (25,000 ha) and the largest area in the South region (54,000 ha). 3.2. Drained non-productive peat-covered land Peat-covered land, which in spite of drainage has remained non-productive with respect to tree production, makes up 267,000 ha (Table 5). About 80% (215,000 ha) of this is peatland. Thus, the total area of non-productive wet mineral soils is about 52,000 ha (Table 5), and 10,000 ha of those had non-functioning ditches (data not shown).

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Table 5 Drained non-productive peat-covered land categorized according to soil type, dominant field vegetation and region, in ‘000 ha Soil type

Dominant field vegetation Herbs

Rich shrubs

Low sedges

Poor shrubs

Total

19 7 5 4

23 13 4 4

39 18 7 13

26 12 12 10

107 50 28 31

Whole country

33

44

77

61

215

Wet mineral soil North North Central South Central South

5 5 1 0

6 1 2 0

16 3 1 0

8 2 1 0

35 10 5 1

Whole country

11

9

21

21

52

24 11 5 4

29 13 6 5

56 21 9 13

34 14 13 10

142 60 33 32

44

53

98

72

267

Peatland North North Central South Central South

Total North North Central South Central South Whole country

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More than half (142,000 ha) of the drained but still non-productive peat-covered land was found in the North region. The northern dominance is especially great regarding the wet mineral soils which are rare in South Sweden. Two-thirds (180,000 ha) of all drained non-productive peat-covered land was dominated by either low sedges or poor shrubs, falling outside the potential ash fertilization area (Table 5). The area dominated by herbs and rich shrubs is mostly peatland (about 80%). 3.3. Peat harvesting fields and after-use Only a small percentage of the current peat harvesting area (about 15,000 ha) is completely cut away and eligible for after-use, in total about 500 ha. About half of these fields are located in the South Central region. According to Larsson (2003) and interviews made with persons responsible for the peat harvesting, after-use measures will be required for 3000–5000 ha within the following 5-year period. The harvesting fields that will be completed in this period are fairly well spread over the country.

4. Discussion 4.1. Factors affecting tree growth The 1.3 million ha of peat-covered land reported in Tables 3–5 can be regarded as potential sites for forest fertilization with biofuel ash in Sweden. Accurate area calculations on the most suitable sites cannot be made owing to insufficient knowledge on the relation between ash fertilization and stand growth responses in various sites. Most reported results from ash fertilization originate from Swedish and Finnish trials. These do, however, leave much to be desired regarding soil and site type representation as well as statistical reliability. The first reported ash fertilization experiments were established in 1918 and 1926 on South Ha¨ llmyren and North Ha¨ llmyren, respectively, both open, drained sites with thick peat in the province of Va¨ sterbotten (Malmstro¨ m, 1935, 1943, 1952; Bjo¨ rkman, 1941). Several of the other old Swedish ash fertilization experiments were also laid out on ditched treeless mires. The same is true for most of the first

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forest fertilizations with wood-ash in Finland which were carried out on mires where the forest drainage had failed (Lukkala, 1951; Silfverberg and Issakainen, 1996). Ash fertilization trials in mid-rotation and old forests are more recent. Most extensive is a series of experiments, about 170 in all, established by the Finnish Forest Research Institute between 1977 and 1985 (Silfverberg, 1996). In spite of shortcomings in the experimental foundation, existing knowledge can support a rough selection of the most suitable sites for biofuel ash forest fertilization. Such a selection should be made on the basis of information on climatic conditions, site drainage status, peat nitrogen content, peat depth, and stand maturity class (age, volume, and growth). 4.1.1. Climate The forest growth capacity of a site decreases with increasing latitude and height above sea level. Conversely, under the more favourable climatic conditions for tree growth, greater tree growth response can be expected after drainage and ash fertilization. The most commonly used expression of the climatic influence on tree growth is the temperature sum (threshold value: 5 8C), which is a function of both latitude and altitude for the site (Odin et al., 1983). The climate varies from extremely cold (<800 day degrees) in the upper and NW parts of the North region, to very favourable (>1500 day degrees) in the coastal areas of the South region. This north–south gradient is reflected in the region-wise division of the data used in this study. It may be hypothesized that the growth response from ash fertilization in a forest in the South region generally will be higher than the response from the same ash dose applied to a similar forest in the North region. For reasons of transport economy, however, a choice between regions can rarely be made regarding where a specific amount of ash should be used as fertilizer. Thus, in this study the selection of sites suitable for ash fertilization has been made regardless of geographical location. 4.1.2. Drainage status of site The experiences from forest fertilization in sites with high ground water levels are clear—a good growth response can only be achieved on well-drained sites. The nutrient uptake by trees is not effective when the tree roots are hampered by water. Experiments

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show that the growth response following repeated fertilization was much greater when the ground water level was kept at 30–70 cm from the soil surface compared to raising the ground water level to only 10 cm below the surface (Silfverberg, 1984). This suggests that sites with functioning drains should be the first choices for ash fertilization. 4.1.3. Peat nitrogen content Biofuel ash contains all the elements except for nitrogen needed for tree growth (Paavilainen and Pa¨ iva¨ nen, 1995). The basic ash (pH 9–13) has a strong neutralizing effect and increases the microbial activity in the surface peat which in turn changes the composition of the flora towards more easily decomposable herbs and grasses (Malmstro¨ m, 1952; Karsisto, 1979). These effects of the ash as well as its positive effect on tree growth increase with the peat nitrogen content. According to Holmen (1969), the nitrogen content of the surface peat should be at least 1.3–1.5% to satisfy forest growth. This approximate limit coincides with the results from an older study on 55 ash-fertilizing trials well distributed over Finland in which Silfverberg and Huiukari (1985) found that the growth response was always small when the total nitrogen content of the peat was lower than 1.0%. The chemical properties of peat are primarily determined by the chemical composition of the peatforming plant societies and the peat decomposition rate (Kivinen, 1933). Therefore, a clear correlation exists between peat nitrogen content, peat type, and site type. For example, the concentration of N in Carex peat is higher than in Sphagnum peat (Bohlin et al., 1989). Furthermore, the N content in peat on herbdominated sites is greater than on sites characterized by tall sedges, and the peat N content in tall sedge sites is higher than in sites dominated by low sedge plants (cf. Westman, 1981). Practical guidelines on forest fertilization, based on the nutrient conditions in peat and peatlands as described by Huikari (1952), were reported by Paavilainen (1979). Sites characterized by nutrient demanding mosses, grasses, and herbs, should not be subject to nutrient amendments, but less nutrient-rich sites where potassium and phosphorus deficiencies are common should be fertilized. It is underlined that N must also be added in nutrient poor, ombrotrophic sites in order to increase forest growth. The essence of these recommendations is supported by

results from a large number of studies, cf. Tamm (1965), Braekke (1977, 1983), Carlsson and Mo¨ ller (1985), Almqvist (1990), Wells (1991), and Paavilainen and Pa¨ iva¨ nen (1995). On the basis of present experience and knowledge, information on nutrient status in general and nitrogen availability in particular can assist in the selection of the most suitable sites for forest fertilization with bioash. Thus, a selection of sites from the most nutrientrich to the most nutrient-poor in the country means a reduction from both ends of this continuum of sites. The most nutrient-rich sites should be rejected because the amounts of available nitrogen and other mineral elements there are adequately large and ash supplement will probably not result in significant stand growth responses. From that point of view, Holmen (1985) claimed that an estimate of the potential area of peatlands suitable for biofuel ash fertilization should be based on less nutrient-rich sites only. In the present study, the most nutrient poor sites, which have too little available nitrogen and other mineral elements to satisfy tree growth, should be rejected, because biofuel ash supplement alone does not result in increased forest growth, due to prolonged microbial nitrogen immobilization. Significant growth responses would require the addition of nitrogen. For these reasons, sites which in this study have been grouped into ‘‘herbs’’ and ‘‘poor shrubs’’ should be excluded from those most suitable for ash fertilization. As for the remaining site groups, rich shrubs and low sedge, the first mentioned could be included in the selection of suitable sites. The low sedge group however, the most extensive site type on peatlands in Sweden (Ha˚ nell, 1984), also includes a significant proportion of sites where additions of both mineral elements and nitrogen are required for increased forest growth. Low sedge-dominated sites with relatively high concentrations of nitrogen in the surface peat should be included in the selection and can probably be identified with guidance from the stand data. 4.1.4. Peat thickness It is well known that the effects of biofuel ash on tree growth are poor on mineral soil sites (Bo¨ rjesson, 1992; Campbell, 1990; Eriksson, 1990; Levula, 1991). To the extent that any effects can be registered, there is a general tendency towards stand growth decreases on low productive sites and increases on highly produc-

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tive sites (Jacobsson, 2001). The reason for these effects is probably the indirect influence of the ash supplement on both soil nitrogen and carbon turnover in a similar way to liming (Ho¨ gbom et al., 2001). Another experience is that forest growth response from ash is poor on soils with shallow peat. The likely reason for this is that tree roots in these soils still can reach below to the mineral soil where the nutrient supply usually is sufficient, as discussed in Silfverberg and Issakainen (1987). In the present study, the area of wet mineral soil forest land (with peat cover <30 cm) was estimated to be slightly less than 0.5 million ha (Table 3). With respect to the fact that about 90% of the Scots pine roots on drained peatlands is found within 20 cm from soil surface (Paavilainen, 1966), it is questionable if all the wet mineral soil area should be rejected from the selection of sites most suitable for ash-fertilization. A growth increase following ash amendment in forests with 20–30 cm peat cover is reasonable to expect. On the assumption that the wet mineral soil area is evenly distributed on peat thickness up to 30 cm, the area within the thickness interval 20–30 cm represents about 160,000 ha.

4.1.5. Stand development phase (age, volume, and growth) The response of a tree to fertilization, particularly regarding Scots pine, depends on the developmental stage of the tree. The absolute nutrient demand increases with tree size (and in that respect also with age) and is greatest in mid-rotation stands. In very young stands, there is a nutrient demand to maintain currently active tissues and also to build new biomass of active tissues (e.g., needles/leaves). In older stands, if well stocked, where the leaf area (crown size) no longer increases so much, nutrients are mainly needed for maintenance of the active biomass. Much of this latter demand can be accounted for by internal cycling of nutrients between old tissues and new tissues. The absolute nutrient demand, per unit of ground area, will be small in the seedling phase. It will increase and be very large approximately from stand crown closure to the peak of the annual increment curve, and become somewhat smaller again during later phases. Silfverberg and Issakainen (1987) emphasize that the growth response in old stands, which primarily need nitrogen, can be small. It should be said that our knowledge on

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growth responses after ash fertilization in dense stands of different development stages is inadequate. The growth response from fertilization has often been reported to be correlated to stand volume at the date of fertilization (Paavilainen and Pa¨ iva¨ nen, 1995), and therefore stand volume, basal area, as well as current annual increment have been used to predict the response (Keltikangas and Seppa¨ la¨ , 1973; Paavilainen, 1979; Almqvist, 1990). In contrast, Silfverberg (1996) did not find a clear correlation between stand volume and stem wood increase following ash fertilization. The reason for this could be that his data encompassed observations only from the first half of the rotation period. To a certain extent, it is true that the better the tree growth before fertilization, the better the growth response after fertilization. Small responses are achieved only in the most nutrient-rich sites characterized by herbs where tree growth is not limited by a nutrient deficiency. Several Finnish studies of commercial fertilizer amendments show that the fertilization profitability depends on the stand development phase and that the well growing stands ready for harvesting are the most suitable for fertilization (Keltikangas and Seppa¨ la¨ , 1973; Ha¨ ma¨ la¨ inen and Laakkonen, 1983; Ha¨ ma¨ la¨ inen et al., 1985). 4.2. Selection of suitable sites It can be concluded that all thinned stands as well as stands ready for thinning, that is, unthinned midrotation stands, should be given high priority in the selection of most suitable sites for ash fertilization. Older stands designated for final cut have a greater basal area and timber volume than the thinning stands and should also be given priority in spite of the warning from Silfverberg and Issakainen (1987) regarding N as the most needed element in old stands. Bare forest land and areas occupied by seedling and sapling stands should probably be rejected. This is motivated partly by the concern for losses of valuable ash nutrients such as potassium, and partly by the risk for nitrate leaching into surface waters. Data for evaluating the risk for nitrification and subsequent nitrate leaching, as a response to ash fertilization of peat soils, are scarce. It is, however, well known that the net mineralization is related to the substrate C/N ratio and that the nitrification process responds

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strongly to increases in pH, why the probability for nitrate leaching would be high, e.g., in relatively nitrogen-rich fen peats. The selection of most suitable sites for biofuel ash fertilization can be described step-by-step starting from the total peat-covered area in the country (Ha˚ nell, 1990), about 10 million ha (Table 6). A strict application of the selection criteria discussed suggests that it is the drained, productive peatland sites covered by mid-rotation or older stands characterized by rich shrubs or low sedges that are most suitable for ash fertilization. This grouping

comprises about 190,000 ha, of which 60,000 ha are located in the North region, 30,000 ha in each of the North Central and South Central regions, and about 70,000 ha in the South region (Fig. 2). In addition, the area of abandoned peat harvesting fields where the after-use plan is afforestation should be included. Assuming that half of the after-use area will be subject to afforestation, this addition would presently represent only a few hundred hectares. However, an increase to several thousand hectares is expected within the next 5 years. 4.3. Ash amounts for full-scale land use

Table 6 Process of selection of sites for forest fertilization using biofuel ash, in ’000 ha Sites

Total

Peat-covered area in Sweden Peatlands Wet mineral soils

10000 6300 3700

Selection 1: Reject non-productive areas Forest land with peat cover Peatlands Wet mineral soils

5000 1800 3200

Selection 2: Reject undrained sites and shallow peat Drained forest land, peatland Non-functioning drains Functioning drains

560 120 440

Selection 3: Reject non-functioning drains Drained forest land, peatland, functioning drains Herbs Rich shrubs Low sedge plants Poor shrubs

440 199 191 25 20

Selection 4: Reject herbs and poor shrubs Drained forest land, peatland, functioning drains, rich shrubs, low sedges Open forest land Seedling and sapling stands Unthinned mid-rotation stands Thinned stands Old stands Selection 5: Reject open forest land, seedling, and sapling stands Drained forest land, peatland, functioning drains, rich shrubs, low sedges, mid-rotation, and old forests Unthinned stands Thinned stands Old stands

217 4 23 61 56 73

190

61 56 73

The amount of ash that should be used in peatland forest fertilization can be calculated on the basis of the recommended dose of phosphorus, approximately 40– 50 kg P per ha (Paavilainen and Pa¨ iva¨ nen, 1995). Paarlahti (1980) reports a variation in P content and bulk density in wood-ash of different origins of 10.0– 12.3 g kg 1 and 390–923 kg m 3, respectively. Based on this variation an ash dose of 3000–5000 kg dw per ha should be used. Eriksson and Bo¨ rjesson (1991), Eriksson (1993) and Jo¨ nsson and Nilsson (1996) report similar P median/mean concentrations in woodash, i.e., 1.2, 1.0, and 0.9%, respectively, but considerably greater variation. Thus, an application of 5000 kg wood-ash per ha on about 200,000 ha requires about 1 million tones ash to fertilize the whole area the first time. This means that the amount of ash needed for fertilizing all selected peatland forests and peat-harvesting fields corresponds to three to four times the present annual production of biofuel ash. 4.4. Research needs A more optimal selection of sites for biofuel ash forest fertilization requires improved knowledge on the following issues: (i) the accuracy regarding the selection based on site type. Some herb-dominated sites should perhaps be included in the selection, (ii) ash fertilization responses in old-drained sites with middle-aged stands), (iii) ash fertilization responses in sites where the peat cover is shallower than 30 cm and (iv) stand density, preferably expressed as Leaf Area Index before fertilization and its importance for the growth response.

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Fig. 2. Selection of most suitable sites for biofuel ash forest fertilization in Sweden—drained, productive peatlands covered by thinned and unthinned mid-rotation stands or older forests characterized by rich shrubs (filled bars) or low sedge plants (open bars) in the field layer.

Acknowledgements This study was financed by Va¨ rmeforsk Service AB, Stockholm. The authors would like to thank Hans Toet, Go¨ ran Kempe, and Per Nilsson at the Department of Forest Resource Management and Geomatics, Swedish University of Agricultural Sciences, Umea˚ , for valuable support in the selection of data from the Swedish National Forest Inventory. References Almqvist, C., 1990. Go¨ dslingseffekter pa˚ dikade torvmarker—prognoskurvor fo¨ r tall (Prediction equations for estimation of Scots pine (Pinus sylvestris) response to fertilizer on drained peatland in Sweden), Inst. f. Skogsfo¨ rba¨ ttring, Uppsala. Inform. Va¨ xtna¨ ring – skogsproduktion, 1989/90, pp. 1–6. Anon, 2000. Skogsdata 2000. Aktuella uppgifter om de svenska skogarna fra˚ n Riksskogstaxeringen. Tema: Tillva¨ xt och avga˚ ng. Institutionen fo¨ r skoglig resurshusha˚ llning och geomatik, SLU, Umea˚ . 110 pp. Anon, 2001a. Rekommendationer vid uttag av skogsbra¨ nsle och kompensationsgo¨ dsling. Skogsstyrelsen, Jo¨ nko¨ ping, Meddelande nr 2, 21 pp. ˚ tga¨ rder mot markfo¨ rsurning och fo¨ r ett utha˚ lligt Anon, 2001b. A brukande av skogsmarken. Skogsstyrelsen, Jo¨ nko¨ ping, Meddelande nr 4, 37 pp. Anon, 2002a. Skogsva˚ rdsorganisationens utva¨ rdering av skogspolitikens effekter, SUS 2001. Jo¨ nko¨ ping, Meddelande nr 1, 275 pp.

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Silfverberg, K., Hotanen, J.P., 1989. Puuntuhkan pita¨ aikaisvaikutukset ojitetulla mesotrofisella kalvakkanevalla Pohjois-Pohjanmaalla (Long-term effects of wood-ash on a drained mesotrophic Sphagnum papillosum fen in Oulu district). Metsa¨ ntutkimuslaitos, Helsingfors. Folia For. 742, 23 pp. Silfverberg, K., Issakainen, J., 1987. Tuhkan ma¨ a¨ ra¨ n ja laadun vaikutus neulasten ravinnepitoisuuksiin ja painoon ra¨ mema¨ nniko¨ issa¨ (Nutrient contents and weight of Scots pine needles in ash-fertilized peatland stands). Metsa¨ ntutkimuslaitos, Helsingfors. Metsa¨ ntutkimuslaitoksen tiedonantoja 271, 29 pp. Silfverberg, K., Issakainen, J., 1996. Skogstillva¨ xt pa˚ en askgo¨ dslad, nordfinsk kalmyr –40-a˚ rigt perspektiv pa˚ asktillfo¨ rsel i praktisk skala (Forest growth on an ash-fertilized oligotrophic fen in northern Finland). Suo 47, 137–139. Svensson, J., Ha˚ nell, B., Magnusson, T., 1998. Naturlig beskogning av utbrutna torvmarker genom insa˚ dd fra˚ n omgivande skog (Three and shrub colonization of abandoned peat winning fields by seeding from adjacent forests). SLU, Uppsala. Rapporter i skogsekologi och skoglig markla¨ ra, Rapport 78, 46 pp. Tamm, C.O., 1965. Some experiences from forest fertilization trials in Sweden. Silva Fenn. 117 (3), 1–24. Wells, E.D., 1991. Effects of refertilization of an 18-year-old Japanese larch (Larix leptolepis) peatland plantation in western Newfoundland, Canada. In: Jeglum, J.K., Overend, R.P. (Eds.), Peat and Peatlands—Diversification and Innovation, vol. 1. Can. Soc. Peat Peatlands, pp. 129–138. Westman, C.J., 1981. Fertility of surface peat in relation to the site type and potential stand growth. Acta For. Fenn. 172, 1–77.