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Supplying Biomass for Small Scale Energy Production

Written By

Tord Johansson

Submitted: October 28th, 2010 Published: September 9th, 2011

DOI: 10.5772/18581

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1. Introduction

Our sources of energy are constantly changing. In Sweden the focus is on nuclear and hydro power for producing electricity and total Swedish energy production amounts to about 612 TWh (Anon, 2010). Since Sweden has a cold climate, there is a high demand for energy to heat homes and energy sources other than oil and coal are required. Currently, fuel systems are based on oil and electrical power but there has been an increase in the use of biomass during recent decades. The support of biomass for heating provides 19% of the total Swedish energy output, (Fig. 1).

For centuries trees have been used in a domestic context for firewood and charcoal production. In Sweden, conventional forest management combined with bioenergy production has been practiced for the last 40-50 years. Currently, for economic reasons, bioenergy harvesting is mainly based on large areas of forest land. Tops and branches are harvested from clear cut areas and this biomass contributes greatly to the production of bioenergy. Special equipment is used to harvest biomass, which is used for energy production in direct heating plants. The infrastructure is well established. Most of the harvested material goes to heating plants close to cities, although some is used by individual households.

Figure 1.

Total energy use in Sweden in 2007 (Anon, 2010)

The management of forests is mainly directed towards producing pulpwood and timber. The remaining parts of the tree – branches and tops – represent raw material for bioenergy production. Over the last twenty years there has been an increased willingness to make use of these parts of the tree.

Biomass production on former farmland, using willows, poplar and hybrid aspens, is another option for energy production. In general, the Swedish people look favorably on such land use, as well as forest biomass production. There is strict regulation of the management of forest land to minimize the risks of nutrient loss, but no such regulations exist for farmland. Farmers and some sections of the public wish to maintain farmland as an open landscape and to continue with agricultural cultivation.

The Swedish government has twice proposed a reduction in farmland available for the production of cereals, in 1969 and 1986. The plan was to reduce the area by about one million hectares, out of the total of three million hectares. Both attempts failed, although since 1968 350,000 ha have been taken out of production. Some areas of this former farmland have been planted, mostly with Norway spruce and birches, but more than 200,000 hectares which were taken out of production in the period 1970-1980 have received no subsequent management. Today these areas are covered by broadleaved trees with a range of numbers of stems per hectare (Johansson, 1999a), but they are not managed to generate forest products.


2. Small-scale production of biomass

Currently, there are standard practices for the management and harvesting of biomass from large forest stands, used in state forests and by forestry companies. It is much more challenging, however, for small-scale forest owners to utilize forest biomass for bioenergy. The amount of biomass that can be harvested from forest land or farmland depends on various factors including site condition, species and management intensity. Few practical recommendations for small-scale owners have been published, and land owners may be unaware of appropriate practice. More information would enhance the use of resources available for bioenergy production.

Herein I present examples of activities and the management of farmland and forest land demonstrating how an owner can undertake small scale biomass production for their own consumption or to supply a local market (neighbors etc.).

The examples presented are:

  • ingrowth, i.e. natural establishment of broadleaved trees on former farmland via seeds, sprouts or suckers;

  • direct seeding on farmland;

  • management of existing mixed stands;

  • harvesting tops and branches after clear cutting; and

  • establishing and using fast-growing species.

Finally, some recommendations for small scale bioenergy production are presented.


3. Ingrowth

The most important factors affecting the colonization of open areas by plants are: the year and season of abandonment; the physical state of the site; climate; soil; the existing flora and fauna; proximity and position of source material; opportunities for vegetative regeneration; and the presence, within a range possible for seed dispersal, of an efficient generative reproduction and a rapid, rich and long-distance dispersal of seeds (Falinski, 1980; Harmer et al., 2001). Reviews by Osbornova et al. (1990) and Myster (1993) report many studies of tree generation on abandoned farmland. Natural colonization by trees and other species have been recorded since 1882 at the Broadbalk Wilderness, UK, which has established on former farmland (Harmer et al., 2001). The first tree plants were recorded 30 years after abandonment, i.e. in 1913. The main species regenerating in the area were: common ash (Fraxinus excelsior L.); sycamore (Acer pseudoplatanus L.); field maple (Acer campestre L.); suckers of wild cherry (Prunus avium L.); blackthorn (Prunus spinosa L.); pedunculate oak (Quercus robur L.) and hazel (Corylus avellana L.). In 1998 the dominant and most frequent tree species were pedunculate oak, common ash, wild cherry and sycamore.

Figure 2.

Naturally seeded birch (left), sucker from aspen (right) and naturally seeded grey alder (below)

The area of farmland no longer in agricultural production increases as land owners cease activities or direct their energies towards other forms of management. When farmland is abandoned it is invaded by herbs and broadleaved tree species (alder, aspen and birch). In general, one species dominates in the new stand. Most such farmland areas are owned by private individuals. In Sweden, Johansson (1999a) found up to 10,000 broadleaved tree stems ha-1 on about 100,000 hectares of former farmland.

Natural tree establishment in an open area is a slow process, and it may be 5-10 years before trees 2-5 old are seen (Werner and Harbeck, 1982). Most such areas in Northern Europe are small, amounting to 0.5-2.0 ha. In the initial phase, the areas are not noticeable from the surroundings, but later a dense stand is established and the landscape is changed. In general, these areas continue to develop unnoticed by the owner or the public. Eventually, former open areas become covered by forest. Such ingrowth can be the result of natural seeding, sprouting or suckering (Fig. 2).

3.1. Natural seeding

To produce conditions that will encourage establishment of a wide range of seedlings through natural seeding, and avoid revegetation failing, an understanding of certain abiotic and biotic factors is required. The main factors that affect establishment through natural seeding are: species present, soil type, moisture, competition by grasses and herbs, available seed trees, and weather conditions (heat, dryness etc). It is important to know the timing and periodicity of seed production and dispersal. Basic knowledge about the period for the high rates of seed dispersal is necessary when practicing natural regeneration. In order to encourage natural seeding, ground preparation must be undertaken prior to seed dispersal.

Specific characteristics of a species, such as number of seeds per tree, seed weight and frost resistance, greatly influence the establishment of seedlings. Seeds from some species are wind dispersed (e.g. birch and sallow (Salix caprea L.)) and others water dispersed (e.g. alder); a combination of methods may be used. Studies of wind-mediated seed dispersal for different species indicate the following order of decreasing dispersal:

birch>elm=maple>alder>hornbeam>beech>oak (Augspurger and Franson, 1987; Okubo and Levin, 1989; Willson, 1990; Karlsson, 2001). Table 1 contains data on birch and alder seed dispersal.

Distance from forest stand, mCountryReference
"/>400"/>100Sweden1Fries (1982)
"/>200<100Sweden2Björkroth (1973)
58 % of total10 % of totalUSA3Björkbom (1971)
10,4504,200400USA4Hughes and Fahey (1988)
78-94 % of totalSweden5Johansson and Lundh (2006)
90 % of totalSweden5Karlsson (2001)

Table 1.

Dispersal of birch and alder seeds into open areas, number of seeds m-2 year-1

Both downy (Betula pubescens Ehrh.) and silver (Betula pendula Roth) birch produce many seeds. In Estonia, Uri et al. (2007) recorded 3060-36,200 8-year-old birches ha-1 that had been produced by natural seeding on farmland. Seeds from a birch growing at the edge of a clear cut area have been found to spread at a rate of about 100 seeds m-2 up to 200 m from the tree (Fries, 1984). Most of these birch seeds were dispersed during September, although the process continued until December. In a study of sweet birch (Betula lenta L.), Matlack (1989) reported seed were dispersed 3.3 times further than the distance measured by Fries (1984). In a study of silver birch in Estonia, 21 % of the seeds were dispersed in July, 77 % in August and 2 % in September (Kohh, 1936). Heikinheimo (1932, 1937), who reported the same dispersal periods, commented that the weather during summer and autumn is the main factor affecting the period of seed dispersal. Graber and Leak (1992) presented a study on seed fall for broadleaved species in New Hampshire. The mean seed fall (million ha-1) in a study lasting 11 years was: 6.58 for yellow birch (Betula alleghaniensis Britton); 6.38 for paper birch (Betula papyrifera Marsh.); 4.11 for sugar maple (Acer saccharum Marsh.); and 0.17 for American beech (Fagus grandifolia Ehrh.). The seed viability was 30-50 %, depending on species.

Besides wind dispersal, there are some reports of secondary dispersal of seeds (Hesselman, 1934; Matlack, 1989; Greene and Johansson, 1997). The most common is by movement on snow, but for this to occur, seed fall must happen during winter months when snow is on the ground. The seeds can be damaged by friction on frozen snow, thus reducing viability.

The level of seed production by alder depends on the number of hours of sunshine in the period April-September in the year before fruiting, the number of hours of sunshine in the seeding year and the level of seed production in the preceding year (MacVean, 1955). According to MacVean (1955), common alder (Alnus glutinosa (L.) Gaertner) seeds are generally dispersed within a radius of 30-60 m of the mother tree. Karlsson (2001) found that 50 % of the total number of alder seeds produced fell within 5 m and 90 % within 20 m of the stand. In a study by Johansson and Lundh (2006), 50 % of the common alder seeds were found to have fallen before December and 75 % before February. Alder seeds can also be transported by water in spring at the time of snow melt.

Seeds from European aspen (Populus tremula L.) are extremely small (low weight) with a limited growing capacity (Blumenthal, 1942, Latva-Karjanmaa et al., 2006). A large aspen growing close to Tartu city, Estonia, produced 49 kg or 54 million seeds (Reim, 1930). Only a small proportion of the aspen seeds produced will grow; success depends on site conditions, seed size and the level of competition. Aspen seeds can grow on poor sandy sites, burned areas and small patches without vegetation (Blumenthal, 1942). Seeds of sallow are also small and have a plume to aid dispersal (Grime et al., 1988). Seeds of both species can be dispersed over long distances.

The most favorable soil types for rapid establishment of seedlings are fine sand, silt and light clay, sandy-silty till and light clay till. Even peat soils can provide an ideal site, providing there is sufficient water. A mixture of mineral soil and humus is common on farmland, where the area has been cultivated for many years.

Birch seeds establish well on undisturbed sites with a high level of moisture (Mork, 1948; Fries, 1982). During the first part of the growing season in Nordic countries (April-May) soil moisture tends to be low. The lack of rain combined with the sunshine during this period results in a dry soil. Therefore any soil treatment (plowing, harrowing or screefing) should be undertaken in autumn or very early in spring. Studies to determine the best soil treatment to ensure limited cover of competitive vegetation indicate that removal of topsoil is preferable (Karlsson, 1996).

3.2. Sprouting and suckering

The main difference between sprouting and suckering is that sprouts emerge from a stump whilst suckers originate from roots, (Fig. 3). Both types of regeneration result in fast-growing individual stems. In studies of dormant buds on birch, most have been found close to the ground: 0-10 cm above or 0-5 cm below ground level (Kauppi, 1989; Kauppi et al., 1987; 1988 Johansson, 1992a). The number of sprouts per living birch stump has been found to vary between 1 and 52, mean 10±8, decreasing to 3-8 sprouts per stump after five years (Johansson, 1992 b, c). Rydberg (2000) found the number of birch sprouts had decreased by >40 % of the initial number two years after stump creation nine years after cutting, Johansson (2008) found that the initial number of sprouting birch stumps had decreased to 61 and 55 % respectively for downy and silver birch stumps. In a study of downy birch growing in central Finland, the number of sprouts decreased from an average of 9.5 one year after cutting to 5 after three years and 3 after seven years. The sprouting abilities of red oak (Quercus rubra L.), white oak (Quercus alba L.), black cherry (Prunus serotina Ehrh.), sugar maple and yellow poplar (Liriodendron tulipifera L.) growing in West Virginia were studied by Wendel (1974). After ten years the number of sprouts per living stump was 15-20 % of the initial number produced. In another study of yellow poplar, the average number of sprouts recorded six years after cutting was 7.0 per stump (Beck, 1977). Sprouting capacity is highest when a tree is young (Johansson, 1992c). Kauppi et al. (1988) reported the poorest sprouting results from old (40 year) downy birch stumps. Older trees have thicker stem bark, so the buds cannot penetrate the bark and develop into sprouts (Mikola, 1942). Sprouting capacity may depend on carbohydrates in the roots. However, Johansson (1993) found no pronounced peaks in the carbohydrate content in birch roots during the year. Sprouting capacity may also depend on the cutting date. Johansson (1992b) found the highest number of living birch stumps producing sprouts cut in all months but June-October. Etholén (1974) found no effect of cutting time on the sprouting ability of young downy birch stumps.

Figure 3.

Sprouts of birch (left) and suckers of aspen (right)

In southeastern New York, Kays and Canham (1991) studied the sprouting ability of four hardwood species: red maple (Acer rubrum L.), gray birch (Betula populifolia Marsh.), white ash (Fraxinus Americana L.) and black cherry (Prunus serotina Ehrh.). They reported that gray birch had the highest mortality (87 %) of stumps after cutting in May but the other species only had mortalities of 10-20 % depending on cutting date. In a study of the suckering capacity of parent trees of American beech, a mean of 41,365 (3,924-89,765) suckers ha-1 was found (Jones and Raynal, 1986).

European aspen and trembling aspen (Populus tremuloides Michx.) are two Populus species with a high capacity for sucker production. The number of suckers after cutting the mother tree differs depending on the cutting date (Johansson, 1993) and on site, stand and management factors (Frey et al., 2003). The age of the mother tree also influences the suckering ability (Brinkman and Roe, 1975). A trembling aspen stand was found to produce 8000 suckers ha-1 after cutting (Tew, 1970). In a study by Alban et al. (1994) of trembling aspen growing in Minnesota, the number of suckers the first year after disturbance was >250,000 per hectare. The number had decreased to 40,000 after five years (Stone and Elioff, 1998). Trembling aspen stands growing on similar soils in Minnesota and British Columbia produced 50,000 suckers ha-1 after five years and the mean sucker height was 2.1 m (Stone and Kabzems, 2002). The root system of an individual aspen is widely spread, with root lengths up to 20 m (Reim, 1930). In a Swedish study, about 70 % of the suckers occurred within 10 m of the parent aspen tree (Bärring, 1988). In a study by Johansson (1993) the content of starch in roots of European aspens fluctuated during the year with the lowest levels in May-July. The same pattern has been reported for trembling aspen by Baker (1925), Zehngraff (1946), Tew (1970) and Brinkman and Roe (1975).The lowest content has been recorded in late May and early June. When aspen is cut in the winter the highest numbers of suckers are produced (Stoeckler and Macon, 1956; Steneker, 1976; Peterson and Peterson, 1992). In other studies (Shier and Zasada, 1973; Fraser et al., 2002) on trembling aspen, no relationships have been identified between carbohydrate content in roots and the number of suckers initiated.

Alder regenerate vegetatively by sprouts or suckers depending on species. In a study of red alder (Alnus rubra Bong.), the number of sprouts per living stump ranged between 5 and 9 (Harrington, 1989). In another study of the same species, the number of sprouts was in the range 9-13 (DeBell and Turpin, 1989). According to Rytter (1996), young grey alders (Alnus incana (L.) Moench) produce sprouts after cutting, but the old trees produce suckers. In a Finnish study, grey alder stumps sprouted within three weeks of cutting (Paukkkonen and Kauppi, 1992). Sucker production by grey alder is the main means of vegetative regeneration when the trees are more than 25-30 years old (Schrötter, 1983). In a study of seasonal variation of carbohydrates in the roots of common and grey alders, levels were found to be highest during September-November (Johansson, 1998). In a study of the influence of felling time on sprout and sucker production by common and grey alder, the carbohydrate content in the roots was found to influence biomass production (Johansson, 2009). The highest number of sprouts from common alder stumps was produced after cutting in August-October (23-24 sprouts stump-1). Ten years later, the number of sprouts had decreased to 1.3-2.3 sprouts stump-1. The average number of sprouts on living grey alder stumps was highest after cutting in March (3.0), August (3.4) and September (3.4), with a reduction to an average of 2.0 after five years. The number of grey alder suckers per m2 was highest, 21.0, after cutting in September with a reduction to 1.5 after five years. The recommendation, therefore, is to cut grey alder in August and September ad common alder in August-October when the largest number of sprouts and suckers will result. In a study on the initial sprouting of 4-year-old red alders, the percentage of sprouting stumps was highest when the alders were cut in January (Harrington, 1984).

In a study of the spouting ability of Eucalyptus in plantations, the number of sprouts per living stump varied, but the highest number was 5-6 sprouts stump-1 (Sims, 1999). The stumps have the capacity to resprout several times, depending on their vigor.


4. Direct seeding

When practicing direct seeding on forest land there are practical recommendations considering among others Norway spruce (Picea abies (L.) Karst.), Scots pine (Pinus sylvestris L.), birch, beech (Fagus sylvatica L.) and oak in relation to the target species. There are, however, few recommendations available for seeding on farmland, although the factors associated with successful establishment are the same as for natural seeding (species, mineral soil, moisture, competition by grasses and herbs, and weather conditions).

The success of establishment of seedlings after direct seeding depends on the nature of the soil treatment and the date of seeding. The critical phase is the emergence of seedlings during the first days or weeks after seeding and the moisture conditions in the treated spots. Generally, precipitation is low in late spring and therefore seeding must be undertaken early in spring.

High quality seeds are expensive and therefore a natural seed source close to the planting site can allow collection from mature seed trees of the appropriate species. Birch and alder are suitable species for producing stands for bioenergy harvest, with subsequent vigorous sprouting or suckering. Depending on seeding method the amount of seeds is 0.5-1.0 kg ha-1.


5. Management of mixed stands on farmland

Using a mixture of species in forest management has been common in Europe for the last three centuries. Hegre and Langhammer (1967) and Stewart et al. (2000) have presented overviews of the importance of mixed stands and their management in different countries worldwide.

Figure 4.

Mixed stand of alder and Norway spruce (left), aspen and Norway spruce (middle) and birch and Norway spruce (right)

In Finland and Norway, a forest stand is defined as being mixed if 20 % of its basal area is made up of broadleaved species, with conifers comprising the dominant species (Frivold, 1982). In Sweden, the proportion is 30 % and in Italy 10 % of the basal area. The Swedish definition of a mixed broadleaved and coniferous stand is “a type of stand in which the total percentage of broadleaved species is 30-70 % of the growing stock” (Anon., 2010). In Nordic countries mixed stands are the most frequent type of stand.

Mixed stands mostly establish spontaneously i.e. a planted or naturally regenerated conifer stand is mixed with naturally regenerated broadleaves. Areas of clear felling that are moist are readily colonized by broadleaves, which can establish from seeds, sprouts or suckers. The number of stems can amount to 5000 to 50,000 per hectare. However there is a conflict between broadleaf cover preventing frost damage to young spruce trees and the strong competition between broadleaves and conifer seedlings. In older stands, both species become established, competition is stabilized and the risk of frost damage declines (Johansson, 2003).

Mostly, Nordic forestry is focused on the management of stands for the production of softwood. A large number of young broadleaves are likely to compete with the conifer seedlings in such stands. In the past, the broadleaves were cut or treated with herbicides. Nowadays, with increasing interest in the supply of biomass for bioenergy production, other management systems have been introduced.

When managing mixed forest stands, a stratified mixture of shade-tolerant, late-successional species in the lower stratum and early successional species in the upper stratum is recommended (Assmann, 1970; Kelty, 1992). Mixed stands may contain alder, aspen or birch and Norway spruce (Johansson, 2003), (Fig. 4). The management of mixed stands is often based on stands which have not been cleaned at the correct time. The spontaneous establishment of broadleaved trees takes up to10 years.

5.1. Mixed forest management

A number of methods are practiced in the Nordic countries, most commonly the shelter method (Tham, 1988; Johansson and Lundh, 1991) and the “Kronoberg” method (Anon., 1985). The descriptions in the sections below are based on a mixed stand of birch and Norway spruce, since this is the most common situation, but the same techniques can be used for other broadleaved species with Norway spruce.

When managing this type of stand it is important that the density of the broadleaved stems is not too high once the spruces have been established. According to Braathe (1988), the competition is too strong for spruces if there are more than 1200 birches ha-1 and they are >3 m tall. In that case, he postulated a 30 % decrease in the height increment of the spruce.

5.1.1. The shelter method

This method is common in Finland, Norway and Sweden. It was introduced in Sweden by Tham (1988) with some modifications by Johansson and Lundh (1991). Currently, the same technique is used for birch and Norway spruce in Finland, Norway and Sweden. The principal aim is to create an initial mixed stand with an optimal density of birch.

The method involves two or three steps:

  1. When the spruces are 1.5-2 m tall, the density of birch is reduced by cleaning to 800-1000 stems ha-1.

  2. The “birch shelter” is cut when the birches are 30-35 years old with a diameter at breast height (dbh) of 15-20 cm.

  3. An alternative is to cut all 30-35-year-old birches except 50-100 stems ha-1. The remaining stems should be evenly spread through the stand. These birches will produce high-quality timber during the following 20 years.

5.1.2. The “Kronoberg” method

This method was first introduced in southern Sweden (Anon., 1985). The aims are to avoid frost damage to Norway spruce plants and to control the number of sprouts that are able to establish after the removal of birch in each step.

The method involves three steps:

  1. When the birches are 3-4 m tall the stand is cleaned. A total of 3000-4000 birch stems ha-1 should be retained. The Norway spruce is not cleaned.

  2. When the birches are 6-9 m tall the stand is cleaned again. A total of 1000-1500 birch stems ha-1 should be retained; the dbh of the birches should be about 5 cm.

  3. When the birch stand is 20-25 years old the birches are felled. They will be 8-12 m tall with a dbh of 8 cm. The mean height of the Norway spruce will be 3-4 m. The spruce stand should be thinned to 2000-2500 stems ha-1.

Alternatively, instead of felling all the birches, 600-800 birches ha-1 could be left for 10-15 years. When the birches are finally cut, their mean dbh will be 15-20 cm.

5.1.3. Mixed stands of birch and Norway spruce

The most common type of young stands in Nordic countries is mixed birch and Norway spruce, Fig. 5. Many reports describe how to manage birch and Norway spruce. In Finland, Norway and Sweden the management of mixed stands is common (Mielikänen, 1985; Braathe, 1988; Tham, 1988; Mård, 1997; Klang and Ekö, 1999). Frivold and Groven (1996) discussed the importance of managing mixed stands for future high timber quality. The competition between the taller birches and Norway spruce may adversely affect spruce growth. Therefore the birches must be carefully managed with respect to both numbers of stems removed and controlling competition. A common recommendation is to leave 500-1000 stems ha-1 when the birches are 10-15 years old. A Finnish study of a mixed stand of birches (downy and silver) and Norway spruce examined the influence of competition (Valkonen and Valsta, 2001). A reduction of 7-15 % by volume production was reduced by 7-15 % in mixed stands with 1000 birches ha-1 compared to pure spruce stands.

Figure 5.

Managed mixed stand of birch and Norway spruce.

Below an experiment in mixed stands of birch and Norway spruce is described (Johansson, 2000b). The experiment was started in 1983 and was based on trials established at eight localities in central and southern Sweden. The experimental stands were 20-30 years old. They were dense, 1520-20,280 stems ha-1, and self regenerated.

The experiment included three thinning regimes:

  • Thinning of the birch overstory to create a shelter of 500 stems ha-1.

  • Total removal of the birch trees

  • Only Norway spruces

At the first cutting, to create the shelter and the pure Norway stands, 1520 to 20,280 birch stems ha-1 with a mean diameter of 5.2 cm were removed. After 5 years, 373 to 507 birch stems ha-1 with a mean diameter of 15.7 cm were recorded.

Data collected five years after the experiment started are presented in Table 2. The competition by the birch shelter did not influence the growth of Norway spruce. As shown in the table, the mean diameter of the Norway spruce trees was almost the same in the shelter as in the pure stands, 7.6 and 7.0 cm respectively.

dbh, cmHeight, mStocking level,
stems ha-1
Mean ± SE13.3±0.414.2±0.5499±5
Norway spruce
Mean ± SE7.6±0.39.7±0.52811±110
No shelter
Norway spruce
Mean ± SEMean ± SE7.0±0.18.5±1.02517±154

Table 2.

Stand characteristics of the trees remaining five years after cutting

Figure 6.

Managed mixed stand of European aspen and Norway spruce

5.1.4. Mixed stands of aspen and Norway spruce

Mixed stands of European aspen and Norway spruce are usually established on rich soils, (Fig. 6). Hegre and Langhammer (1967) and Langhammer (1982) presented results from a Norwegian experiment on farmland that involved planted European aspen and Norway spruce. Aspens and Norway spruces were planted each at a density of 2000 stems ha-1. The aspens were thinned 30 years later and 580 stems ha-1 were retained. Recommendations based on the study stated that planting densities of 2000 Norway spruce and 1000 aspen ha-1 would avoid strong competition by the aspens.

5.1.5. Mixed stands of alder and Norway spruce

Naturally established mixed stands of alder are common on wet or moist sites, (Fig. 7). Few studies have examined mixed stands of alder and Norway spruce; those which do exist are based on stands that were not managed correctly during the first ten years after establishment (Lines, 1982; Johansson, 1999d).

Figure 7.

Managed mixed stand of grey alder and Norway spruce


6. Harvesting tops and branches after clear cutting

After clear cutting, tops and branches from felled trees are traditionally left on site together with small trees (Fig. 8). On nutrient-limited sites this slash should not be removed because that would reduce the nutrients present on site. The amount of biomass present in tops and branches is estimated to amount to 20-30 % of the total harvest. The supply of biomass from tops and branches is the main source of bioenergy production in Sweden.

Figure 8.

Clear cut area with branches and tops (left) and stacks of branches and tops (right)


7. Fast-growing species

Besides conventional forestry management, there is increasing interest in management of so-called fast-growing species. Depending on geographical location, different species can be considered fast-growing. There are at least three types of tree suitable and frequently used for management in Europe, the USA and Canada: Salix clones, poplar and hybrid aspen. In areas with higher temperatures than northern Europe, species of Eucalyptus are also planted.

7.1. Salix

In Sweden research on short rotations using Salix began in the end of 1900. Today 10,000-15,000 hectares of short rotation Salix stands have been established and are actively managed using advanced technology. The management is based on small-scale plots, where the farmer owns the stand and manages it. Harvesting is undertaken using machinery owned by entrepreneurs and the harvested material is sold to be used for district heating. Common rotation periods are 4-5 years with 5-6 repeated rotations; a plantation lasts a total of 20-30 years before a new one must be established. The plantations must be fertilized and in some cases treated with herbicides. Pathogens (fungi and insects) damaging the leaves and shoots will cause a reduction in growth. As the seedlings represent attractive wildlife habitat, the plantations must be fenced.

Figure 9.

Harvested area of Salix (left) and a stack of harvested coppice (right)

7.2. Poplar

Worldwide, and for a long time, poplars have been used for, inter alia, pulpwood and timber production. Currently, short rotation plantations intended for biomass production are being established. In Sweden poplars have been planted in experiments or plots for practical survey for the last 20 years. Poplar plantations covering small areas of 0.5-2 ha on former farmland can produce 80-100 tonnes ha-1 of wood in ten years (Mean annual increment (MAI): 8-10 tonnes ha-1 years-1). If rotations are longer than 10 years, some of the material harvested will be suitable for use as pulpwood. Nowadays short rotation plantations aiming biomass production has been established. In Sweden poplars have been planted in experiments or plots for practical survey the last 20 years. After harvesting, regeneration of older trees by suckers or sprouts is limited. Certain clones and species produce no or only a few sprouts or suckers. This may be because poplars must be young when they are cut for sprouts to be initiated. The bark on the poplar stems is thick already when the alders are 15 years old, preventing any buds from growing into sprouts.

Figure 10.

Hybrid poplar stand

7.3. Hybrid aspen

Hybrid aspen is a hybrid between European aspen and trembling aspen (Wettstein, 1933). The hybrid was introduced into Sweden in 1939. Today plantations of hybrid aspen are a potential source of bioenergy, pulpwood and timber. The MAI for hybrid aspen is the same as for poplar, 10 tonnes ha-1 year-1. A German study compared the biomass production in repeated five-year rotations of European, trembling and hybrid aspen (Liesebach, et al., 1999). After harvest of the 5-year-old plantation the biomass was: 7 tonnes ha-1 year-1 from European aspen, 18 from trembling aspen and 16-34 from the four clones of hybrid aspen that were examined. The plants were then allowed to produce suckers, resulting in 165,000 suckers ha-1 during the first year and 45,000 suckers ha-1 five years later. During the second rotation, the production was 18 and 20 tonnes ha-1 for European and trembling aspen and 27-41 for the hybrid aspen clones. The amount of biomass after 5 and 10 years could amount to 50 and 100 tonnes ha-1 respectively. If longer rotations are preferred, the focus should be

Figure 11.

Hybrid aspen stand

on pulpwood and timber production, with bioenergy derived from tops and branches. After harvesting the trees, the stumps produce 50,000-100,000 suckers ha-1. During the subsequent 5-10 year period the sucker biomass will amount to 50-100 tonnes ha.-1. However biomass production during a 10-year-old rotation was found to amount to 47, 51 and 87-124 tonnes ha-1 respectively for the aspen stands.


8. Biomass characteristics

The biomass fractions of a tree are the stump (including roots), stem, branches and foliage (needles and leaves). Broadleaved trees and conifers have different fractions of these aboveground components (Johansson 1999a, b). For birches, the mean aboveground fractions are: stem, 75 %; branches, 18 %; and leaves, 7 %. For conifers, the mean values are 63 %, 23 % and 14 % respectively (Johansson, 1999b, c). The percentage represented by needles is higher in young than old conifers, Fig. 12.

Figure 12.

Percentage biomass fractions by total d. w. %, of a tree at different diameters (DBH), mm

The effect of repeated harvesting on biomass production and sprouting of downy birches growing in central and northern Finland has been studied by Hytönen and Issakainen (2001). Different harvesting cycles of 1, 2, 4, 8, 12 and 16 years were examined. The main results were that downy birch is not suitable for biomass production using short rotations. Most of the stumps, 87 %, did not sprout in the one year rotations, but 8-year rotations produced the same number of sprouting stumps as the longer rotations.

Reim (1929) reported that European aspen growing along the borders of farmland may produce large numbers of suckers when cultivation ceases. In a study of repeated short rotations of aspen, the number of suckers per hectare decreased with every additional rotation (Perala, 1979). The study included rotations of four or eight years and, in both cases, the number of suckers decreased over the three rotations studied.


9. Conclusions

There are several establishment and management techniques available that can be applied to small-scale plots for biomass production on farmland and forest land.

The management methods presented here rely on the land owner having extensive and detailed knowledge of biological processes. The changes in growth of individual species and mixed stands must be known. Some of the methods are based on optimal rotation periods and adequate management of the stand, including cleaning and thinning at the correct time. Severe competition could drastically decrease tree growth. Besides the need for the site to be suitable for tree cultivation, the skill of the owners is important. The most important factor, however, is the enthusiasm and curiosity of the owner; without this, most of the methods will not produce the yields suggested in the present study.

Table 3 lists possible future management models for trees established on farmland and forest land. When operating on a small-scale, there are many alternatives and the owner can be more flexible than is possible in large-scale operations. As the possible rotation periods range from 5 to 40 years it is important to have stands of different ages to ensure a continuous supply. Efficient management of such small areas would make it possible to produce a certain amount of biomass for personal use or to sell to neighbors or local heating plants..

Figures for potential energy supply from different stand types and management options allow us to make comparisons and select appropriate ways to use available land.

Most of the methods are cheap, need a short time to establish and involve relatively straightforward management. The raw materials produced can be used to generate energy for the landowner or can be sold.

ActivityRotation period,
tonnes ha-1
Next generation
Natural seeding10-2050-110115-255Sprouts or suckers
Sprouting, suckering5-1550-120115-275Sprouts or suckers
Direct seeding10-1540-8090-185Sprouts or suckers
Mixed stands35-40100-150230-345
Harvesting tops and branches-50135
Fast-growing species5-2530-30070-690Sprouts or suckers

Table 3.

Small-scale management of tree stands on farmland and forest land and possible biomass production


  1. 1. AgestamE.1985 En produktionsmodell för blandskog av tall, gran och björk i Sverige. Summary: A growth simulator for mixed stands for pine, spruce and birch in Sweden. Swedish University of Agricultural Sciences. Department of Forest Yield Research. Report 15, 150 pp.
  2. 2. AlbanD. H.HostG. E.ElliofJ. D.ShadisD.1994 Soil and vegetation response to soil compaction and forest floor removal after aspen harvesting. U.S.D.A. Forest Service Research Paper NC-315, 8 pp.
  3. 3. Anon.1985 Lövröjning med skärmmetoden- Skötsel av granföryngringar med tätt lövsly. (Cleaning of broadleaves- Management of Norway spruce regenerations with a dense broadleaved stands). Skogsvårdsstyrelsen i Kronobergs län. Information sheet, 4 pp. (In Swedish).
  4. 4. Anon.2010 Swedish statistical yearbook of forestry 2010. Swedish Forest Agency. Jönköping. 337 pp.
  5. 5. AssmannE.1970 The principles of forest yield study. Oxford. Pergamon Press, 506
  6. 6. AugspurgerC. K.FransonS. E.1987 Wind dispersal of artificial fruits varying in mass, area and morphology. Ecology 68(1), 2742
  7. 7. BärringU.1988 On reproduction of aspen (Populus tremula L.) with emphasis on its suckering ability. Scandinavian Journal of Forest Research 3, 229240
  8. 8. BeckD. E.1977 Growth and development of thinned versus unthinned yellow-poplar sprout clumps. U.S.D.A. Forest Service Research Paper SE-173, 16 pp.
  9. 9. BjörkbomJ. C.1971 Production and Germination of Paper birch Seed and its Dispersal into a Forest opening. U.S.D.A. Forest Service. Research Paper NE-209, 14 pp.
  10. 10. BjörkrothG.1973 Sådd och självsådd med björk på inägomark- resultat från ett 1rigt försök i övre Norrland. (Seeding and natural seeding with birch on farmland- results from a 1-year-old trial). Royal College of Forestry. Department of Forest Regeneration. Note 1973-03-12, 4 pp. (In Swedish).
  11. 11. Blumenthal-EB.1942 Studier angående aspens förekomst och egenskaper i Finland. Referat: Untersuchungen über des Vorkommen und die Eigenschaften der Espe in Finnland. Silva Fennica 56, 163
  12. 12. BraatheP.1988 Utviklingen av gjenvekst och ulike blaningsforhold mellom barrtræer of løvtræer- II: Summary: Development of regeneration with different mixtures of conifers and broadleaves II. Norwegian Forest Research Institute. Report 8, 50 pp. (In Norwegian).
  13. 13. BrinkmanK. A.RoeE. I.1975 Quaking aspen silvics and management in Lake States. U.S.D.A. Forest Service. Agriculture Handbook 486pp.
  14. 14. BørsetO.1956 Rotskudd hos osp. Summary: Suckers i aspen. Tidskrift for Skogbruk IV, 219240In Norwegian).
  15. 15. EtholénK.1974 Kaatoajankohdan vaikutus koivun ja haavan vesomiseen taimistonhoitoaloilla Pohjois-suomessa. Summary: The effect of felling time of Betula pubescens and Populus tremula in the seedling stands in Northern Finland. Folia Forestalia 213, 116In Finnish).
  16. 16. De BellD. S.TurpinT. C.1989 Control of red alders by cutting. U.S.D.A. Forest Service. PNW-RP-414, 10 pp.
  17. 17. FalinskiJ. B.1980 Vegetative dynamics and sex structure of the populations of pioneer dioecious woody plants. Vegetatio 43, 2338
  18. 18. FraserE. C.LieffersV. J.LandhäusserS. M.FreyB. R.2002 Soil nutrition and temperature as drivers of root suckering in trembling aspen. Canadian Journal of Forest Research 32, 16851691
  19. 19. FreyB. R.LieffersV. J.LandhäusserS. M.ComeauP. G.GreenwayK. J.2003An analysis of sucker regeneration of trembling aspen. Canadian Journal of Forestry Research 33, 11691179
  20. 20. FriesC.1984 Den frösådda björkens invandring på hygget. (The establishment of naturally seeded birch on clear cut areas). Sveriges Skogsvårdsförenings Tidskrift 82 (3-4), 3549In Swedish).
  21. 21. Frivold-HL.1982 Blandingsskogens status i europeisk skogbruk. Summary: Status of mixed forest in European forestry. Tidskrift for Skogbruk 90, 250261In Norwegian).
  22. 22. Frivold-HL.GrovenR.1996 Yield and management of mixed stands of spruce, birch and aspen. In. J. Dietrichson (Ed.). Silviculture for fuelwood. Norwegian Journal of Agricultural Science. Supplement 24, 2128
  23. 23. GraberR. E.LeakW. B.1992 Seed fall in an old-grown northern hardwood forest. U.S.D.A. Forest Service. Northeastern Forest Experiment Station. Research Paper NE-663, 11 pp.
  24. 24. GreeneD. F.JohnssonE. A.1997 Secondary dispersal of tree seeds on snow. Journal of Ecology 85, 329340
  25. 25. GrimeHodgson. J. G.HuntR.1988 Comparative Plant Ecology.- A functional approach to common British species. Oxford University. Press, UK.
  26. 26. HarmerR.PeterkenG.KerrG.PoultonP.2001 Vegetation changes during 100 years of development of two secondary woodlands on abandoned arable land. Biological Conservation 101291304
  27. 27. HarringtonC. A.1984 Factors influencing initial sprouting of red alder. Canadian Journal of Forest Research 14, 357361
  28. 28. HegreA.LanghammerAa.1967 Et bidrag til diskusjonen om blandingskog. Zusammenfassung: Beitrag zur Diskussion über Mischwald. Scientific Report from the Agricultural College Norway 46 (9), 30 pp. (In Norwegian).
  29. 29. HeikinheimoO.1932 Skogarnas naturliga föryngring. (Natural regeneration in forest stands). Centralsällskapets förening för skogskultur. Skriftserie nr 3. Helsinki, 90 pp. (In Swedish).
  30. 30. HeikinheimoO.1937 Über die Besamoungsfähigkeit der Waldbäume II. Communicationes Instituti Forestalis Fenniæ, 24 (4), 67 pp. (In German).
  31. 31. HesselmanH.1934 Några studier over fröspridningen hos gran och tall och kalhyggets besåning. (Some aspects about seed dispersal by Norway spruce and Scots pine and the seeding of the clear cut area)Meddelande från Statens Skogsforsöksanstalt, 27. (In Swedish).
  32. 32. HughesJ. W.FaheyT. J.1989 Seed dispersal and colonization in a disturbed northern hardwood forest. Bulletin of the Torrey Botanical Club 115 (2), 8999
  33. 33. HytönenJ.1994 Effect of cutting season, stump height and harvest damage on coppicing and biomass production of willow and birch. Biomass and Bioenergy 6 (5), 349357
  34. 34. HytönenJ.IssakainenJ.2001 Effect of repeated harvesting on biomass production and sprouting of Betula pubescens. Biomass and Bioenergy 20, 237245
  35. 35. JohanssonT.1992a Dormant buds on Betula pubescens and Betula pendula stumps under different field conditions. Forest Ecology and Management 47, 245259
  36. 36. JohanssonT.1992b Sprouting of 2- and 5-year-old birches (Betula pubescens Ehrh. and Betula pendula Roth) in relation to stump height and felling time. Forest Ecology and Management 53, 263281
  37. 37. JohanssonT.1992c Sprouting of 10- and 50-year-old Betula pubescens in relation to felling time. Forest Ecology and Management 53, 283296
  38. 38. JohanssonT.1993 Seasonal changes in contents of root starch and soluble carbohydrate in 4-6-year old Betula pubescens and Populus tremula. Scandinavian Journal of Forest Research 8, 94106
  39. 39. JohanssonT.1998 Seasonal changes in contents of root starch and soluble carbohydrates in young Alnus incana and Alnus glutinosa. Swedish University of Agricultural Sciences. Department of Forest Yield Research. Report 44, 20 pp.
  40. 40. JohanssonT.1999a Förekomst av självföryngrade lövträd på nedlagd åkermark. Summary: Presence of self-regenerated broad-leaved trees growing on abandoned farmland. The Swedish University of Agricultural Sciences. Department of Forest Management and Products. Report 2, 83 pp. (In Swedish).
  41. 41. JohanssonT.1999b Biomass production of Norway spruce (Picea abies (L.) Karst.). Silva Fennica 33 (4), 261 EOF280 EOF
  42. 42. JohanssonT.1999c Biomass equations for determining fractions of pendula and pubescent birches growing on abandoned farmland and some practical implications. Biomass and Bioenergy 16, 223238
  43. 43. JohanssonT.1999d Dry matter amounts and increment in 21-91-year-old common alder and grey alder and some practical implications. Canadian Journal of Forest Research 29, 16791690
  44. 44. JohanssonT.2000a Biomass equations for determining fractions of common and grey alders growing on abandoned farmland and some practical implications. Biomass and Bioenergy 18, 471480
  45. 45. JohanssonT.2000b Regeneration Norway spruce under the shelter of birch on good sites might increase the bioenergy supply I Sweden. New Zealand Journal of Forestry Science 30 (1/2), 1628
  46. 46. JohanssonT.2003 Mixed stands in Nordic countries- a challenge for the future. Biomass and Bioenergy 24365372
  47. 47. JohanssonT.2008 Sprouting ability and biomass production of downy and silver birch stumps of different diameters. Biomass and Bioenergy 32, 944951
  48. 48. JohanssonT.2009 Influence of felling time on the vegetative reproduction of 15year-old Alnus glutinosa and 8-year-old Alnus incana: Implications for biomass production. Swedish University of Agricultural Sciences. Department of Energy and Technology. Report 8, 27 pp.
  49. 49. JohanssonT.LundhJ.-E.1991Anläggning av blandskog. (Cultivation of management of mixed forests), Skog och Forskning 2, 1118In Swedish).
  50. 50. JohanssonT.Lundh-EJ.2006 Seed dispersal from a common alder (Alnus glutinosa (L.) Gaertner) stand. Swedish University of Agricultural Sciences. Department of Bioenergy. Report 10, 49 pp.
  51. 51. JonesR. H.RaynalD. J.1986 Spatial distribution and development of root sprouts in Fagus grandifolia (Fagaceæ). American Journal of Botany 73 (12), 1723-1731.
  52. 52. KarlsssonA.1996 Site preparation of abandoned fields and early establishment of naturally and direct-seeded birch in Sweden. Studia Forestalia Suecica 199, 25 pp.
  53. 53. KarlssonM.2001 Natural regeneration of broadleaved tree species in southern Sweden: Effects of silvicultural treatment and seed dispersal form surrounding stands. Acta Universitalis Agriculturœ Sueciœ. Silvestria 196, 44 pp. Doctoral dissertation.
  54. 54. KauppiA.1989 Sprouting in birches. Acta Universitas Ouly. A209, 32 pp.
  55. 55. KauppiA.RinneP.FermA.1987 Initiation, structure and sprouting of dormant buds in Betula pubescens. Flora 179, 5583
  56. 56. KauppiA.RinneP.FermA.1988 Sprouting ability and significance for coppicing of dormant buds on Betula pubescens Ehrh. stumps, Scandinavian Journal of Forest Research 3, 343354
  57. 57. KaysJ. S.CanhemC. D.1991 Effects of time and frequency of cutting on h and wood root reserves and sprout growth. Forest Science 37 (2), 524539
  58. 58. KeltyJ. M.1992 Comparative productivity of monocultures and mixed-species stands. In. M.J. Kelty and C.D. Oliver (Eds). Dordreht. Kluwer Academic Publishers, 125141
  59. 59. KlangF.Ekö-MP.1999 Tree properties and yield of Picea abies planted in shelterwoods. Scandinavian Journal of Forest Research 14, 262269
  60. 60. KohhE.1936 Beobachtungen über Reifen und Fallzeit der Samen im Lehr- und Versuchs-Forsamt v.j. 19301035Mitteilungen der Forstwissenshaftlichen Abteilung der Universität. Tartu, 124 pp.
  61. 61. La BastideJ. G. A.van VredenburchC. L. H.1970 The influence of weather conditions on the seed production of some forest trees in the Netherlands. Mede. Bosbouwproefstation, nr 102, 12 pp.
  62. 62. AaLanghammer1982 Refleksjoner omkring et plantefelt med osp (Populus tremula) and spruce (Picea abies) in Norway. Tidskrift for Skogbruk 90, 102110In Norwegian).
  63. 63. Latva-KarjanmaaT.SuvantoL.LeinonenK.HannuR.2006 Sexual reproduction of European aspen (Populus tremula L.) at prescribed burned site: the effects of moisture conditions. New Forests 31, 545548
  64. 64. LinesR.1982 Mixture experiments. Forestry Commission. Report on Forest Research. HMSO. London, 1314
  65. 65. Lundh-EJ.2003 Direct seeding of alder on farmland.-Effects of seed stratification on seedling emergence and height development. The Swedish of Agricultural Sciences. Department of Bioenergy. Licentiate thesis, 32 pp.
  66. 66. MacVean. D. N.1955 Ecology of Alnus glutinosa (L.) Gaertn. Seed distribution and germination. Journal of Ecology 43, 6171
  67. 67. MacVean. D. N.1956 Ecology of Alnus glutinosa (L.) Gaertn. III. Seedling establishment. Journal of Ecology 44, 195218
  68. 68. MatlackG. R.1988 Secondary dispersal of seed across snow in Betula lenta, a gap-colonizing tree species. Journal of Ecology 77, 853869
  69. 69. MårdH.1997 Stratified mixtures of young Norway spruce and birch as an alternative to pure stands of Norway spruce. Acta Universitatis Sueciæ. Silvestria 35, 29. Doctorial thesis.
  70. 70. MielikäinenK.1985 Koivuskoituksen vaikutus kuusikon rakenttseen ja kehitykseen. Summary: Effect of an admixture of birch on the structure and development of Norway spruce stands. Communicationes Institutii Forestalis Fenniæ 99 (3), 99 pp.
  71. 71. MikolaP.1942 Koivun vesomisesta ja sen metsänhoidollista merkityksestä. Referat: Über die Ausschlagbildung bei der Birke und ihre forstlige Bedeutung. Acta Forestaia Fenniœ 50, 0pIn Finnish)
  72. 72. Mork,E.1944 Om björkfruktens bygning, modning, og spiring. (The anatomy and germination of birch seeds). Meddelser fra det Norske Skogsfosøgsvesen 30, 423471In Norwegian).
  73. 73. MysterR. W.1993 Tree invasion and establishment in old fields at the Hutcheson Memorial Forest. Botanical Review 59251278
  74. 74. OkuboA.LevinS. A.1989 A theoretical framework for data analysis of wind dispersal of seeds and pollen. Ecology, 70(2), 329338
  75. 75. OsbornovaJ.KovarovaM.LepsJ.PrachK.1990 Succession in abandoned fields. Kluwer Academic Publishers, Dordrecht.
  76. 76. PaukkonenK.KauppicA.1992 Root and stump buds as structural faculties for reinvigoration in Alnus incana (L.) Moench. Flora 187, 353367
  77. 77. PeralaD.1979 Regeneration and productivity of aspen grown on repeated short rotations. U.S.D.A. Forest Service Research Paper NC-176, 7 pp.
  78. 78. PetersonE. B.PetersonN. M.1996 Ecology and silviculture of trembling aspen. In. P.G. Comeay, G.J. Harper, M.E. Blache, J.O. Boateng and K.D. Thomas (Eds.). Ecology and management of B.C. hardwoods. F.R.D.A. British Columbia. Ministry of Forests. Research Branch. Victoria, B.C. Report 255, 3152
  79. 79. ReimP.1929 Die Vermehrungsbiologie der Aspe auf Grundlage des in Estland und Finnland gesammelten Untersuchungsmaterials. Ph. D. Thesis. The University of Tartu, Estonia, 60 pp. (In German).
  80. 80. ReimP.1930 Haava paljunemis-bioloogia. Zusammenfassung: Die Vermehrungsbiologie der Aspe auf Grundlage des in Estland und Finnland gesammelten Untersuchungsmaterials. Mitteilungen der Forstwissenschaftlichen Abteilung der Universität. Tartu 16, 188 p.
  81. 81. RydbergD.2000 Initial sprouting, growth and mortality of European aspen and birch after selective coppicing in central Sweden. Forest Ecology and Management 130, 2735
  82. 82. RytterL.1996 Grey alder in forestry: a review. Norwegian Journal of Agricultural Sciences. Supplement 24246584
  83. 83. SchrötterH.1983 Waldbaulichertragskundliche Untersuchungen an Weisserle (Alnus incana (L.) Moench) im Jungpeistozän der DDR. Beiträge für die Fortwisschaft 17, 89102
  84. 84. ShierG. A.ZasadaJ. C.1973 Role of carbohydrate reserves in the development of root suckers in Populus tremuloides. Canadian Journal of Forest Research 3, 243250
  85. 85. SimsR. E. H.SenetwaK.MaiavaT.BullockB. T.1999 Eucalyptus species for biomass energy in New Zealand.- Part II: Coppice performance. Biomass and Bioenergy 17, 333343
  86. 86. StenekerG. A.1976 Guide to the silvicultural management of trembling aspen in the prairie provinces. Canadian Information Report. NOR-X-164, 6 pp.
  87. 87. StewartJ. D.LandhäusserS. M.StadtK. J.LieffersV. J.2000 Regeneration of white spruce under aspen canopies: seeding, planting and site preparation. Western Journal of Applied Forestry 15 (4), 177182
  88. 88. StoneD. M.ElioffJ. D.1998 Soil properties and aspen development five years after compaction and forest floor removal. Canadian Journal of Soil Science 785158
  89. 89. StoneD. M.KabzemsR.2002 Aspen development on similar soils in Minnesota and British Columbia after compaction and forest floor removal. The Forest Chronicle 78 (6), 886891
  90. 90. TewR. K.1970 Root carbohydrate reserves in vegetative reproduction of aspen. Forest Science 16318320
  91. 91. ThamÅ.1988 Yield prediction after heavy thinning of birch in mixed stands of Norway spruce (Picea abies (L.). Karst.) and birch (Betula pendula Roth and Betula pubescens Ehrh.). University of Agricultural Sciences. Department of Forest Yield Researh. Report 33, 36 pp. Doctoral Thesis.
  92. 92. UriV.VaresA.TullusH.KanalA.2007 Above-ground biomass production and nutrient accumulation in young stands of silver birch on abandoned agricultural land. Biomass and Bioenergy 31195204
  93. 93. ValkonenS.ValstaL.2001 Productivity and economics of mixed two-storied spruce and birch stands in Southern Finland simulated with empirical models. Forest Ecology and Management 140133149
  94. 94. WendelG. W.1975 Stump sprout growth and quality of several Appalachian hardwood species after clearcutting. U.S.D.A. Forest Service. Research Paper NE-329, 9 pp.
  95. 95. WernerP. A.HarbeckA. L.1982 The pattern of tree seedling establishment relative to staghorn sumac cover in Michigan old fields. The American Midland Naturalist 108 (1), 124132
  96. 96. WillsonM. F.TraversetA.2000 The ecology of seed dispersal. In. M. Fenner (ed.), Seeds-The Ecology of Regeneration in Plant Communities. 2nd edition. CABI Publishing. UK, 85110
  97. 97. ZehngraffP. J.1946 Season of cutting affects aspen sprouting. Lake States Forest Experimental Station. Technical Note 250pp.

Written By

Tord Johansson

Submitted: October 28th, 2010 Published: September 9th, 2011