The Effect of Whole Tree Harvesting on Fuel Quality and Coppicing Ability of SRIC Willow Crops

Juha Nurmi and Jyrki Hytönen, Finnish Forest Research Institute, Kannus Research Station, PL 44, 69100 Kannus, Finland

ABSTRACT

A seven-year-old stand of willow (Salix 'Aquatica') stand was manually felled by using a chain saw and a clearing saw. Uncomminuted stems were piled and stored for 18 months. Heating value and basic density of the material remained unchanged. Moisture content dropped from the initial 54 percent to 20 percent by the end of the second summer of storage. Bark content dropped from 24.1 percent to 14.9 percent. Stool damage caused by forwarding did not have a significant effect on the height growth or biomass production of the following crop.

INTRODUCTION

The purpose of this study was two-fold. First, to determine how storage will affect SRIC fuelwood characteristics when it is stored as whole-trees over an extended period of time. Secondly, to find out how the second generation biomass production is affected by the stool damage caused by harvesting machinery. The opportunity to carry out this study arose when a seven-year-old, second generation willow crop was made available for research.

The common problems of wood storage include dry matter losses and uneven moisture content. Numerous studies on wood storage from conventional forestry operations have shown that if comminution of wood by chipping, shredding or hammermilling is followed by storage in piles the possibilities of self- ignition and health risks by fungi spores will eventually emerge. Although much emphasis has been put on the breeding, growth and yield of SRIC willow crops, the storage problem has been totally neglected. With this in mind a storage study was set up at the Finnish Forest Research Institute. The aim was to monitor moisture content, wood density, heating value and bark content over a period of 18 months. It is a well accepted fact that the comminution of the SRIC crops should be integrated into the harvesting operation. However, if it is necessary to store the feedstock for any length of time this should be done as whole trees. This is because chip storage has been shown to cause extensive dry matter losses in numerous studies (Thornquist 1987; Thornquist 1988; Nurmi 1990).

A factor critical to the success of willow plantations is the sustainability of the coppice system over successive harvests. Several factors, both internal and external, influence regeneration from stumps described by Sennerby-Forsse and others (1992). Many external factors and practical management measures such as cutting season, stump diameter, stump height, cutting method, fertilization, site quality, rotation length, spacing have been shown to influence coppicing vigor.

Knowledge of the influence of factors affecting the coppicing ability and biomass production of short rotation plantations is necessary for the determination of cutting schedules and the development of harvesting techniques. The aim was to investigate the effect of harvest damage on willow coppicing.

MATERIAL AND METHODS

The experimental area was established in spring 1982 on a sandy mull field situated in southern Finland (60^o32' N, 24^o37' E). The willows (Salix 'Aquatica, clone V769) were planted at a density of 36,000 plants ha-1 (80 x 35 cm) in early summer 1982. One-year-old rooted cuttings, the shoots of which had been cut back to the stem, were used for stocking. Weeds were controlled mechanically using a tractor- pulled harrow during the first summer. Willows were first harvested after three growing seasons (1985) and again after a seven year long rotation in 1992. This time harvesting was done with a chainsaw and a brushsaw to provide material for this study. This second crop also formed the material for the storage study.

The storage study was composed of 13 dry tons of non-comminuted willows. They were forwarded and piled after felling in a single, uncovered stack. Moisture content was determined on green weight bases by drying the samples at 102 degrees C to a constant weight. Calorimetric heating value was determined with a Leco AC-300 calorimeter. Basic density was calculated on the basis of oven dry weight and water saturated volume. Bark content was calculated from the dry weight of the sample. Sampling was done first during the piling of the fresh material and from there on once every three months over the 18 month storage period. It was done in such a manner, that each time two stems were extracted from the bottom, center and top of the pile. These six stems were further sampled at 10, 30 and 80 percent lengths, i.e. pile depth. Furthermore, every time the pile was sampled, two live trees were also sampled for the same characteristics.

To study the effects of forwarding damage and the choice of cutting method on the coppicing ability a willow stand was cut with a chainsaw (Experiment 1) and with a brushsaw (Experiment 2). The treatments consisted of four-meters-long tracks laid out along the rows of planted willow. The treatments included a control (A), light weight Farmi Trac mini-forwarder driving on the row of stumps (B) and manual damage of the stumps using a sledge-hammer (C). In both experiments, the treatments were replicated three times in a randomized block design. Height and the number of sprouts per stool were measured on the experimental plots after one growing season in autumn 1992. The leafless above-ground biomass was determined using allometric dry-mass equations based on sample trees (Hytönen 1988, Hytönen and others 1987).

THE EFFECTS OF STORAGE ON FUEL QUALITY

The moisture content of the fresh material at the beginning of the trial was 54.0 percent on green weight basis. As Hytönen and Ferm (1984) reported moisture content for a one year old stand to be 63.7 percent and 55.9 percent for a five year old stand, this might be an indication that the moisture content of fresh material decreases with stand age. The standing trees showed some variation with the season. This, however, was not significant and is in accordance with previous knowledge (Hakkila 1962). Wood and bark in the storage dried over the first summer to an average moisture content of 34.6 percent gaining some moisture over winter. During the second summer further drying took place proving the time factor to be statistically significant. The final moisture content was 20.0 percent. When the significance of the location in the pile (top, center, bottom) was tested for moisture content no significance was found. But, when the height of the stem, i.e., pile depth, was tested the 80 percent height was found to be significantly higher in moisture content than the other parts of the pile.

Figure 1. Moisture content (%, green weight basis) of willow stock over time and the significance of time in the analysis of variance

Figure 2. Calorimetric heating value (MJ/kg) of willow wood over time and the significance of time in the analysis of variance

The calorimetric heating value of wood and bark were determined separately. Heating value of both standing trees and those in the pile had very little fluctuation with time. However, this fluctuation in the pile was statistically significant. This was strictly caused by the fluctuation in the samples taken at 80 percent height. The cause of this behavior is not known.

The heating value of willow bark was higher than that of wood. The calorimetric heating value of bark ranged between 20.1 to 20.3 MJ/kg and of wood 19.8 to 20.2 MJ/kg. Season was a significant factor both among standing trees and those from the pile

Figure 3. Calorimetric heating value (MJ/kg) of willow bark over time and the significance of time in the analysis of variance

Height of the pile did not turn out to be a significant factor. The pile depth was significant, however, as the heating value of willow bark increases from the base of the stem to the top. Similar or opposite transitions along the stem have been recorded previously on other tree species by Nurmi (1993).

The wood density of the fresh material in the beginning of the study was 411 kg/m³. This is somewhat higher than what was reported for a 5-year-old stand (382 kg/m³) by Hytönen and Ferm (1984). This might be an indication of increased density with time. The density dropped during the storage to 399 kg/m³. This reduction proved to be significant . Similarly location, both in height and depth, proved to be significant factors. Wood density was significantly lower in the bottom of the pile and at the 80 percent depth than in other parts of the pile.

Figure 4. Weight density (kg/m³) of willow wood over time and the significance of time in the analysis of variance

The low density of the tops also proved to be evident at the end of the study when they suffered from much breakage during handling and comminution. Although this loss was not measured dry matter losses looked substantial.

Figure 5. Bark content (%) over time

As tops contain more bark than the rest of the stem the average bark content fell from 25 percent to 15 percent . Consequently, the average heating value of the remaining material was lowered.

As a result of the storage study we can conclude that willow stems dried well in uncovered piles. Although the lowest moisture content was reached during the second summer it is recommended that the material should be comminuted and burned after the first summer. This is to reduce material loss through breakage of the tops and dry matter losses by fungi and bacteria.

HARVESTING DAMAGE

Figure 6. The effect of harvesting damage on the leafless above ground dry mass(g/stool) and the number of sprouts per living stump of S. 'Aquatica'

The effects of harvesting damage on the biomass, mean height and number of sprouts per stool were measured one growing season after treatment. In both experiments (1 and 2) the biomass per stool and the number of sprouts per stool was lower following harvest damage caused by the mini-forwarder . Mean shoot height was not affected by the treatment. Manual damage, thought to be more severe, caused similar effects. However, the differences between the treatments were not statistically significant.

Similar to birch (Mikola 1942, Ferm and Issakainen 1981) harvest damage did not have a significant effect on older, well established plantation. However, in young plantations, harvest damage has had a negative effect on survival, height growth and biomass production of S. 'Aquatica' (Hytönen 1994).

Differences between species in relation to effects of harvest damage may be due to the location of the sprout producing buds. About 90 percent of birch's basal buds are located below ground (Kauppi and others 1987, 1988) while most of the buds of Salix 'Aquatica' are above ground level (Paukkonen and others 1992). In coppiced S. viminalis most (85 to 90 percent) of the sprouts originate from the axillary bud groups located on the remaining basal parts of the previously harvested stems (Sennerby-Forsse and others 1992). Thus, harvesting damage may have more serious effects on willow than on birch regeneration. Iin the design of willow harvesters, their effects on the sustainability of the coppice system should be taken into account.

LITERATURE

1. Ferm, A.; and Issakainen, J. 1981. Kaatoajankohdan ja kaatotavan vaikutus hieskoivun vesomiseen turvemaalla. Metsäntutkimuslaitoksen tiedonantoja 33. 13 pp.
2. Hakkila, P. 1962. Polttohakepuun kuivuminen metsässä. Summary: Forest seasoning of wood intended for fuel chips. Communicationes Instituti Forestalis Fenniae 54(4). 82 pp.
3. Hytönen, J. 1994: Effect of cutting season, stump height and harvest damage on coppicing and biomass production of willow and birch. Biomass and Bioenergy. In print.
4. Hytönen, J. 1988. Biomass production of Salix 'Aquatica' on an abandoned field in South Finland. Metsäntutkimuslaitoksen tiedonantoja 304:74-90.
5. Lumme, I.; and Törmälä, T. 1987. Comparison of methods for estimating willow biomass. Biomass 14:39-49.
6. Hytönen, J.; and Ferm, A. 1984. Vesipajun vesojen puuteknisiä ominaisuuksia. Abstract: On the technical properties of Salix 'Aquatica' sprouts. Metsäntutkimuslaitoksen tiedonantoja 163. 20 pp.
7. Kauppi, A.; Rinne, P.; and Ferm, A. 1987. Initiation, structure and sprouting of dormant basal buds in Betula pubescens. Flora 179: 55-83.
8. Kauppi, A.; Rinne, P.; and Ferm, A. 1988. Sprouting ability and significance for coppicing of dormant buds on Betula pubescens Ehnr. stumps. Scand. J. For. Res 3: 343-354.
9. Mikola, P. 1942. Koivun vesomisesta ja sen metsänhoidollisesta merkityksestä. Referat: Über die Ausschlagsbildung bei der Birke und ihre forstliche Bedeutung. Acta For. Fenn. 50(3):1-102.
10. Nurmi, J. 1990. Polttohakkeen varastointi suurissa aumoissa. Summary: Longterm storage of fuel chips in large piles. Folia Forestalia 767. 18 pp.
11. Nurmi, J. 1993. Heating values of the above grown biomass of small-sized trees. Acta For. Fenn. 236. 30 pp.
12. Paukkonen, K.; Kauppi, A.; and Ferm, A. 1992. Origin, structure and shoot-formation ability of buds in cutting-origin stools of Salix 'Aquatica'. Flora 186: 53-65.
13. Sennerby-Forsse, L.; Ferm, A.; and Kauppi, A. 1992. Coppicing ability and sustainability. In: C. P. Mitchell, J. B. Ford-Robertson, T. Hincklye, and L. Senerby-Forsse (eds.), Ecophysiology of short rotating coppice. Elsevier Applied Science, 146-184.
14. Thörnqvist, T. 1987. Bränder i stackar med sönderdelat trädbränsle. Summary: Spontaneous combustion in piles with comminuted wood fuel. Sveriges Lantbruksuniversitet, Institutionen for virkeslära. Uppsats 163.

File posted on March 5, 1996; Date Modified: February 21, 1999