A management regime for hybrid aspen stands combining conventional forestry techniques with early biomass harvests to exploit their rapid early growth

Transcription

1 Forest Ecology and Management 236 (2006) Short communication A management regime for hybrid aspen stands combining conventional forestry techniques with early biomass harvests to exploit their rapid early growth Lars Rytter * The Forestry Research Institute of Sweden (Skogforsk), Ekebo 2250, SE Svalöv, Sweden Received 21 October 2005; received in revised form 4 July 2006; accepted 21 September Abstract A management regime that combines early harvests and conventional forestry techniques is proposed for hardwood stands in northern environments and is illustrated by the potential use of hybrid aspen (Populus tremuloides Michx. Populus tremula L.). The high early biomass production rates of hardwoods could be exploited by corridor cleaning and use of the material harvested for biofuels, while the remaining stand could be treated with ordinary forestry measures to produce pulpwood and logs. Regenerating hybrid aspen rapidly produces a large number of suckers. In this study, 76,000 suckers ha 1 on average was found at a site in southern Sweden 2 years after clear felling. After 4 years 38 tonnes of dry matter ha 1 could be extracted, corresponding to 9.5 tonnes ha 1 year 1. However, areas, which were corridor-cleaned already 2 years after felling produced, in total significantly less biomass ( tonnes ha 1 year 1 ) in the 4-year post-felling period. The high early rates of biomass production suggest that the combined forestry concept has interesting potential, but the effects of corridor cleaning and biomass removal on the future growth and quality of the stand should be examined in further studies before it is practiced on a commercial scale. # 2006 Elsevier B.V. All rights reserved. Keywords: Biofuels; Corridor cleaning; Populus tremula Populus tremuloides; Sucker regeneration; Woody biomass production 1. Introduction Pioneer hardwood tree species generally show fast initial development. Height growth, and thus annual increment, peak at an early age (Børset and Langhammer, 1966; Raulo, 1977). Consequently, future stand development is strongly affected by early management practices. In the boreal zone, under Nordic conditions, fast-growing pioneer species of the genera Betula, Populus and Alnus are commonly found (e.g. Rytter, 2004). A particularly fast-growing species is hybrid aspen (Populus tremula L. Populus tremuloides Michx.), a cross between American (P. tremuloides) and European aspen (P. tremula). This cross is also well adapted to the climate in the region and can be grown over large areas. Planted hybrid aspen can be highly productive (Johnsson, 1953; Jakobsen, 1976; Elfving, 1986; Rytter, 2002; Karačić et al., 2003; Rytter and Stener, 2005), giving estimated yields exceeding 20 m 3 of stem wood ha 1 during a years * Tel.: ; fax: address: Lars.Rytter@skogforsk.se. rotation period using currently available improved plant material (Rytter and Stener, 2005). However, hybrid aspen, like both of the parent species, also regenerates quickly from root suckers (Hansson and Palmér, 1990; Liesebach et al., 1999). Thus, the rapid development of hybrid aspen stands is further enhanced by the existing root system from the second generation onwards. The importance of biofuels in the energy sector of Nordic countries is increasing. Today they supply around 25% of the total energy domestically consumed in Sweden (Grunéus, 2005). The main sources are lops and tops collected during the course of ordinary forestry at the end of the rotation period. However, energy forests have been developed on a commercial scale, and other crops like grasses and cereals are also being tested. In addition, there is a shortage of hardwood trees and wood in Sweden for both industrial uses and nature conservation (Sjöberg and Lennartsson, 1995; Ministry of the Environment, 2000; National Board of Forestry, 2003), so an increase in the extent of hardwood forest would be welcome. Aspen and hybrid aspen wood is currently mainly used as pulpwood or matchwood by industries in Sweden. However, some is also used in plywood, and in Norway aspen is used for boarding. Aspen and hybrid aspen are also important /$ see front matter # 2006 Elsevier B.V. All rights reserved. doi: /j.foreco

2 L. Rytter / Forest Ecology and Management 236 (2006) components of high quality pulp produced in Finland. Thus, there are existing markets for aspen pulpwood, saw logs and biofuels. A possible way to improve forestry profitability is to collect large quantities of early biomass from fast-growing hardwood stands, which should have little or no impact on their later development or profitability. The first issues to consider in assessing the viability of this option are the amounts of biomass that could be collected, and the feasibility of collecting it by cleaning. The study reported here focused on these issues, by estimating the amounts of biomass involved. The impact of different corridor cleaning methods and schedules on the productivity of hybrid aspen stands was also investigated. 2. Material and methods The Jordkull study site is situated on medium-productivity agricultural land with clayey moraine soil in southern Sweden (latitude N; longitude E; altitude 85 m). During the winter of a planted 11-years-old stand, consisting of selected clones that grow well but cannot easily be distinguished after harvest, was harvested, and the sucker stand examined in this study started to grow in the spring of The site is divided into four blocks, each of 0.18 ha (42 m 42 m), covering 0.71 ha in total. The blocking was justified by differences in the initial survival rate of the first generation stand over the area, and by suspected differences in soil moisture. The area was not fertilized or irrigated during the hybrid aspen cultivation period. Three different treatments were applied to different areas in the new sucker stand after two growing seasons: (1) no cleaning, (2) corridor cleaning and (3) cross-corridor cleaning. There were four replications of each treatment and the treatments were randomly applied in each block to areas of 42 m (12 15) m. In alternative two, 2 m corridors were cleaned leaving 1 m rows of tree, i.e. 2/3 of the area was cleaned. In alternative three, 2 m corridors were cleaned in two directions, leaving 1 m 1 m squares with trees, i.e. 8/9 of the area was treated. The cleaning was carried out manually by brush saw, so only effects of above-ground spacing could be evaluated. Effects of machinery such as soil compaction and root damage are thus beyond the scope of this study. Three circular plots were placed in each treatment area and block before the cleaning treatment in year 2, giving 36 plots in total. The plots were placed randomly, with at least a 3 m zone between the plot and the border, in every third (14 m (12 15) m) of each treatment and block combination. The area of each of these plots was 10 m 2 on the first measurement occasion after 2 years. The same plots were used in the next measurement after 4 years, but on cleaned areas the circular surrounding 15 m 2 were also investigated giving 25 m 2 in total, to obtain a more even stem number among treatments and thus, more accurate estimates of tree growth between corridors by better covering proportions of cleaned and uncleaned ground. The diameter on bark at breast height of all living trees with heights greater than 1.3 m was measured in each plot. Height was recorded for every 10th tree, and functions describing the relationship between diameter and height were constructed, using the formula: H ¼ ad b (1) where H is the tree height, D the diameter at breast height over bark, and a and b are constants (r 2 = 0.93 in year 2). Different functions were used for cleaned and uncleaned parts in the analyses after 4 years (r 2 = 0.87 and 0.86, respectively). Sample trees of different sizes were collected for each combination of block and treatment. Their dry weights were recorded and functions, forced through the origin, describing the relationship between dry weight (including branches) and volume (estimated using D and H data) were constructed according to the formula: W d ¼ cv 2 þ dv (2) where W d is the tree dry weight, V the tree volume expressed by D 2 H, and c and d are constants. In the sampling after 2 years 40 trees were used (r 2 = 0.96). After 4 years, 60 trees were used in total. The same function could be used for all treatments (r 2 = 0.97). Tree and stand characteristics were compared between treatments by ANOVA (SAS, 1999), using the following statistical model: Y ij ¼ m þ p i þ b j þ e ij (3) where Y ij is the observation ij, p i the fixed effect of the cleaning treatment, b j the random effect of block and e ij is the random error term for observation ij. When significant differences were found between treatments, pair-wise comparisons among treatments were performed with the Tukey method. The null hypothesis, that there was no difference between means, was rejected if the probability level (P) was Results A large number of suckers sprouted after the planted stand was harvested (Fig. 1), resulting in high ground coverage and Fig. 1. Sucker density before cleaning treatment (year 2) and 2 years after treatment (year 4) in the studied hybrid aspen stand. Corridor cleaning was performed after the measurements at 2 years of age. Significant differences within years are indicated by different letters. Bars show the standard errors between blocks for the corresponding treatments and years.

3 424 L. Rytter / Forest Ecology and Management 236 (2006) on average in the not cleaned plots after 4 years. In both cleaning treatments the total production of living suckers amounted to tonnes ha 1 (biomass in year 4 + biomass harvested in year 2, Fig. 2B): significantly lower than in areas that were not cleaned. Although, significantly more biomass was removed in the crosscorridor than the corridor cleaning treatment (Fig. 2B) the mean annual increments after 4 years were similar (Fig. 2C). Thus, mean annual increments were 9.5 and tonnes ha 1 year 1 after 4 years for not cleaned and cleaned areas, respectively (Fig. 2C). New suckers appeared in the cleaned corridors during the first season after cleaning. However, their growth slowed rapidly in the second year and then ceased, contributing very little to standing biomass. Fig. 2. Development of area-related characters in the hybrid aspen sucker stand. (A) Stem basal area development in the three treatments after 2 and 4 years of growth, and amount of basal area removed by cleaning after 2 years; (B) standing woody biomass and amount of biomass removed by cleaning after 2 years and (C) mean annual increment (MAI) of woody biomass estimated after 2 and 4 years of growth under the different treatments. Different letters for a particular character and year indicate significant differences between treatments. Bars show the standard errors between blocks for the corresponding characters, treatments and years. biomass (Fig. 2) within 2 years, although the diameter of the suckers remained at around 1.5 cm (Fig. 3). Cleaning strongly reduced stem number, but stem number also decreased between years 2 and 4 in the areas that were not cleaned. The average woody biomass production of living suckers reached about 15 tonnes of dry matter ha 1 after 2 years (Fig. 2B). The production continued and reached 38 tonnes ha 1 Fig. 3. Development of sucker-related characters in the hybrid aspen stand. (A) Arithmetic mean diameter (D a ) at breast height over bark; (B) arithmetic mean height (H a ) and (C) arithmetic mean sucker dry weight (W a ), in each case after 2 (before cleaning) and 4 years. Bars show the standard errors between blocks for the corresponding characters, treatments and years.

4 L. Rytter / Forest Ecology and Management 236 (2006) In our study, 25 tonnes of dry woody biomass ha 1 could be harvested by corridor cleaning 2/3 of the area after 4 years. In the cross-corridor cleaning the amount was estimated to 34 tonnes by cleaning 8/9 of the area. Corridor and crosscorridor cleaning after 2 years resulted, correspondingly, in biomass harvests of nearly 9 and nearly 15 tonnes ha 1 for the respective treatments (Fig. 2B). The development of sucker diameter, height and weight was similar within blocks before cleaning and was not significantly affected by the treatments during the following 2 years (Fig. 3). However, there were significant differences between blocks for all characters before cleaning treatments were performed (after 2 years: D a, P = 0.003; H a, P = ; W a, P = ). The arithmetic tree height and tree diameter were about 5 m and 2.4 cm, respectively, after 4 years (Fig. 3). However, the ranges of both variables were large and the biggest suckers were over 8 m tall with breast height diameters exceeding 5 cm. 4. Discussion and conclusion The results show that regenerating hybrid aspen stands can provide large amounts of biomass in early stages: over 9 tonnes of dry matter ha 1 year 1 with no fertilizer applications during the first 4 years, similar to estimated yields of fertilized willow stands grown for energy purposes in Sweden (Willebrand et al., 1993; Ledin, 1996; Larsson, 2001). Thus, sucker stands of hybrid aspen seem to be competitive as biomass producers. Treatments involving cleaning after 2 years resulted in significantly lower production figures (Fig. 2). After harvesting this early biomass, conventional forestry regimes directed towards pulpwood and saw log production may continue. The concept proposed here involves harvesting biomass by corridor cleaning and subsequent main stem selection by manual cleaning. It may also be possible to develop costeffective techniques for further biomass collection after the initial cleaning, since the corridors generated using this approach would facilitate the movement and use of machinery. The target tree density after cleaning operations would be ca stems ha 1, the standard planting density for hybrid aspen in the first generation. Practical experiences have shown that this density leads to favourable stem dimensions at the first thinning occasion, and also allows a no-thinning forestry alternative. Since the costs of regeneration and early silvicultural measures are high and have been accounting for increasingly large proportions of total forestry costs in Sweden (National Board of Forestry, 2003), any income from biomass harvested at cleaning should be welcomed. In addition, the regeneration of hybrid aspen involves no costs other than those associated with cleaning, and fencing where recommended (especially in game dense regions). Economic calculations have shown that hybrid aspen is a profitable species even without harvesting the early biomass (Elfving, 1986; Eriksson, 1991; Rytter et al., 2002), but any income from early biomass harvests would further increase the profitability. In the last 2 years suckers were growing in areas that had not been cleaned, and in the non-cleaned rows and squares in the areas subjected to the corridor and cross-corridor cleaning, respectively. The dimensions of suckers did not differ between the treatments at the end of this period, possibly because it was too short for such differences to develop and/or trees in rows and squares may only have been released from light and root competition by neighbouring suckers to a small degree. Otherwise, diameter growth would normally be expected to increase after thinning (cf. Fällman et al., 2003; Rytter and Stener, 2005). Growth of the new suckers that appeared in the cleaned corridors stagnated in the second year, so they are unlikely to represent obstacles in further management operations, or to contribute further biomass. The recorded biomass consisted of living suckers taller than 1.3 m, representing readily obtainable biomass. Substantial quantities of biomass were lost through self-thinning, as shown by the reduction in stem number (Fig. 1) in the not cleaned areas between years 2 and 4. The total woody biomass production was therefore higher than the figures suggest, but the lost biomass may be difficult to collect in any practical silvicultural system. The effects of mechanized corridor cleaning on growth and damage to hybrid aspen roots are unknown and could not be observed in this study since the cleaning treatments were applied manually. However, regeneration felling has not been shown to reduce growth of the next hybrid aspen generation (e.g. Rytter and Stener, 2005). The results show that large amounts of biomass could be available early in hybrid aspen rotations and that early harvest of biofuels is possible. The biomass harvest could be succeeded by conventional forestry to produce pulpwood and logs, for which there are existing markets. The results suggest that further studies should be undertaken, in which the influence of early corridor cleaning, combined with biomass harvest, on future stand development should be investigated as well as the economic consequences of the proposed combined management regime. Acknowledgements This study was financially supported by the Swedish Energy Agency. The author wish to thank the personnel at the Forestry Research Institute of Sweden for their invaluable technical assistance, especially Gudmund Ahlberg, Eva Christensson, and Lars Wremert. Martin Werner and two anonymous reviewers are gratefully acknowledged for valuable comments on the manuscript. References Børset, O., Langhammer, A., Growth and yield in stands of grey alder (Alnus incana). Scientific Rep. Agric. Coll. Norway 45 (24), 1 35 (in Norwegian with English summary). Elfving, B., Odlingsvärdet av björk, asp och al på nedlagd jordbruksmark i Sydsverige. Sv. Skogsv. Förb. Tidskr. 5/86, (in Swedish). Eriksson, L., The economy of afforestation of abandoned fields. Swed. Univ. Agric. Sci., Dept. Forestry-Industry-Market Studies. Report No. 17, Uppsala, 141 pp. (in Swedish with English summary).

5 426 L. Rytter / Forest Ecology and Management 236 (2006) Fällman, K., Ligné, D., Karlsson, A., Albrektson, A., Stem quality and height development in a Betula-dominated stand 7 years after precommercial thinning at different stump heights. Scand. J. For. Res. 18, Grunéus, P., Energiförsörjningen i Sverige. Swedish Energy Agency, Report ER 2005:04. Eskilstuna, 44 pp. (in Swedish). Hansson, P., Palmér, C.H., Självföryngring av hybridasp. Inst. f. skogsförbättring. Information Skogsträdsförädling No /90, 4 pp. (in Swedish). Jakobsen, B., Hybrid aspen (Populus tremula L. Populus tremuloides Michx.). Det Forstlige Forsøgsvæsen i Danmark 34, (in Danish with English summary). Johnsson, H., Hybridaspens ungdomsutveckling och ett försök till framtidsprognos. Sv. Skogsv. Förb. Tidskr. 51, (in Swedish). Karačić, A., Verwijst, T., Weih, M., Above-ground woody biomass production of short-rotation Populus plantations on agricultural land in Sweden. Scand. J. For. Res. 18, Larsson, S., Breeding of Salix. J. Swed. Seed Assoc. 111, (in Swedish, English summary). Ledin, S., Soil characteristics and production potential of willow on farmland. Swed. Univ. Agric. Sci., Dept. Short Rotation Forestry, Report 57, Uppsala, pp Liesebach, M., von Wuehlisch, G., Muhs, H.-J., Aspen for short-rotation coppice plantations on agricultural sites in Germany: effects of spacing and rotation time on growth and biomass production of aspen progenies. For. Ecol. Manage. 121, Ministry of the Environment, The Swedish Environmental Objectives Interim Targets and Action Strategies. Summary of the Government Bill 2000/01:130. Regeringskansliet, Stockholm, 80 pp. National Board of Forestry, Statistical Yearbook of Forestry National Board of Forestry, Jönköping, 345 pp. (in Swedish with English summaries). Raulo, J., Development of dominant trees in Betula pendula Roth and Betula pubescens Ehrh. plantations. Comm. Inst. For. Fenn Rytter, L., Nutrient content in stems of hybrid aspen as affected by tree age and tree size, and nutrient removal with harvest. Biomass Bioenergy 23, Rytter, L., Production potentials of aspen, birch and alder a review on possibilities and consequences of harvest of biomass and merchantable timber. Skogforsk, Redogörelse No. 4/04, Uppsala, 62 pp. (in Swedish with English summary). Rytter, L., Stener, L.-G., Productivity and thinning effects in hybrid aspen (Populus tremula L P. tremuloides Michx.) stands in southern Sweden. Forestry 78, Rytter, L., Stener, L., Werner, M., Hybrid aspen: a cost-effective alternative for new forestry. SkogForsk, Resultat No , Uppsala, 4 pp. (in Swedish with English summary). SAS, SAS/STAT 1 User s Guide, Version 8, vol. 1. SAS Institute, Cary, NC, 1243 pp. Sjöberg, K., Lennartsson, T., Fauna and flora management in forestry. In: Hytönen, M. (Ed.), Multiple-Use Forestry in the Nordic Countries. METLA, The Finnish Forest Research Institute, pp Willebrand, E., Ledin, S., Verwijst, T., Willow coppice systems in short rotation forestry: effects of plant spacing, rotation length and clonal composition on biomass production. Biomass Bioenergy 4,