Original scientific paper
STUDY OF BIOPRODUCTIVE AND ENERGY POTENTIALS OF INDIGOBUSH (Amorpha fruticosa L.)
Ante P. B. Krpan
; Akademija šumarskih znanosti, Zagreb
Željko Tomašić
orcid.org/0000-0003-1864-6371
; Hrvatske šume d.o.o. Zagreb
Igor Stankić
; Šumarski fakultet Sveučilišta u Zagrebu
Abstract
The paper shows the results of the third year of research into biopotential and energy properties of indigobush. The research is planned to last for 6 years. According to the annual plan, the experiments were done in sample plots 1 and 3 in the third year of research. A research block polygon was established in a thirteen-year-old natural stand of indigobush in the compartment 126a of the Management Unit Posavske Šume, Sunja Forest Office, Sisak Forest Administration. The research is based on a project protocol and annual work plans and is carried out in four experimental fields, each containing six 5 x 5 m experimental plots. The basic task of the scientific-research project is to determine trends in bioproductive or bioenergy capacity in naturally planted indigobush at repeated felling operations. The time rhythm of the research was determined by the number of the plot, in combination with the project protocol and annual work plan.
Indigobush, particularly in the lowland systems of the Posavina region, covers large areas of forest soil. In some of these areas indigobush is so widespread that its shade prevents natural regeneration of stands of valuable autochthonous tree species. As a species of light, indigobush invades forest areas after tree cutting and overshadows the desired autochthonous young growth with its dense canopy. For this reason, it is perceived as an aggressive plant and a very dangerous weed, since not only does it considerably hinder the regeneration of Croatia’s most valuable lowland forests but also makes it more costly.
The possibility of using indigobush biomass for energy is limited by a number of factors. The most important ones include the quantity of biomass per surface unit and the profitability of its harvesting, chipping, transport
to the user, storage and drying to the desired level of moisture. The energy value of indigobush wood has been firmly confirmed by various literary sources. Extensive research into bioenergy potentials of different plant species, including indigobush, has been conducted in Hungary. According to Marosvölgyi et al. (2009), experiments related to naturally grown indigobush and energy plants showed that indigobush is an exceptionally suitable material for energy production. Initial moisture of one-year-old sprouts during one-month storage dropped from 47.0 % to 34.2 %. The measured fuel value at W = 34.2 % was 12.7 MJ/kg. In dry condition, the energy value of indigobush is 20.2 MJ/kg. In comparison, dry pine sawdust has a slightly lower value of 19.7 MJ/kg. Moreover, the ash content in indigobush was found to be 1.5 %, while the content of volatile materials was relatively higher. Puljak (2005) burned indigobush in a biomass energy plant in Ogulin to confirm its energy value as fuel by monitoring the temperature of the firebox, smoke gases and water, which satisfied the set criteria.
Figures 2 and 3 show the data for plots 1. A spatial arrangement of the stumps, their form and surface size are shown in the layout. There are from 12 to 20 stumps in the plots. The number of the stumps is not an indicator of indigobush productivity (the case with a plot in field IV with the lowest number of stumps and field 1 with the highest). As seen from table data, indigobush productivity correlates with the number of the sprouts and their dimensions. The number of the sprouts in the plots varies from 278 pcs/plot (111,200 pcs/ha) to 389 pcs/plot (155,000 pcs/ha). The mean heights in the plots range from 2.23 m to 2.48 m and the mean diameters vary from 7.35 mm to 8.58 mm. The lowest sprout mass of 11,000 kg/ha was recorded in sample field IV, and the highest of 18,400 kg/ha was found in field III. The mean value of green biomass bioproduction during one vegetation season amounts to 14,776 kg/ha of green indigobush mass per hectare.
Sample plots 3 were measured for the first time. The results of measurements are given in Figures 4 and 5. The number of the stumps ranged from 13 to 21, and that of the sprouts from 254 pcs/plot to 292 pcs/plot, or from 101,600 pcs/ha to 116,800 pcs/ha. The mean heights in the plots were from 2.69 m to 2.88 m, and the mean diameters were from 10.60 mm to 12.43 mm. The sprout mass ranged from 61.50 kg/plot or 24,600 kg/ha to 89.30 kg/plot or 35,720 kg/ha. The average overall green mass production in plots 3 amounts to 30,700 kg/ha, and the average annual production amounts to 10,233 kg/ha.
Variance analysis was used for indigobush breast diameters and sprout heights in plots 1 and 3. The results are given in Table 1 and 2, and mean values are presented graphically in Figures 6, 7, 8 and 9. It can be concluded from Table 1 and variance analysis that there is a statistically significant difference in mean breast diameters between the analysed sample fields (F = 9.597; df = 3; p < 0.001). The Tukey post hoc test showed a statistically significant difference between field III and fields I and IV, as well as between field I and field II and III. The statistically significant highest breast diameter of 8.58 mm was found in sample field III.
Variance analysis found a statistically significant difference in the average height values for the analysed sample fields (F = 17.38; df = 3; p < 0.001). The Tukey post hoc test showed that sample fields I and IV differed statistically significantly from fields II and III, whereas the former (I and IV) and (II and III) did not differ from one another.
According to the Tukey post hoc test, there was a statistically significant difference among the average values of breast diameters in the three-year-old stand (Table 2) between experimental fields 1 and 4, as well as between 2 and 4. The mentioned average values between exp. fields 3 and 4 did not show any statistically significant difference.
Related to the analysis of the average height values, a statistically significant difference was also found between the sample fields (F = 10.39; df = 3; p < 0.0001). According to the results of the Tukey post hoc test, the average height value in sample field IV was found to differ statistically significantly from the average values in all other sample fields. These values did not differ statistically significantly in sample fields I, II and III.
Table 3 shows data of indigobush bioproduction analysis, both green and dry mass, in plots 1 and 3 in the sample fields, as well as data calculated per surface hectare. Data are also given of the percentage share of moisture calculated on the basis of laboratory research into indigobush wood samples and dry matter, expressed as a difference between the percent share of moisture and 100 % amount. The moisture content of indigobush wood at the moment of harvesting is important in terms of price, which depends on water content and a possible need to dry the chips in the storehouse until they reach the optimal water content. In exp. plots 1 the indigobush wood moisture percentage ranges from 32.78 % to 34.50 % (33.71 % on average), meaning that all percentages are lower than 35 %. Exp. plots 3 contain even lower values, which oscillate from 30.09 % to 31.90 (31.33 % on average). Dry wood matter in plots 1 ranges from 18.21 kg to 30.46 kg, with the mean value of 24.47 kg, and in plots 3 from 41.88 kg to 60.97 kg, with the mean value of 52.77 kg. Dry wood matter in plots 1 is from 7.28 t/ha to 12.18 t/ha, with the mean value of 9.79 t/ha, and in plots 3 from 16.75 t/ha to 24.38 t/ha with the mean value of 21.09 t/ha.
In plots 1, which are harvested every year at the end of vegetation, bioproductivity was found to vary. After the first vegetation in 2008, dry matter amounted to 12 t/ha (Krpan and Tomašić 2009), after the second (2009) it came to 7.87 t/ha (Krpan et al. 2011a,b), and after the third (2010) it was 9.79 t/ha. It can therefore be concluded that, in relation to the first vegetation, bioproduction manifests a downward trend. The average annual production of two-year-old indigobush is 8.19 t/ha, while that of three-year-old indigobush is 7.03 t/ha. Hence, a downward trend in bioproduction is present here as well.
A quantity of 2.68 kg seed or 1,073 kg/ha was collected in exp. plots 3. This result is tentative because a large amount of seed naturally falls off by the time of collection, as well as during collection due to shaking caused by bending the branches. According to our estimates, seed loss may amount to over 50 %. To avoid the possibility of incorrect evaluation, seed loss will not be analysed in more detail here. It is evident from the above that almost all the seed is lost during harvesting after vegetation, handling, chipping and transport and that it cannot be expected to accompany the leaves in the combustion process in energy plants.
The results of indigobush research in the third year of the project show that, despite the established downward trends in bioproductivity, the plant still retains its competitiveness in the field of renewable energy sources, particularly because it occurs and develops naturally. It does not require any agrotechnical measures, nor does it incur any costs (except for harvesting and handling costs), which are otherwise indispensable when establishing and managing energy cultures and short rotation orchards of well-known fast growing tree species.
Keywords
indigobush bioproductivity; energy benefit; lowland forest ecosystems; Croatia
Hrčak ID:
119453
URI
Publication date:
28.2.2014.
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