DIGITALNA ARHIVA ŠUMARSKOG LISTA
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Maximum value of biomass weight was estimated for clone ‘PE 19/66’ (45.267 tha–1). Clones ‘B229’ and ‘B81’, which in the case of young plantations with higher number of seedlings per hectare have the maximum values of biomass weight, have similar values, but about 11 % lower (cl. ‘B81’), or about 19 % lower (cl. ‘B229’) from the maximum estimated values in the experiment with older plants.
The lowest values of biomass weight per unit area in Experimental plot 3 are estimated to clone ‘182/81’ (35.389 tha–1) and clone ‘B229’ (39.108 tha–1), which is in complete accordance with the trends in the early stages of growth of examined clones (Experimental plot 1), as well as with the values in the same experiment on another type of soil (Experimental plot 2). Maximum values of weight of biomass were estimated for clone ‘PE 19/66’ (50.655 tha−1). Mutual comparison of results for the yield of biomass in the Experimental plot 2, and 3, it can be concluded that there are differences in all tested clones, and they are reflected in the higher yield of approximately 20 % in the case of average values.
Differences between the maximum yield for clone ‘PE 19/66’ are about 12 % in favor of Experimental plot 2, which was founded on meadow brown soil on alluvial loess, while the minimum value for the clone ‘182/81’ also differ by about 20 %. The difference in yield is caused by the differences in moistening and content of dust and clay fractions in meadow brown soil and in humofluvisol (Živanov and Ivanišević 1986, Galić 2000). If the annual biomass yields are observed in experimental plot 2, they range from 4.071 tha−1 for the clone ‘Pannonia’ (min), and to the 6.467 tha−1 for clone ‘PE 19/66’. In experimental plot 3, the annual biomass yields range from 5.055 tha–1 for clone ‘182/81’ (min), to 7.236 tha–1 for clone ‘PE 19/66’.
The data in literature presents that the plantation of 10000 plants ha–1 above-ground woody biomass production per year averaged 1.6–9.7 tha–1 (Tharakan et al. 2003) at the end of the second rotation. These production levels range within values found in other studies. According to the data given by Fang (1999), for Euramerican clones, the highest biomass, 78.4 tha–1 and 71.8 tha–1 respectively was achieved under the combination of 1111 plants ha–1 with 6 year rotation length. However, at 6 years P.deltoides the highest biomass occurred at the combination of 833 plants ha–1 with 6 year rotation, i.e. 75.8 tha–1. By the end of 6 years, the biomass productivity averaged over four planting densities was from 10.5 tha–1 to 11.4 tha–1. This large range of biomass yield per hectare, are primarily a reflection of characteristics of poplar clones that were researched and the characteristics of specific habitats on which the experiments were established, and only then the density of plantations, and the possibility of development of plants in defined time periods in dense or rare formations.
Estimated heating energy yield – Procijenjena količina toplotne energije
The energy yield is a relevant criterion for biomass use as fuel. It can be related to land surface, weight or volume of harvested biomass. The mean energy content related to the dry matter of biomass is therefore a stable feature within a particular type of biomass and more or less independent of external factors. Average heating values of the analysed poplar clones ranged in a very narrow interval from 18.261 MJkg–1 (clone ‘182/81’) to 18.656 MJkg–1 (clone ‘129/81’). This agrees fully with the values of our previous research (Klašnja et al. 2006, 2008, 2008a), and the values reported by Ciria (1995) for heating values of SRIC poplar wood (3–5-year old stem and branches) 18.1MJkg–1 to 18.3 MJkg–1. Benetka (2002), for 1–3 year old poplar clones (wood at breast height and basal part, and branches) reported heating value from 18.60 MJkg–1to 19.27 MJkg–1.
The values of estimated energy yield per plantation unit area, and plant age (the first or the second year) in Experimental plot 1, are presented in Fig. 3.
Maximal estimated energy which can be produced by the combustion of the total (aboveground) biomass after the first year (without leaves) was 121.523 GJha–1 (for clone ‘B81’) and 3.4 times higher than the minimum for the clone ‘M1’ (35.579 GJha–1).
After the second year the values for the clones were similar to biomass yield, so the ratio of the maximal energy yield for clone ‘B229’ (408.843 GJha–1), and minimal energy yield 147.243 GJha–1 for clone ‘PE 19/66’ was about 2.8 : 1. If the values of yield are compared, especially the evaluated energy after first and second year of growing, it is evident that the values at the end of second year are significantly higher, for all clones.
Estimated values of energy yield that could be obtained by combustion of biomass of plantations after the seventh year of vegetation in the Experimental plot 2, range from 528.162 GJha–1 for clone ‘Pannonia’ and 535.453 GJha–1 for clone ‘182/81’, to 841.688 GJha–1 for clone ‘PE 19/66’ which reaches the maximum value (Fig. 4).
Estimated values of energy yield that could be obtained by combustion of biomass of plantations after the seventh year of vegetation in the Experimental plot 3 range from minimum 646.236 GJha–1 for clone ‘182/81’, to 941.889 GJha–1 for clone ‘PE 19/66’ that reaches the maximum value (Fig. 4).
Trend that is noticed considering the average annual yield of biomass per hectare is achieved in terms of energy which could be obtained by combustion. The best results are achieved