Fibre length Results and Discussion

14 Sengon and jabon of 5, 6 and 7 years old still showed gradual increase in fibre length until near bark. Shorter fibre length near pith is caused by accelerated rate of anticlinal division in fusiform initial cell while longer fibre length near the bark is due to this rate slowing down Panshin and de Zeuw 1980. Further, average fibre lengths at the dbh from pith to bark for sengon at the age of 5, 6 and 7 years were 1131, 1170 and 1147 μm, respectively, for jabon 5, 6 and 7 years were 1190, 1245 and 1224 μm, respectively. These results also correspond with results from Kiaei et al. 2012 who find that fibre length of Acer velutinum increase along the radial direction from pith to bark. The proportional increase of fibre length from pith to bark proved fibre length as a reliable indicator of the juvenile wood presence. Douglas-fir produced longer average fiber length from pith to bark compare to other species in this study. The average fiber length value for 26-years old Douglas fir was 3177 µm Figure 7c. It correspond to the results presented by Martin et al. 2006, who discovered that the average fiber length of 31 years old Douglas fir is 3360 µm. Figure 7d also shows that ‘koster’ had fiber length less than 1 mm from the first to third segmented rings. The other cultivars had more than 1000 µm of fiber length starting from the first segmented rings. Poplar cultivars did not show gradual increase in fiber length from pith to bark. The average fiber length for ‘lambro’, ‘soligo’, ‘I-214’ and ‘koster’ were 1097, 1311, 1306 and 1403 μm, respectively. These results corresponded to Panshin and de Zeuw 1980 who reported that the poplar fiber length are in range from 1320 and 1380 μm. However, these results did not correspond to Berthelot et al. 2013 who find the average of fiber length for ‘lambro’, ‘soligo’, ‘I-214’ and ‘koster’ were 853, 867, 868 and 979 μm, respectively. The higher values of fiber length from these results were due to the samples were taken after log diameter of 14 cm, otherwise Berthelot et al. 2013 use the samples from pith to bark. Douglas-fir softwood had the longest fiber length, then followed by jabon, poplar cultivars and sengon belongs to hardwood. This is in line with Bowyer et al . 2005 who state that hardwood fibers are shorter than softwood tracheids. These differences gave an indication that the tree species in this study would have different impacts on their utilization. Segmented regression analysis suggested that fibre length was an appropriate trait to determine the demarcation point from juvenile to mature wood in sengon, jabon, poplar cultivars and douglas-fir. The demarcation point according to the fibre length values are presented in Table 3. Using segmented regression analysis, we concluded that juvenility of 5, 6 and 7 years old sengon occurred until the 17 th , 17 th and 16 th ring while that of jabon occurred until the 24 th , 23 rd and 21 st rings. Therefore, based on fibre length trait, we concluded that 5, 6 and 7 years old jabon and sengon were all juvenile. These results suggested that the mature wood for sengon would be occurred after dbh 34 cm and for jabon would be occurred after dbh 48 cm. Unfortunately, sengon and jabon in Indonesia are felled at the ages between 5 and 7 years because the dbh of about 35 cm is large enough for wood industry and selling at shorter cycle will mean more income for the communities. 15 Table 3 Estimated demarcation point transition age from juvenile to mature wood for sengon, jabon, douglas-fir and poplar cultivars based on fibre length and microfibril angle using segmented regression analysis Species Age years Number of segmented rings based on Fibre length Microfibril angle Sengon 5 17 th 19 th 6 17 th 17 th 7 16 th 18 th Jabon 5 24 th 24 th 6 23 rd 22 nd 7 21 st 19 th Douglas-fir 26 20 th 21 st Poplar – ‘soligo’ 18 16 th 18 th Poplar – ‘koster’ 18 16 th 19 th Poplar – ‘I214’ 18 15 th 17 th Poplar – ‘lambro’ 18 16 th 19 th Transition age for the 26 years old douglas-fir occurred at 20 th segmented rings 20 years old. We concluded that the 26-years old douglas-fir contained 77 of juvenile wood portion. Di Lucca 1989 used segmented regression modelling in an effort to identify the transition age for douglas-fir. He found that the transition age is about 20 rings from the pith at breast height. Using segmented regression approach, we concluded that juvenility of ‘lambro’, ‘soligo’ and ‘koster’ occurred until 16 th segmented rings, while for ‘I- 214 ’ occurred until 15 th segmented rings. These results suggested that poplar cultivars contained approximately 52 of juvenile portion at the age around 12- 13 years old. This result is in line with Pezlen 1994, who found that the transition age of Euramerican hybrid poplar [Populus x euramericana Dode Guinier] clones occur on 10-13 years old.

2.4.3 Microfibril angle

The demarcation point transition age could also be determined with microfibril angle by the segmented regression approach. The average value of microfibril angle at 1.3 m sampling height from pith to bark are presented in Figure 8a-d. Different from fibre length, microbril angle values of sengon, jabon, poplar cultivars and douglas-fir decreased proportionally from pith to bark. Microfibril angle values decreased markedly from pith to segmented ring 14 for sengon and segmented ring 15 for jabon, then it decreased gradually toward the bark Figure 8a-b. Microfibril angle values near pith reached over 65° for both wood species. After the 14 th segmented ring, microfibril angle values decreased until less than 30°. The results in Figure 8a-b show that microfibril angles for 5, 6 and 7 years old sengon and jabon would be constant at values less than 30°. Microfibril angles near the pith for 5 years old sengon varied from 56° to 71° average 63° and near the bark, from 29° to 35° average 31° Figure 8a. The 6 years old sengon varied from 55° to 70° average 62° near pith and from 27° to 30° average 29° near bark. The 7 years old sengon varied from 60° to 68° average 65° near pith and from 20° to 28° average 24° near bark. Microfibril angle values of 5 years old jabon near pith Figure 8b varied from 50° to 71° 16 average 65° and near bark, from 28° to 35° average 32°. Microfibril angles of