Taxodium ‘Zhongshansa’ (T. ‘Zhongshansa’) is a hybrid cultivar of the Taxodium Rich. genus. The wood and fiber characteristics and wood chemical compositions of three T. ‘Zhongshansa’ wood specimens were analyzed and compared with their parents, Taxodium mucronatum (T. mucronatum) and Taxodium distichum (T. distichum). It was found that the wood density of the three T. ‘Zhongshansa’ hybrids were slightly lower than the densities of their parents. The average fiber length and length to width ratio of the three samples were similar and greater than their parents. Runkel ratios of the three T. ‘Zhongshansa’ hybrids was smaller than their patents. Compared to the parents, the extractives contents of the three T. ‘Zhongshansa’ hybrids hardly varied, while the lignin content was slightly lower. The result of the kraft cooking and bleaching of T.Z.502 indicated that with a cooking yield of 44.9%, the kappa number of the kraft pulp was 41.8. The T.Z.502 kraft pulp was easier to delignify via oxygen with alkali, and the brightness reached up to 76% ISO after an elemental chlorine free bleaching process. The results provide a reference for the application of T. ‘Zhongshansa’ wood in pulp and paper making in future.
Fiber Characteristics and Chemical Composition of Three Taxodium ‘Zhongshansa’ Woods and Pulping Evaluation
Taxodium ‘Zhongshansa’ (T. ‘Zhongshansa’) is a hybrid cultivar of the Taxodium Rich. genus. The wood and fiber characteristics and wood chemical compositions of three T. ‘Zhongshansa’ wood specimens were analyzed and compared with their parents, Taxodium mucronatum (T. mucronatum) and Taxodium distichum (T. distichum). It was found that the wood density of the three T. ‘Zhongshansa’ hybrids were slightly lower than the densities of their parents. The average fiber length and length to width ratio of the three samples were similar and greater than their parents. Runkel ratios of the three T. ‘Zhongshansa’ hybrids was smaller than their patents. Compared to the parents, the extractives contents of the three T. ‘Zhongshansa’ hybrids
Contact information: a: Jiangsu Co-Innovation Center of Efficient Processing and Utilization of Forest Resources, College of Light Industry and Food Engineering, College of Chemical Engineering, Nanjing Forestry University, Longpan Road 159, Nanjing 210037 China; b: Jiangsu Engineering Research Center for Taxodium Rich. Germplasm Innovation and Propagation, Institute of Botany, Jiangsu Province and Chinese Academy of Sciences, Nanjing 210014 China; *Corresponding author: firstname.lastname@example.org
During the breeding process, the techniques in hybrid breeding and backcrossing are often used to enhance the good growth performance of new species, e.g., Eucalyptus and Populus (Zhang et al. 2006; Yang et al. 2010; Yu et al. 2011). Taxodium ‘Zhongshansa’ (T. ‘Zhongshansa’) is a hybrid cultivar of Taxodium Rich. (Lu et al. 2006; Yu et al. 2008). There are more than 20 varieties of T. ‘Zhongshansa’, which have the following advantages: fast growth, a tall and straight tree trunk, and resistance to waterlogging, saline-alkaline, disease, pest, and storms, as well as high productivity and good wood quality, etc. (Yin et al. 2019). So far, T. ‘Zhongshansa’ trees have been planted in more than 10 provinces in China and are primarily used for urban greening construction and wetland restoration, which commonly plant T. ‘Zhongshansa’ 302, T. ‘Zhongshansa’ 118, and T. ‘Zhongshansa’ 401 (Yu et al. 2008).
Taxodium ‘Zhongshansa’ trees shapes are tall and straight, and its wood has potential value for processing and utilization in various industries. However, the study and production practice in terms of the processing and utilization of T. ‘Zhongshansa’ wood is not very well reported. A few previous studies have indicated that the moisture content and adhesive strength of T. ‘Zhongshansa’ preservative plywood all meet the requirements for Class I outdoor plywood, but the smooth grain compressive strength, bending strength, bending elastic modulus, and wood hardness are lower, and the wood easily dries and cracks, causing warped deformation (Zhang et al. 2012a,b). Researchers are already working on improving the wood processing performance of T. ‘Zhongshansa’. For example, Zhu et al. (2019) used a log-core veneer lathe to obtain peeled veneers of T. ‘Zhongshansa’, whose thickness deviation met the requirements for industrial applications (Chen et al 2019).
There is a shortage of fiber resources for pulp and paper making in China because of the lack of forest resources. Given the fast growth of T. ‘Zhongshansa’ trees, the shortage of these fiber resources can be alleviated in China if they are suitable for pulp and paper making. The work by Chi et al. (2014) showed that under effective alkali conditions of 22% NaOH, a sulfidity of 27%, and a maximum temperature of 170 °C for 2 h, the cooking yield of T. ‘Zhongshansa’ chips was 47.6%, while the kappa number was 42 for the kraft pulp and the pulp brightness reached up to 60% ISO after bleaching. Obviously, as a chemical pulp, this brightness is not attractive enough. The research group of the authors has investigated the kraft pulping performance of two T. ‘Zhongshansa’ species wood; the brightness of the bleached pulp was improved to approximately 76% ISO by using oxygen delignification and an elemental chlorine free (ECF) bleaching process under cooking conditions similar to Chi et al. (2014) (Xu et al. 2021).
Taxodium ‘Zhongshansa’ 302 (T.Z.302) is a hybrid species of Taxodium distichum (T. distichum) and Taxodium mucronatum (T. mucronatum). It has been cultivated since 1973. Previous studies indicated that under soil conditions with a pH of 8.0 to 9.5 and a salinity less than 0.2%, T.Z.302 has an excellent synthetic growing appearance (Yin and Chen 1997; Yin et al. 2019). Although T.Z.302 has advantages, with the increasing age of the tree, various problems, e.g., low rooting rates and breeding difficulties, occurred. To address these problems, T.Z.302 were backcrossed with its parents, T. distichum and some breeding varieties are cultivated, with T.Z.118 being one of them. Previous investigation indicated that the growth characteristics and stress resistance of T.Z.118 was better than T.Z.302 (Yu et al. 2010). Taxodium ‘Zhongshansa’ 502 (T.Z.502) is also a new variety, which was obtained after years of artificial hybrid investigation from T. distichum and T. mucronatum; T.Z.502 not only maintains the excellent characteristics of being semi-evergreen with high landscape value, but also shows remarkable superior characteristics compared to its parents in terms of growth. In addition, it is more resistant to red blight than T. mucronatum (Yu et al. 2011; Yin et al. 2019).
In order to understand the characteristic differences between the varieties and their feasibility as a pulp and paper making raw material, the chemical composition, wood properties, and fiber properties of the above three T. ‘Zhongshansa’ woods were analyzed and compared with their parents. In addition, the pulping and bleaching performance were evaluated for T.Z.502 wood, in order to provide a reference for the use of T. ‘Zhongshansa’ in the pulp and papermaking industries.
The T.Z.502, T.Z.302, and T.Z.118 wood as well as their parents, T. mucronatum and T. distichum were all plantation trees with 3 m×2 m spacing. They were supplied by the Institute of Botany, Chinese Academy of Sciences. All samples were 11 years old, and were harvested in Yancheng, Jiangsu province, China. The sampling and preparation of the wood for analysis was performed according to TAPPI standard T257 sp-14 (2014).
Chemical composition analysis of the wood
For the determination of the ash content, water extract, and benzene-alcohol extractive, the procedures outlined in TAPPI standards T211 om-12 (2012), T207 cm-08 (2008), and T 204 cm-07 (2007) were used, respectively.
The lignin and polysaccharides contents were analyzed according to the Laboratory Analytical Procedure from the National Renewable Energy Laboratory (Sluiter et al. 2010). The hydrolysate from the acid hydrolysis was used to analyze the sugars and acid-soluble lignins (ASL). The sugar contents were measured via a high-performance liquid chromatography (HPLC) system equipped with an Aminex HPX-87H column (300 mm × 7.8 mm) and a refractive index (RI) detector; a 5 mM H2SO4 solution was used as the eluent at a flow rate of 0.6 mL/min. The acid soluble lignin content was determined via absorbance at 205 nm with a UV-Vis spectrometer (TU-1810, Puxi, Beijing, China). The Klason lignin (KL) content was taken as the ash residue after acid hydrolysis. All experiments were tested using three replications.
Fiber morphology analysis
The wood sticks (1 mm × 2 mm × 30 mm) were obtained by cutting small wood block with a sharp blade and were soaked in an acetic acid and hydrogen peroxide (30% ± 0.1%) solution (1 to 1 by vol) at a temperature of 60 °C for 48 h. After thoroughly washing, 40 mg of oven-dried fibers were dispersed in 1000 mL of water. The fiber length, width, coarseness, and fines content were measured via a Morfi Compact fiber morphology analyzer.
Determination of the wood density
The wood samples were cut into 20 mm segments. The water-saturated volume and oven dry mass of the segments were measured. The basic density was defined as the ratio of the oven-dry mass to the water-saturated volume.
A digester with an oil bath was used with a batch capacity of oven dry wood chips (20 mm × 15 mm ×2 mm) of 80 g. The wood chips were directly subjected to cooking using sodium hydroxide and sodium sulfide with total titratable alkali (TTA) charge of 19% (as Na2O) and sulfidity of 25% at a temperature of 170 °C for 90 min. After cooking, the pulp was thoroughly washed and dewatered via centrifugation, then refrigerated for usage.
Oxygen delignification was carried out in an electric rotary digester with a water bath. The conditions were as follows: an oxygen pressure of 0.7 mPa, a NaOH dosage of 2.5%, a MgSO4 dosage of 0.1%, and a pulp consistency of 10%, at a temperature of 100 °C for 60 min.
Each stage of the D0ED1 bleaching process were carried out in plastic bag with a water bath. The conditions of each stage are shown in Table 1.
Table 1. Bleaching Conditions of the T.Z.502 Oxygen Delignified Pulp
RESULTS AND DISCUSSION
Density of the Wood Samples
Although the five trees were basically the same age and had the same growth environment, the apparent size of the individuals varied. The tree height and diameter at breast height of the T.Z.502 trees were similar to its parents, while the T.Z.302 and T.Z.118 trees had greater tree heights and diameters compared to T.Z.502. The basic density of the three T. ‘Zhongshansa’ ranged from 0.28 kg/m3 to 0.30 kg/m3, which was slightly lower than the basic density of their parents. All the three T. ‘Zhongshansa’ species are hybrids of T. distichum and T. mucronatum, and the differences in appearance between the trees might be due to the different microenvironments in which they grew. Based on the average of the three trees it can be concluded that T. ‘Zhongshansa’ wood hybrids have certain advantages over than their parents, as with other hybrid trees, e.g., Liriodendron (Rehfeldt 1977) and Douglas-fir (Pan et al. 2014).
Table 2. Appearance
Previous studies have indicated that the fiber length values measured via a fiber quality analysis (FQA) were lower than the values measured via microscopy, either the arithmetic average length (Ln) or the weighted average length (Lw). The Ln especially was considerably lower due to the influence of fines content, whereas the weight weighted average length (Lww) was closer to the value measured via microscopy. Table 3 shows the fiber morphology of the three T. ‘Zhongshansa’ trees and their parents.
The Lww values of the three T. ‘Zhongshansa’ trees were longer the Lww of the T. mucronatum and T. distichum trees, with T.Z.118 having the longest (2.02 mm). The fiber width of the three T. ‘Zhongshansa’ trees were similar, and all were close to the fiber width of T. distichum and slightly larger than the fiber width of T. mucronatum. However, the length to width ratios of the three hybrids were greater than the length to width ratios of T. mucronatum and T. distichum. Especially T.Z.118, which had the highest length to width ratio (46), was higher than its parent, either T.Z.302 or T. mucronatum. The fiber width of the three T. ‘Zhongshansa’ trees were greater than the fiber width of their parents, but the cell wall thickness (Table 3) did not increase proportionally; all three hybrids had a lower cell wall thickness compared to T. mucronatum and T. distichum. This indicated that the T. ‘Zhongshansa’ species had a smaller Runkel ratios (Table 3), which is why the T. ‘Zhongshansa’ species have lower wood densities (Table 2). Similar phenomena have occurred in other tree species (Xu et al. 2002). However, a smaller Runkel ratios for fiber is conducive to the physical strength when they are used for papermaking.
The chemical compositions of the T. ‘Zhongshansa’ hybrid woods and parents are listed in Table 4. The total polysaccharide contents of the three T. ‘Zhongshansa’ woods were similar, all were approximately 60% of the dried weigh of wood, while the lignin contents (31% to 32%) were slightly lower than their parents. The lignin content of T.Z.302 wood was 31.9%, which was lower than the lignin content of its parentals; the lignin content of T. mucronatum and T. distichum 34.1% and 33.8%, respectively. However, the lignin content of the T.Z.118 wood was close to that of its parent T.Z.302. The water soluble extractive contents of the three T. ‘Zhongshansa’ woods were similar and all were close to the water soluble extractive contents of T. distichum and slightly higher than T. mucronatum. However, all the benzene-alcohol extractive contents were lower in the three hybrids compared to their parents. The 1% NaOH extractive contents of T.Z.302 and T.Z.118 were all considerably lower than the parents, while the T.Z. 502 1% NaOH extractive content was slightly higher, somewhere between the parents.
The ash content of three T. ‘Zhongshansa’ woods were higher than the ash contents of their parents, more than twice that of T. distichum. The three T. ‘Zhongshansa’ trees used in this work were all grown in the same saline-alkali environment, and there was no yellowing phenomenon, which meant that T. ‘Zhongshansa’ can assimilate inorganic salt ions from the soil during the growth process, which is the reason T. ‘Zhongshansa’ had a higher ash content.
The lignin content of the three T. ‘Zhongshansa’ hybrids were similar and close to the lignin contents of Cryptomeria fortunei Hooibrenk (34.2%) and Cunninghamia lanceolata (33.5%), but higher than the lignin contents of Picea jezoensis var. (28.5%) and Picea koraiensis Nakai (27.0%), which are also grown in China (An et al. 1993; Lu et al. 1994). As the primary substance of wood growth, lignins maintain the firmness of the trunk. However, a higher lignin content will lead to lower chemical pulping yields, because lignins are hard to degrade and remove from the raw material during the cooking and bleaching process.
Table 4. Main Chemical Composition of T. ‘Zhongshansa’ and Their Parents Woods
Table 5. Pulping Performance of T. ‘Zhongshansa’ 502
Pulping Performance Evaluation of T.Z.502
Given the superior characteristics in terms of growth and resistance to red blight, T.Z.502 has high productivity (Yu et al. 2011), and therefore was evaluated for pulping performance in this work. Under the conditions of a TTA charge of 19% (as Na2O) and a sulfidity of 25% at a temperature of 170℃ for 90 min, the cooking results are listed in Table 5.
The pulp yield was 44.9% when Kappa number of the pulp was decreased to 41.8. The kraft pulp was subjected to oxygen delignification, and the kappa number of the pulp decreased to 17.0; the corresponding overall yield was 40.4%, while the extent of oxygen delignification was 59.3%. The oxygen delignified pulp was bleached using the DED process with a chlorine dioxide usage of 3.5% (on oven dried pulp). The resulting pulp brightness reached up to 76.8% ISO, and the brightness stability was a PC (post color) number of 0.51. After bleaching, the DP of the pulp was 902.
The lignin content of T. ‘Zhongshansa’ wood is similar to the lignin content of Liushan wood. Liushan is also a species of fir grown in China, whose lignin content is close to that of T. ‘Zhongshansa’, which has also been evaluated for good pulping performance (An et al. 1993). Under the same cooking conditions as above, the pulp kappa number of T.Z.502 (41.8) was much higher than the pulp kappa number of Liushan wood, which was approximately 33. The kraft pulp of T.Z.502 is easily delignified via oxygen with alkali and has a good delignification selectivity, given the high yield of 96%. Since the kappa number of the oxygen delignified pulp was still as high as 17, the brightness only reached 76% after the ECF bleaching process, which is obviously not high enough for usage as a chemical pulp.
Of course, by strengthening the cooking conditions, e.g., increasing the alkali usage, the kappa number of the kraft pulp can be further reduced, which needs to be further studied. However, the wood density of T. ‘Zhongshansa’ is relatively low, cooking conditions that are too intense may result in a lower cooking yield, which is not economical. Although the kappa number of T.Z.502 kraft pulp is relatively high at the acceptable cooking yield, and the brightness of the bleached pulp is not particularly high, it does not impact the application of the pulp in paperboard and some paper products requiring normal brightness. Based on the characteristics of the relatively low wood density of T. ‘Zhongshansa’, chemical mechanical pulping may be more suitable for it, which will be studied as the next step.
- The morphological characteristics of the wood and fibers of three hybrid Taxodium ‘Zhongshansa’ woods and their parents, T. mucronatum and T. distichum, were analyzed and compared. The wood densities of the three T. ‘Zhongshansa’ hybrids were slightly lower than the wood densities of T. mucronatum and T. distichum. The fiber length and length to width ratio of both T. Z.302 and T. Z.502 were similar and greater than their parents, while T.Z.118 was greater than its parent, T.Z.302 and T. distichum. The cell wall thicknesses of the three T. ‘Zhongshansa’ hybrids were thinner than their patents and therefor had a greater cavity to diameter ratio.
- Compared with the parents, the extractives contents of the three T. ‘Zhongshansa’ hybrids were not much different. The lignin contents of the three T. ‘Zhongshansa’ hybrids were slightly lower than their parents.
- At the acceptable cooking yield, the kappa number of the T.Z.502 kraft pulp was higher than the kappa number of the softwood, which currently used in the pulping industry. The T.Z.502 kraft pulp was easier to delignify via oxygen with alkali, but it was harder to reach a higher brightness via the ECF bleaching process. T.Z.502 can be used for pulping, which is used in the production of paperboard and certain paper products whose high brightness is not a particular requirement.
The authors are grateful for the support of the Jiangsu Long-term Scientific Research Base for Taxodium Rich. Breeding and Cultivation (Grant No. LYKJ (2021) 05).
An, G. (1993). “Study on the characteristics of pulping and bleaching of Liushan wood,” China Pulp & Paper 12(5), 48-55.
Chen, Y., Zhu, Y., Yang, B., Guo, Y., Pan. B., and Zhu, Y. (2019). “Study on axial compression behavior and ring stiffness of spiral pipe wound by Taxodium ‘Zhongshansa’ veneer,” Journal of Forestry Engineering 4(2), 43-47. DOI: 10.13360/j.issn.2096-1359.2019.02.007
Chi, X., Qi, B., Lyu, J., Lyu, S., Cheng, J., and Yin, Y. (2014). “Preliminary study on the pulping characteristics of Taxodium ‘Zhongshansa’ wood,” Forestry Science and Technology Development 28(5), 96-98. DOI: 10.13360/j.issn.1000-8101.2014.05.023
Lu, X.-Q., and Chen, Y.-H., Li, X.-C., Huang, L.-B., and Li, Y.-Y. (2006). “Directional cultivation techniques of superior clones of Taxodium ‘Zhongshansa 302’ and Taxodium ‘Zhongshansa 401’,” Practical Forestry Technology 7(6), 15-17. DOI: 10.3969/j.issn.1671-4938.2006.06.005
Pan, W., Yao, J., and Li, H. (2014). “Analysis of selfing depression in Liriodendron with SSR markers,” Scientia Silvae Sinicae 50(4), 32-38. DOI: 10.11707 /j.1001-7488.20140405
Rehfeldt, G. E. (1977). “Growth and cold hardiness of intervarietal hybrids of Douglas-fir,” Theoretical and Applied Genetics 50(1), 3-15. DOI: 10.1007/BF00273790
Sluiter, A., Hames, B., Ruiz, R., Scarlata, C., Sluiter, J., Templeton, D., and Crocker, D. (2010). Determination of Structural Carbohydrates and Lignin in Biomass (Technical Report NREL/TP-510-42618), National Renewable Energy Laboratory, Golden, CO, USA.
TAPPI T204 cm-17 (2017). “Solvent extractives of wood and pulp,” TAPPI Press, Atlanta, GA.
TAPPI T207 cm-08 (2008). “Water solubility of wood and pulp,” TAPPI Press, Atlanta, GA.
TAPPI T211 om-16 (2016). “Ash in wood, pulp, paper and paperboard: Combustion at 525 degrees C,” TAPPI Press, Atlanta, GA.
TAPPI T257 sp-14 (2014). “Sampling and preparing wood for analysis,” TAPPI Press, Atlanta, GA.
Xu, S., Wan, S., Wu, S., Yu, C., and Yin, Y. (2021). “Wood and fiber properties, and pulping performance of four Taxodium hybrid ‘Zhongshansa’,” Chemistry and Industry of Forest Products 41(2), 93-102. DOI: 10.3969/j.issn.0253-2417.2021.02.013
Xu, Y., Zou, M., and Tang, W., Su, F., Hu, X., Ban, L. H., and Liu, W. (2002). “Difference analyses of tracheid morphological characteristics among provenances of exotic loblolly pine in southern districts of Henan province in China,” Nanjing Forestry University (Natural Science) 26(5), 15-20. DOI: 10.3969/j.issn.1000-2006.2002.05.004
Yang, M., and Yin, Y., Fang, Y., and Yu, C. (2010). “Study on the external morphological variation and genetic relationship of Taxodium associated with species, cultivars and hybrids,” Journal of Plant Resources and Environment 19(2), 40-47. DOI: 10.3969/j.issn.1674-7895.2010.02.007
Yin, Y., and Chen, Y. (1997). “Through survey, the plantation of Taxodium ‘Zhongshansa’ , T. ascendens, T. distichum and Metaseqoia glyptostroboides was compared and evaluated,” Journal of Plant Resources and Environment 6(3), 24-29.
Yin, Y., Yu, C., and Hua, J. (2019). “Progress in the breeding and utilization of Taxodium ‘Zhongshansa’,” Journal of Plant Resources and Environment 28(4), 99-106. DOI: 10.3969/j.issn.1674-7895.2019.04.11
Yu, C., and Yin, Y. (2008). “Renew on cross-breeding between species of Taxodium,” Journal of Jiangsu Forestry Science Technology 35(2), 39-46. DOI: 10.3969/j.issn.1001-7380.2008.02.012
Yu, C., Yin, Y., and Xu, J. (2011). “Four hybrid varieties of Taxodium,” Science Silvae Sinicae 47(5), 181-182. DOI: 10.11707/j.1001-7488.20110531
Zhang, D., Zhang, Z., and Yang, K. (2006). “QTL analysis of growth and wood chemical content traits in an interspecific backcross family of white poplar (Populus tomentosa × P. bolleana) × P. tomentosa,” Canadian Journal of Forest Research 36(8), 2015-2023. DOI: 10.1139/X06-103
Zhang, X., Luo, Z., Pan, B., and Wang, X. (2012a). “Study on the plywood processing process of Taxodium ‘Zhongshansa 302’ wood preserved by ACQ-D,” Forest Products Industry 39(4), 28-30. DOI: 10.19531/j.issn1001-5299.2012.04.008
Zhang, X., Luo, Z., Wang, X., Jin, Y., Hu, Y., and Pan, B. (2012b). “Effect of preservative treatment of Taxodium ‘Zhongshansa 302’ veneer on plywood properties,” Forestry Science and Technology Development 26(5), 123-125. DOI: 10.3969/j.issn.1000-8101.2012.05.032
Zhu, Y., Pan, B., Yu, C., Yin, Y., and Zhang, Y. (2019). “Quality evaluation of rotary peeled veneer from Taxodium ‘Zhongshansa’ wood,” Journal of Forestry Engineering 4(4), 47-52. DOI: 10.13360/j.issn.2096-1359.2019.04.007
Article submitted: November 24, 2021; Peer review completed: January 22, 2022; Revised version received: January 26, 2022; Accepted: January 27, 2022; Published: January 31, 2022.