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Abstract [Objectives]The aim was to reveal the accumulation characteristics and differences of biomass productivity of Taiwania flousiana plantation and successive rotation plantation of Cunninghamia lanceolata.
[Methods]The biomass and productivity of the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata were studied at Nandan Shankou Forestry Farm of Guangxi.， China.
[Results]There were differences in the biomass distribution of different organs of T. flousiana plantation and successive rotation plantation of C.lanceolata. The biomass for the T. flousiana plantation was distributed in the order of stem > branch > leaves > bark， and the successive rotation plantations of C. lanceolata was stem> root > branch > bark > leaves. The biomass of tree layer of 23yearsold T. flousiana plantation and successive rotation plantations were 195.21 and 136.82 t/hm2， respectively， including 113.32 and 87.91 t/hm2 of economic biomass （stem）. The annual net productivity of tree layer of the 2 plantations were 8.49 and 5.95 t/（hm2·a）， respectively， including 5.14 and 3.82 t/（hm2·a） of stem. Therefore， T. flousiana plantation has higher biomass accumulation ability than that of successive rotation plantations of C.lanceolata， and can be used as an excellent substitute tree species for the regeneration of C.lanceolata cutover plantation.
[Conclusions]
Key words Taiwania flousiana； Cunninghamia lanceolata； Successive rotation； Biomass； Productivity
Cunninghamia lanceolata is one of the main afforestation tree species in the mountainous areas of southern China， and plays a decisive role in the construction of forestry production and strategic reserve wood bases in China[1]. However， the successive rotation of C. lanceolata can makes the problems such as the decline in productivity of forest lands become more serious[2-3]. In particular， most of current C. lanceolata forests in China have been cut and restocked， faced with the restocking problems of the second and even the third generations. If we cannot find the measures and methods to solve the above problems， it will seriously affect the sustainable development of forestry sustainable development and the construction of 2 forestry systems. Taiwania flousiana is a rare plant in the world unique to China. The tree is tall with majestic tree performance. It can adapt to the climate and soil in the middle， low mountains and part of the hilly areas in the south of China， and it has the advantages of fast growth， long fastgrowing time， few pests and diseases， good tree texture， and high stocking per unit area and merchantable volume， which make it become an excellent alternative tree species for resolving land degradation caused by successive rotation of C. lanceolata in some regions[4-6]. At present， there have been many reports on the growth characteristics， biological productivity， and nutrient element characteristics of the T. flousiana plantations[7-12]. However， there are few studies involving the successive plantations of C. lanceolata， and ages of the studied C. lanceolata plantations and T. flousiana plantations are less than 12 years. To this end， the study object of this paper was the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata constructed over the cutover land of the 1stgeneration C. lanceolata plantation in Shankou Forest Farm， Nandan County in northwestern Guangxi. The biomass， productivity and their distribution characteristics of the 2 plantation types were compared and analyzed in a systematic way to further reveal the accumulation characteristics of the biological productivity of T. flousiana plantation and successive rotation plantation of C. lanceolata， as well as their differences， with the aim to provide scientific bases and technical supports for the sustainable management and development of artificial T. flousiana plantation. Materials and Methods
Overview of the study area
The research area is located at the Shankou branch of Shankou Forest Farm in Nandan County， Guangxi. Located at 107°1′-107°55′E， 24°42-25°37′N， Nandan County has a subtropical mountain climate with an annual average temperature of 16.9 ℃， an annual rainfall of 1 498.2 mm and an altitude of 600-1 100 m， mainly lowland landform. The soil parent material is mainly composed of sandshale containing greygreen slate. The soil is yellow mountainous soil with an average soil layer thickness of more than 80 cm， and humus layer thickness of about 16-23 cm. The standard plot was located in the middle of the hillside， on the northeast slope， with a slope of 27-34°. The soil （0-40 cm layer， the same below） pH value of T. flousiana plantation was 4.52， and the contents of organic matter， total nitrogen， available phosphorus， and available potassium were 52.56 g/kg， 2.70 g/kg， 1.65 mg/kg and 83.0 mg/kg， respectively. The soil pH value of C. lanceolata plantation was 4.56， with the contents of organic matter， total nitrogen， available phosphorus， and available potassium of 49.16 g/kg， 2.70 g/kg， 1.65 mg/kg and 83.0 mg/kg， respectively.
In the test area， the former stand was C. lanceolata plantation of the first generation， which was chopped in October 1992. After the mountain was cleaned up and the woodland was cleared， the land was well prepared. The planting holes were all 0.4 m×0.4 m×0.3 m， which were planted with the oneyearold seedlings of T. flousiana and C. lanceolata. The initial planting density was 2 500 plants/hm2. After the afforestation， weeding was conducted once in May-June in the year of the afforestation. Thereafter， it was conducted for 2 consecutive years once every May-June， September-October， respectively. Then， in the fourth year， thorough weeding was conducted in September， and the plantation was performed with intermediate cutting in the ninth and fourteenth year. In the survey conducted in May 2016， the 23yearold T. flousiana plantation had relatively uniform forest form after intermediate cutting and stand selfthinning， with a canopy density of 0.8， reserve density of 930 plants/hm2， stand average tree height of 18.3 m， average DBH （diameter at breast height） of 25.2 cm. The understory vegetation was dominated by Eurya ciliate and Maesa japonica. In addition， there are also other sparsely distributed species like Osmunda japonica， Woodwaordia japonica， Sarcandra glabra， and Pyrrosia lingua. The coverage of the plantation was about 70%， and the average thickness of the litter layer was 2.5 cm. The 23yearold C. lanceolata plantation had uniform forest form， with a canopy density of 0.7， reserve density of 950 plants/hm2， stand average tree height of 18.9 m， average DBH （diameter at breast height） of 20.3 cm. The understory vegetation was mainly Cinnamomum porrectum， Alangium chinense， Eurya ciliate， Dicranopteris dichotoma， Miscanthus floridulus， Osmunda japonica， and Oplismenus compositus. The average thickness of litter layer was 1.5 cm. Research Methods
Forest stand growth and biomass survey
Areas with basically the same site conditions （including slope direction， slope position， slope and altitude） were selected from the adjacent parts of T. flousiana plantation and successive rotation plantation of C. lanceolata， and 3 standard sample plots with the size of 20 m × 20 m were set up in each of the plantation to measure the tree heights and DBH of the trees in each sample plots. Moreover， 3 standard trees were selected from the standard sample plots of T. flousiana plantation and successive rotation plantation of C. lanceolata using the average stand tree height and average DBH as the indexes surrounding the sample plots. Monsic stratified cutting method was used for the aboveground parts， including leaves， branches， stems， barks and roots， and whole root digging method was used for the underground parts to measure the weights of each parts. The quadrat harvesting method was used to measure the biomass of the understory vegetation layer and litter layer in the following way： first， set up 3 quadrats of 1 m × 1 m diagonally in each plot， collect and dentine the herbs， shrubs and herbs in the plot with their weights measured； and then collect some plant samples from each group， which were dried at 80 ℃ to measure the moisture content and dry weight， and the biomass of each structure layer （component） was calculated. The annual average growth rate was used as the estimate index of the productivity of forest stand[7].
Results and Analysis
Biomass and distribution of average tree of T. flousiana plantation and successive rotation plantation of C. lanceolata
As shown in Table 1， the average biomass per tree was 209.90 and 144.02 kg/plant， respectively for the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata. The former was 1.46 times greater than that of the latter. The distribution ratio of biomass in various organs of forest trees differed depending on the type of stand. As for the T. flousiana plantation， the distribution ratio was in the order of stems （60.61%） > branches （15.46%） > roots （14.46%） > leaves （5.62%）> barks （3.84%）， and for the successive rotation plantation of C. lanceolata， the order was stems （64.25%） > roots （15.74%） > branches （8.84%） > barks （6.99%） > leaves （4.18%）. Thus， although proportion of stem biomass of T. flousiana plantation was slightly lower than that of successive rotation plantation of C. lanceolata， the biomass of the stems of T. flousiana plantation was 1.35 times that of successive rotation plantation of C. lanceolata， and there was a significant difference between the two （P<0.05）. Moreover， the biomass of the canopy （branches + leaves） of T. flousiana plantation was significantly higher than that of successive rotation plantation of C. lanceolata， which was beneficial to the photosynthesis of the trees in the plantation. It was also the reason why the biomass of the T. flousiana plantation was significantly higher than that of the successive rotation plantation of C. lanceolata. Biomass of T. flousiana plantation and successive rotation plantation of C. lanceolata
As shown in Table 2， the stand biomass was 206.82 t/hm2 and 146.13 t/hm2， respectively for the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata. The biomasses for the arborous layers of the 2 types of plantations were 195.21 and 136.32 t/hm2， respectively， accounting for 94.39% and 93.29% of the stand biomasses； the biomasses for the shrub layers were 4.57 and 5.14 t/hm2， which accounted for 2.41% and 3.52%， respectively； the biomasses of the litter layers were 7.04 and 4.67 t/hm2， respectively， accounting for 3.40% and 3.20%. As for the distribution of biomass in the arborous layer of the T. flousiana plantation， the stem （economic） biomass was 118.32 t/hm2， which was 1.35 times of that of successive rotation plantation of C. lanceolata； the canopy （leaves + branches） biomass was 44.51 t/hm2， which was 2.5 times of that of successive rotation plantation of C. lanceolata； the root biomass was 24.88 t/hm2， which was 1.6 times of that of successive rotation plantation of C. lanceolata； only the bark biomass （7.50 t/hm2） was slightly less than that of successive rotation plantation of C. lanceolata （9.56 t/hm2）.
Estimation of the net productivity of the tree layer of T. flousiana plantation and successive rotation plantation of C. lanceolata
As shown in Table 3， the net productivity was 8.49 and 5.95 t/（hm2·a）， respectively for the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata. The former was 42.69% higher than that of the latter. There was certain difference in the order of net productivity of different organs between the 2 types of plantations. As for the T. flousiana plantation， the order was stems [5.14 t/（hm2·a）]> branches [1.46 t/（hm2·a）]> roots [1.08 t/（hm2·a）]> leaves [0.48 t/（hm2·a）]> barks [0.33 t/（hm2·a）]， and for the successive rotation plantation of C. lanceolata， the order was stems [3.82 t/（hm2·a）]> roots [0.94 t/（hm2·a）]> branches [0.53 t/（hm2·a）]> barks [0.33 t/（hm2·a）]> leaves [0.25 t/（hm2·a）]. Although both plantations had the stems had the highest net productivity， the net productivity of the stems of T. flousiana plantation was 34.55% higher than that of successive rotation plantation of C. lanceolata. Therefore， the biomass accumulation speed was much faster in the stems of T. flousiana plantation than that of successive rotation plantation of C. lanceolata， which was more favorable for the management of artificial commercial plantations with the main economic objective of stem cultivation. Conclusions and discussions
The stand biomass was 206.82 and 146.13 t/hm2， respectively for the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata. The biomasses for the arborous layers of the 2 types of plantations were 195.21 and 136.32 t/hm2， respectively， accounting for 94.39% and 93.29% of the stand biomasses； the biomasses for the shrub layers were 4.57 and 5.14 t/hm2， which accounted for 2.41% and 3.52%， respectively； the biomasses of the litter layers were 7.04 and 4.67 t/hm2， respectively， accounting for 3.40% and 3.20%. As for the arborous layer of the T. flousiana plantation， the stem （economic） biomass was the largest of 118.32 t/hm2 among all the organs， which was 34.59% higher than of that of successive rotation plantation of C. lanceolata （87.91 t/hm2）， and it accounted for 60.61% of the biomass of the arborous layer； the biomass of the canopy （leaves + branches） was 44.51 t/hm2， which increased by 149.92% from that of successive rotation plantation of C. lanceolata （17.81 t/hm2）， accounting for 22.80% of the biomass of the arborous layer. Higher tree crown biomass is beneficial to the photosynthesis of forest trees. This is also an important reason that T. flousiana plantation can keep high biomass accumulation speed even reaches the ages of nearmature forest or mature forest of C. lanceolata. Moreover， the stem biomass of T. flousiana plantation is significantly higher than that of successive rotation plantation of C. lanceolata， indicating T. flousiana plantation is more favorable for stem （economic） biomass accumulation than successive rotation plantation of C. lanceolata， thereby increasing the timber production and operating efficiency.
It is reported that the first generation of 23yearold C. lanceolata plantation in north Guangxi has a net productivity of 5.71 t/（hm2·a）[13]， the first generation of manure C. lanceolata plantation （21-25 years old） in Rongshi County of Guangxi has a net productivity of 7.20 t/（hm2·a）[14]； the net productivity of the firstgeneration 20yearold C. lanceolata plantation in Huitong of Hunan is 10.48 t/（hm2·a）[15]， the net productivity of the first， second and third generations of 20yearold C. lanceolata plantation in Youxi county of Fujian is 7.84， 6.26， and4.86 t/（hm2·a）， respectively[16]； the 26yearold C. lanceolata plantation in Nanjing of Jiangsu has a net productivity of 2.89 t/（hm2·a）[17]. In this study， the results show that the net productivity of the 23yearold T. flousiana plantation in northwest Guilin is not only higher than that of successive rotation plantation of C. lanceolata at the same site conditions， but also higher than that of the first generation C. lanceolata plantations and Pinus massoniana plantations with close or the same tree ages in in Youxi County of Fujian Province， Jianou City of Fujian Province and Wuxuan County of Guangxi Province， indicating that T. flousiana plantation has strong ecological adaptability and high level of bioproductivity in northwest Guilin. Therefore， T. flousiana is an excellent alternative tree species in the cutover land of C. lanceolata in northwestern Guangxi Province. The rational extension and development of the plantations of T. flousiana have an important role and significance to increase the productivity of the forestland in the region and promote the sustainable development of forestry. References
[1]CHEN DX， CHEN XM， LAN X， et al. Fullsib progeny genetic test and selection of superior germ plasm of Cunninghamia lanceolata[J]. Guangxi Forestry Science， 2016， 45（4）： 347-351.
[2]QIN ZY， TANG J， CAO JZ， et al. Assessment of soil fertility of continuous plantation of Chinese fir based on principal component and cluster analysis[J]. Forestry Resources Management， 2015（5）： 81-87.
[3]WEI XX， CHEN AL， WANG SY， et al. A comparative study of soil microbial carbon source utilization in different successive rotation plantations of Chinese fir[J]. Chinese Journal of Applied and Environmental Biology， 2016，22（3）：518-523.
[4]HE B， HUANG HC， HUANG CB， et al. Concentrations， accumulation and distribution characteristics of nutrient elements in Taiwania flousiana plantation[J]. Journal of Natural Resources， 2008， 23（5）： 903-910.
[5]HE B， LU WP， TANG GW， et al.Soil fertility change in Taiwania flousiana plantation in northwest Guangxi[J]. Forest Research 2015， 28（1）： 88-92
[6]MENG YH， TAN ZY， HE B， et al. Analysis on tree growth and economic benefit of different intercropping patterns of Taiwania flousiana young plantation[J]. Journal of West China Forestry Science， 2015，44（3）： 133-137.
[7]HE B， DIAO HL， HUANG HC， et al. Changing regularity of biomass and productivity of Taiwania flousiana plantation[J]. Journal of Northeast Forestry University， 2008， 36（9）： 17–18，27.
[8]HUANG CB， CAO JZ， WU QB， et al. Comparative analysis on soil physicchemical properties and the tree growth in Taiwania flousiana plantations and successive rotation plantations of Cunninghamia lanceolata[J]. Forestry Science， 2010，46（4）： 1-7.
[9]LIN RY. Study on the difference of growth and soil fertility between the mixed forest of Taiwania flousiana and pure Taiwania flousiana plantation[J]. Auhui Agricultural Science Bulletin， 2015， 21（Z1）：103-105.
[10]TANG GW， HE B， LUO HF， et al. Growth regularity of Taiwania flousiana plantation in northwest Guangxi[J]. Journal of West China Forestry Science， 2015， 44（2）： 96-99.
[11]HUANG JL. The biomass and productivity of Taiwania flousiana plantation on South Mountain[J]. Journal of Fujian Forestry Science and Technology， 2016， 43（3）：161-164.
[12]SUN Y. The study on the growth rule and productivity of Taiwania flousiana[D]. Central South University of Forestry and Technology， 2016.
[13]YU YF， SONG TQ， ZENG YP， et al. Dynamic changes of biomass and its allocation in Cunninghamia lanceolata plantations of different stand ages[J]. Chinese Journal of Ecology， 2013， 32（7）： 1660-1666.
[14]DU H， ZENG YP， WANG KL， et al. Dynamic changes of biomass and productivity of three main plantation types in southern China[J]. Acta Ecologica Sinica， 2014，34（10）： 2712-2724.
[15]HUANG ZH， TIAN DL， KANG WX， et al. Dynamics of biomass distribution in first rotation of Chinese fir plantations in Huitong County， Hunan Province[J]. Journal of Central South University of Forestry & Technology， 2011， 31（5）： 37-43.
[16]MA QX， FAN SH， CHEN SS， et al. Study on biomass productivity of Chinese fir plantations after successive planting[J]. Scientia Silvae Sinicae，2003，39（2）：78-83.
[17]DIAO JJ， XIAO WY， FEI F， et al. Short effects of thinning on the growth and carbon storage of Cunninghamia lanceolata plantation[J]. Journal of Southwest Forestry University （Natural Science）， 2017， 37（3）： 134-139.
Editor： Na LI Proofreader： Xinxiu ZHU
[Methods]The biomass and productivity of the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata were studied at Nandan Shankou Forestry Farm of Guangxi.， China.
[Results]There were differences in the biomass distribution of different organs of T. flousiana plantation and successive rotation plantation of C.lanceolata. The biomass for the T. flousiana plantation was distributed in the order of stem > branch > leaves > bark， and the successive rotation plantations of C. lanceolata was stem> root > branch > bark > leaves. The biomass of tree layer of 23yearsold T. flousiana plantation and successive rotation plantations were 195.21 and 136.82 t/hm2， respectively， including 113.32 and 87.91 t/hm2 of economic biomass （stem）. The annual net productivity of tree layer of the 2 plantations were 8.49 and 5.95 t/（hm2·a）， respectively， including 5.14 and 3.82 t/（hm2·a） of stem. Therefore， T. flousiana plantation has higher biomass accumulation ability than that of successive rotation plantations of C.lanceolata， and can be used as an excellent substitute tree species for the regeneration of C.lanceolata cutover plantation.
[Conclusions]
Key words Taiwania flousiana； Cunninghamia lanceolata； Successive rotation； Biomass； Productivity
Cunninghamia lanceolata is one of the main afforestation tree species in the mountainous areas of southern China， and plays a decisive role in the construction of forestry production and strategic reserve wood bases in China[1]. However， the successive rotation of C. lanceolata can makes the problems such as the decline in productivity of forest lands become more serious[2-3]. In particular， most of current C. lanceolata forests in China have been cut and restocked， faced with the restocking problems of the second and even the third generations. If we cannot find the measures and methods to solve the above problems， it will seriously affect the sustainable development of forestry sustainable development and the construction of 2 forestry systems. Taiwania flousiana is a rare plant in the world unique to China. The tree is tall with majestic tree performance. It can adapt to the climate and soil in the middle， low mountains and part of the hilly areas in the south of China， and it has the advantages of fast growth， long fastgrowing time， few pests and diseases， good tree texture， and high stocking per unit area and merchantable volume， which make it become an excellent alternative tree species for resolving land degradation caused by successive rotation of C. lanceolata in some regions[4-6]. At present， there have been many reports on the growth characteristics， biological productivity， and nutrient element characteristics of the T. flousiana plantations[7-12]. However， there are few studies involving the successive plantations of C. lanceolata， and ages of the studied C. lanceolata plantations and T. flousiana plantations are less than 12 years. To this end， the study object of this paper was the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata constructed over the cutover land of the 1stgeneration C. lanceolata plantation in Shankou Forest Farm， Nandan County in northwestern Guangxi. The biomass， productivity and their distribution characteristics of the 2 plantation types were compared and analyzed in a systematic way to further reveal the accumulation characteristics of the biological productivity of T. flousiana plantation and successive rotation plantation of C. lanceolata， as well as their differences， with the aim to provide scientific bases and technical supports for the sustainable management and development of artificial T. flousiana plantation. Materials and Methods
Overview of the study area
The research area is located at the Shankou branch of Shankou Forest Farm in Nandan County， Guangxi. Located at 107°1′-107°55′E， 24°42-25°37′N， Nandan County has a subtropical mountain climate with an annual average temperature of 16.9 ℃， an annual rainfall of 1 498.2 mm and an altitude of 600-1 100 m， mainly lowland landform. The soil parent material is mainly composed of sandshale containing greygreen slate. The soil is yellow mountainous soil with an average soil layer thickness of more than 80 cm， and humus layer thickness of about 16-23 cm. The standard plot was located in the middle of the hillside， on the northeast slope， with a slope of 27-34°. The soil （0-40 cm layer， the same below） pH value of T. flousiana plantation was 4.52， and the contents of organic matter， total nitrogen， available phosphorus， and available potassium were 52.56 g/kg， 2.70 g/kg， 1.65 mg/kg and 83.0 mg/kg， respectively. The soil pH value of C. lanceolata plantation was 4.56， with the contents of organic matter， total nitrogen， available phosphorus， and available potassium of 49.16 g/kg， 2.70 g/kg， 1.65 mg/kg and 83.0 mg/kg， respectively.
In the test area， the former stand was C. lanceolata plantation of the first generation， which was chopped in October 1992. After the mountain was cleaned up and the woodland was cleared， the land was well prepared. The planting holes were all 0.4 m×0.4 m×0.3 m， which were planted with the oneyearold seedlings of T. flousiana and C. lanceolata. The initial planting density was 2 500 plants/hm2. After the afforestation， weeding was conducted once in May-June in the year of the afforestation. Thereafter， it was conducted for 2 consecutive years once every May-June， September-October， respectively. Then， in the fourth year， thorough weeding was conducted in September， and the plantation was performed with intermediate cutting in the ninth and fourteenth year. In the survey conducted in May 2016， the 23yearold T. flousiana plantation had relatively uniform forest form after intermediate cutting and stand selfthinning， with a canopy density of 0.8， reserve density of 930 plants/hm2， stand average tree height of 18.3 m， average DBH （diameter at breast height） of 25.2 cm. The understory vegetation was dominated by Eurya ciliate and Maesa japonica. In addition， there are also other sparsely distributed species like Osmunda japonica， Woodwaordia japonica， Sarcandra glabra， and Pyrrosia lingua. The coverage of the plantation was about 70%， and the average thickness of the litter layer was 2.5 cm. The 23yearold C. lanceolata plantation had uniform forest form， with a canopy density of 0.7， reserve density of 950 plants/hm2， stand average tree height of 18.9 m， average DBH （diameter at breast height） of 20.3 cm. The understory vegetation was mainly Cinnamomum porrectum， Alangium chinense， Eurya ciliate， Dicranopteris dichotoma， Miscanthus floridulus， Osmunda japonica， and Oplismenus compositus. The average thickness of litter layer was 1.5 cm. Research Methods
Forest stand growth and biomass survey
Areas with basically the same site conditions （including slope direction， slope position， slope and altitude） were selected from the adjacent parts of T. flousiana plantation and successive rotation plantation of C. lanceolata， and 3 standard sample plots with the size of 20 m × 20 m were set up in each of the plantation to measure the tree heights and DBH of the trees in each sample plots. Moreover， 3 standard trees were selected from the standard sample plots of T. flousiana plantation and successive rotation plantation of C. lanceolata using the average stand tree height and average DBH as the indexes surrounding the sample plots. Monsic stratified cutting method was used for the aboveground parts， including leaves， branches， stems， barks and roots， and whole root digging method was used for the underground parts to measure the weights of each parts. The quadrat harvesting method was used to measure the biomass of the understory vegetation layer and litter layer in the following way： first， set up 3 quadrats of 1 m × 1 m diagonally in each plot， collect and dentine the herbs， shrubs and herbs in the plot with their weights measured； and then collect some plant samples from each group， which were dried at 80 ℃ to measure the moisture content and dry weight， and the biomass of each structure layer （component） was calculated. The annual average growth rate was used as the estimate index of the productivity of forest stand[7].
Results and Analysis
Biomass and distribution of average tree of T. flousiana plantation and successive rotation plantation of C. lanceolata
As shown in Table 1， the average biomass per tree was 209.90 and 144.02 kg/plant， respectively for the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata. The former was 1.46 times greater than that of the latter. The distribution ratio of biomass in various organs of forest trees differed depending on the type of stand. As for the T. flousiana plantation， the distribution ratio was in the order of stems （60.61%） > branches （15.46%） > roots （14.46%） > leaves （5.62%）> barks （3.84%）， and for the successive rotation plantation of C. lanceolata， the order was stems （64.25%） > roots （15.74%） > branches （8.84%） > barks （6.99%） > leaves （4.18%）. Thus， although proportion of stem biomass of T. flousiana plantation was slightly lower than that of successive rotation plantation of C. lanceolata， the biomass of the stems of T. flousiana plantation was 1.35 times that of successive rotation plantation of C. lanceolata， and there was a significant difference between the two （P<0.05）. Moreover， the biomass of the canopy （branches + leaves） of T. flousiana plantation was significantly higher than that of successive rotation plantation of C. lanceolata， which was beneficial to the photosynthesis of the trees in the plantation. It was also the reason why the biomass of the T. flousiana plantation was significantly higher than that of the successive rotation plantation of C. lanceolata. Biomass of T. flousiana plantation and successive rotation plantation of C. lanceolata
As shown in Table 2， the stand biomass was 206.82 t/hm2 and 146.13 t/hm2， respectively for the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata. The biomasses for the arborous layers of the 2 types of plantations were 195.21 and 136.32 t/hm2， respectively， accounting for 94.39% and 93.29% of the stand biomasses； the biomasses for the shrub layers were 4.57 and 5.14 t/hm2， which accounted for 2.41% and 3.52%， respectively； the biomasses of the litter layers were 7.04 and 4.67 t/hm2， respectively， accounting for 3.40% and 3.20%. As for the distribution of biomass in the arborous layer of the T. flousiana plantation， the stem （economic） biomass was 118.32 t/hm2， which was 1.35 times of that of successive rotation plantation of C. lanceolata； the canopy （leaves + branches） biomass was 44.51 t/hm2， which was 2.5 times of that of successive rotation plantation of C. lanceolata； the root biomass was 24.88 t/hm2， which was 1.6 times of that of successive rotation plantation of C. lanceolata； only the bark biomass （7.50 t/hm2） was slightly less than that of successive rotation plantation of C. lanceolata （9.56 t/hm2）.
Estimation of the net productivity of the tree layer of T. flousiana plantation and successive rotation plantation of C. lanceolata
As shown in Table 3， the net productivity was 8.49 and 5.95 t/（hm2·a）， respectively for the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata. The former was 42.69% higher than that of the latter. There was certain difference in the order of net productivity of different organs between the 2 types of plantations. As for the T. flousiana plantation， the order was stems [5.14 t/（hm2·a）]> branches [1.46 t/（hm2·a）]> roots [1.08 t/（hm2·a）]> leaves [0.48 t/（hm2·a）]> barks [0.33 t/（hm2·a）]， and for the successive rotation plantation of C. lanceolata， the order was stems [3.82 t/（hm2·a）]> roots [0.94 t/（hm2·a）]> branches [0.53 t/（hm2·a）]> barks [0.33 t/（hm2·a）]> leaves [0.25 t/（hm2·a）]. Although both plantations had the stems had the highest net productivity， the net productivity of the stems of T. flousiana plantation was 34.55% higher than that of successive rotation plantation of C. lanceolata. Therefore， the biomass accumulation speed was much faster in the stems of T. flousiana plantation than that of successive rotation plantation of C. lanceolata， which was more favorable for the management of artificial commercial plantations with the main economic objective of stem cultivation. Conclusions and discussions
The stand biomass was 206.82 and 146.13 t/hm2， respectively for the 23yearold T. flousiana plantation and successive rotation plantation of C. lanceolata. The biomasses for the arborous layers of the 2 types of plantations were 195.21 and 136.32 t/hm2， respectively， accounting for 94.39% and 93.29% of the stand biomasses； the biomasses for the shrub layers were 4.57 and 5.14 t/hm2， which accounted for 2.41% and 3.52%， respectively； the biomasses of the litter layers were 7.04 and 4.67 t/hm2， respectively， accounting for 3.40% and 3.20%. As for the arborous layer of the T. flousiana plantation， the stem （economic） biomass was the largest of 118.32 t/hm2 among all the organs， which was 34.59% higher than of that of successive rotation plantation of C. lanceolata （87.91 t/hm2）， and it accounted for 60.61% of the biomass of the arborous layer； the biomass of the canopy （leaves + branches） was 44.51 t/hm2， which increased by 149.92% from that of successive rotation plantation of C. lanceolata （17.81 t/hm2）， accounting for 22.80% of the biomass of the arborous layer. Higher tree crown biomass is beneficial to the photosynthesis of forest trees. This is also an important reason that T. flousiana plantation can keep high biomass accumulation speed even reaches the ages of nearmature forest or mature forest of C. lanceolata. Moreover， the stem biomass of T. flousiana plantation is significantly higher than that of successive rotation plantation of C. lanceolata， indicating T. flousiana plantation is more favorable for stem （economic） biomass accumulation than successive rotation plantation of C. lanceolata， thereby increasing the timber production and operating efficiency.
It is reported that the first generation of 23yearold C. lanceolata plantation in north Guangxi has a net productivity of 5.71 t/（hm2·a）[13]， the first generation of manure C. lanceolata plantation （21-25 years old） in Rongshi County of Guangxi has a net productivity of 7.20 t/（hm2·a）[14]； the net productivity of the firstgeneration 20yearold C. lanceolata plantation in Huitong of Hunan is 10.48 t/（hm2·a）[15]， the net productivity of the first， second and third generations of 20yearold C. lanceolata plantation in Youxi county of Fujian is 7.84， 6.26， and4.86 t/（hm2·a）， respectively[16]； the 26yearold C. lanceolata plantation in Nanjing of Jiangsu has a net productivity of 2.89 t/（hm2·a）[17]. In this study， the results show that the net productivity of the 23yearold T. flousiana plantation in northwest Guilin is not only higher than that of successive rotation plantation of C. lanceolata at the same site conditions， but also higher than that of the first generation C. lanceolata plantations and Pinus massoniana plantations with close or the same tree ages in in Youxi County of Fujian Province， Jianou City of Fujian Province and Wuxuan County of Guangxi Province， indicating that T. flousiana plantation has strong ecological adaptability and high level of bioproductivity in northwest Guilin. Therefore， T. flousiana is an excellent alternative tree species in the cutover land of C. lanceolata in northwestern Guangxi Province. The rational extension and development of the plantations of T. flousiana have an important role and significance to increase the productivity of the forestland in the region and promote the sustainable development of forestry. References
[1]CHEN DX， CHEN XM， LAN X， et al. Fullsib progeny genetic test and selection of superior germ plasm of Cunninghamia lanceolata[J]. Guangxi Forestry Science， 2016， 45（4）： 347-351.
[2]QIN ZY， TANG J， CAO JZ， et al. Assessment of soil fertility of continuous plantation of Chinese fir based on principal component and cluster analysis[J]. Forestry Resources Management， 2015（5）： 81-87.
[3]WEI XX， CHEN AL， WANG SY， et al. A comparative study of soil microbial carbon source utilization in different successive rotation plantations of Chinese fir[J]. Chinese Journal of Applied and Environmental Biology， 2016，22（3）：518-523.
[4]HE B， HUANG HC， HUANG CB， et al. Concentrations， accumulation and distribution characteristics of nutrient elements in Taiwania flousiana plantation[J]. Journal of Natural Resources， 2008， 23（5）： 903-910.
[5]HE B， LU WP， TANG GW， et al.Soil fertility change in Taiwania flousiana plantation in northwest Guangxi[J]. Forest Research 2015， 28（1）： 88-92
[6]MENG YH， TAN ZY， HE B， et al. Analysis on tree growth and economic benefit of different intercropping patterns of Taiwania flousiana young plantation[J]. Journal of West China Forestry Science， 2015，44（3）： 133-137.
[7]HE B， DIAO HL， HUANG HC， et al. Changing regularity of biomass and productivity of Taiwania flousiana plantation[J]. Journal of Northeast Forestry University， 2008， 36（9）： 17–18，27.
[8]HUANG CB， CAO JZ， WU QB， et al. Comparative analysis on soil physicchemical properties and the tree growth in Taiwania flousiana plantations and successive rotation plantations of Cunninghamia lanceolata[J]. Forestry Science， 2010，46（4）： 1-7.
[9]LIN RY. Study on the difference of growth and soil fertility between the mixed forest of Taiwania flousiana and pure Taiwania flousiana plantation[J]. Auhui Agricultural Science Bulletin， 2015， 21（Z1）：103-105.
[10]TANG GW， HE B， LUO HF， et al. Growth regularity of Taiwania flousiana plantation in northwest Guangxi[J]. Journal of West China Forestry Science， 2015， 44（2）： 96-99.
[11]HUANG JL. The biomass and productivity of Taiwania flousiana plantation on South Mountain[J]. Journal of Fujian Forestry Science and Technology， 2016， 43（3）：161-164.
[12]SUN Y. The study on the growth rule and productivity of Taiwania flousiana[D]. Central South University of Forestry and Technology， 2016.
[13]YU YF， SONG TQ， ZENG YP， et al. Dynamic changes of biomass and its allocation in Cunninghamia lanceolata plantations of different stand ages[J]. Chinese Journal of Ecology， 2013， 32（7）： 1660-1666.
[14]DU H， ZENG YP， WANG KL， et al. Dynamic changes of biomass and productivity of three main plantation types in southern China[J]. Acta Ecologica Sinica， 2014，34（10）： 2712-2724.
[15]HUANG ZH， TIAN DL， KANG WX， et al. Dynamics of biomass distribution in first rotation of Chinese fir plantations in Huitong County， Hunan Province[J]. Journal of Central South University of Forestry & Technology， 2011， 31（5）： 37-43.
[16]MA QX， FAN SH， CHEN SS， et al. Study on biomass productivity of Chinese fir plantations after successive planting[J]. Scientia Silvae Sinicae，2003，39（2）：78-83.
[17]DIAO JJ， XIAO WY， FEI F， et al. Short effects of thinning on the growth and carbon storage of Cunninghamia lanceolata plantation[J]. Journal of Southwest Forestry University （Natural Science）， 2017， 37（3）： 134-139.
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