Chinese fir plantation-the second most important timber species

in China: problems and solutions

 

1)Liping Liao, 2)Shidong Zhao

1)Professor, Institute of Applied Ecology, Chinese Academy of Sciences,

Shenyang 110015, China

2)Professor, Institute of Geographic Science and Natural Resources,

Chinese Academy of Sciences, Beijing 100101, China

 

 

Abstract

 

Chinese fir is the most important conifer species for commercial timber with huge distribution area in southern China, covering 24% of plantation forests and providing 1/4 of total wood materials in the country. At present, however, the plantation forests have faced serious problems of soil degradation. We review the main processes concerning soil degradation in terms of nutrient bio-cycles, soil nutrients, soil biochemical activity and productivity of the plantations. We also proposed and discussed the possible countermeasures to solve the problems.

Contents of N, P, and K of 0-60cm soil under pure Chinese fir plantation with the age of 20 declined by 43.4%, 24.3% and 43.2% respectively compared to those of 1 year-old plantation. Soil microbes populations changed markedly, bacteria decreasing by 19.2%, fungi and actinomyces  increasing 10.4 and 42.7 times respectively.  When Chinese fir plantations were managed in successive rotations, the problems turned more serious in soil fertility decline and soil microbes change. Long-term localized investigations on two permanent plots of Chinese fir plantation showed that the soil content of available N, K and P declined by 40.5%, 47.5% and 42.3% respectively when the plantation turned into second rotation from the first, and declined by 17.5% , 51.5% and 34.1%  when established into the third rotation from the second . The total amount of soil microbes of third rotation of Chinese fir plantation was reduced to 45.0% below of that of the first rotation. The biochemical activities of soil microbes were inhibited obviously by the management practice of successive rotations, with ammonification declining by 80.0%, nitrification by 53.9%, and fiber decomposition by 58.3%, when Chinese fir plantation was repeated on the site for three rotations.

The deteriorated soil properties were well mirrored in the reduction of the plantation growth.   The mean diameter at breast height (DBH) decreased by 13.3% and 21.7% respectively for the second and third rotation, and the average tree height by 11.0% and 35.5% . The volumes of three rotations of Chinese fir plantations were as follows: First, 383.0 m3.hm-2; Second, 262.5 m3.hm-2; Third, 173.6 m3.hm-2.

 

 

Keywords: Chinese fir, plantation, soil fertility, productivity, soil degradation, ecosystem management

 

 

INTRODUCTION

 

China is one of the countries with limited forest resources. Although the Chinese government has spent much effort on reafforestation and plantation projects since 1949, the forest cover has only reached 16.55% up to now. The situation is getting more serious because the forest resource has been continuously decreasing. This has been caused by the pressure of a fast growing population, the development of the economy and the poor management of resources (1).

Chinese fir (Cunninghamia lanceolata (Lamb) Hook.) is a fast growing ever-green conifer, and native to China, with extensive distribution from 220 N and 1020 E to 340N and 1220 E, and is the second most important timber species in China (2). It produces good quality timber, with straight shape, high resistance of bending and cracking, and easily processing trait. Its timber is most popular with local people, widely used in furniture manufacturing and construction building. It produces 24% of the total timber grown in China each year. The Chinese have more than 1000 years experience in managing its plantations. The local people have been using shifting cultivation approach in order to maintain the soil fertility for a long time. Three rotations, lasting about 70 years, are grown on a site and then the land is left under natural conditions for another 70 years. At the end of this period the natural growth is cut and burnt. The land is then replanted with Chinese fir(3). Because of rapid population growth and limited land, this system is becoming more and more difficult to maintain.

The purposes of this article are (1) to review the problems with replanting Chinese fir plantations for consecutive rotations in terms of soil nutrient depletion, change in soil biochemical activity and decline in stand productivity; (2) to discuss the solutions to those problems from three aspects: reformation of conventional management system, cultivation of Chinese fir mixture and utilization of fertilizers.

 

 

PROBLEMS

 

I. Soil fertility decline over the same rotation

 

Nutrient content

 

The species prefers rich soil with thick depth, which implies that it consumes large amount of nutrients to meet its fast growth. Over the same rotation of Chinese fir plantation, soil nutrient contents decreased greatly. When the plantation got its age of 20 years, contents of soil total N, P and K  in the depth of  0- 20 cm were only 50.8% , 14.1% and 36.6% of those before the establishment of the plantation( Table 1). The contents in the depths of 20- 40cm and 40-60 cm declined  markedly as well. The serious decline in nutrient contents could be caused by the imbalance between uptake and return. The ratios of return to uptake were 0.43, 0.46 and 0.25, respectively, for N, P and K (Table 2), due to low rate of litter decomposition (4).  It returned much less nutrients than uptook even when the plantation got over-mature up to sixties years of age(Table 2). The ratios of return to uptake were still low, and were 0.26, 0.29, 0.16, 0.25 and 0.26 respectively for N, P, K, Ca and Mg (Table 2).

 

 

Table. 1. Changes of soil nutrient contents from the establishment of Chinese fir plantation to the age of 20 years (g.kg-1).

Sampling time

Sampling depth (cm)

N

P

K

Pre-establishment

0-20

1.79

0.311

10.44

At the age of 20

0.91

0.044

 3.82

Pre-establishment

20-40

1.33

0.311

 9.78

At the age of 20

0.74

0.089

 3.82

Pre-establishment

40-60

1.07

0.308

 8.63

At the age of 20

0.17

0.089

 4.81

 

 

Table. 2. Nutrient uptake and return in Chinese fir plantations of different ages (kg.hm-2.a-1).

Stand age(yr)

Items

N

P

K

Ca

Mg

 

Uptake

40.97

15.09

18.47

79.01

42.86

20

Return

17.79

7.0

4.61

25.82

14.35

 

Return/Uptake

0.43

0.46

0.25

0.33

0.34

60

Uptake

73.85

48.94

33.48

139.27

103.35

Return

19.17

14.06

5.43

34.82

26.83

Return/Uptake

0.26

0.29

0.16

0.25

0.26

 

 

Soil microbe population

 

As the important decomposers of plantation ecosystem, soil microbes play important role in nutrient transformations, and meanwhile are the good indicator of habitat change. The total number of soil microbes decreased, as the plantation grew old. When the plantation became 21 years old from one-year, the total number of soil microbes decreased by 8.4% due to the decrease in the number of soil bacteria (Table 3), but the numbers of fungi and actinomyces increased 532.4% and 4173.5%, respectively. The numbers of soil microbes and three component populations changed in similar pattern, but for total microbes and bacteria the numbers declined in greater magnitude, by 58.1% and 59.9%, respectively, and increased in less extent for fungi and actinomyces, by 20.6% and 111.6%, respectively (Table 3).

 

 

Table 3 Variation of soil microbe population under different ages of Chinese fir plantations (103.g-1 soil)

Age(yr)

Total microbes

Bacteria

Fungi

Actinomyces

19

39859

39350

165

344

39

16687

15760

199

728

1

17008

16900

 74

 34

21

15581

13660

468

1453

 

 

II. Contents of soil nutrients and biochemical activities of soil microbes in different rotations

Soil nutrient

 

We chose three sites where Chinese fir plantations grew in first, second and third rotations, respectively, and were not far from each other. The contents of soil humus carbon, N, P and K were analyzed by taking soil samples from the three sites. As the sampling depth got deeper, the contents of all the elements declined for all the three sites (Table 4). There were severe decreases in the contents of all the elements when the rotation times of Chinese fir plantation increased from one to three.  The average content of soil humus carbon decreased by 16.2% for the second rotation and by 33.7% for the third rotation, N, 20.0% and 35.0%, P, 9.0% and 63.5%, and K, 2.8% and 10.3%, respectively (Table 4).

To further confirm the above results, two permanent plots F1 and F2, in one of which the first rotation of Chinese fir plantation was grown and in the other the second rotation was there, were set up on the experiment site of Huitong Research Station in 1961. 15 soil samples were taken respectively from each plot, and the ten soil pits were marked with stone pillars for each and the adjacent characteristics of each pit were recorded as well. In 1991, the site of first rotation had been reafforested with Chinese fir and developed into second rotation of Chinese fir plantation, and the second into the third. All the soil pits were re-visited and a soil sample was collected from each pit. Every 15 samples were mixed into one sample for each plot, and used to analyze the contents of available N, P and K. When the plantation was regenerated from the site of first rotation of Chinese fir plantation, the contents of soil available N, P and K decreased respectively by 43.2%, 43.0% and 38.6% in the depth of 0-20cm. When the plantation was replanted on the site of second rotation, the contents of soil available N, P and K by 25.0%, and 48.8% and 6.3%(Table 5). The contents of available N, P and K in the soil depth declined by different degrees as well.

 

 

Table 4 Relationship between rotation times and contents of soil nutrients (g.kg-1)

Rotation times

Sampling depth (cm)

Humus Carbon

N

P

K

 

5-15

39.3

4.8

0.266

17.9

 

16-26

32.6

3.5

0.266

15.4

First

30-40

31.4

3.3

0.266

12.4

 

55-65

19.1

4.4

0.178

12.1

 

Average

31.5

4.0

0.244

14.5

 

5-15

34.6

3.9

0.266

16.2

 

16-26

32.3

3.5

0.266

14.5

Second

30-40

21.1

2.9

0.178

13.9

 

55-65

17.5

2.6

0.178

11.6

 

Average

26.4

3.2

0.222

14.1

 

5-15

32.0

3.3

0.133

13.4

 

16-26

19.8

2.7

0.089

13.3

Third

30-40

16.8

2.2

0.089

13.3

 

55-65

15.0

2.0

0.044

11.8

 

Average

20.9

2.6

0.089

13.0

 

 

Table 5 Variations of soil contents of available nutrients under different rotations (mg.kg-1)

Plot No

Sampling depth (cm)

Time

Rotation

Times

N

P

K

F1

0-20

Pre-30 years

First

103.5

56.3

102.5

Post-30 years

Second

58.7

32.1

62.9

20-40

Pre-30 years

First

83.5

55.5

90.6

Post-30 years

Second

52.6

26.6

48.5

F2

0-20

Pre-30 years

Second

93.5

52.5

56.9

Post-30 years

Third

70.1

26.9

53.3

20-40

Pre-30 years

Second

47.5

49.7

65.9

Post-30 years

Third

46.3

22.7

27.6

 

 

The variations of available nutrients under the first, second and third rotation of Chinese fir plantations were monitored during a growing season. Over the whole growing season, contents of soil NH4+-N for the different rotations were in the order that first rotation the highest, the second the middle, the third the lowest (Fig.1). Generally speaking, the contents of available P and K followed the similar pattern to NH4+-N, with one exception, respectively, for P, that the second rotation was lower than third rotation, and for K, that the first rotation was lower than the second rotation, in June (Fig.1).

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 


Figure 1.  Dynamics of soil NH4+-N, available P and K within growing season of different rotations of Chinese fir plantations.

Biochemical activities of soil microbes

 

The pattern of soil nutrient decline over different rotations could be mirrored in the biochemical activities of soil microbes. Ammonification declined by 80%, ranging from 1.0 mg N.g-1 soil to 0.2 mg N.g-1 soil, when continuous cropping of Chinese fir plantation took place for twice,  nitrogen fixation by 53.9,  fibre decomposition by 58.3%, and respiration by 40.0%(Table 6).

 

 

Table 6 Effects of successive rotation practice of Chinese fir plantations on biochemical activities of soil microbes.

Rotation times

Ammonification

(mg N. g-1soil)

Nitrogen fixation

(%)

Fibre decomposition

(CO2 mg.g-1 soil)

Respiration

(%)

    First

1.0

14.1

3.6

1.5

Second

0.7

12.8

0.7

1.2

    Third

0.2

 6.5

1.5

0.9

 

 

III. Productivity of Chinese fir plantations over different rotations

 

The above mentioned soil nutrition consumption due to continuous cropping could lead to the impact on the growth of Chinese fir trees. The investigation on Chinese fir plantations of different rotations but with same ages of 20 years showed that the average values and annual increments of the diameter at breast height (DBH), height and volume decreased whilst the times of repeated rotations increased. The average DBH, height and volume of the second rotation declined by 13.3%, 11.0% and 31.5%, respectively, and for the third rotation by 21.7%, 35.5% and 54.7%, respectively (Table 7).

After the above results were firstly published by researchers of  Huitong Research Station of Forest Ecology, Chinese Academy of Sciences, Hunan Province, China, many other researchers reported the similar results. Fang (5) found in the same area that the tree height of the third rotation of Chinese fir plantation was 11.0 m while the first was 14.2 m, declining by 22.5%. The investigation on Chinese fir plantation continuously cropped for one time in Guangxi Zhuang Nationality Autonomous Region by Wu (6) showed that the decrease in the volume of the second rotation ranged from 10% to 50% compared with the first rotation. Yu & Zhang s (7) in Fujian Province and Xu (8)s research in Anhui Province suggested the similar conclusion that continuous cropping practice of Chinese fir plantation lead to decline in productivity. Productivity decline due to the continuous cropping practice became a general phenomenon for the Chinese fir plantation.

Since the end of 1970s, a lot of research interest was put into solving the problems caused by the continuous cropping of Chinese fir plantation. Different management practices were surveyed, such as turning mono-culture into mixture of Chinese fir plantations, utilization of fertilizers, cultivation of mixtures, shifting culture, etc (9).

 

 

Table 7 Relationship between rotation times and productivity of Chinese fir plantations

 

Rotation times

DBH

Height

Volume

Average

(cm)

Increment

(cm.a-1)

Average

(m)

Increment

(m.a-1)

Average

(m3.hm-2)

Increment

(m3.hm-2.a-1)

First

14.3

2.3

15.5

1.35

383.0

129.5

Second

12.4

2.0

13.8

1.20

262.5

89.5

Third

11.2

1.8

10.0

0.95

173.6

58.5

 

 

Table 8  Comparison of growth of tree species on degrading soil

Tree species

Stand age

(a)

Site preparation practice

Survival rate

(%)

Height

(m)

DBH

(cm)

Biomass

(t.hm-2)

Cunninghamia lanceolata

6

Complete plowing

35

2.19

2.45

 3.05

Pinus elliottii

6

Complete plowing

95

4.50

7.62

37.45

Pinus massoniana

6

Complete plowing

92

3.49

4.52

27.63

Cunninghamia lanceolata

6

Terraced field

58

3.34

4.34

 6.99

Pinus elliottii

6

Terraced field

98

4.86

8.58

44.67

Pinus massoniana

6

Terraced field

98

4.11

5.97

35.90

 

 

SOLUTIONS

 

I. Reforming of conventional management system

 

The conventional management system on Chinese fir plantation has been used for over 1000 years, including slash and burning, complete plowing for site preparation, clear cutting over a large area, remove of felling residues, and monoculture. The system was blamed to be responsible for leading to site degradation in replanting woodland of Chinese fir plantation. Slash and burning was adopted to facilitate the replanting operation, but it burned out organic matter and some nutrients, and most minerals in the ash were lost when a rainfall happened. Complete plowing significantly increased soil erosion and nutrients loss comparing with other site preparation methods such as pit plowing and terrace preparing (10).  Clear cutting caused soil and water loss as well. In mountain areas of southern China, local peasants used the residues from tree cutting as fuels, which increased the nutrient loss. All the procedures of the management system tended to decrease stand growth of the following rotation. Table 8 showed growth of different stands after the stumpy land of Chinese fir plantation was either replanted with Chinese fir again or shifted to other tree species-Pinus elliotti and Pinus massoniana under different site preparation of either complete plowing or terraced field. The survival rate, mean height, mean diameter at breast height (DBH) irrespective of site preparation were generally lower when the woodland was replanted with Chinese fir (9).  Biomass production of Chinese fir was much less than the shift species, the former was only 1/12-1/9 of the latter for the site preparation of complete plowing, 1/6-1/5 for the site preparation of terraced field (Table 8)(9). As of the shift species, site preparation also affected their survival rate, height and DBH growth and biomass production.  The conventional management system on Chinese fir plantation has to be abandoned, and recently a rational and science-based management system, which includes mild site preparation, clear-cutting in small area, burying of cutting leftovers and multi-culturing, has been introduced.

 

II. Cultivation of the mixture

 

The critical problem in pure Chinese fir plantation is litters recalcitrance to decomposition because of its high C/N ratio, and other serious problems also exist such as bad stability and low resistance to pest attack due to its low biodiversity. The mono-culturing practice has been gradually replaced with silviculture of mixtures. The Chinese fir plantation mixtures could have higher productivity and show positive improvement on soils if associated species was properly selected and the mixing mode and proportion was rationally set up. A carefully designed mixture of Chinese fir with Michelia macclurei Dandy had 13.7% higher timber productivity than the pure Chinese fir plantation, and simultaneously soil chemical and physical properties were improved to certain degrees when the mixture was 8-years old(11). The improvement mechanism has been explored. In the mixture, the amount of litterfall and nutrient content were 2.3 and 1.8 times of those in the pure, respectively(4,11). The fine-root turnover in the mixture was increased by 8.5%(12). The weight loss of decomposing Chinese fir litter was accelerated by existence of M. macclurei leaf litter (13,14).

Cultivation of Chinese fir mixture also means culturing of understorey in pure Chinese fir plantation. Shrubs and herbs were rich in nutrients and their litter decomposed easily. The development of understoreys could be achieved by properly thinning the pure plantation. For instance, at medium site quality, if only 1050 –1200 trees.ha-1 remained after final thinning, the understorey coverage could reach up to over 80%(15). The great quantity input of shrub and herb litter could improve soil nutrient status at a high level, and soil biochemical activity was also enhanced markedly.

 

III. Application of fertilizer

 

Fertilization is a direct approach of nutrient input to the soil and a relatively expensive way of soil improvement compared with the way of mixture cultivation. The fertilization trials on Chinese fir plantation over large areas have been undertaken for over ten years in China, most of which however were done on young Chinese fir plantation below the age of 6 years. The effect of fertilization on growth of Chinese fir plantation was not quite confirmative, and even some contradictory results were found based on fertilizer trials of different areas, which hindered the utilization of fertilizers in the field over large areas. Based on the fertilization experiment on young Chinese fir plantation of 5-years old in Jiangxi Province, China, Li et al(16) concluded that application of phosphorus and potassium fertilizers could increase stand growth. In another fertilizer trial on Chinese fir seedling in Zhejiang Province of southern China, Ye(17) found that phosphorus fertilizer increased growth at the beginning of fertilization, nitrogen fertilizer had no effect, and potassium fertilizer showed negative effect.  The fact that no long-term and well-designed fertilization trials have up to now been conducted across the distribution area of Chinese fir plantation could explain somewhat the reason why the results of fertilization effect on Chinese fir growth was not convincing.

Both the above-mentioned trials were on fertilizers of single nutrient element. The effect of compound fertilizers of two or three nutrient elements of N, P and K in certain rates was proved to be a different profile. When the amount of N, P, K in the compound NPK fertilizer was in the proportion of 1:3:1, the compound fertilizer increased tree height increment of Chinese fir growing in soil of the third rotation by 75.4% and net production by 52.6% during one years trial in the pot culture, comparing with the control of non-fertilization (9). The results were supported by other fertilization experiments (18, 19, 20). Generally speaking, application of compound fertilizer containing phosphorus shows more pronounced effects because soils in the distribution area of Chinese fir plantation are in deficiency of phosphorus, and site index is also a limiting factor (21).

 

 

 

 

CONCLUSIONS

 

The decline of forest soil quality is a complex ecological problem, varieties of factors contributing to it. First of all, the species has its own intrinsic properties, such as restricted site quality for growing, recalcitrant litter, etc., which hindered sustainable development of Chinese fir plantation ecosystems. Secondly, the unreasonable management practices of replanting of monoculture have aggravated the process of soil degradation. Thirdly, socioeconomic factors in that the huge demand for the timber and good price in the market led to the existence of conventional management system for hundreds of years. All the factors have made the solution to the soil degradation much more difficulty as well as the exploring of the mechanism of soil degradation, which will necessitate a long term and comprehensive study to understand the dynamics of soil quality of plantation forests under extensive management. The ecosystem-based management approaches will probably provide promising solutions to the problems.

 

 

Acknowledgements

 

The authors were grateful to Prof. Chen Chuying for providing quite a lot of information concerning the subject. This study was financially supported by Chinese Academy of Sciences at grants No. KZ952-J1-202, KZ951-A1-301 and KZ95T-04.

 

 

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