Sustainable management of teak plantations on acidic soil in China - Mr. Kuang Bingchao and Mr. Bai Jiayu

Kuang Bingchao and Bai Jiayu
Research Institute of Tropical Forestry,
Chinese Academy of Forestry, Longdong, Guangzhou.

A teak plantation in Hainan Island, China.

ABSTRACT

Research in promoting sustainable teak plantation management under acidic soil conditions in China has concentrated on: selection of acidic soil tolerant genotypes; intercropping with nitrogen-fixing tree species, and inoculation with nitrogen-fixing Rhizobium species (root nodule bacterium), VA mycorrhizal fungi and other nitrogen-fixing microbes. Preliminary results indicate that teak tolerance of acidic soil varies distinctly at both population and individual levels. Selected promising provenances, families, trees and clones can be grown routinely on soil types with pH values ranging from 4.2 to 5.9, and have a genetic gain of 14.4 - 51.3% in volume production. In strongly acidic soil, the moderate application of lime (2.5-5.0 kg/plant) at establishment can assist growth in the juvenile stage. Inoculation with nitrogen-fixing root nodule bacteria can also increase growth markedly, although the combining performance in nitrogen-fixing of different microbes requires further study. Inoculation by VA mycorrhizal fungus at seedling phase gave satisfactory infection percentages in all provenances from 76.7-100%. Future efforts should foster the application and evaluate the efficiency of VA mycorrhizal fungus, nitrogen-fixing Rhizobium and nitrogen-fixing intercrops in teak plantations, to develop a suitable model for routine sustainable management.

Key Words: Tectona grandis, China, acidic soil, VA mycorrhizal fungi, Rhizobium, nitrogen-fixing tree species.

INTRODUCTION

Tropical southern Chinese man-made forests are largely planted to pure Chinese fir (Cunninghamia lanceolata Lamb, Hook.) or Pinus massoniana, which has increasingly degraded the soil and lowered productivity. Studies on the transformation and improvement of pure Cunninghamia lanceolata and Pinus massoniana plantations commenced in the early 1980s. Objectives include: rebuilding an ideal multi-story model plantation, or climax plantation of mixed tree species; and developing technologies and theories in tropical China favourable to sustainable management. The major species employed is teak (Tectona grandis), which is accepted by the local farmers because of its wide introduction in over eighty counties and cities representing seven provinces, its ease of intensive management, and its high economic value. Research was conducted with the ultimate objective of establishing plantations of teak in relatively stable structures in sustainable management. Resulting techniques, assumed as solutions to the problems of management of teak plantations, are:

1. Selection and breeding of superior provenances, families and clones tolerant of acidic damage, and tolerant to aluminium damage in particular.

2. Establishment with application of fertilizers, lime, and other minerals to increase the soil pH value and improve juvenile growth.

3. Creating plantation mixtures by introducing legume and non-leguminous nitrogen-fixing tree species, and establishing mixed multi-story plantations of teak.

4. Inoculating with nitrogen-fixing Rhizobium and Actnomyces and identification of other possible microbes effective in reducing or eliminating the negative influence of excessive soil acidity and aluminium levels.

Financial support was received from FORSPA in 1992, and the two-year period from June 1992 to June 1994 saw remarkable achievements on the above four aspects. The preliminary results are detailed in this paper.

GENERAL SURVEYS

Early trials

A project, “Selection of provenances of teak tolerant of acidic soil”, was carried out in Guangdong, Guangxi, and Hainan Provinces during 1982-1988, testing 92 provenances from 212 families. The trial site at Guangzhou, Guangdong Province, was also the site of another project, “Amelioration of strongly acidic soil, and selection of acid-tolerant teak provenances”. The selection of acid-tolerant provenances, families and plus trees of teak mentioned herein are based on these projects.

Field trial sites

The sites designated for field trials are Longdong, Guangzhou City, and Hepai, Enping City in Guangdong Province. Both locations have a tropical monsoon climate with an annual average temperature of 20.3°C and 22.8°C respectively, and 1,400-2,300 mm annual rainfall. The sites were both previously under Chinese fir and Pinus massoniana. The soil type is a well-developed deep laterite from granite (Table 1).

Table 1. Soil properties of the two trial sites⁽¹⁾

Notes:

(1) The chemical data are the average of A, AB and B layers of the soil profile;

(2) There are four sites as planned, but the rest two have not been investigated due to time and fund limitation.

MATERIALS AND METHODS

Materials

Ninety-two provenances from 212 families have been tested in the trials, of which 22 provenances were provided by the DANIDA Forest Seed Centre (Denmark) and the Teak Improvement Center of Thailand, and the remaining 70 provenances were collected from plantations previously established in China (Table 2).

The 1992-1994 field trials were conducted on 106 families representing 25 provenances, the seeds of which were collected from plus trees in the plantations established earlier, and from the gene pool formed in the 1970s. In addition, over 3,800 terminal bud branches derived from 256 plus trees were used as explants for tissue culture (Table 2).

Other species were mixed within the teak plantations, including: Shorea robusta, Casuarina junghuhniana, Acacia melanoxylon, A. oraria, A. harpophylla, A. implexa, A. mangium, A. crassicarpa, A. neriifolia, A. fasciulifera, Daemonorops margritae and Stylosanthes guyanemsis. The two species of VA mycorrhizal fungi inoculated are Glomus caledonium and G. rersiforme, representing the family Endogonaceae, with two isolates of each species, namely, No. 90068 and 90036 of the former and No. 90080 and 89404 of the latter. Both of these two fungi species were selected from Klebsiella pneumoniae by the Microbiological Institute, Guangdong Academy of Agriculture.

Table 2. Number and origin of provenances, families and clones tested

Notes: * Provenances/families obtained from provenance trials in China. Number in brackets indicates provenance(s) provided by DANIDA/FSC and/or TIC.

Methods

The steps adopted for the selection of teak acid-tolerant provenances, families, and clones were as follows:

1. On the basis of the early trials, analyzing the pH value of soil of the trial sites;

2. On those sites with a pH value of less than 5.8, selecting superior provenances and families with the criteria of acid tolerance expressed by biomass and adaptability performance;

3. Selecting individual plus trees based on the former selection of superior provenances and families; and,

4. Collection of seed and stem apices, followed by raising seedlings, tissue culture, and progeny tests in the field, and determining acid and aluminium tolerance in laboratory conditions.

The field trial was conducted in random blocks, with 2-4 trees per block and 6-8 replicates. A MS medium for tissue culture in five pH treatments, namely in pH values 4.0, 4.5, 5.0, 5.5 and 6.0 adjusted with HCl, was used in the laboratory to determine the degree of plant acid tolerance. Forty clones were tested in the laboratory experiment (three bottles per clone; three seedlings per bottle). Twenty-four tissue culture clones were successfully free from contamination. The period for culturing is 30-60 days, during which the culturing fluid is renewed weekly. The ingredients of the fluid used for testing aluminium tolerance were:

The culture fluid was divided into five treatments with different doses of AlCl₃ (Keltjens, 1990), namely, 0, 10 uMol/L (aluminium activity 4.66 uMol/L), 30 uMol/L, 100 uMol/L and 500 uMol/L. Forty clones were tested, with five bottles per clone per treatment, and two seedlings per bottle. Because the foam plastic cubes used as containers for non-root explants could not be sterilized completely at high temperatures, heavy contamination occurred and only ten clones gave good performance in the culture process.

Parameters investigated and statistical methods

The characters investigated were tree height, diameter (at breast height or ground level), height of stem free from damage, survival percentage, and degree of stem degrade and death as well as infection percentage of VA mycorrhizal fungus and the degree of infection. The volume calculation was based on the formula:

V = 0.4787 D² H

where V is the volume, D the diameter, and H the height.

The variance was analyzed with the block average data. The transfer between percentage (P) and absolute value (X) was done through the formula:

P = Sin^-1 (X^1/2)

The calculation of heritability, interaction, stability, and integrated heritability were made with references to Wang Mingxiu (1988) and Keiding, Wellendorf & Lauridsem (1986), in which the weighted coefficient was obtained on the basis of the characteristic vector and contribution of the major element.

RESEARCH ACTIVITIES

Application of lime and organic matter to improve the growth of young trees

In strongly acidic soil, lime (pure CaO) was applied in the first two years after planting. On the Hepai, Enping site, the influence of superphosphate was tested in five treatments: 0, 250, 500, 750, and 1,000 grams per plant and with five replicates. At the Longdong site, a double factorial trial of 9 provenances at five lime levels (0, 2.5, 5.0, 7.5, and 10 kg/plant), was made (eight replicates). On both sites the trials were designed in completely random blocks.

Inoculation with nitrogen-fixing root nodule bacteria

The exploratory test was conducted with 40 potted seedlings, one seedling per pot, in a 5 × 8 arrangement, which could be divided into 5 blocks and 8 replicates, or 8 blocks and 5 replicates. The inoculation method used 50 ml of fungus suspension, at a concentration of 10⁷ fungi/ml poured into each pot (seedling). The control was made with 35 single seedling planted pots.

Inoculation with VA mycorrhizal fungi

Twelve provenances of teak and four isolates of VA mycorrhizal fungus were tested, plus one control. The fungal isolates were firstly inoculated with Trifolium L. for reproduction purposes, and then the infected roots and rhizosphere soil were taken as inoculum to the teak seedlings at a dosage of 50 g per plant (planted in pots). The organization was such that the completely random blocks contained two factors, namely provenance and VA fungus, 8 replicates in provenance terms and 24 replicates in VA fungus respectively, with two plants per block.

Mixed species in multi-story plantation trials

The arrangement of the trial was a random block pattern, with an area of 270-450 m² per block and 3-6 replicates. The allocation of tree species, mixing types and forest structures were as follows (Diagram 1):

Diagram 1. Mixture models of teak with nitrogen-fixing tree species.

Note: A row spacing is 3m.

Legends:

T = Teak (spacing 6m × 3 m);
Mg = Acacia mangium (6m × 3 m);
Ao = Acacia oraria (6m × 3m);
Sr = Shorea robusta (6m × 3 m);
Am = Acacia melanoxylon (6m× 3 m);
C = Casuarina junghuhniana (6m × 3 m);
Sg = Stylosanthes guyanemsis (3m × 3 m);
D = Daemonorops margritae (0.5m × 3 m).

PRELIMINARY RESULTS

Selection of acid-tolerant provenances

Preliminary results are summarized as follows:

1. Tree growth in the five treatments varied significantly by provenance at 0.1 or 0.05 level (with a F value ranging from 1.65^* to 4.55^***);

2. Heritability values of the main characters are mostly above 0.50, with the highest reaching 0.96, indicating that the heritability of growth and adaptability of teak provenances tested on acidic and strongly acidic soil is mainly governed by genetic background;

3. Eleven superior acid-tolerant provenances (9) and families (2) selected amongst 44 provenances and 13 families of the five trials have a genetic gain ranging from 14.4 to 51.3% in volume (Table 3);

4. On the Longdong site (trial No TP8630), the growth of the 16 provenances involved was impeded at the juvenile stage and only 8 provenances resulted in statistical data; 6 provenances died in the first two years and the other provenances died in the first three years. However, by application of lime and organic matter, the site could sustain normal teak growth; and

5. In trial No TP8630, provenances 3072, 7794, and 3054 showed better performance than others, either with or without lime application. In particular, provenance 7794 is moderately adaptable to strongly acidic soil and shows the least variance of interaction. Its regression coefficient of tree height is nearly 1.00. Its volume accumulation rises in direct proportion with the degree of amelioration of the acidic soil (Table 4).

Table 3. Selection of acid-tolerant provenances and families of teak*

Note:

* Only volume data are listed; quality and adaptability to be reported elsewhere;

** Number in the brackets indicates the number of provenances tested.

Meng = Menglum, Jian = Jianfeng, Long = Longchuan, BPL = Ban Pha Lai, Manl = Manladu, Bang = Bangba, Mang = Mangshi, Masa = Masale, Paks = Pakse. M = Myanmar, T = Thailand, L = Lao PDR, C = China, I = India.

Table 4. Stability and adaptability of some provenances in height (H) and breast-high diameter (D) to lime application

Note: Only part of the tested provenances are presented in the table.

Selection of acid-tolerant plus trees

The results of Tables 3 and 4 indicate that:

1. There are 37 plus trees among the acid-tolerant provenances; 14 are from the pH 4.0-4.5 provenance trial and the other 23 from the pH 4.3-5.4 trial. Individual plus tree volume is 83-663% higher than the total average of the plantation, and 27-380% higher than that of all the superior provenances. In average volume terms, the plus trees selected are 294.2% higher than the trial plantation and 248.5% than the selected provenances.

2. Twenty seven plus trees have been selected among the acid-tolerant families grown on pH 4.2-4.7 soil. The individual volume of the selected plus trees ranges from 144 to 445% of the forest average, and 116-359% of the average of the superior families selected. The mean volume of the plus trees is 221.1% of the forest total average.

3. Selection made in other provenance/family trial resulted in 58 plus trees under pH 4.2-5.4 soil conditions, 22 of which are selected from provenance trials and have a mean volume 132.4% higher than that of the total forest, and the remaining 36 trees from family trials are 104% higher as regards mean volume (Table 5).

4. Besides their excellent performance in increment, all selected plus trees have the best growth in adaptability and qualitative terms.

In 1994, a progeny test was established on the two sites (3.3 ha); in total, 53 families were involved. The test remains to be surveyed and data analyzed.

Table 5. Selection of plus trees (PT) tolerant of acidic soil conditions*

Notes:

(1) * This paper deals only with the selection of plus trees with the criteria of acid-tolerance and volume, and selection with the criteria of adaptability and other qualitative characters will be reported in another paper.

(2) ** Number in the brackets gives the age (Year) of the forest.

(3) Abbreviations: PT = plus tree; and Prov.= provenance.

DETERMINATION OF ACID AND ALUMINIUM TOLERANCE OF TISSUE CULTURE EXPLANTS OF SELECTED CLONES

Acid-pH-effects

Laboratory determinations were conducted on acid tolerance in November 1993. Two months of testing concluded that the height and root length of the culturing explants vary with the clone and pH value significantly at 0.1 or 0.05 level. When the pH value changes from 4.0 to 5.0, only the root length differs significantly with clone and pH treatment, except for the control (Table 6). The best performance was shown by clone 72, followed by clones 715 and 1010 (Table 7). Height and root length of the same clone also vary significantly among individual seedlings, probably as a result of position effect of the explant, which disturbed the genetic difference of clones at treatment level.

Table 6. Variance analysis of height and root length of clonal explants of teak at different pH level

Notes:

(1) The number of ‘*’ indicates the degree of significance.
(2) NS = not significant.

In addition, the results indicate that, under laboratory conditions, when the pH level is below 4.0, the height and root length of the culture plants are not impeded or affected, which is very different from the field conclusion (juvenile trees could not survive on pH 4.5 and lower in the degraded soil of provenance trials in Guangdong Province). That is to say, the factors causing the death of juvenile teak are much more complicated, and pH value at 4.0 and lower is not the only reason for the failure of teak on such a soil.

The analysis of the correlation between height, root length of tissue culture explants, and pH treatment indicates that height is correlated significantly with the pH level (r = 0.9660^**). The correlation may be interpreted with the following single linear regression equation:

where H = height of the culture seedling, and X = pH value of the culture medium.

The results indicate that the root length has no significant correlation with the pH level, whereas the root length at pH 4.0 level is distinctly higher than that of the pH 4.5. Whether it is caused by the position effect of the explant, or experimental error, or the common growth performance of the culture explants is not clear and should be identified in future studies.

Aluminium damage to tissue culture explants

Laboratory results indicate that the explant height and sprout numbers vary significantly at 0.1 or 0.05 levels with aluminium concentration and clone treatment. The survival rate shows no significant variance (Table 8). Comparing the aluminium concentration with the control, only treatment No. 5 having a concentration 233.2 uMol/L of active aluminium, shows a significant difference in explant height and sprout number (Table 9). When the aluminium concentration of the culturing fluid reaches 100 uMol/L (with an activity 46.62 uMol/L) and 500 uMol/L (an activity 233.2 uMol/L), the only clone to die was 70107, while clones 715, 72-72, 7059, 7018, 7013 and 70118 survived with no heavy damage observed, though growth is distinctly delayed. With reference to the three indices mentioned above, promising clones are 7272, 715 and 7059 (Table 9). Aluminum activity has no significant correlation with the survival percentage of tissue culture teak.

The relation of aluminium activity with stem height and sprout number, with a correlation coefficient -0.8890^* and -0.9791^** respectively, can be implied from the following equations:

where H = stem height of culture explant, X = aluminium activity, and S = sprout number of culture explant.

Table 7. Average of height and root length of culture explants at different pH and clone levels and the multiple range Duncan’s test

Note: a) = *, b) = **, c) = ***, d) = ****, e) = *****. Those mean sharing the same letter does not differ significantly at 5% level, Duncan’s test.

Table 8. Variance analysis of explant stem height, sprout number and survival at aluminium different activity of the culture medium

Note: (1) Two blocks not available for study.

Table 9. Averages of seedling stem height, sprout number and survival under different aluminium activity and clone conditions; Duncan’s multiple range test

AMELIORATION OF ACIDIC SOIL CONDITIONS AND TEAK GROWTH

Impact of lime on soil properties

It is concluded that soil pH value and properties change considerably in chemical composition after the application of lime, at a dosage of 7,500 kg/ha, coupled with compound fertilizers (12% N : 15% P : 21% K) 250 kg/ha (Table 10). The organic matter and total nitrogen content declines, due to soil erosion and by the teak consumption. Additionally, the understory of the plantation changes from a Dicranopteris dichotoma - Rhodomyrtus tomentosa - Adiantum spp. acid favourable population, into a weed-shrub one (Table 11).

Table 10. Impact of lime on strongly acidic soil chemical composition

Table 11. Comparison of plant species on strongly acidic soil before and after lime application

Note: Assessed in Drude’s system evaluating ‘abundance’, in which soc, cop², cop¹, sp, sol, and un are referred to the abundance in a decreasing order.

Lime application and teak growth

In terms of volume, each lime-added provenance on pH 4.5 soil at 1.5 years age is over 50% higher than the control of the same provenance, which is free of lime and organic matter. Particularly so in provenance 7794, in which the treatment has a volume 675% higher than the control. When at 4.5 years of age, the volume of provenances treated with lime and organic matter is over 100% higher, compared with the control (excluding provenance 3072); this is especially so in the case of provenance 7794, which was 13.9 times more (Table 12). The optimum amount of lime and organic matter for volume production varies with teak age. Incidentally, Treatment 2 with 2.5 kg/plant is the best treatment selection at age of 0.5-1.5 years; however, when 4.5-10 years old, Treatment 3 with 5 kg/plant shows maximum volume (Table 13 and Diagram 2). Their relation is reflected in the equations:

Y = 1.7100 - 0.8632X - 0.1584X² - 0.0147X³ + 0.0021X⁴ (0.5 year old)

Y = 2.0501 + 2.1677X - 0.3104X² - 0.0169X³ + 0.0036X⁴ (1.5 years old)

Y = 3.6101 + 6.3427X - 0.6254X² - 0.0747X³ + 0.0082X⁴ (4.5 years old)

where Y = volume, and X = weight of lime applied. It is clear that each of the above three equations has a correlation coefficient of 1.0000 and is in accordance with the trial data.

Table 12. Volume accumulation (10^-4m³) of provenances influenced by lime application

Note: A mean sharing the same letter or group of letters does not differ significantly at 0.05 level in Duncan’s test.

Table 13. The growth of teak at different levels of lime application

A mean sharing the same letter or group of letters does not differ significantly at 0.05 level in Duncan’s test.

Diagram 2. Relation of lime addition and teak volume growth

Note: 0.5a, 1.5a, 4.5a = 0.5 year, 1.5 year, 4.5 year

The impact of nitrogen-fixing tree species on soil properties and on teak growth

The tree height, diameter at breast height and survival were studied in the concurrent year and 1.5 years after planting. The initial results indicate that Acacia mangium, A. implexa and A. crassicarpa, without lime and fertilizer additions, have better growth in height than teak under treatment of 2.5 kg lime and 625 grams per plant. These three species have a survival rate of 94.0%, 60.0% and 93.3% respectively. It appears that the three Acacia species are well matched to the soil type. A pruning and/or pollarding operation is essential for these Acacia trees, which would otherwise shade the smaller teak and retard growth. The other Acacia species tested - excepting A. melanoxylon - are inferior to teak height growth and have a survival of no more than 53% (Table 14). It is to be noted that the final evaluation of the impact of mixed nitrogen-fixing trees on acidic soil amelioration and teak growth will be made 3-5 years after planting.

Table 14. Growth and survival of mixed species in teak plantations

Notes:

(1) The 3.3 ha mixed teak plantation established in Longdong in 1994 will be assessed in 1995.

(2) Mixtures: A: Teak - Acacia - Stylosanthes guyanemsis; B: Teak - Acacia - rattan, C: Teak - Acacia mangium, and D: Trial for selection of adaptable tree species.

INOCULATION OF TEAK SEEDLINGS WITH NITROGEN-FIXING RHIZOBIUM AND VA MYCORRHIZAL FUNGI

Inoculation of teak seedlings with nitrogen-fixing Rhizobium

Results show that 0.5-year old seedlings inoculated with nitrogen-fixing Rhizobium have a significant increase in height and ground girth at 0.1 or 0.05 levels, compared to that of two controls. Biomass is 21.7-40.5% and 16.1% higher respectively than that of each of the two controls (Tables 15 and 16).

Table 15. Variance analysis of the nitrogen fixing Rhizobium inoculation and teak growth

Table 16. The influence of nitrogen fixing Rhizobium on teak growth

Inoculation with VA mycorrhizal fungus on teak seedlings

Microscopic observation of infection status indicates that the 4 isolates can cause effective infection of teak seedlings. The infection rate varies with provenance from 76.3 to 91.0% and the highest infection index is 11.20 observed in provenance 1006 from Thailand, which is 242.4% of that of the lowest provenance 77109 from Yingjian/Myanmar. In an individual provenance, the infection average of a block varies from 300 to 630%, so the individual has a greater difference in infection the provenance, and there is, therefore, no significant variance at 0.1 level, either in treatments or in provenances. Hence, in selection of VA-mycorrhizal-fungus-preferring genotypes of teak, more attention should be paid to the selection of plus trees and clones, followed with inoculation and field testing, and then propagating the best for large-scale afforestation activities.

RESULTS AND DISCUSSIONS

Acid tolerance

Preliminary results in the selection of acid-tolerant provenances, families and trees indicate that:

• Variations exist at 0.1 or 0.5 significance level between and in both provenances and families, either in field acidic soil trials or in different pH value and aluminium activity in laboratory determinations, and this provides a broad basis on which selection can rely.

• When pH value of MS medium in laboratory experiments is in the range 5.5-4.0 and aluminium activity from 4.66-233.2 uMol/L, stem height and sprout number of culture explants have significant relevance with pH and aluminium levels at 0.1 or 0.5 levels. This relevance can be represented in a single linear regression equation. However, root length and survival are not significantly relevant in pH and aluminium activity treatments. It was concluded that treatments with the lowest pH value or highest aluminium activity did not cause the heaviest death of the teak tissue culture explants.

• The preliminary selection is successful and provenances 7794 (Mangshi, China), 3072 (Masale, India), 3054 (Pakse, Lao), 8204 (Longchuan, Myanmar) and 1008 (Ban Pha Lai, Thailand), as well as families 7549 and 7504 (Longchuan, Myanmar), can survive on pH 4.2-4.9 laterite and have a genetic gain of 16.4-51.3% in volume terms.

Lime application on strongly acidic soil

Preliminary results indicate:

• Lime addition to strongly acidic soil can change the understory plant structure and soil chemical properties, and improve the growth of juvenile teak plantations.

• Optimal lime application may be 2.5-5.0 kg/plant, or 2,100 to 4,166 kg/ha in case of pH 4.5 soil. Different treatments of lime applied had different effects on teak growth at different ages. The best growth in the first year and at 1.5 years is shown with 2.5 kg/plant and at 4.5 years with 5.0 kg/ plant.

• Teak growth influenced by lime addition varies markedly with provenance. Incidentally, provenance 7794 (Mangshi, China), is moderately adaptable to strongly acidic soil and has the highest increase in growth and should be a promising selection for future use.

Mixture of nitrogen-fixing tree species and teak

The preliminary results obtained of survival and juvenile growth indicate that:

• Most Acacia tree species, for example, A. mangium, A. crassicarpa and A. implexa, have better height growth than teak and this impedes the growth of the lower teak story; measures are required to resolve this problem. By contrast, A. fasciulifera and A. harpophylla have poor survival rates and are not suitable for acidic soil planting.

• There are many complicated problems in mixed species planting that need further study to identify them.

Inoculation of nitrogen-fixing Rhizobium and VA mycorrhizal fungi to teak seedlings

The results show:

• Nitrogen-fixing root nodule bacteria inoculation can increase seedling teak growth considerably.

• Preliminary results indicate that VA mycorrhizal fungus can infect teak seedling root tissue at a ratio of 60-100%. There was no significant difference at provenance or fungus isolate level. However, individual differences in infection rate in a provenance is greater, which indicates that teak-VA mycorrhiza fungi combinations should be selected at individual tree levels.

PROBLEMS

Selection of aluminium tolerant genotypes of teak

Edmeades (1985) and Bruce (1989) hold the viewpoint that the plant root system would be impeded when the monomer aluminium activity in soil reached 4-15 uMol/L. In the treatments with aluminium activity between 4.66-233.2 uMol/L, after 20 days of tissue culturing, only one clone (No. 70107) has a death rate 100% under the lowest and the highest aluminium concentrations. When the aluminium activity is 13.99 uMol/L, there are three clones, 30% of the total, dying (over 40%). That is to say, aluminium activity has a great influence on the survival, growth, and sprout number of tissue culture explants of teak, but the relationship requires further study. As the field performance of teak growth greatly differs from that in the laboratory culturing (Hang, 1984), the tolerant clones selected - No. 715, 7013 and 70118 - which survived 100% after two months’ culturing in the laboratory experiment, should be tested through field trials.

Some limitations in the experiment included: a) foam plastic cubes could not be sterilized completely and this caused heavy contamination; thus, this material is not suitable for use in tissue culturing; b) due to the employment of non-root tube-explants, the relationship between root length and aluminium activity is not available; c) the gap of aluminium activity between different treatments indicates the experiment was not properly designed, partly due to lack of experience and reference data. In the highest aluminium activity cases, the explants did not die to any extent. On the other hand, the gap between the three treatments from concentration 4.66 to 46.6 uMol/L is very small and from 46.6 to 233.2 uMol/L overly large, most probably the reasons why the experiment did not reach a satisfactory conclusion.

Inoculation of nitrogen-fixing root nodule bacterium to teak

The test concludes that bacteria can improve the growth of teak seedlings, making them 16-40.5% higher in height and ground level girth. But investigation of the test seedlings following transplanting in the field was neglected. Whether increased seedling growth is caused by the nutrients carried in the fungus suspension, or the nitrogen fixed by the fungus inoculated, could not be identified. Further studies are needed to investigate the possibility of teak-fungus combining fixing of nitrogen in acidic soil, and its functions.

Inoculation of teak with VA mycorrhizal fungus

Unexpected satisfactory results were obtained. The application of VA mycorrhizal fungus is of potential importance for teak to make effective utilization of P fertilizer in acidic soil. The mycelia of VA mycorrhizal fungus is available for absorbing N, Zn, Cu, S, Ca, K and the other nutrients and, therefore, upgrade the pH value of the rhizosphere soil. In addition, the fungus can alleviate the damage of heavy metal elements, for example, Mn to plants (especially to trees) in acidic soil (Li and Chao, 1994).

The success in VA mycorrhizal fungus inoculation on teak gives great promise for establishing multi-story plantations of teak on poor acidic soil, and the difference in infection rate and index at provenance and individual tree levels provides an important basis for further selection of promising genetic material for use on degraded acidic soil sites. On the other hand, there are two points that should be understood:

1. The seed of six provenances, half of the provenances tested, was collected in the plantations and gene pool established in China in the 1970s from seed imports. Under these conditions, where natural pollination occurs and thereby inter-provenance crossing takes place readily, the seed of a certain provenance is no longer pure, some, even most, being half-sibs. So the difference in infection is much less smaller at provenance level than that of individual trees.

2. Without careful consideration of practical nursery isolation of the inoculation blocks (only a 30 cm distance), the inter-isolates’ infection with fungus is possible due to water splashing and/or percolation; this disturbs the normal infection of a certain fungal species/isolate-provenance combination. The high infection of the control could be evidence of this. What is more, owing to technical limitations, the mycelia of a certain fungal species/isolate and its inter-infection with others could not be distinguished in microscopic observation. So the infection of different fungal species/isolate treatment shows no significant difference. The test could not reach the planned result to select optimal teak genotypes suitable for a particular fungal species/isolate.

It may be concluded that the impact of VA mycorrhiza fungus on the nutritive cycle and growth of teak plantations requires further study and interpretation.

PROSPECTS

In southern China, a large area, some 480,000 km², is suitable for teak planting. However, in practical terms the strong soil acidity and degradation in such a widespread range are constraints to teak planting extension. Therefore, strenuous efforts, and a rather long time, have to pass before preferred models of climax stable sustainable plantations, with teak being the major tree species, come into being. It is to be hoped that the success of studies on some related issues in recent years indicate a promising future to this end point. Further efforts are proposed in the coming years:

Selection of acid-tolerant genotypes of teak

Based on the previous selections of provenances and families tolerant of acidic soils, the next step to be made is to select superior individual trees, with an emphasis on VA-mycorrhizal-fungus-compatible genotypes. It is anticipated that after clone tests, the selected genetic resources will be propagated on a scale appropriate to the requirements of large-scale afforestation in tropical China under its acidic soil conditions.

Inoculation of teak

Inoculation of teak with nitrogen-fixing bacteria and/or VA mycorrhizal fungi, mixed together with leguminous or non-leguminous crops, would be used to establish multi-story plantations. This would help form a symbiotic combination of a major tree (teak), mixed tree(s), and the inoculated fungi/ bacteria, and so improve the status of bio-fertilization, nutritive cycle, and ecological stability of the forest land.

REFERENCES

Blamey, F. P. and Asher. 1990. Aluminium tolerant mechanism of plants on acid soil. (Lu Wenxin et al. translated from Transactions of 14th International Congress of Soil Science, 1990, (4): 44-48.) Tropical and Subtropical Soil Science (Chinese). 1993, 2(1): 50-53.

Keiding, H., Wellendorf, H. and Lauridsen E. B. 1986. Evaluation of an international series of teak provenance trials. DANIDA FSC. Humlevack. 7-27.

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Li Xiaoling and Chao Yiping. 1994. Mechanism of VA mycorrhizal fungus for absorbing mineral nutrients. Acta Pedologica Sinica. 31 (Supplementary): 274-277.

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