2. RESOURCE PROPERTIES

The emergence of rubberwood (also referred to as heveawood or parawood) as an internationally established wood product has often been termed a `success story.' Various factors have contributed to this development, first and foremost the fact that rubberwood represents a relatively sustainable alternative to tropical woods extracted from natural forests. Furthermore, rubberwood has proven to be very versatile in its use in furniture manufacturing and the wood-based panels industry. Where forests are scarce, particularly in South Asia, the use of rubberwood as fuelwood continues to mitigate pressure on natural forest resources. This section provides an overview of the origin of rubber trees (hevea brasiliensis) in Asia, their cultivation, the environmental impacts of rubber plantations, rubberwood yields and rubberwood properties.

2.1 Asian origins

Hevea brasiliensis arrived in Asia in 1877 from its native Brazil by way of the British Colonial Office. Initially, rubber trees were grown experimentally in Sri Lanka, from where they were brought to Singapore and Malaysia. Much of the pioneering work was carried out at the Singapore Botanical Garden, where Henry Ridley, the Garden's director from 1888-1911, discovered a sustainable harvesting method to obtain continuous yields over a period of years. By the turn of the century, rubber tree production had spread to what was then Indochina (Vietnam and Cambodia), the Dutch East Indies (Indonesia) and Thailand; the British in Malaysia and the Dutch in Indonesia cleared large areas of rainforest to create rubber plantations. Although cultivation at first took place on plantations, smallholders rapidly adopted hevea as a source of income (Killmann & Long, 2000; International Rubber Research and Development Board (IRRDB, 2000; Goldthorpe, 1993). Today, rubber trees are cultivated in more than 30 countries in Asia, Africa and Latin America.

2.2 Rubber tree cultivation

As a tropical tree, Hevea brasiliensis grows best under conditions of temperatures between 20-28°C, well-balanced annual rainfall of 1,800-2,000 mm and protection from high winds. It develops reasonably well up to 600 meters above sea level (but is capable of growing to at least 1000 meters near the Equator) and will perform on most adequately drained soils. Its prime growing area is between 10° latitude on either side of the equator, although it is also found further north, as in China, and south (IRRDB, 2000). The current distribution of rubber production is shown in Figure 1.

Mature trees on rubber plantations are commonly 20-30 meters tall with a relatively slim trunk of up to 30 cm diameter at breast height, an average branch-free bole of 3 meters and upwards-extending branches. Young trees have a smooth brown-green bark. Rubber trees flower once a year. Insect cross-pollination results in large fruits containing several thimble-sized seeds with hard outer coats (in some countries, such as Malaysia, a second round of seed production may occur). If satisfactorily germinated and planted within 2-3 weeks (at about 500 trees per hectare, although some clones are planted at much higher densities), seeds grow to produce seedling plants. Depending on conditions, these take 5-10 years to reach 'maturity', which is defined as the stage when tapping can be started. In practice, this is when the trunk has about 500 mm circumference at 1 meter above ground level.

Figure 1 World production of natural rubber in 1999

Source: FAOSTAT.

When the rubber plantation industry began in Southeast Asia the main source for propagation was unselected seeds. It was soon found, however, that using selected seeds from higher yielding trees could lead to appreciably higher yields. This method attained commercial acceptability in the early 1920s. Among the many possible vegetative methods of propagation, only bud grafting has been adopted commercially.

At least once a year the leaves of the tree die and fall off in wintering, after which new leaves are formed. The sixteen-week wintering period greatly affects the tree's metabolism, latex constitution and yield, which together with other climatic factors, causes distinct seasonal variations in natural rubber production, with important repercussions for the primary rubberwood processing industry (IRRDB, 2000).

Tapping of rubber trees starts in the fifth to seventh year after planting and continues for 25 to 30 years. The classical method for tapping is the removal at each tapping of only a thin layer of bark from the cut end, thus permitting a smooth flow of latex and allowing the bark to regenerate. However, improper tapping in smallholdings often has negative consequences for wood recovery. After 30 years, a decline in latex production renders further tapping of the trees uneconomical, although smallholders may continue for many years. The trees are then removed and replaced with new seedlings.

The age at which rubber trees are actually replaced can vary considerably, depending on the health of the tree, prevailing rubber prices and access to replanting funds. Table 12 on page 21 provides information on harvesting ages in Thailand.

The rubber plant is infected by many diseases. Some are common to many countries while others are restricted to a region or only a few countries. A 1998 survey by the International Rubber Research and Development Board found the occurrence of 22 severe diseases (Hashim, 1998); the survey also noted the emergence of many new diseases since a similar study was carried out eight years earlier. The most significant are Colletotrichum anthracnose (reported in Indonesia), Colletotrichum leaf fall (China, India, Indonesia, Malaysia and Vietnam), Oidium leaf fall (China, India, Indonesia, Malaysia, Vietnam), Corynespora leaf fall (India, Indonesia, Malaysia, Sri Lanka), Black stripe (China, India, Indonesia, Malaysia, Thailand and Vietnam), Pink disease (India, Indonesia, Malaysia and Vietnam) and white root disease (Indonesia, Malaysia, Sri Lanka, Thailand). In certain countries, Corynespora leaf fall has been considered the most severe disease, causing substantial economic loss. The IRRDB has repeatedly warned producers of this disease and the risk of outbreaks remaining undetected for a long time.

2.3 Environmental considerations

Rubberwood discussions make frequent reference to the product's environmental sustainability, due primarily to the fact that it is procured as a by-product of a tree plantation crop. In view of the potential availability of rubberwood from existing plantations (see next section) and the increasing scarcity of tropical woods from natural forests, there is little doubt that rubberwood relieves pressure on remaining forest areas.

Agroforestry researchers are also paying increasing attention to the role of smallholder cultivation (sometimes called `jungle rubber agroforestry') as an alternative to certain types of unsustainable food crop-based shifting cultivation systems. Jungle rubber agroforestry is widely practiced in Indonesia (Sumatra and Kalimantan) and Southern Thailand; similar approaches are being introduced in Vietnam and are being considered for Myanmar.

When hevea arrived in Sumatra at the end of the 19^th century, farmers and shifting cultivators quickly adopted the cultivation of hevea in their fields. Realizing the value, as well as the ease and flexibility of its management, they transformed their fallows to a semi-permanent form of agroforestry. Following slash and burn of existing vegetation in secondary or primary forests or other land types, rubber trees are planted in between upland rice. Secondary forest species are allowed to regenerate with some selection pressure on type and intensity of species dominance. This practice has create complex agroforestry systems yielding diverse harvestable products, including timber, fruits, rattan, bamboo, vegetables and medicinal plants in addition to rubber. Rubber cultivation thus assisted the transition from shifting cultivation to a more permanent settled form of agriculture, which has been recognized as a `best bet' land use system for the humid tropics when local and global impacts are both considered (van Noordwijk et al., 1995; Tomich et al., 2000).

Aside from the role of rubberwood plantations vis-à-vis other land use forms, numerous studies have been carried out to evaluate the environmental impact of rubber plantations as such. While many of these may exaggerate in their favorable comparison of hevea ecosystems to primary forest, they all present convincing evidence of positive effects.

Research on the ecological impact of rubber plantations on soils degraded by shifting cultivation in Northeast India has demonstrated an improvement of soil properties after the establishment of Hevea. Rubber plantations adopting proper agroforestry management practices (including terracing; silt pitting and bunding; and the growth of leguminous cover plants between the rows to assist with nitrogen fixation) were found to help in the enrichment of organic matter, which consequently improved soil physical properties, such as bulk density, soil porosity, moisture retention and infiltration. An increase in organic matter was also observed. (Krishnakumar et al., 1990).

Similarly, a review of Malaysian research argues that of all the agroforestry cropping systems rubber plantations approximate closest to the rainforest system, in terms of canopy, leaf litter and in nutrient cycling (see Figure 2 and Joseph (1991) quoted in Goldthorpe, 1993). Fertilizer inputs are considered very low and soil surrounding rubber trees appears to be enriched by abundant leaf fall.

Figure 2 Nutrient removal and yield of different crops

Note: parentheses behind crop names indicate yield in kg/ha. Source: Sethuraj et al (1996).

According to some researchers, the most understated aspect of Hevea cultivation is that of its role as a carbon sink. Physiological studies have shown that Hevea is more effective than teak grown in plantation conditions in taking up carbon dioxide (Sethuraj et al., 1996). This is thought to be due to the extra energy required to produce the latex inside the tree: in contrast to a synthetic rubber plant which consumes energy and produces carbon dioxide to convert pure energy (crude oil) into elastomers, the natural rubber plant converts carbon dioxide into an elastomer. The leaf area created by a mature rubber tree is also sizeable: the leaf area index of a mature rubber plantation can be as high as six or seven. Because of the high photosynthetic rate and leaf area index, the biomass production per unit land area within a given time is very high in Hevea. With a planting density of 450 trees per hectare, the canopy closes in less than five years.

Environmental considerations in the context of rubberwood plantations have also attracted the attention of certification/labeling schemes. In 1994, a United Kingdom do-it-yourself retailer contracted SGS Silviconsult and Certification to undertake a Forest Audit of a Malaysian firm's Hevea plantations in Johor (southern tip of Peninsular Malaysia). The audit, the first of its kind on sustainable management of rubber plantations in Malaysia, was carried out using the principles and criteria for forest management of the European Forest Stewardship Council. While the auditors recommended certification, they also found areas in need of improvement, including the storage and use of herbicides; health, safety and environmental issues at sawmill sites; biodiversity conservation; and the use of pesticides and fertilizers (Chan et al., 1995).

In Indonesia, Forest Stewardship Council/SmartWood certification has been granted to a company sourcing naturally occurring Pulai wood (Alstonia spp.) for its pencil slat processing plant from rubberwood agroforestry systems in Musi Rawas District, South Sumatra (SmartWood, 2000).

Finally, strides toward certification are also made in Thailand. In 1998, the Ministry of Agriculture announced that it would apply ISO 14000 environmental standards to all farm products, starting with rubberwood, one of the country's top five exports. The Ministry expected at least 30 percent of all rubberwood would be certified by 2001 (Bangkok Post, 1998).

2.4 Rubberwood yield

Rubberwood yields per tree vary according to clone, site conditions and management. The global rubberwood study carried out by Indufor under the auspices of the International Trade Centre estimated yield at 140 to 200 m³/ha, with the higher ranges observed in countries where plantations are carefully managed, i.e. Malaysia, Thailand, India and Sri Lanka (Indufor, 1993).

A 1995 study on using Malaysian rubber plantations as timber resources used as the following as a basis for its calculations (Arshad et al., 1996):

Accordingly, estates and smallholdings can yield 190 and 180 m³ of greenwood per hectare, respectively. In the case of usable logs, estates recuperate about 57 m³ and smallholdings about 54 m³ per hectare. After sawing, the estates and smallholdings produce about 18.1 m³ and 10.8 m³ of sawnwood, respectively.

In another study, gross yield in 1994 for estates in Peninsular Malaysia was quoted at 180 m³/ha, which included branches greater than 5 cm diameter. In smallholdings, where trees are generally of poorer form, average yields were found as low as 100 m³/ha (Khoo et al. (1987) quoted in Ismariah & Norini, 1994). Net volumes suitable for sawnwood processing were 20% and 15% of total volumes for estates and smallholdings, respectively.

Research on the development of more productive varieties (clones) has been carried out in a only a few countries, where trials for identifying clones as suitable for large-scale introduction often last ten to fifteen years. In 1998, for instance, Malaysia launched latex-timber clones that can produce timber in a shorter period of time compared to other tropical species and can be densely planted. The clones RRIM 2023, 2024, 2025 and 2026 have been reported capable of producing 0.81 to 1.87 m³ of wood per tree, significantly higher than the 0.68 to 1.33 m³ of the earlier 2000 series clones (Malaysian Timber Bulletin, 1998). Some more detailed statistics on rubberwood yield in Malaysia are also presented in Table 1

In 1992, Ramli Othman reported that some of the Hevea spp. found in South America are potential rubber trees for timber production. The species are H. guianensis, H. nitida, H. pauciflora and H. benthamiana. These species are known to have high diameter and height growth with reasonably good stem straightness. Efforts were initiated to import the materials for trial and testing.

Table 1 Usable and available trunk volume/ha from nine Hevea cultivars before felling

* Measured from ground level to the first branch from the ground.

** Estimated by _r²I, where _ = 3.142, r = mean of tree radius (diameter - 2) at 0.6 and 1.8 m, I = clear bole height.

*** Estimated from mean trunk volume/tree x stand.

Figures in parenthesis are standard deviations. Source: S. K. Khoo, 1993.

2.5 Rubberwood properties

The natural color of rubberwood is one of the principal reasons for its popularity. The air-dry density is between 560-640 kg/m³ and it has good overall woodworking and machining qualities for sawing, boring, turning, nailing and gluing. It also takes finishes and stains well. Its strength and mechanical properties are comparable to traditional timbers used for furniture making and woodworking. However, there are more than 20 clones of rubber trees used in commercial plantations and some of the variations between clones are reflected in wood characteristics.

Rubberwood can substitute for several timber species that are essential for the primary and secondary industries in tropical and sub-tropical countries. They include the following species: ramin, meranti, sersaya, merbau, kapur, tangile and teak. A comparison with other species is shown in Table 2. In appearance, it can substitute for well-known African species (sapelli, iroko, and kosipo), South American species (imbuia) and heavily traded Asian species (ramin, meranti, mersawa, seraya, merbau, kapur, tangile, and teak). In Japan, rubberwood is increasingly used to replace more traditional timbers such as buna (Fagus spp) and nara (Quercus serrata).

Rubberwood is easy to saw and causes no significant blunting of the saw teeth. The presence of latex in rubberwood tends to clog the saw teeth, which can be reduced by using router bits with larger than standard clearance angles. Rubberwood slices or peels well when converted into veneer. Rubberwood can be turned without burn marks or tear outs on standard lathes and the wood is easy to drill or bore. There are two common methods used for the primary breakdown of rubberwood logs. One is to break the logs into two halves, each of which is then converted into sawnwood. The other method is to cut a slab from one side of the log, then turn it 90 degrees to cut the sawnwood.

Table 2 Strength properties of (air-dried) rubberwood and other species

MOR: Modulus of rupture; MOE: Modulus of Elasticity; CPaG: Compression parallel to grain; CPeG: Compression perpendicular to grain (stress at limit of proportionality); Side hardness: load to embed 0.0113 diameter steel sphere to half its diameter, N. Source: Lee et al. (1965) quoted in Hong, 1995

While rubberwood exhibits a number of physical weaknesses, many have been overcome through technological advances in processing. Where rubberwood makes a significant economic contribution, research institutions continue their search for feasible remedies. Below is a list of commonly known deficiencies and associated treatments:

_ Non-durability, i.e. the susceptibility to insects and fungi attacks. This is by far the most emphasized weakness of rubber trees, caused by the high starch content that attracts a range of insects and fungal diseases, especially blue stain. Logs must be milled within a few hours of felling. Sawnwood cannot be air dried but is kiln dried immediately after sawing, when it has a moisture content of about 60 percent. Pentachlorophenate (PCP) compounds and copper-chrome-arsenic (CCA) compositions are used to treat the wood, but many mills prefer to use boron-based preservatives to avoid restrictions on PCP and CCA in the USA, Japan and Europe.

_ Smaller sizes compared to other leading timber species. Commonly harvested commercial sizes of rubberwood rarely exceed 50 mm in thickness and 1800 mm in length (Kollert, 1994). This problem is usually avoided by laminating or finger-jointing techniques, as in manufacturing table-tops.

_ Seasoning physical defects, such as cupping, twisting, bowing and checking. Cutting the timber into short lengths of 0.3-1.2 meters and narrow widths reduces the effects of twist, while finger jointing or laminating is used to obtain larger pieces.

_ Clogging of saw teeth caused by remaining latex.

_ Low conversion rate. This is generally compensated by the relatively low cost of rubberwood logs.

_ Low productivity rate, particularly in regards to smallholders of rubber plantations. Where proximity allows, extension agents introduce high-yielding clones and assist in improving tapping methods.

Although it is considered a perfect plan to manage the rubber tree for both latex and rubberwood, such perfection rarely exists because under normal circumstances tapping rubber tree for latex affects plant growth significantly (Kollert, 1994). Hence, a focus on either one of them is considered to be prudent.