Hevea brasiliensis is one of the most economically important plantation crops in the world. It is rapidly expanding into both climatically optimal and sub-optimal environments in almost all rubber growing countries across the world.
Rubber tree belongs to the genus Hevea of family Euphorbiaceae. It is the major source of ‘natural rubber’, an agricultural produce used to manufacture thousands of products including the pneumatic tyres, which is the most common (Chandrasekhar et al., 2012). Other than medical, industrial and household gloves, industrial products like hoses, auto parts, industrial components, tubes, automotive and aviation tyres and general rubber products like rubber floorings, floor mats, carpets, sports goods, foot wears and hot water bottles some other products manufactured using natural rubber (Anon, 2015). H. brasiliensis is the major tree species commercially planted to obtain natural rubber. In general, it is a fast growing erect tree with straight trunk which grows over 40 m in height and live for more than 100 years. In commercial cultivation, they rarely exceed 25 m because they are budded plants, growth retardation due to tapping and subsequent felling after 25-35 years when yield fall to uneconomic levels (Chandrasekhar, 2012).
Rubber trees mature in six to eight years after the establishment (Chandrasekhar, 2012). Growth rate during the immature period is one of the key parameters for evaluating the performance of rubber clones (Sangsing et al., 2004). The trees reach for a suitable girth (about 50 cm girth at a height of 120 cm from the bud union) for tapping after about seven to eight years. During this period the girth of trunk is widely used as the parameter of growth. It is a variable of major interest to planters and is the main factor taken into consideration for evaluating tree growth and realization of maturity for crop harvesting (Chandrasekhar et al., 2005).
The rate of growth depends upon the nature of soil, climate and care which has been exercised in selecting budded plants and in planting operations. During the first few years the growth is mainly in length, and the rapid increase in girth is most noticeable after trees are a few years old. The potential growth of novel clones as measured by the girth of the plants at 120 cm above ground level.
In Sri Lanka, Rubber is mainly grown in the low country wet zone (LCWZ). Currently, the rubber extent in the country is around 133,668 hectares out of which 85,442 hectares (64%) belongs to the rubber small holder sector (Anon, 2015).
Due to the low yield in Sri Lankan rubber plantations and to the rising amounts of domestic consumption, Sri Lanka’s raw rubber exports have diminished from 120,900 metric tons in 1980 to a mere 16,300 metric tons in 2014 (Anon, 2017). The climatic changes evident in rubber growing areas of the country, i.e. extreme rainfall and temperatures is associated with changes in growth and economic performance of rubber (Withanage et al., 2005).
Sri Lanka could expand and replant rubber to increase the production to meet the demands of the local and global markets (Anon, 2017).
The Rubber Research Institute of Sri Lanka (RRISL) has introduced possible solutions to increase production by introducing good agricultural practices and recommending environmentally stable clones, which can yield in a wide range of climatic conditions. In addition to that identification of highly adaptable clones for particular environmental conditions and higher survival level to infections are other approaches in maximizing the returns from these rubber plantations (Withanage et al., 2005).
It is very difficult to identify high-yielding and vigorous clones with all secondary characters such as tolerance to all the foliar diseases, white latex, tolerance to wind damage etc. Most of the new clones are high-yielding and vigorous. But some clones, though having these two important traits, may be susceptible to some diseases and tolerant to others. Before choosing a clone for planting in a particular area should be consider some factors such as elevation, rainfall, susceptibility/tolerance of clones to diseases and wind damage. In Sri Lanka, clonal material for conventional planting is classified into three main classes, depending on the amount of experimental and commercial experience that has been developed on the clones.
Group I – each clone to be planted up to 10% of the extent
The performance of the clones in this group has been confirmed from commercial areas.
RRIC 100, RRIC 102, RRIC 121, RRIC 130′, PB 217, PB 28/59
Group II – Each clone to be planted up to 3% of the extent
This group consists of promising planting material where the performance has not yet been confirmed by commercial plantings.
RRIC 117, RRISL 201, PB2351, RRIC 131, RRISL 202, PB 2601, RRIC 133, RRISL 203, BPM 24
Group III – to be planted as Estate/RRI collaborative clone trials each clone to be planted up to 2 ha
This group includes experimental planting material identified for testing in large scale with the collaboration of the growers.
RRISL 200, RRISL 204, RRISL 208, RRISL 218, RRISL 219, RRISL 220 (Attanayaka, 2001)
The aim of this study is to analyze growth performance of novel Hevea clones planted in Low Country Wet (LCW) and Low Country Intermediate (LCI) Zones.