Timber production in high density planting of Hevea brasiliensis

The demand of natural rubber has increased continuously with the increase in population and living standards of the human being. Rubber plantations are also a major resource of timber and fuel wood. In order to meet the continuous increase in demand for latex, timber and fuel wood, the productivity of rubber plantations should be increased. Whilst producing high yielding clones for improved latex and timber yield per tree which is a long-term process in perennial crops, planting density could be adjusted to obtain high productivity in rubber plantations. The present level of planting density of rubber in Sri Lanka has been decided on the experiments conducted with the genotypes which are not in common use at the moment. Also, the optimum density should vary with different socio-economic conditions. Therefore, the present study was aimed to identify the suitable planting density for the recently developed and commonly used genotypes of rubber. This paper is focused to assess the timber production of rubber with respect to high density planting. The experiment was set up in Ratnapura district of Sri Lanka in 1992. Rubber was planted in three high densities, i.e., 600, 700 and 800 trees per hectare, with the presently recommended level of 500 trees per hectare. Also, three genotypes (clones) i.e., RRIC 100, RRIC 110 and RRIC 121 were incorporated with the statistical design of split plot where the planting densities were laid as the main plots whilst clones were in the sub plots. Five trees in each sub plot were selected randomly and were used for the measurements of total tree height (TH), crown height (CH), thickness of the untapped bark (BT) and tree diameter at breast height at 11 years after planting (11 YAP). Thereafter, stem volumes were determined using Newton’s formula. Both TH and CH did not vary significantly among planting densities tested. Though not statistically significant, there was a marginal decrease in tree diameter with the increase in planting density. Irrespective of the clone used, BT and mean merchantable timber volume per tree decreased significantly with increase in planting density. Nevertheless, this decline was compensated by increased number of trees in high densities resulting in comparable levels of merchantable volume per hectare among different densities. Total stem volume per tree remained same among four densities tested with that total stem volume per hectare increased significantly with the increase in planting density. Therefore, higher densities are more useful in the industries of fuel wood, pulp, MDF boards etc. Among the clones tested, the clone RRIC 121 outperformed in growth and timber production. The clone RRIC 110 was infected with Corynespora leaf disease hence showed poor performance in all densities. Despite the increase in total timber production with the increase in planting density, overall financial viability of different densities is to be assessed considering all cost components and valuing both timber and latex produced before making any firm recommendation