T U K Silva, V H L Rodrigo, S M C U P Subasinghe
Rubber Research Institute of Sri Lanka,
Department of Forestry and Environmental Science, University of Sri Jayewardenepura, Sri Lanka.
Despite short-term fluctuation of rubber prices, the demand of natural rubber has increased continuously with the increase in population and living standards of the human being. Nevertheless, urbanization results in the fast depletion of forests as well as the land area under rubber. Therefore, in order to meet the continuous demand on latex, the productivity of rubber plantations should be increased. While producing high yielding clones for improved 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. Specifically, this paper is focused to assess the overall productivity of rubber with respect to latex yield.
The experiment was set up in Rathnapura district of Sri Lanka in 1992. Rubber was planted in three high densities, i.e., 600, 700, 800 trees per hectare, with the presently recommended level of 500 trees per hectare. Also, 3 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. Growth and yield parameters in terms of girth, bark thickness (BT), and the incidences of tapping panel dryness (TPD) were assessed yearly and the latex volume (LV), % dry rubber content (DRC), number of trees in tapping (TIT) were assessed on daily basis up to 2004.
Irrespective of the clone used, mean yield per tree per tapping decreased with increase in planting density. However, it was vice versa in the case of yield per hectare (YPH) due to the increase in TIT with increase in planting density. The percentage of trees with TPD was not significantly affected by the planting density. The clone RRIC 110 was infected with Corynespora leaf disease hence poor performance was shown in all densities. The trend of increasing YPH was similar in both other clones, i.e. RRIC 100 and RRIC 121. However, the rate of increase in TIT showed a decline of 700 trees per hectare resulting in a lower YPH at this level than expected. Although latex productivity could generally be increased with increase in planting density, overall economic profitability of the system will also depend on cost of production, amount and value of timber and carbon produced. Therefore, study warrants further investigations on above issues before making any changes to the presently recommended planting density.