Timber production in high density planting of Rubber

Timber production in high density planting of Hevea brasiliensis (Rubber)

Kelum Silva, Upul Subasinghe and Lakshman Rodrigo

Paper presented for the 14th International Forestry and Environmental Symposium 2009

The demand of natural rubber has increased continuously with the increase of population and living standards of the human being. Rubber plantations are also a major source of timber and fuels wood. In order to meet the continuous increase in demand for latex, timber and fuelwood, 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. 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 Rathnapura District of Sri Lanka in 1992. Rubber was planted in three high densities, i.e., 600, 700 and 800 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 plot whilst clones were in the sub-plots. Five trees in each sub-plot were randomly selected and used for the measurements of total tree height (TH), crown height (CH), thickness of the untapped bark (BT) and tree diameter at breast height. Thereafter, stem volumes were determined using the Newton’s formula.

Both TH and CH did not vary significantly among the planting densities tested. Although 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 decrease was compensated by the increased number of trees in high densities resulting in comparable level of merchantable volume per hectare among different densities. Total stem volume per tree remained same among four densities tested with that total stem per hectare increased significantly with the increase of planting density. Therefore, higher densities are more useful in the industries of fuelwood, pulp, MDF boards etc. Among the clones tested, the clone RRIC 212outperformed in growth and timber production. The clone RRIC 110 was infected with the Corynespora 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.

Change of crown diameter with dbh of mahogany

Change of crown diameter with dbh of Swietenia macrophylla (mahogany)

even-aged monocultures

Shanika Lakmali and Upul Subasinghe

Paper presented for the 14th International Forestry and Environment Symposium 2009

Mahogany is one of the renowned timber species for its beautifully grained, hard and red-brown wood. It is an introduced timber species, dominated in Sri Lankan forest plantations with fast growth rates and higher timber value. Mahogany plantations are widely established in the intermediate zone and low country wet zone in Sri Lanka. Recently, private sector has started to establish of mahogany monocultures with shorter rotations to gain high income.

Due to the lack of research studies on mahogany, the objective of the present study was to establish an empirical model to predict crown diameter accurately using dbh. For this purpose, data were collected from 16 mahogany monocultures in Kalutara, Ratnapura and Matale districts. In order to represent the whole plantation, sixty trees were selected from good, moderate and poor areas from each plantation.

In order to build a reliable model, theoretical basic structures were developed assuming the crown diameter is a function of tree dbh. This basic structure was fitted to the data as linear, exponential, and logistic form separately for different growth types. In addition to the untransformed variables, transformations were also made whenever possible. Suitable candidate models were preliminary selected by R2 and residual distributions. After further analysis, it was proven that the best results were given by the logistic model structure for good, moderate and poor site types (R2 = 92.0%, 71.4%, 65.9% respectively). In order to eliminate the difficulty of using separate models for different growth types, the possibility of using a common model for all growth types were tested. For this reason, one way ANOVA was used for residuals of different growth types generated after fitting respective models. Results indicated that it was possible to use a common model and therefore the logistic form was re-fitted to pooled data.

The final model was “crown diameter = 0.645 + 2.682 / (1+exp (-0.356 (dbh –7.749)))” and it had a R2 of 60.9%.