Construction of a complete yield table for Eucalyptus torelliana using empirical relationships

Abstract of a research study

conducted by Menaka Thirunadarajah and Upul Subasinghe

Yield table is one of the useful ways of predicting yield and growth of forests which shows expected volume and other important parameters per unit area by the combination of measurable characteristics of the forest stand. Although the yield and growth of trees can be predicted using more advanced modelling tools in forestry, the information contained in traditional yield tables is still considered valuable.

Eucalyptus torelliana is one of the eucalyptus species planted in Sri Lanka especially in the low country wet zone. Due to the unavailability of growth prediction methods for E. torelliana in Sri Lanka, a complete yield table and a two parameter volume table was constructed for this species using height and age relationships.

Five E. torelliana plantations from Matara and Rathnapura districts were used for this study. These plantations are 10 to 14 years old at the time of measurements taken and vary in extent from 1.14 to 20.00 ha. In order to represent the whole area of each plantation, stratified random sampling was used and 5 to 10 circular plots of 0.02 ha were used.

For the trees located in the plots, total height and dbh were initially measured. For the volume determination, each tree stem was divided into sections and sectional volumes were separately calculated using Newton's formula. The stem volume was determined by adding section volumes together.

Due to the unavailability of re-measured data, it was decided to to develop a relationship between height and tree age. In order to build that relationship, it was assumed that the tree produces one node per year. This concept was statistically tested later on in the present study and proven to be correct.

For the construction of the yield table in the present study, three relationships were mathematically constructed, i.e., height with age, dbh with height, volume with height and dbh. For the development of height age relationship, 10 trees were selected from each plantation and number of inter-nodes was counted using a high-quality binocular and the distance between consecutive nodes were accurately measured.

All three relationships were mathematically built using regression analysis and for each relationship over 15 models were tested. The best modes were selected based on R2 values and distribution of standard residuals. The selected best model for height-age relationship was non-linear. The selected linear models to predict dbh and volume had 75.9% and 73.9% R2 values respectively.

By using the finally selected models, a complete yield table and a two-parameter volume table was constructed. As it was found that there were no significant difference among the selected sites for this study, it was decided to construct one yield table for all site types with the planting density of 1,100 trees per ha.

Complete yield table for Eucalyptus torelliana

Construction of a complete yield table for Eucalyptus torelliana using the height-age relationship
by
Menaka Thirunadarajah and Upul Subasinghe

Yield table is a useful way of predicting yield and growth of forest plantations. It shows the expected volume and other important parameters per unit area according to a combination of measurable characteristics of the forest plantation and age. Although the yield and growth of trees can be predicted using advanced techniques in forestry, the information contained in traditional yield tables is still considered as valuable.

Eucalyptus torelliana is one of the eucalypt species planted in Sri Lanka especially in the low country wet zone. Due to unavailabiity of growth prediction methods for this species in Sri Lanka, a complete yield table and a two parameter volume table were constructed in this study using height-age relationship.

Data were collected from five plantations selected in Matara and Rathnapura districts. These even-aged plantations were 10 to 14 years old and varied in extent from 1.2 to 20.0 ha. In order to represent the whole area of each plantation, stratified random sampling was used and 5-10 circular plots (0.02 ha) were used for data collection. Total height and dbh were measured primarily from the trees located in the sample plots. For the volume estimations, each tree stem was divided into sections and for each section, log length, end, mid and top diameters were measured without felling the trees. Then the stem volume was calculated by adding the section volumes estimated using the Newton's formlula.

Due to the unavailability of re-measured data for the selected species, it was decided to develop a relationship between height and tree age. In order to build this relationship, it was assumed that a tree produced one node per year. That was statistically proven to be correct later in the same study.

For the construction of the yeild table, three relationships were mathematically constructed, i.e., height-age, dbh-height and height, dbh-volume. 10 trees were selected from each plantation and nodes were counted to build the height-age relationship. Moreover the distance between nodes were also measured.

By using the above constructed models, a complete yield table and a two parameter volume table (volume = f dbh, height) were constructed. As it was found that there were no significant differnces among the selected plantations in growth, the yield table was recommeded for the use of E. torelliana plantations growing in the low country wet zone of Sri Lanka.