Lumber recovery percentage of two indigenous tree species of Ethiopia : A case study at Ambo sawmill

The study was done on lumber recovery percentage of two indigenous tree species of Ethiopia, namely, Olea capensis subsp. Welwitschii and Prunus africanus at Ambo sawmill. the aim of the study was to compare the lumber recovery percentage of the two indigenous tree species and logs with different diameter classes. The study consists of 100 logs of Olea capensis subsp. wehwitschii and 100 logs of Prunus africanus. Sample stratification for each log was based on the diameter size classes of 20-24.9, 25-29.9, 30-34.9, and 35-39.9 cm. The diameter of each log under bark at bottom, middle, and top in cm and length in m were measured for volume calculation. Volume was calculated using Newton's formula. Each log was processed according to normal production rate and standard lumber dimension of the sawmill. Then lumber recovery percentage was calculated for each log. The lumber recovery percentage information is presented per diameter classes for both indigenous tree species. Volume recovery from the input log differed by diameter class. The average lumber recovery per diameter class ranges from 44 up to 57% for Olea capensis subsp. welwitschii logs and 38 up to 55% for Prunus africanus logs. For both species the lower diameter classes logs yielded lower volume recovery percentage compared to upper diameter classes logs. The main reasons attributed to the low lumber recovery were very irregular shape and heavy defects on smaller logs, saw kerf, and lack of quick assessment and position the log before sawing. More than 50% of lumber obtained from the 4m logs wasn't longer than 2.5m. The only exception was lumber obtained from the diameter class 35-39.9 cm. for Olea capensis subsp. welwitschii. The result of the study of lumber recovery percentage indicate that more lumber recovery achieved from Olea capensis subsp. welwitschii than Prunus africanus. The possible explanation to the differences is that the logs of Prunus africanus contain more heavy defects, very irregular in shape, the more slabs and off-cuts were made to avoid the defects than Olea capensis subsp. welwitschii. The main factors that affect lumber recovery percentage were, the tendency of the sawmill operator to cut more slabs and not to evaluate the log before it is placed on the carriage, the defects found on the log, and the very old and welded sawblade that was used for sawing logs. All slabs produced per diameter class were collected and measured by stacking. A rough estimation of solid volume of slab in m3 is presented per diameter class. The lumber recovery percentage at the sawmill could be improved by close supervision of loggers during harvesting operation, providing training to workers, assessment of the suitability of standing trees for lumber production, crosscutting logs into convenient length, storing logs properly, and appropriate maintenance and adequate supply of equipment.