STUDY ON VERIATION OF MODULUS OF RUPTURE AND MODULUS OF ELASTICITY WITHIN Eucalyptus grandis TREES GROWING IN DIFFERENT SITE CLASSES.

Abstract

Only few studies have been done on the relationship of mechanical properties with growth rate ofEucalyptus grandis. Increase ingrowth rate and reduction in the harvesting age can be achieved bysilvicultural and management practices. However these management practices may affect woodquality by reducing technical performance and commercial acceptance of timber. One of the mainobjectives here was to determine whether wood mechanical properties such as, Modulus of rupture(MOR) and Modulus of elasticity (MOE) are primarily affected by growth rates. Another objectiveof this study was to collect MOR and MOE values in a systematic manner, which are essential incommercial utilization of this species.

In this study, diameter of breast height (dbh) and total height of the trees of 30 years-oldEucalyptus grandis at plantations in Nuwara-Eliya district were compared. Based on height andage, trees in the respective plantations were categorized into "Slow Growth Site" (SGS), "MediumGrowth Site" (MGS) and "Fast Growth Site" (FGS). Tree height difference between SGS (33.8meters) and FGS (45 meters) was significantly different at (PS 0.05) level but not between SGSand MGS (38.7). Five trees with straight and non-leaned boles were selected from each site forsampling. Five stem bolts (about 40 em length) were eut and removed at breast height, 20%, 40%,60% and 80% of the total tree height of each sample tree. Four sides of the stem bolts weremarked as north, south, east and west respectively. Two linear sections were cut across thediameter of every sample disc, from north to south and east to west respectively, avoidingcompression or distorted grain around the knots. 30cmx2cmx2cm size of samples was cut from thislinear section. 399 samples were tested by a Huns field universal testing machine and Load-Deflection diagrams were used to calculate the MOE and MOR values.

It was found that variation of modulus of rupture and modulus of elasticity increasedfrom pith outwards at different height levels in the three growth sites. Mean MaR valuesat Breast height, 20%, 40%, 60% and 80% height levels in SGS were 51.48 ,42.48,48.08, 54.77, 57.46 (Nmm'") respectively. MaR values for these levels in MGS were5~.29, 53.01, 59.06, 60.78, 65.19 and MaR values were 62.7,59.24,59.03,58.96,56.67for FGS respectively. Mean site MaR values increased with growth rate (slow: 50.85,medium: 59.27, fast: 59.32). These values were significantly different between the sitesexcept at medium and fast growth sites. Mean MaE values at Breast height, 20%, 40%,60% and 80% height levels in SGS were 7200 ,7040, 7680, 8696, 7865 (Nmrn ")respectively. MOE values for these levels in MGS were 8407, 8760, 8826, 9208, 8886and MOE values for same height levels were 8437, 8614, 8954, 9649, 9330 for FGSrespectively.

Mean site MOE values increased with gro •.•1•h rate (slow:7696, medium:8817, fast:8997) and these values weresignificantly different between sites except between medium and fast growth sites at PS 0.05 level.

it can be concluded that fast growth trees produced wood with higher mechanical properties (MOR and MOE)compared with slow growth trees, Therefore silvicultural practices aiming to accelerate growth rate of Eucalyptusgrandis do not affect MOR and MOE values negatively.