Effect of quantity and quality of light on survival, growth and morphology of mahogany (swietenia macrophylla (king)) seedlings

Abstract

Of the five introduced tree species of the subfamily Swietenoideae grown in Sri Lanka, big leaf mahogany (Swietenia macrophylla King) has become the most important timber tree. It has been recommended to establish mahogany plantations under shade to overcome attack by the major pest Hypsipyla robusta Moore. However, mahogany being a pioneer and light demanding species, planting it under shade may reduce the survival and growth of seedlings. Therefore it is important to determine shade conditions that provide optimum growth during establishment. A study was carried out to monitor survival, growth and morphology of S. macrophylla during the initial growing period in shade houses with different light regimes. Shade houses providing three shade regimes were replicated thrice and planted with 18 seedlings. Photosynthetically active radiation (light quantity) and R:FR ratio (light quality) in each shade regime were measured. Overall seedling mortality was low and not significantly different between shade treatments. No significant differences in height were observed under the three shade regimes. Root collar diameter was 20% higher in the low shade and significantly (P=0.02) longer internodes were observed under low shade. Results indicate that with the increase in the quantity of light reaching the seedlings, there is an increase in plant height, root collar diameter and internodal length while the leaf area index is reduced. The quality of light reaching the seedlings in different shade houses remained similar.