Seedling growth of co-occurring Shorea species in the simulated light environments of a rain forest

Four tree species of Shorea section Doona (Shorea megistophylla, Shorea trapezifolia, Shorea disticha, Shorea worthingtonii) were investigated for differences in seedling growth among related species. These species occur together as canopy emergents in rain forest of southwest Sri Lanka. Seedlings of each species were grown in light treatments that simulated a range of light environments found in the groundstorey of the rain forest. Seedlings of S. megistophylla grew most in the full-sun treatment. The other species grew tallest in treatments that provided partial shade or lower amounts of direct sunlight. For two of these species, S. disticha and S. worthingtonii, greatest proportions of seedling dry mass allocated to roots were in either dark-understorey shade or long durations of direct sunlight. Seedlings of S. megistophylla allocated the greatest proportion of dry mass to roots in the full-sun treatment. In the dark-understorey shade seedlings of S. trapezifolia had the least proportion of dry mass allocated to roots compared with the other species. This might predispose S. trapezifolia to more stress during drought periods. Under treatments providing high amounts of light S. megistophylla increased leaf area by increasing the size of its leaves. For S. worthingtonii and S. trapezifolia, leaf area increased by the production of more leaves rather than by an increase in area of individual leaves. Leaf area increase by number was less marked for S. disticha. Findings suggest an explanation for the co-existence of these Shorea species in this rain forest. High light environments like that in the centre of a large canopy disturbance promotes growth of S. megistophylla seedlings. Smaller canopy openings that expose seedlings to some direct sunlight favour growth of S. trapezifolia and S. disticha. Light regimes of the forest understorey and gap edge favour seedling growth of the more shade-tolerant S. worthingtonii.