The ecophysiology of pioneer tree species in relation to their disturbance ecology in a wet lowland rainforest, Sri Lanka

This study conducted in a lowland rainforest in Sri Lanka showed that co-occurring pioneer species differentiated in relation to light availability and disturbance characteristics. Observations, experimental plantings in the field and controlled shade house experiments demonstrated progressively more refined differences among the pioneer species in their response to the disturbance condition broadly and to their canopy openness condition more specifically.;The observational field study showed that seedlings and saplings of Trema orientalis and Macaranga peltata had stronger associations with higher canopy openness conditions than the four species--- Macaranga indica, Dillenia triquetra, Wendlandia bicuspidata and Schumacheria castaneifolia, which were restricted to smaller gaps. Alstonia macrophylla and Melastoma malabathricum were common across a wide range of disturbance environments. The larger trees of small gap restricted species had more plastic crown dimensions that were closely correlated with canopy openness conditions.;A field planting experiment across replicated gaps showed differential seedling survival among species. D. triquetra had the greatest probability of survival followed by S. castaneifolia, W. bicuspidata, M. indica, M. peltata and T. orientalis in all gap-understory positions. Survival probability was greatest at the gap edge and least in the understory. W. bicuspidata exhibited a slightly positive effect from herbivory due to extensive re-sprouting following damage while height growth rate was strongly negatively affected by herbivory for all species.;The experimental shade treatment study demonstrated that the adaptability to changes in light environment can be species-specific and can depend on the severity and direction of the change. T. orientalis showed the greatest relative growth rates and photosynthetic rates in the high, medium and low light conditions. This is the first study to demonstrate that pioneers can differ amongst each other as dramatically as later-successional species in their shade tolerance and hence should not necessarily be grouped together as shade-intolerant generalists. This has profound implications for developing more sophisticated models for forest restoration and reforestation by establishing these species using edge in gap environments, or their equivalent environmental analogs, because only several pioneers demonstrated a capacity to grow in the full open condition of large gaps.