Disturbance and tropical pioneer species: Patterns of association across life history stages

Pioneer tree species of tropical rain forests in the seedling and sapling size classes are generally associated with disturbances to the forest canopy. Pioneers have usually been lumped together within the same broad guild. Many studies have demonstrated differentiation of late successional shade-tolerant species within a forest but only a few studies have shown pioneers to be complex in differentiation. Most assume pioneers to be generalist and similar in growth and establishment. It is therefore not well understood, how sympatric pioneer species differ across life history stages in relation to disturbance characteristics such as canopy openness, type, intensity and historical condition of disturbance. Here we studied seedlings, saplings and larger trees of eight pioneer species common in and around the Sinharaja World Heritage Reserve in Sri Lanka to characterize species differences in relation to canopy openness, type and intensity of disturbance. We analyzed canopy hemispherical photographs and conditions of disturbance and assessed the probability of species' occurrence and the relative importance of disturbance characteristics by fitting progressively more complex multinomial logistical regression models. Canopy openness demonstrated the greatest association with seedling and sapling distribution. . Trema orientalis, was found in the highest light conditions, while all other species showed a decrease in the probability of occurrence with increasing canopy openness. For larger trees, the historical condition of disturbance was shown to be a more important factor than the light characteristics. Irrespective of size classes, . T. orientalis and . Macaranga peltata were restricted to high light environments and large disturbances whereas . Macaranga indica, . Dillenia triquetra, . Schumacheria castaneifolia, and . Wendlandia bicuspidata were found in relatively lower light environments and smaller disturbances. . Alstonia macrophylla and . Melastoma malabathricum were able to exist across wide ranging light conditions. In seedlings and saplings, disturbance type was more influential than both the historical condition of disturbance and the intensity of the disturbance, with some evidence that species do occur differentially in certain disturbance types. In summary this study provides evidence that pioneers of all life history stages differentiate in relation to amount of light and disturbance characteristics and that such differentiations are important mechanisms for the coexistence and diversity of early seral tropical tree species. Findings from our study suggests that pioneers used for reforestation and site amelioration need to be selected carefully to match degree of exposure, history and type of disturbance of the site that is to be reforested.