Ecological significance of crown functional traits across size classes and disturbance environments in eight pioneer species in a Sri Lankan rain forest

Information on tree crown characteristics has been used in forestry to assess tree and ecosystem health and productivity. Although there are well known differences in crown functional traits between light-demanding pioneer species and later successional species, data is limited usually to regeneration phases. Further, information on variation between species within the pioneer guild is needed in order to select pioneer species for facilitation of reforestation, more shade demanding species, and for monitoring and evaluation of such efforts. In this study we assessed three crown functional characteristics - crown surface area, crown volume, and live crown ratio - in eight pioneer species commonly found in the tropical rain forests of Sri Lanka: Alstonia macrophylla Wallich ex G. Don, Dillenia triquetra (Rottb.) Gilg, Macaranga indica Wight, Macaranga peltata (Roxb.) Muell, Melastoma malabathricum L., Schumacheria castaneifolia Vahl, Trema orientalis (L.) Blume, and Wendlandia bicuspidata Wight & Arn. We modeled the influence of species, tree size, and environmental factors on these crown functional traits in seedling, sapling, and larger tree size classes using generalized linear models. We found that the disturbance environment was only influential at the larger tree size class. For smaller size classes, the majority of the variation in crown characteristics was explained by tree size characteristics. In larger trees, among the three functional traits, live crown ratio showed the strongest relationship with the disturbance environment and crown surface area and crown volume had higher correlation with tree height and diameter. While crown characteristics were influenced by species irrespective of size class, in the larger trees species varied in their relationship between tree crown functional traits and the disturbance environment. In larger trees, four species - M. indica, D. triquetra, S. castaneifolia, and W. bicuspidata, which are more commonly found in small gaps - had stronger relationships between crown surface area, live crown ratio and canopy openness than T. orientalis and M. peltata, which are more commonly found in large canopy openings, and A. macrophylla and M. malabathricum, two generalist species. The small gap specialists may develop deeper crowns to overcome the short duration of direct light in small gaps and more efficiently utilize lateral light which is a larger fraction of total light in these environments compared to large canopy openings. The differences among species must be considered in selecting pioneers for reforestation efforts and designing silvicultural treatments. We recommend a step wise introduction of species starting with the more high light restricted and generalist pioneers followed by the small gap restricted species and finally the late successional species.