Nutrient addition to a white sand tropical forest results in greater growth of saplings of canopy tree species than understorey specialist tree species

In tropical rain forest understories, canopy tree saplings coexist with understorey specialist trees, in a highly competitive environment for light, space, and soil nutrients. To investigate the impact of increasing nutrient pollution from human activity, we assessed differences between canopy trees saplings and understory specialists in their crown structures, allometries, and growth rates in response to experimental nutrient fertilization. In a nutrient-poor white sand forest in Sabah, Malaysian Borneo we measured tree size and crown dimensions for understorey specialists and canopy tree saplings (≤ 10 cm diameter) before and after experimental fertilization with nitrogen and calcium carbonate. Results showed that saplings of canopy species were taller than understorey specialists for a given stem diameter, but crown allometries were similar. Three years of fertilization did not affect allometric relationships between diameter and tree height or crown dimensions, suggesting that these allometries are stable and resource-independent. However, relative crown radius growth rate and crown volume growth rate for canopy tree saplings were three times faster after N + CaCO3 fertilization when compared to controls, demonstrating canopy tree saplings’ crown plasticity to maximize access to light. This study highlights different strategies employed by canopy and understorey species in the competitive forest understorey. Canopy saplings showed greater crown plasticity, investing added soil nutrients into crown expansion for enhanced light interception. Nitrogen deposition could thus advantage canopy species through increased competition with understory specialists resulting in potential species loss. The stability of the allometric relationships, when forests are subjected to nitrogen deposition supports their use for biomass estimation.