How is the functioning of saplings limited by soil nutrients in tropical rainforests?

Abstract. This study was conducted in an unmanaged tropical rainforest in French Guiana, in which either nitrogen (N), phosphorus (P) or a combination of N and P fertilisers were experimentally added to the soil. Here, we focused on the effects of fertilisation on photosynthetic capacity and growth of saplings in the understorey of the rainforest. We measured sapling photosynthesis and stem dimensions (height and diameter) after eight years of fertilisation, and compared these with pre-fertilisation data. In addition, we measured overstorey leaf area index, soil nutrients, foliar leaf nutrients, leaf morphology, total sapling leaf area and the leaf area/wood volume ratio. The results showed that an increase in available soil nutrients contributed to an increase in leaf area of tree canopies, reducing the light availability for saplings in the understorey, eliciting shade adaptations, such as an increase in specific leaf area and an increase in height increment. There was no fertilisation effect on the growth and photosynthesis of saplings that overrides the increase in light limitation. Nonetheless, the effect of fertiliser was significantly different among species, making it likely that fertilisation on the long-term will lead to a change in the composition of tropical rainforests.