Soil and light effects on the sapling performance of the shade-tolerant species Brosimum alicastrum (Moraceae) in a Mexican tropical rain forest

Many studies conclude that light is the most important resource that determines plant performance of tree saplings in tropical rain forests, and implicitly suggest that soil resources are less important. To provide a quantitative test for soil versus light effects on sapling performance, we studied how saplings of the shade-tolerant tree species Brosimum alicastrum responded to contrasting levels of light availability and soil fertility in a Mexican tropical rain forest. Therefore saplings were selected from ten low-light exposure (crown position index < 1.5) and ten high-light exposure (crown position index >= 2.5) sites either on productive alluvial soils, or on poor sandy soils on adjacent hills. Annual growth responses were scored for 58 saplings. The soil-light interaction had a strong positive effect oil branching rate and leaf area production, rather than light or soil atone. Height growth only increased at higher light availability. Herbivore damage was higher on the more productive soil and, to a lesser extent. at higher light availability. Our results suggest that saplings (1) responded differently to soil and light availability. (2) expanded in overall size when both soil and light limitations are released, but only increased in height with increasing light intensity (3) and faced a trade-off between expansion and defence along both soil and light gradients. This study emphasizes the role of soil resources, in interaction with light availability, on the plastic responses in saplings living in a tropical forest understorey.