Seedling growth responses to water and nutrient augmentation in the understorey of a lowland moist forest, Panama

We irrigated and fertilized (with nutrients) seedlings of Doliocarpus olivaceus (Dilleniaceae, a shade-tolerant liana), Faramea occidentalis (Rubiaceae, a shade-tolerant understorey tree) and Tetragastris panamensis (Burseraceae, a shade-tolerant canopy tree) growing in the understorey of an old-growth tropical moist forest in Panama to assess the impact of seasonal water availability and nutrient-poor soils on seedling growth rates. In control plots, height growth rates were greater in the dry season than in the wet season for Doliocarpus (21%) and for Faramea (89%), whereas Tetragastris had similar seasonal rates. For numbers of leaves, Faramea had 3.5-fold greater relative growth rates in the dry season than in the wet season, while Doliocarpus and Tetragastris lost leaves (semi-deciduous) during the same period. Irrigation and nutrient augmentation increased height growth rates for all three species (45% to 272%,). For Doliocarpus, irrigation and nutrient augmentation prevented leaf fall during the dry season. For Faramea in the dry season, irrigation and nutrient augmentation when applied independently reduced the growth of new leaves by 65%, to 87%, but relative growth rates for number of leaves were the same as the control rates in the combined irrigation and nutrient augmentation treatment. The growth of new leaves for Tetragastris responded to dry-season irrigation but not nutrient augmentation. Although all measurements occurred beneath the forest canopy, during the dry season, Tetragastris had a negative relationship between canopy openness and relative growth rate for number of leaves, whereas the other two species had a positive relationship. Our results show that soil resources influence growth rates even in the deep shade of the forest understorey, and demonstrate different responses to soil resources among species that might contribute to niche differentiation and species coexistence.