The response of tropical tree seedlings to nutrient supply: meta-analysis for understanding a changing tropical landscape

Current literature on the response of tropical tree seedlings to fertilization was analysed. Fertilization effects on biomass accumulation, relative growth rate and tissue nitrogen (N) and phosphorus (P) were examined for seedlings grown under relatively high light. Higher light conditions in gaps are expected as human disturbance expands in tropical landscapes. Two questions were asked: (1) Does nutrient addition enhance growth or result in nutrient accumulation in the tissues of tropical tree seedlings grown in pots under high light? (2) Are these responses conditional on their adaptations to light availability, seed mass or growth form? Most of the species (73% of light-demanding and 60% of shade-tolerant) demonstrated a positive growth response to fertilization. The magnitude of the response of light-demanding species was more than twice that of shade-tolerant species. A higher proportion of shade-tolerant species accumulated nutrients in their tissues (up to 95% for P), but over half the light-demanding species did so as well. For both types of species, a higher percentage accumulated P than N. Responses in growth and in P accumulation were negatively correlated with seed mass. Shrubs responded more than trees in biomass accumulation, but there was a trend toward greater response by trees in relative growth rate and N and P accumulation. The results suggest that variation in nutrient availability could affect the structure of tropical forests regenerating from large-scale disturbance. In fertile sites, competitive exclusion may occur within the light-demanding species, resulting in a decline in local tree diversity. Depending on the effects of human disturbance on nutrient availability, loss of tree diversity at a local scale may compound diversity loss due to habitat destruction at the landscape scale.