Seedling responses to nutrient addition in the tropics: lessons for restoration projects

Given the current need to speed up forest recovery through tree planting in the tropics, it is important to understand how nutrient addition affect seedling performance. Multiple studies conducted tropical forests have suggested that lowland species performance is limited by soil phosphorus (P), while species growing in high elevation are limited by nitrogen (N). Therefore, we hypothesized adding P to lowland seedlings will increase their performance, while seedlings growing in high-elevation ecosystems will have positive responses to N addition. To test these hypotheses, we conducted a pan-tropical data synthesis of seedling responses (i.e., growth and survival) to N, P, or NP additions using studies conducted in shade houses or field plantings.

We found data for 145 species growing across the tropics. The dataset was biased towards studies conducted in shade houses (72% of studies) located in lowland areas in the neotropics and Asia, and not all studies reported both growth and survival. Few studies were conducted in Africa and in high elevation forests. Seedling survival and growth responded to N (11 studies), P (15 studies), and NP (five studies). In studies conducted in lowlands, P addition resulted in increased biomass and height in the neotropics and Africa, and decreased it in Asia. The opposite trend was found when N was added to seedlings in these three regions. On the other hand, species from high elevation forests in the neotropics showed positive responses to N and mixed results to P and NP. These results suggest that seedlings in lowland forests can be either limited by N or P depending on the region they are native to. Seedlings growing in high elevation forests in the neotropics seem to be limited by N. Finally, our data set highlights that management decisions about fertilization in restoration projects need to be based on region-specific knowledge.