Is fertilization essential for taking care of the next generation of tropical trees?

Abstract

Regrowing tropical forest is one of the main challenges of this decade. However, thischallenge goes beyond tree planting at small scales, ensuring that this practice can be scaled up, that the planted trees will grow, that native species will colonize, and that these plantations will become a functional forest is the main goal. Understanding plant-soil interactions in restoration settings can aid accelerate the regeneration process in degraded areas where active restoration initiatives are needed (tree planting or apply nucleation), and it can also help implement more costeffective restoration strategies. Some studies have found that P is the only nutrient limiting plant performance, while other nutrient manipulation studies and meta-analyses suggest that at the ecosystem scale there is colimitation by N, P, and other macronutrients. However, plant species from different functional groups respond in different ways to P availability. N-fixing species for example have higher responses to P addition, while non-N-fixing species have higher responses when the combination of NP is added. Even though, several nutrient manipulation experiments and meta-analysis have tested these hypotheses at the tree level, few studies have extensively evaluated how this costly practice influences seedling growth and survival across the tropics and how variable the cost-effectiveness of this practice is across species. Therefore, the main goal of this research was to determine how nutrient addition impacts the growth and survival of tree seedling species (i.e., Nitrogen (N)-fixing and non-N-fixing species) across the tropics. To do that, I first investigated how tropical dry forest N-fixing and non-N-fixing seedlings growing in shade house conditions in Costa Rica respond to P addition and how P acquisition strategies vary with P addition. I found that N-fixing seedlings had higher responses to P addition compared to non-Nfixers, and that P acquisition strategies were not down-regulated with P addition. Then, I tested this hypothesis at a larger scale where I established a 7-hectare tropical dry forest active restoration project in Southwestern Colombia. There I tested if P or the combination of multi-nutrients (N, P, and potassium) and water had an impact on seedling growth and survival. I also calculated the costeffectiveness of the different management practices implemented after two years of plant establishment. I found that the management strategies implemented did not have an effect on seedling growth and survival, instead the species identity had the highest influence over these two variables. Additionally, the most cost-effective management strategy was the unamended control. Finally, I conducted a meta-analysis to look at how nutrient addition impacts seedling growth and survival in shade-house and field studies across the tropics and explored the responses of N-fixing and non-N-fixing seedlings. This analysis showed that nutrient addition impacts the growth but not the survival of seedlings growing in both field and shade house studies. Additionally, seedlings grown in shade houses had higher responses to nutrient addition than seedlings growing in the field. v The magnitude of the increase in growth in shade houses double the one reported in field studies. Finally, non-N-fixers were more responsive to nutrient addition than N-fixer seedlings in shade houses and field studies. Collectively, these three studies help us to understand how nutrient addition impacts the survival and growth of N-fixing and non-N-fixing seedlings across the tropics and provide insight into when this practice should be implemented in restoration settings.