Browsing by Subject "Tropical forests"

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    Biological nitrogen fixation in tropical dry forests of Costa Rica: patterns and controls
    (2014-06) Gei, Maria Gabriela
    In tropical forests, new nitrogen (N) inputs fuel a large proportion of global net primary productivity. However, global estimates of tropical N fixation are biased towards wet forests and other areas such as tropical dry forests are understudied. In the dry forests of Guanacaste, Costa Rica, N fixing legume trees are highly abundant throughout forest successional stages, thus I hypothesized that in tropical dry forests legume trees are critical regulators of ecosystem level N dynamics. I addressed this question from multiple approaches that included a shade house experiment and field surveys of N fixing legume trees in plantations or in diverse secondary forests using a common set of species: Acosmium panamense, Dalbergia retusa, Enterolobium cyclocarpum, Gliricidia sepium, and Lysiloma divaricatum. Individual legume species had measurable influences on a number of soil properties, but this effect is more pronounced than the influence of legumes as a functional group. I observed species-specific variation in belowground foraging strategies and in the timing and degree of nodulation. In the shade house experiment, species differed in their nodulation effort and in how they regulated N fixation with respect to available resources. These five legume species could be arrayed along a continuum defined by strategies of nutrient conservation and nutrient acquisition, which coincided with degrees of fine-tuning of N fixation. In the field study, I did not find evidence of down-regulation of fixation with soil N. I hypothesized that the adjustment of N fixation to soil nutrients occurs indirectly and is mediated by water availability and its effects on nutrient pulses. My stand-level estimates N fixation by legumes showed that legumes are responsible for the largest contribution of new N inputs to this ecosystem relative to other inputs such as free-living fixation or wet deposition, but which are modest relative to N recycling through leaf litter and fine root decomposition. Different legume strategies could represent different ways of dealing with the transient and seasonal water availability of this ecosystem. Collectively, my results suggest that the conceptual models of how N fixation works in tropical wet forests may not necessarily be the same in seasonally dry forests.
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    Is fertilization essential for taking care of the next generation of tropical trees?
    (2022-12) Toro-Gonzalez, Laura
    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.