Browsing by Subject "Seedlings"

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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.
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    Surveying genetic and phenotypic variation for response to density stress in maize wheat and barley.,
    (2010-04) St. Pierre, Summer Lea
    Increasing plant population density can increase biomass and grain yield on a per area unit basis, however this relationship is curvilinear where eventually a high plant density will overwhelm the tolerance to density stress and yield per unit area will decrease. The main objective of this thesis is to gain an understanding of the genetic and morphological responses to density stress. In the first part of this thesis, we examined differential gene expression in seedlings of four barley genotypes and five maize genotypes grown at low and high densities. A microarray analysis approach was employed and identified 219 and 35 transcripts differentially expressed in barley and maize, respectively, with little gene expression patterns overlap among genotypes, indicating that these genotypes respond in very different ways and may have different mechanisms to deal with density stress. In the second part of this thesis, we examined multiple genotypes of maize, wheat, and barley planted at three different densities with the highest density being up to three times the normal density. A combined genotype analysis for each species showed that the highest plant density had the greatest biomass yield m-2 for wheat and maize while barley had no significant difference among the three densities for biomass yield m-2. For grain yield m-2, barley had the lowest yield at the highest plant density, wheat had no difference among the densities and maize had the greatest grain yield m-2 in the high plant density.