Undergraduate Honors Theses

Thesis Defended

Spring 2011

Document Type


First Advisor



Tropical soils are generally nutrient poor, even though they support high biodiversity and productivity. Most tropical plants are able to thrive in these soils because they form a mutualistic relationship with Arbuscular Mycorrhizae (AM), the latter of which increases nutrient absorption, and therefore the fitness of the host plant (though some species are more closely associated with AM than others). As a now-abandoned cattle pasture is allowed to passively regenerate (to re-grow forest without human intervention), edaphic properties (characteristics of the soil) and AM spore abundance may change in response to alterations in the plant community and structural changes (like increased leaf litter deposition). The present study examines spore abundance, nitrogen (N), phosphorus (P), potassium (K), pH, and bulk soil density for plots of various stages of regeneration from pasture (ages 0-45, and primary forest, N=15) in the Premontane Wet Forest near San Luis, Costa Rica. The data show substantial variation within and between sites in spore number and other soil characteristics. Regeneration age did not significantly affect AM spore number, macronutrient levels (NPK), or bulk density. pH was positively correlated with increasing site age, though this trend was not significant following the removal of two outliers. Spore abundance was not significantly correlated with N, P, K, pH, or bulk density. These findings suggest that AM spores may not be evenly distributed throughout the soil, instead exhibiting patchy distributions. It is possible that edaphic and AM trends with regeneration exist, but are confounded by other variables; however, it may also be that AM are so widespread throughout San Luis that AM infectivity is unlikely to direct forest regeneration.