With increasing awareness of large scale tropical deforestation and the ecological importance of mycorrhizae, incorporating fungal-plant symbioses into tropical restoration strategies has attracted considerable attention in recent years. In nutrient-poor landscapes, mycorrhizal fungi can help trees alleviate soil nutrient constraints to growth, and in particular, ectomycorrhizal fungi (EMF) may be more advantageous for nutrient capture than arbuscular mycorrhizal fungi (AMF) due to EMF’s saprophytic capabilities and potential for increased nutrient uptake for their host trees. Establishing diverse mycorrhizal fungal communities is considered to be important to forest recovery, yet mycorrhizae may have complex effects on tree growth depending on the fungal composition of species present. Thus, the relative benefits that mycorrhizal fungi confer to host trees may be sensitive to changes in fungal type, fungal diversity and/or composition, and the soil nutrient environment. In this dissertation, I combined field and greenhouse-based experiments to examine fungal-tree-soil interactions and test the role of mycorrhizal fungi in restoring tropical trees in nutrient poor soils in southern Costa Rica. I tested: (1) the importance of fungal type (AMF vs. EMF) on tree growth, (2) the role of fungal type in nutrient acquisition (as reflected in tissue nutrient concentrations) under different soil nutrient availabilities, and (3) the effect of AMF diversity and fungal composition on tree growth in multiple reforested sites. My results showed variation in tree responses to fungal symbionts, depending on tree species, fungal type, and soil nutrient status. In the field, I found that differences in growth and tissue nutrient concentrations among four tree species were greater than fungal type differences (AMF vs. EMF) after two years of growth, highlighting the importance of tree species selection in reforestation. Soil nutrient availability via site variation (i.e., initial base cation availability) and nutrient treatments also were associated with differences in tree growth. After isolating fungal from tree species effects in the greenhouse, I did not find consistent effects of fungal type on plant growth and tissue nutrient content; AMF had greater tissue nutrient content compared to EMF- or non-inoculated plants in one tree species whereas another tree species’ growth responded more to EMF symbioses. In reforested agricultural lands, I found that increasing AMF diversity and spore numbers negatively correlated with reduced tree growth after 5 to 7 years, suggesting that more diverse and productive fungal communities could consume more tree carbon. In general, this dissertation provided greenhouse and field-based support for both positive and negative tree growth responses to mycorrhizal fungal community and demonstrated the importance of mycorrhizal fungi in reforestation efforts. This research also highlights that understanding specific fungal-tree symbioses, soil nutrient status, and site effects are crucial to making broader management recommendations for reforestation success.
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