Format: ORAL

Lina Aragn1 Eloisa Lasso2,3 Kenneth J. Feeley1

1 Biology Department, University of Miami, Miami, USA 2 Departamento de Ciencias Biológicas, Universidad de los Andes, Bogotá, Colombia 3 Smithsonian Tropical Research Institute, Balboa, Ancón, Panama.

Despite rising temperatures, treeline is stable (i.e., not shifting upslope) in many montane areas. Indeed, recent studies have shown that treeline has remained unchanged in Peru and other parts of the tropical Andes for at least the last 40-50 years even as many trees are shifting their distributions upslope within the forested areas below treeline. Treeline stability suggests that factors other than temperature must play essential roles in determining the upper elevational limit of tree growth and the dynamics of the forest-grassland ecotone in these systems. One factor that could influence treelines is the composition and abundance of symbiotic soil fungi, which are known to play important roles in determining tree species performance in different habitats and shaping forest ecosystems. To explore the possible role of soil fungi in determining treeline location and stability, we are studying the composition, diversity, and abundance of soil arbuscular and ectomycorrhizal fungi in the Eastern Andes Cordillera of Colombia from 3,000 to 3,500 masl. At each of our sites, we collected soil samples in each habitat: the High Andean Forest (~3,000 - 3,200 masl), treeline ecotone (~3,200 - 3,400 masl), and Pramo (3,400 masl). At each sampling site, we also measured the soils pH, mineral nitrogen, total nitrogen, available phosphorous, cationic exchange capacity, exchangeable acidity, potassium, calcium, magnesium, and oxidizable organic carbon. Microbial DNA was extracted to determine soil fungi composition, diversity, and abundance, and the ITS and SSU regions were sequenced for ectomycorrhizal and arbuscular mycorrhizal fungi, respectively. By April 2024, we will have used HMSC models to assess what are the environmental drivers of arbuscular and ectomycorrhizal fungi communities from the High Andean Forest to the Paramo. These data will provide valuable insight into the role of microbes in determining tropical montane tree species distributions and the dynamics of the Andean treeline.