DISSERTATION RESEARCH: Effects of nitrogen addition on ectomycorrhizal communities in tropical montane forest

热带森林以其高度的生物多样性而闻名，成百上千的树种生活在一个地点。然而，在一些热带森林中，单一树种的数量可能超过所有其他树种的总和。最近试图解释这种单一物种优势的尝试主要集中在土壤微生物在决定哪些树种常见，哪些树种稀有方面所起的作用。人们一直关注树木和根部真菌之间的有益联系，这种联系被称为菌根。关于为什么一些热带树木如此常见的一个主要观点是，它们独特地能够与特定类型的菌根真菌形成联系，这些真菌提供了不太常见的树种所不能获得的氮。在巴拿马西部的一片森林中进行的实验工作与这一假说是一致的，这就引出了这样一个问题：大气中氮沉积速率的增加可能会如何破坏这种植物-菌根联合，从而破坏树木群落的组成。这种新的和相对丰富的氮形式会让菌根真菌变得不重要吗？氮素对优势树种的植物-菌根联合的影响将通过比较过去八年施氮肥和不施氮肥处理林地中的菌根真菌群落来解决。该项目还将为研究生提供真菌遗传学和生态学方面的培训，并为本科生提供研究指导。与主要树冠树Oreomunnea miciana相关的外生菌根真菌群落将在巴拿马福图纳的下山森林的八个林地中进行描述。自2008年以来，其中四个地块每年接受四次氮肥添加处理。在每一块地中，根样本将从Oreomunnea树和外生菌根群落中收集，并使用Illumina测序进行表征。据预测，氮素的添加将降低外生菌根类群的多样性和侵染频率，并导致外生菌根群落的组成和系统发育结构的变化，有利于已知含有亚硝化类群的谱系。

Tropical forests are renowned for their high biodiversity, with hundreds or even thousands of tree species living together at a single site. In some tropical forests, however, a single tree species can outnumber all other species combined. Recent attempts to explain this one-species dominance have focused on the role of soil microbes in determining which tree species are common and which are rare. Attention has been on beneficial associations between trees and fungi in their roots, called mycorrhizas. A leading idea about why some tropical trees are so common is that they are uniquely capable of forming associations with particular types of mycorrhizal fungi that provide access to nitrogen that is unavailable to less common tree species. Experimental work from a forest in western Panama is consistent with this hypothesis, leading to the question of how this plant-mycorrhizal association, and therefore tree community composition, may be disrupted by increasing rates of nitrogen deposition from the atmosphere. Will this new and relatively plentiful form of nitrogen make mycorrhizal fungi unimportant? Effects of nitrogen on the plant-mycorrhizal association of a dominant tree will be addressed by comparing mycorrhizal fungal communities in forest plots with and without a treatment of nitrogen fertilization over the last eight years. This project will also provide training in fungal genetics and ecology to a graduate student, and research mentoring for undergraduate students. The ectomycorrhizal fungal community associated with a dominant canopy tree, Oreomunnea mexicana, will be characterized in eight forest plots located in lower montane forest at Fortuna, Panama. Four of the plots have received a nitrogen addition treatment four times per year since 2008. In each plot, root samples will be collected from Oreomunnea trees and ectomycorrhizal communities characterized using Illumina sequencing. It is predicted that nitrogen addition will reduce the diversity and infection frequency of ectomycorrhizal taxa and result in a shift in the composition and phylogenetic structure of ectomycorrhizal communities, favoring lineages known to contain nitrophilic taxa.