Soil Pathogen-Mediated Coexistence of Tree Species

智力优点：Janzen（1970）和Connell（1971）认为，天敌在同种成虫附近和/或同种幼苗密度高时减少幼苗补充，同时允许异种补充发生，这是热带树木多样性得以维持的一种解释。许多研究记录了与Janzen-Connell（J-C）假设一致的死亡率模式，但没有确定具体原因。土壤真菌病原体可能是密度和距离依赖性死亡率的原因，因为宿主专化性强，世代时间短，孢子在土壤中持久存在，繁殖力高。此外，疾病在构建植物群落中的潜在重要性现在才得到更广泛的认识。本研究以土壤真菌病原体为主要研究对象，将检验J-C的基本假设，包括：1）在温带和热带系统中，寄主特异性的土传真菌病原体是否会引起幼苗的距离和密度依赖性死亡; 2）光照的有效性通过补偿生长和/或更快的木质化在介导病原体效应中的作用;（3）耐荫性不同的物种对病原菌的敏感性不同：（4）地方丰度不同的物种对病原菌的敏感性不同，病原菌可能抑制常见物种，促进物种多样性，或者稀有物种由于对病原菌更敏感而保持稀有; 5）热带和温带物种对病原体的敏感性不同。更广泛的影响：如果疾病在其宿主的种群动态以及它们栖息的群落的结构和多样性中发挥作用，那么拟议的研究与保护区的管理高度相关。此外，没有足够的知识，植物-土壤病原体的相互作用在“自然”系统中，它是更难以打击外来的，入侵的病原体的传播。这项研究促进了代表性不足的群体（一名女博士候选人和一名少数民族女实习生）参与科学，为本科生在哥斯达黎加实习和在密歇根州就业提供了机会，并为哥斯达黎加实地助理提供了培训。

Intellectual merit: As an explanation for the maintenance of tropical tree diversity, Janzen (1970) and Connell (1971) proposed that natural enemies reduce seedling recruitment in the vicinity of conspecific adults and/or at high conspecific seedling density while allowing for heterospecific recruitment to occur. Numerous studies have documented mortality patterns consistent with the Janzen-Connell (J-C) hypothesis, without identifying particular causes. Soil fungal pathogens are likely causal factors of density- and distance-dependent mortality because of strong host specialization, short generation times, long persistence in soil as spores, and high fecundity. Furthermore, the potential importance of disease in structuring plant communities is only now becoming more widely appreciated. The proposed experiments, with a focus on soil fungal pathogens as the mortality agent, will test fundamental assumptions of J-C, including: 1) if host-specific soil-borne fungal pathogens cause distance and density-dependent mortality in seedlings in both temperate and tropical systems; 2) the effects of light availability in mediating pathogen effects through compensation in growth and/or more rapid lignification; 3) differential pathogen sensitivity among species varying in shade tolerance; 4) differential pathogen sensitivity among species varying in local abundance, with the possibility that pathogens keep common species in check and promote diversity or alternatively that rare species arekept rare because they are more susceptible to pathogens; 5) differential pathogen sensitivity between tropical and temperate species.Broader impacts: The proposed research is highly relevant to managing conservation reserves if diseases play a role in the population dynamics of their hosts and the structure and diversity of communities that they inhabit. Also, without adequate knowledge of plant-soil pathogen interactions in 'natural' systems it is more difficult to combat the spread of exotic, invasive pathogens. The research enhances participation of underrepresented groups in science (a female doctoral candidate and a minority female student intern), provides opportunities for undergraduate internships in Costa Rica and employment in Michigan, and provides training to Costa Rican field assistants.