Soil Pathogen-Mediated Coexistence of Tree Species

智力优势：作为对热带树木多样性维持的解释，Janzen（1970）和Connell（1971）提出，天敌减少了同种成虫附近和/或同种幼苗密度高时的幼苗补充，同时允许异种补充发生。大量的研究记录了与Janzen-Connell （J-C）假说一致的死亡模式，但没有确定具体的原因。土壤真菌病原体可能是密度和距离依赖性死亡的原因，因为它具有很强的寄主专门化、较短的繁殖时间、作为孢子在土壤中长期存在和高繁殖力。此外，疾病在构建植物群落中的潜在重要性直到现在才得到更广泛的认识。拟开展的实验将重点关注土壤真菌病原体作为致死率，将测试J-C的基本假设，包括：1)在温带和热带系统中，宿主特异性土壤传播的真菌病原体是否会导致幼苗的距离和密度依赖性死亡；2)通过补偿生长和/或更快的木质化，光有效性在介导病原体效应中的作用；3)不同物种对病原菌的敏感性差异；4)不同物种对病原体的敏感性存在差异，病原体可能抑制常见物种并促进多样性，或者稀有物种因对病原体更敏感而保持稀有；5)热带和温带物种对病原菌的敏感性差异。更广泛的影响：如果疾病在其宿主的种群动态以及它们所居住的群落的结构和多样性中发挥作用，那么拟议的研究与管理保护区高度相关。此外，如果不充分了解“自然”系统中植物与土壤病原体的相互作用，就更难以对抗外来侵入性病原体的传播。这项研究加强了代表性不足的群体在科学领域的参与（一名女博士候选人和一名少数族裔女实习生），为哥斯达黎加的本科生实习和密歇根州的就业提供了机会，并为哥斯达黎加的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.