diversity & ecosystem function at multiple spatial and genetic scales in a key plant-microbe symbiosis
This project is funded under the NSF Dimensions of Biodiversity program and will be a major focus of my lab in the coming years. This goal of this research is to illuminate the linkages between taxonomic, genetic and functional diversity across large spatial scales. This project in particular represents a synthesis between my previous research themes (see below) and a major step towards building a comprehensive roots-to-biome picture of mycorrhizal symbiosis.
The project is a collaborative effort with UC Berkeley (Tom Bruns, John Taylor) and Rytas Vilgalys (Duke University). We are combining next-generation DNA sequencing, population genomics, transcriptomics, and functional enzyme assays with a sampling design covering multiple spatial and phylogenetic scales. This study will, (1) provide the first continental scale perspective on ectomycorrhizal taxonomic diversity, (2) identify genetic markers under positive selection in multiple ectomycorrhizal taxa across North America, (3) measure a broad spectrum of functional trait expression on individual mycorrhizal root tips and in soils, and (4) use a hierarchical genetic sampling design to measure variation in functional enzyme production across individuals, populations and species of ectomycorrhizal fungi. Our goal is to synthesize these parts to test one overarching hypothesis regarding the general nature of EMF communities, namely that they are taxonomically diverse but have high functional redundancy.
Macroecology of microorganisms
Dispersal is at the heart of a number of important ecological theories, such as neutral theory, island biogeography, metapopulation & metacommunity theory, and historical contingency theory. However, application of these theories to field systems is complicated by the logistical difficulties of studying dispersal at large scales. One of my major research interests is looking at the role of dispersal and other stochastic processes in community assembly. My thesis work used an island biogeography framework to study ectomycorrhizal “tree-islands” – patches of host trees embedded in a non-host matrix – as a way to look for evidence that immigration and extinction affect ectomycorrhizal assemblages. These tree-islands generally conformed to the expectations of island biogeography theory – larger tree islands housed more species of ectomycorrhizal fungi, and more isolated tree islands had fewer species of ectomycorrhizal fungi. This work led to one of the few published species-area relationships for fungi and provided evidence for a competition-colonization tradeoff in ectomycorrhizal communities. We are continuing this work with manipulative experiments to measure the scale of dispersal and colonization using next generation sequencing and quantitative PCR.
context dependence of plant-fungal interactions
Understanding how environmental conditions mediate the outcome of biotic interactions is a key question in community ecology. Many studies in this area have focused on how environmental context affects competitive interactions between members of the same guild. However, less work has been done on the effects of environmental context on other interactions, such as mutualism or parasitism. This question is particularly interesting because changing environmental conditions have the potential to move mutualistic interactions along the mutualism-parasitism continuum and may be a source of evolutionary instability. Mycorrhizal symbiosis is often depicted as a biological market, with the plants trading photosynthetic carbon for fungal derived nutrients and water. Under such a scenario, changes in resource availability (e.g. through anthropogenic nitrogen deposition) may decrease the value of fungal services, and in some agricultural settings there is evidence that high levels of fertilization can select for strains of mycorrhizal fungi that may actually become parasitic, decreasing plant growth.
Ectomycorrhizal symbiosis in lowland tropical forests
Global patterns of fungal biodiversity and their drivers are currently poorly understood. One reason for this is the relatively small amount of research that has been done in southern hemisphere and tropical forests. Ectomycorrhizal fungi are thought to be important primarily in temperate, nitrogen-limited ecosystems. While most neotropical trees associate with arbuscular mycorrhizal fungi, the dominant tree family in southeast Asia, the Dipterocarpaceae, is ectomycorrhizal. Dipterocarp seedlings do not grow successfully without ectomycorrhizal associations, however almost nothing is known about the ectomycorrhizal assemblages of mixed-dipterocarp forests. These forests are among the most diverse in the world, however they are under tremendous pressure from commercial logging and there is a clear need to understand the fungal dimension of diversity at risk and their potential importance in forest regeneration.