Orchid  Symbioses

Symbiotic associations are critical to life on Earth, from the incorporation of micro-organisms as organelles to adaptation to life on land. Making up 10 percent of the world's plant species, orchids are excellent models for studying symbiotic associations because of their own dependence on pollinators and diverse sets of fungi called mycorrhizal fungi. An orchid's relationship to these organisms can range from mutualistic to exploitative. We use both field and laboratory experiments, high throughput sequencing, stable isotopes, PCR probes and quantitative real-time PCR to investigate how orchid distribution and rarity are shaped by mycorrhizal associations.

In the earliest stages of their lives, orchids rely entirely on their mycorrhizal fungi for all nutrients, including carbon. This reliance continues to varying degrees later in life. In an NSF-funded project in collaboratoin with Dennis Whigham (SERC), D. Lee Taylor (University of Alaska) and Timothy Filley (Purdue University), we separated the effects of land use history, carbon source (wood or leaves), and availability of appropriate mycorrhizal fungi on the distributions of three orchid species. We found that both carbon source and land use history impacted the abundance of the necessary mycorrhizal fungi, which in turn impacted the germination of the orchids. This work was recently published in Molecular Ecology and was the subject of articles in Scientific American and Science.

In another orchid project with Dennis Whigham and Jay O'Neill, we combined long-term population measurements and mapping of the mycoheterotrophic orchid Corallorhiza odontorhiza with DNA sequence identification and measurements of the abundance of appropriate mycorrhizal fungi in the soil and on tree root tips. By doing so, we sought to separate the effects of climate and biotic factors on orchid distribution. We found that only plants associating with a particular mycorrhizal fungus were able to emerge and flower during drought years. In wetter years, plants utilizing a wide range of other fungi over a wider area were also able to flower and fruit. This work was published in Ecological Monographs. We continued research on this population in collaboration with Richard P. Shefferson (University of Georgia) to examine the effects of environmental variation on flowering and dormancy in this mycoheterotrophic plant. This work was published in Oikos.