Identifying Drivers of Orchid Population Dynamics: An Example in the Federally-Listed Threatened Orchid, Isotria medeoloides

Melissa K. McCormick, Dennis F. Whigham, Rachel Rock-Blake, Hope E.A. Brooks

North American Orchid Conservation Center (NAOCC) and Smithsonian Environmental Research Center (SERC)

Orchids are widely threatened and endangered worldwide, but efforts to conserve and restore them has been limited by not knowing about the pollinators and fungi they need to grow and reproduce. The absence of appropriate mycorrhizal fungi can limit where orchids grow, but little is known about how the abundance and diversity of appropriate mycorrhizal fungi can affect orchid growth and population dynamics. Light availability is also expected to affect population dynamics, but with orchid life stages occurring predominantly above- or below-ground it seems reasonable to hypothesize that different life stages would be driven by above- or below-ground factors. In particular, emergent, green, above-ground stages would be most impacted by light, while seed, protocorm, and dormant stages would be most affected by mycorrhizal fungi. We hypothesized that the distribution, abundance, and emergence of the globally rare temperate, terrestrial orchid, Isotria medeoloides, would be driven at least partly by their mycorrhizal fungi. We combined the use of specific PCR primers, quantitative real-time PCR, and spatially nested soil samples to measure the distribution and abundance of mycorrhizal fungi that associate with I. medeoloides and measurement of light availability and orchid growth in three distinct studies. We found that I. medeoloides distribution and emergence were affected by the distribution and abundance of their mycorrhizal fungi in the soil. In contrast, plant growth during the growing season and the likelihood of flowering the subsequent year were more affected by light availability. We conclude that orchid conservation and studies of the drivers of orchid population dynamics need to consider both the mycorrhizal fungi and light resources they require.

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