Conservation of Isotria medeoloides, a tale of dormancy and fungi

Background/Question/Methods

Orchids are widely threatened or endangered worldwide and Isotria medeoloides is considered one of the most endangered orchids in the United States. Challenges to conservation of this orchid are many. Early in life, orchids rely on mycorrhizal fungi for all nutrition. Many orchids supplement their nutrition by digesting mycorrhizal fungi throughout their lives, but growth of most mature orchids is also affected by light. However, the life cycles of many orchids, including I. medeoloides, also include periods of vegetative dormancy. High rates of vegetative dormancy are associated with declining populations in many orchids, yet little is known about what factors cause orchids to enter or emerge from dormancy.  During dormancy, orchids do not produce above-ground structures, though they are not physiologically dormant. Dormant orchids are expected to rely completely on fungi, though this has not been demonstrated. Because orchids are affected by different factors during different life stages, we hypothesized that the extent to which different life stages contribute to population decline in threatened orchids can be used to identify the factors that are driving decline. We investigated population dynamics and mycorrhizal fungi in 14 populations of I. medeoloides in the mid-Atlantic area of the US.

Results/Conclusions

We found that lack of seed germination and high rates of plants going into dormancy were the primary drivers of population decline in mid-Atlantic populations of I. medeoloides. Eighty-six percent of plants that entered dormancy remained dormant the following year. Ninety-nine percent of plants that produced an overwintering bud emerged the following year, suggesting that dormancy persistence was characterized by the failure to initiate a bud, rather than by bud damage. This suggested that nutrition during dormancy was insufficient to support bud development as only 14% of the plants that went into dormancy have re-emerged during our studies. While availability of light also contributed to mature plant persistence, dormancy and seed germination were expected by be strongly dependent on mycorrhizal fungi. We have identified I. medeoloides mycorrhizal fungi as members of Russula and Lactarius in the Russulaceae, a family of obligately ectomycorrhizal fungi. This implies that management to conserve I. medeoloides may need to promote trees that host the mycorrhizal fungi needed by the orchids. We are working to identify which trees host these fungi and what factors drive their abundance.