Using phenological monitoring in situ and historical records to determine environmental triggers for emergence and anthesis in the rare orchid Platanthera praeclara Sheviak & Bowles

Abstract

Platanthera praeclara (western prairie fringed orchid (WPFO)) is a rare terrestrial perennial orchid found in wet to mesic prairies in the central Great Plains of the United States. Although much is known about this species, conservation efforts are hindered because the specific factors that contribute to its emergence and anthesis are unknown. We used long-term monitoring data from three sites in the central part of WPFO's range to determine which environmental variables contribute to the plant's emergence and anthesis. We then compared our anthesis model to 110 years of herbarium and iNaturalist.org records over WPFO's entire range. From our monitoring data, we found that accumulated growing degree days (GDD, 0 °C baseline and no maximum temperature) best explained plant emergence (199.05), while anthesis was best explained by GDD accumulation (1561.8), adequate precipitation, and the passage of the summer solstice. In our historical data set, anthesis occurred post-solstice in 95% of the records, which suggests that the strongest driver of anthesis in WPFO is day length. Understanding those conditions that contribute to WPFO phenology is critical for its conservation, and management, such as prescribed fire, which should be avoided during active growth periods. Although WPFO evolution occurred in a variable climate, it appears to have a fixed anthesis trigger, day length, which increases its vulnerability to climate change. WPFO is also sensitive to drought conditions, which are expected to increase in the future. Therefore, intervention may be necessary to ensure the species' long-term viability. Keywords: Tallgrass prairie, Climate change, Great plains north America, Museum collections, iNaturalist.org