A Mycoheterotrophic Orchid, Tomentelloid Fungi, and Drought in an Arizona Sky Island

Abstract

A large population of the fully mycoheterotrophic orchid, Corallorhiza striata var. vreelandii, was monitored for nine years, 2009—2017, in the Santa Catalina Mountains of southeastern Arizona. High elevation slopes were chosen for an unusually high density of plants. Orchid stems were counted annually, and the number of flowering stems steadily decreased by 78% during the first seven years (2009–2015) in drought conditions. Following a partial return to average rainfall on the seventh through ninth years, the number of stems dramatically rebounded on the eighth and ninth years. Overall the total number of flowering stems decreased by 35% during the study. Precipitation from the previous winter and summer strongly correlated with the number of flowering stems observed. Years of extreme drought, with less than half of annual averages, resulted in a decline of flowering stems for two consecutive years, even when the following year had average rainfall. Two years of near average rainfall resulted in an increase on the second year. Orchid numbers were observed to fluctuate as its endophyte was dynamically affected by changes in annual precipitation. This study highlights the need for research on the impact of drought to ectomycorrhizal fungi and affiliated plant species.