DEMOGRAPHY OF THE RARE ORCHID, ISOTRIA MEDEOLOIDES (ORCHIDACEAE) AND ITS RESPONSE TO A CANOPY THINNING TREATMENT

Small whorled pogonia, Isotria medeoloides, is a rare terrestrial orchid of temperate forests in eastern North America. It is federally threatened, and 1992 recovery criteria in place for this species need to be evaluated in light of current data. Study of I. medeoloides demography is complicated by the potential for belowground dormancy lasting one to three or more years. From 1986 to 2000, we observed 404 individuals in monitoring plots at four sites in Maine, USA, with up to 241 additional plants followed less intensively outside those plots. We used a life cycle diagram to summarize stage distribution data regarding emergence, recruitment, fruiting, and dormancy. A frequency analysis of the stage dynamics of individuals revealed no consistent patterns. Within plots, 17.9% of individuals bore fruits. Of plants that flowered, mean fruit set overall was 55%. Plant abundance within plots declined over time, possibly due to increasing shade and weather-related effects. In 1993, we removed 33% of the tree basal area over subsections of the populations at two sites while also leaving a portion of both sites untreated. We used mixed-effect regressions to test for increased recruitment and rate of capsule production as a result of the canopy treatment. As of 1993, for 187 plants in the treated zones mean capsule production per flowering plant did not decline, while it did decline in the untreated zones. In a subset of 100 paired plants matched by year and site, 95.8% of those impacted by severe herbivory were dead within 4 years, compared to 58% of counterparts with no herbivory. We used these data to evaluate the recovery criteria and recommend a change from a focus on percent flowering to a minimum of at least two capsules produced per year on average over a ten-year period.