Life stages, demographic rates, and leaf damage for the round-leaved orchids, Platanthera orbiculata (Pursh.) Lindley and P. macrophylla (Goldie) PM Brown in a northern hardwood forest in New Hampshire, USA

Terrestrial orchids are especially susceptible to population declines, and hence are the focus of plant conservation efforts worldwide, but the life history and demography of heterotrophic orchids is not well understood. Our objective was to quantify life stage transition rates for a species pair of rare round-leaved orchids (Platanthera spp.) in a northern hardwood forest. Marked individuals were measured over 5 years, and metrics included: leaf area, damage by herbivores and pathogens, flowering, and seed production. Germination trials were also conducted. Vital rates were very similar for the two species, which were about equally abundant despite large differences in seed production. Mortality was twice as high for juvenile as for adult stages, and juvenile abundance was markedly lower than for adults. Both species are vulnerable to leaf damage by invertebrates and fungi and exhibit clear leaf area thresholds for progression across life stages, both suggesting a delicate carbon balance for these mixotrophic orchids. The likely role of mycosymbiont distribution in facilitating seedling establishment deserves further detailed study to inform conservation efforts as these and other orchid species face an uncertain future owing to rapid environmental change and increasing pressure from white-tailed deer herbivory in the northern part of their range.