Differences in reproductive performance traits of two Platanthera species

The species Platanthera bifolia and P. chlorantha (Orchidaceae) are both declining in Europe, but P. bifolia has been diminishing relatively more. Loss of habitats and landscape heterogeneity due to agricultural intensification or abandonment of less fertile areas are primary drivers of decreasing orchid populations. We measured reproductive traits (fruit and flower count per plant, fruit weight, seed number and weight, the proportion of seeds with embryos and germination success) to compare the reproductive success of the two species in different habitats across Estonia. Platanthera chlorantha tends to reach its full fruiting capacity in smaller size classes, despite a relatively low base level. Germination success was low for both species but twice as high for P. chlorantha than its congener. We propose that higher proportion of embryos in fruits of P. chlorantha in shade and woodland growing conditions is related to higher availability of diverse pollinators. Platanthera chlorantha , albeit growing on its northern distribution limit in Estonia, seems to be tolerating the abandonment of its habitats more effectively than P. bifolia , at least with respect to reproductive performance traits.