The impact of recent colonization on the genetic diversity and fine-scale genetic structure in Orchis militaris (L.)

Many terrestrial orchid species have undergone a serious decline in their ranges in Estonia and in other countries, mainly due to land-use changes. Some species have found a suitable habitat in anthropogenically disturbed areas, such as abandoned quarries and gravel pits. Using AFLP, we studied genetic variation and its structuring on a fine scale in three recently established and three old populations of Orchis militaris (L.) in Estonia. Our aims were to determine whether the colonization process resulted in a decline of genetic variation due to a created bottleneck or an increased spatial structuring due to seedling establishment around maternal plants. To evaluate plant reproductive success, the fruit set and the proportion of seeds containing embryos were measured. Our results revealed that O. militaris maintained high levels of genetic variation in recently colonized areas, even outside of the main distribution area, with a limited number of potential source populations. Similarly, no differences in reproductive success were observed between old and recently established populations. The extent of fine-scale genetic structure showed substantial variation: significant spatial structure was detected in all of the recently established populations and in one mature mainland population, whereas very weak genetic structuring on a fine scale was detected in two old populations in the main distribution area. Our results suggest that the conservation of recently established populations in anthropogenic areas should be considered in addition to the protection of indigenous calcareous grasslands to prevent the further decline of this species in Estonia.