Identifying risk factors for the rare, endangered fen orchid Liparis loeselii in NE Germany with a fresh approach

Fen mires are one of the most threatened habitats in Central Europe and harbor numerous specialized plant species. One of these species is the fen orchid Liparis loeselii , which is threatened and declining in most European inland fens. However, the risk factors driving the species’ decline are poorly understood. We therefore studied vegetation structure, species composition, and abiotic conditions at 17 current sites with extant and 11 former sites with extinct populations in north-eastern Germany. The methodological challenges posed by the patchiness and rarity of the species were tackled with a fresh approach that combined systematically random plots (assessing common, average habitat conditions) with selectively placed plots (targeting L. loeselii microhabitats). Compared to current sites, former sites of extinct populations were characterized by lower moisture, higher peat degradation, higher nutrient availability, lower moss and herb cover, higher abundancies of nitrophytes and common grassland species, and less fen specialist species. Similar differences emerged between average habitat conditions and L. loeselii microhabitats; and the latter furthermore featured lower vegetation height, higher light availability, and higher proportions of bare soil. Our study indicates that the main risk factors for L. loeselii in our study region resulted from enhanced fen degradation, where lowered water tables due to drainage or drought degraded the upper peat layers and released nutrients. This changed the species composition toward taller, more competitive species which increased shading, litter cover, and limited open patches and brown mosses, ultimately leading to local extinctions. It is therefore crucial that permanently high water tables are maintained or restored to protect L. loeselii – and fen mires in general. Our approach proved successful with only moderate sample size and may facilitate investigating other rare species.