Ecology and survival of Nigritella nigra, a threatened orchid species in Scandinavia

Abstract

Nigritella nigra ssp. nigra (Orchidaceae) is the only taxon of the genus in Scandinavia, where it occurs as an endemic with a disjunct distribution, mostly in upland areas of central Norway and Sweden. Nigritella nigra is a boreal‐alpine, basiophilous species of open grasslands and dry fens. Both the number of localities and the number of individuals in a majority of the localities, including the study area at Sølendet, Røros, central Norway, have declined in recent decades. The most important threat is the change in agricultural practices followed by overgrowing of land earlier harvested for hay or intensively grazed. The total population of N. nigra at Sølendet is estimated to be about 3000 individuals, the largest known. Flowering individuals have been counted annually and all 225 individuals in nine permanent plots have been recorded for twenty years. The population has a large proportion of long‐lived individuals with a half‐life of up to 8 years in particular cohorts. Flowering behaviour varies greatly and flowering rarely occurs several years in succession. Together with the reduced size of individuals after flowering, this indicates a high cost of flowering. Correlation analyses indicate that high precipitation in autumn and winter negatively affects flowering (in July) on dry grasslands. On damp grasslands, warmth in autumn and in May of the flowering year correlates positively with flowering density. Many individuals are killed in years with heavy microtine rodent grazing. Experimental mowing as a management measure, and apparently also cattle grazing, have positive effects on the population. Some small, shallow valleys with a good supply of base‐rich water on its flat, sandy floor is the main N. nigra biotopes at Sølendet.