Population size and reproduction in the declining endangered forest plant Chimaphila umbellata in Sweden

The rare forest plant Chimaphila umbellata (Ericaceae) has decreased drastically during the last century, approximately by 80 % in some regions in Sweden. We examined associations between various biotic and abiotic conditions related to changes in forest management and nitrogen deposition, and C. umbellata population size, flowering frequency, fruit set and seed production. Environmental conditions at 38 C. umbellata sites in the provinces of Uppland and Sodermanland, Sweden, included light inflow, cover of competitive species, soil nitrogen, continuity of forest cover and soil texture. The results suggested that population size was negatively affected by cover of competitive species. Population size was not related to light availability although increased shading was associated with decreased flowering frequency. Fruit set was negatively affected by cover of competitive species, and seed production decreased with increasing soil nitrogen content. Fruit set and seed production increased with increasing population size. This study shows that denser forest stands and increased abundance of Vaccinium myrtillus and graminoid species may have strong negative effects on C. umbellata. This species' longevity and clonal propagation may buffer some negative impacts, which in turn might contribute to an extinction debt. To maintain viable populations of Chimaphila umbellata in Swedish forests, there is a need for targeted management in forest habitats, i.e. reintroducing moderate disturbance regimes to reduce competition and increase light inflow.