Fatal interactions between Scots pine and Sphagnum mosses in bog ecosystems

In this study, we explore how Sphagnum mosses and Scots pine, Pinus sylvestris, interact on different spatial and temporal scales in a boreal bog ecosystem. We were particularly interested in relationships between the occurrence of Sphagnum-dominated habitats and the occurrence of Scots pines of different age and size. Juvenile and adult pines occurred in different habitats. While juveniles mainly occurred in Sphagnum-dominated habitats, predominantly with Sphagnum rubellum, adult pines were found in habitats dominated by lichens, or with a sparse vegetation cover. Examination of surface peat cores sampled close to adult pines revealed that almost all pines (97%) had established in a Sphagnum-dominated environment and that the habitat had changed since pine establishment. Scots pine is thus capable of changing and exterminating the Sphagnum-dominated environment preferred for germination and establishment. Pines impede Sphagnum growth and peat accumulation significantly once they have reached a stem diameter of approximately 20 mm. It takes from 30 to 90 yr for a pine to reach that size. Our results show the importance of interactions between Scots pine and Sphagnum mosses in bog ecosystems. We conclude that interactions between trees and Sphagnum mosses are important driving forces behind the vegetation change that has characterised boreal bogs during the Holocene.