Epiphyte mosses are adapted to the specific microhabitats found on tree trunks. The conditions of the trunk depend on its physical and chemical qualities as well as its surrounding vegetation. Orthotrichum mosses are a diverse group of epiphytes and there are four endangered species that occur in the Archipelago Sea: Nyholmiella gymnostoma, Orthotrichum stramineum, Lewinskya striata and Orthotrichum patens. Although much is known of Orthotrichum mosses in general, there is no recent research on the ecology of these four species. Thus, the purpose of this thesis is to study their habitat requirements so that they can be taken into consideration in conservation management. In addition, the thesis aims to find out which aspects in the Archipelago Sea define the community composition of epiphyte mosses. The material was collected from 49 islands from which 260 tree trunks were sampled, meaning that the occurring epiphyte moss species were listed and that environmental variables were collected.

The habitat requirements were analyzed with HOF-models and model selection. The goal was to find models that could explain the occurrence of the species. With Nyholmiella gymnostoma, variables relating to the trunk, such as height and moss cover, stood out in the model selection. The best model for Orthotrichum stramineum included these variables as well as basal area and number of potential host trees. The models created for Lewinskya striata were the most complex and many variables related to the capability to withstand dry conditions. Unfortunately, the sample size for Orthotrichum patens was too small for statistical analysis. Multivariate analyses were used to answer the community ecology question, but no answer was found - none of the measured variables seemed to significantly explain the epiphyte moss community composition in the archipelago.

The results highlight the differences between the species. Nyhomiella gymnostoma is limited to large aspen trunks and the features of the trunk seem more important than surrounding vegetation. Orthotrichum stramineum was typically found on short, slim, and smooth trunks growing in half-shade, often together with hazel. Lewinskya striata seems to have the widest tolerance to drought. It was found on surprisingly dry environments, which raises the question of whether it is truly as endangered as previously estimated or just poorly known. The thesis significantly increased our knowledge of these species and almost doubled the number of known occurrences in the Archipelago Sea. It also led to the discovery of Pulvigera lyellii, which had not been found from Southwest Finland before. This further emphasizes the importance of the area for epiphyte mosses.