Influence of habitat history on the distribution of Usnea longissima in boreal Scandinavia: a methodological case study

Abstract

Changes in habitat affect the distribution of species at different spatial and temporal scales. Our aim was to assess the use of several retrospective methods when determining habitat history and its influence on species distribution, using the pendent lichen Usnea longissima Ach. as a model organism. The study was carried out in a Norway spruce dominated forest, located in the middle boreal zone of northern Sweden. Historical sources (including forest inventories and maps) and dendrochronology were combined with field surveys to determine past stand characteristics with a high spatial precision. Also, present stand characteristics were thoroughly surveyed and the relationship between distribution of U. longissima and past and present habitat characteristics were evaluated. Our results showed that, despite the fact that historical logging was unevenly distributed within the study area, similarities in stand structure exist today. Several essential factors affecting the distribution of U. longissima could be related to changes in past stand structure, especially the extent and intensity of previous logging operations and the subsequent stand development during the last c. 150 years. Our results also showed that U. longissima was favoured by a continuous old age structure with canopy openness preserved. The importance of detailed reconstructions of specific habitat components is highlighted when relating species presence to habitat change. It is suggested that the use of retrospective methods presented in this study can be helpful in clarifying possible causes for the complex distribution of certain organisms.