Predicting growth of mat-forming lichens on a landscape scale - comparing models with different complexities

During the 20th century, forestry practices has adversely affected lichen-rich habitats. Mat-forming lichens are important components of the vegetation of boreal and arctic ecosystems and are the main reindeer forage during the winter. To support the long-term management of lichens in such habitats we developed models for predicting the growth of two common species. The lichens were transplanted across northern Scandinavia along a west-east gradient varying in precipitation, temperature and irradiance. Growth was recorded seasonally over 16 months and ranged from -4.8 to 34.6% and -12.7 to 34.7% dry weight change for Cetraria stellaris and Cladina islandica, respectively. Growth was light limited below canopies with more than ca 60% cover and highest at the more humid sites when light levels were optimal. The models were based on various meteorological parameters, irradiance, physiological data and lichen hydration status; the latter was derived from a recently developed lichen hydration model. Our models' abilities to predict growth, both annually and seasonally (i.e. in summer), were evaluated in relation to their complexity and their potential usefulness from a management perspective. One parameter related to irradiance (the logarithm of site openness) was valuable in the prediction of annual growth for both species and could, in combination with precipitation, explain 52% of the variation in annual growth for C. stellaris and, in combination with total wet time and the irradiance received while wet, explain 66% of the variation in annual growth for C. islandica. The best simplified model explained 43% of the variation in annual growth for C. stellaris, using stem basal area and the annual normal temperature, and 24% for C. islandica using basal area alone. It is concluded that ensuring sufficient irradiance below the forest canopy is of crucial importance in the long-term management of mat-forming lichens and that simplified models can be used to identify appropriate habitats.