Temporal trends in shade sensitive epiphytic cryptogams residing on old trees

Abstract

Question: How does abandoned management affect species assemblages of shade sensitive bryophytes and lichens residing on old trees in traditionally light- open environments? Method: I examined compositional change over 20 years in the epiphytic vegetation of old pollarded trees of Fraxinus excelsior in a cultural landscape of western Norway through re- sampling. The nature of changes in the epiphytic composition in relation to environment was identified through 1) Quantifying temporal turnover in species compositions along DCA axes; 2) Calculation of relative changes in Ellenberg indicator values for gradients of light and moisture through a weighted average technique and 3) Testing the effect of aspect, stem inclination/ height and the historical management regime on the rate of temporal turnover. This was done by mixed effect models with forward selection. Results: Species composition of the epiphytic vegetation has changed significantly during the last 20 years. Species with lower indicator values for light (shade tolerant species) and higher indicator values for moisture (draught sensitive species) increased in relative abundance. An environmental trend towards more shady and moist conditions was detected. Among other shade sensitive species, shade and draught sensitive cyanolichens are negatively affected, whereas the shade and draught tolerant chlorolichens and liverworts residing throughout the stem within sheltered local environments, like Lepraria spp, Phlyctis argena and Metzgeria conjugata gain advantage. Synthesis: The results imply a negative impact of abandoned management on temporal trends in assemblages of shade sensitive epiphyte bryophytes and lichens. The results demonstrated that the combination of approaches employed is operational and conceptually relevant for detecting temporal trends in cryptogam epiphytic communities in relation to environment at the scale of the landscape.