Abundance and seasonal growth of epiphytic ferns at three sites along a rainfall gradient in Western Europe

Abstract

Vascular epiphytes in temperate zones are allegedly rare due to climatic constraints, such as drought and freezing temperatures during winter. Here, we investigate how abundance and seasonal growth of epiphytic polypod ferns in Western Europe are influenced by annual rainfall and temperature. We recorded abundances and growth of epiphytic polypod ferns over two years at three sites along a rainfall gradient from Southwest Ireland to Northwest Germany, spanning a two-fold difference in annual rainfall. We also investigated plant maturation and recorded key climate characteristics (rainfall and temperature). Abundance of epiphytic polypod ferns strongly decreased from the wetter end of the rainfall gradient in Ireland towards the drier end in Germany. Yet, frond turnover was equally high at all study sites during summer months, and equally low during winter. Plants showed only little to no expansive growth during the study period, except for the study site in France in the middle of the gradient, where the total number of fronds strongly increased. Similarly, we found an increase in the proportion of spore-bearing fronds at the French study site, whereas there was hardly any change between years at the other two sites. The observed growth patterns are inconsistent with the current differences in abundance of epiphytic ferns. We suggest that these differences are due to poor germination of spores under drier conditions and that small differences in annual growth have contributed to the current abundances over an extended time period. Surprisingly, seasonal frond productivity appeared to be unaffected by the amount of annual rainfall and average temperatures, although it seems likely that these factors do play a role during gametophyte establishment.