Vascular epiphytes at the treeline – composition of species assemblages and population biology

• We studied key aspects of epiphyte demography and community structure at the tree line.
• The epiphyte community at the tree line is less diverse than in comparable lowland habitats.
• Some species may only form sink populations, which depend on constant input from lower elevations.
• Many species, however, seem to form viable local populations

Do epiphytes drop out above the treeline because of climatic reasons or due to lack of appropriate substrate? We set out to address this question by studying various key aspects of their biology at a natural treeline on a tropical mountain top in Panama: species composition of assemblages, host preferences, population structure and dynamics, individual growth and reproductive investment. Lack of relevant climate data caused us to monitor microclimatic conditions in tree crowns for 12 months. At our study site we found 15 epiphyte species, mostly ferns, although in terms of abundance orchids were highly dominant. Compared to lowland vegetation of similar stature, the epiphyte assemblages were less species-rich and abundances lower. Some species may only be present at this treeline because of continuous immigration from lower elevations (i.e. represent sink populations). Many species, however, were rather abundant with many reproductive individuals, arguably forming viable local populations. The results of a detailed demographic study with the orchid Pachyphyllum hispidulum over three years also support this notion, being comparable to studies from lowland habitats. Our descriptive study does not allow unambiguous conclusions on mechanisms, but our findings suggest that lack of suitable substrate (=trees) rather than climatic reasons are responsible for the disappearance of epiphytes immediately above the treeline. This notion needs rigorous verification by experimental physiological studies.