Life in the canopy: community trait assessments reveal substantial functional diversity among fern epiphytes

The expansion of angiosperm-dominated forests in the Cretaceous and early Cenozoic had a profound effect on terrestrial biota by creating novel ecological niches. The majority of modern fern lineages are hypothesized to have arisen in response to this expansion, particularly fern epiphytes that radiated into the canopy. Recent evidence, however, suggests that epiphytism does not correlate with increased diversification rates in ferns, calling into question the role of the canopy habitat in fern evolution.
To understand the role of the canopy in structuring fern community diversity, we investigated functional traits of fern sporophytes and gametophytes across a broad phylogenetic sampling on the island of Moorea, French Polynesia, including > 120 species and representatives of multiple epiphytic radiations.
While epiphytes showed convergence in small size and a higher frequency of non-cordate gametophytes, they showed greater functional diversity at the community level relative to terrestrial ferns.
These results suggest previously overlooked functional diversity among fern epiphytes, and raise the hypothesis that while the angiosperm canopy acted as a complex filter that restricted plant size, it also facilitated diversification into finely partitioned niches. Characterizing these niche axes and adaptations of epiphytic ferns occupying them should be a priority for future pteridological research.

Methods
Morphological traits of ferns from Moorea, French Polynesia were measured from voucher specimens or obtained from literature when plant material was unavailable. Sporophyte traits include: stipe length, frond length, frond width, rhizome diameter, frond dissection, number of pinna pairs, and specific leaf area. Gametophyte traits include: morphotype and presence or absence of gemmae, glands, and hairs.

Temperature (celsius) and relative humidity (%) were measured every 15 minutes (Moorea) or 60 minutes (Tahiti) with 33 dataloggers on Moorea and Tahiti, French Polynesia from 2012-07-18 to 2015-02-06. On Moorea, Hobo Pro v2 data loggers with the RS3 Solar Radiation Shield (Onset Corporation, Bourne, Massachusetts, USA) were used. On Tahiti, RHTemp 1000 data loggers (MadgeTech, Warner, New Hampshire, USA) protected with custom radiation shields made from plastic circuit boxes were used.