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

Abstract

Summary 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 noncordate 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.

Leaf water relations in epiphytic ferns are driven by drought avoidance rather than tolerance mechanisms

“…Lower water availability in the canopy habitat likely selected for reduced xylem tissues to prevent embolism, as well as fewer stomata to lower leaf water loss. As ferns rely entirely on primary xylem due to lack of secondary growth and limited leaf venation to supply lamina tissues, reductions in xylem area could constrain frond morphology and leaf hydraulic function (Brodribb, Feild, & Jordan, 2007;Nitta et al, 2020;Pittermann, Brodersen, & Watkins, 2013;Pittermann et al, 2011Pittermann et al, , 2015. Shorter stipes, however, appear to compensate for selection of reduced xylem tissue by decreasing hydraulic path length.…”

“…Therefore, evolutionary selection for physiological and anatomical traits to adapt to these variable environmental conditions should have coincided with the opportunistic diversification of ferns into new epiphytic niches. Indeed, Nitta, Watkins, and Davis (2020) found that, at the community level, epiphytic species were significantly more diverse functionally than terrestrial taxa.…”

Ecophysiological differentiation between life stages in filmy ferns (Hymenophyllaceae)

“…We used microclimate data collected by Nitta et al (2020). This dataset includes relative humidity (RH) and temperature measured every 15 min from 2013-07-07 to 2014-07-05 at the fern survey sites with Hobo ProV2 dataloggers (Onset Computer Corporation, Bourne, Massachusetts, USA), and vapor pressure deficit (VPD) calculated from these two values.…”

Data_Sheet_1.docx

“…Anatomical and physiological traits described by the leaf economic spectrum are often studied in plants (Bruelheide et al, 2018), although difficult to sample in largescale studies. Among ferns, morphological traits such as lifeform and plant size are widely used for describing functional diversity (Kluge and Kessler, 2007;Tanaka and Sato, 2015;Carvajal-Hernández et al, 2018;Nitta et al, 2020).…”

“…Previous studies on fern functional diversity along elevational gradients found that morphological trait diversity mirrors species richness patterns. In Costa Rica and Mexico, species richness and trait diversity show a hump-shaped pattern with maximum values at mid-elevation, Japan a monotonic decline, and the island of Moorea in French Polynesia a linear increase for epiphytes (Kluge and Kessler, 2007;Tanaka and Sato, 2015;Carvajal-Hernández et al, 2018;Nitta et al, 2020). Using a null model to exclude the influence of species richness, Kluge and Kessler (2007) found that for epiphytic ferns in Costa Rica, mid-elevations had higher than expected trait diversity, whereas upper and lower elevations maintained lower diversity.…”