Hymenophyllaceae

Dubuisson, Jean-Yves, Pechon, Timothée Le, Bauret, Lucie, Rouhan, Germinal, Reeb, Catherine, Boucheron-Dubuisson, Elodie, Selosse, Marc-André, Chaussidon, Cécile, Dajoz, Isabelle, Pynee, Kersley, Grangaud, Edmond, Robert, Yannis, Tamon, Jean- Maurice & Hennequin, Sabine, 2018, Disentangling the diversity and taxonomy of Hymenophyllaceae (Hymenophyllales, Polypodiidae) in the Mascarene archipelago, with ecological implications, Phytotaxa 375 (1), pp. 1-58 : 42-44

Hymenophyllaceae and elevational distribution

We discuss here only the La Réunion data for which we have more or less complete transects and because Mauritian data are too fragmented and lack montane ecosystems. For La Réunion, Cadet (1980) linked the different ecosystems and their elevational range to thermal preferences and ranges (as also reviewed by Blanchard 2000). As such, the lowland rainforest, montane rainforest, and ericoid thickets were respectively named megathermous rainforest, mesothermous rainforest, and oligothermous thickets. On the windward side, the megathermous status corresponds to annual temperature averages between 24°C and 17°C, the range for mesotherm is 17–11°C and oligotherm shows temperature averages below 11°C (as reported in Fig. 20 ). We can thus classify Hymenophyllaceae taxa according to such thermal ranges or preferences. Our data allow defining at least three classes. The megathermous class groups the twelve taxa restricted to lowland forests or mostly present in lowland forests with possible occurrence up to the lowest part of the montane forests (and below 1,500 m): C. bonapartei , C. bipunctatum , C. frappieri , C. inopinatum var. tamonii , C. minutum var. mascarenense , D. barklyanum , D. cuspidatum , D. lorencei (thus all the Didymoglossum species), H. fumarioides , H. hirsutum , H. sibthorpioides , and V. gigantea . The mesothermous class groups the seven taxa restricted to montane forests with possible occurrence in the upper part of the lowland forests (foothills) and on the lower part of the oligotherm close to the tree-line: A. tamarisciforme , C. inopinatum var. inopinatum , C. minutum var. minutum , H. capillare , H. digitatum , H. peltatum (restricted to the upper part of the mesotherm zone), and P. diaphanum . The opportunistic class groups the three taxa more or less present on the whole gradient and up to the oligotherm zone: A. parviflorum , H. hygrometricum , and H. inaequale , the latter species having the widest distribution, from almost the sea-level up to 2,400 m. Hymenophyllum inaequale is also the most abundant species on the whole gradient, being observed in 63 of the 107 plots (by excluding the leeward semi-dry forest; 56%). The secondmost abundant species is H. hygrometricum observed in 49 plots (45.8%) and the third one is A. parviflorum with 42 plots (39.3%). However, the most abundant within their typical elevational ranges are some megathermous species such as D. cuspidatum (from sea-level to 1,250 m) with occurrences in 69.4% of plots on the windward side and C. bipunctatum in 62% of such plots (here from 85 m to 800 m). In addition, the mesothermous A. tamarisciforme is observed in 50% of plots on the windward side (from 270 to 1,520 m). Except for the single C. trinerve population, the rarest species appears to be the megathermous C. bonapartei , observed in 3 localities only, including one population in a wet ravine on the leeward side. There is no oligotherm class and thus no species restricted to ericoid thickets. Actually, the presence of Hymenophyllaceae in the oligotherm is exceptional and anecdotal. The specimens are found in rare places, often as lithophytic on wet ravines close to streams or as low epiphytes on rare small trees at high elevations in sheltered areas (pers. obs.). We can note that A. tamarisciforme is mostly observed as epiphytic on tree-fern trunks ( Alsophila spp. ). Hence, its distribution is likely related to that of such phorophytes. The specific relationship between epiphytes and phorophytes was not been tested here, but should be taken into consideration in further studies, as suggested here for A. tamarisciforme and as evidenced by Moran & Russel (2004) on a neotropical species. Lehnert et al. (2017) further suggested that the connections of the root mantle of tree ferns to the soil allows the upward growth of mycorrhizal fungi which may also be important in determining the growth of epiphytic Hymenophyllaceae .

On the leeward side, the elevational ranges of wet ecosystems are shift upwards, especially for the megathermous rainforest which is thus restricted to the foothills. One consequence could be that elevational ranges of hygrophilous taxa are also modified. But as shown in Fig. 20 , apart from a few examples ( A. tamarisciforme and the single population of C. minutum var. minutum ), the distribution of each taxon present on the leeward side is more or less embedded into their distribution on the windward side. This could suggest that the elevational distribution of Hymenophyllaceae species is mainly conditioned by the thermal preferences. The presence of some species in wet ravines within the leeward semi-dry forests more or less at the same elevations as populations in windward lowland rainforests would add support to this hypothesis.

The existence ot the three elevational species groups might explain the elevational distribution of species richness ( Fig. 5 ), which displays three modes: the first and highest one in the megatherm zone (~ 450–600 m), a second one in the mesotherm zone (~ 1,200 –1,500 m) below the ‘Tamarinaie’, and a small third one close to the tree-line (~ 1,950 – 2,100 m). The third mode is actually due to three opportunistic species. Similar peaked richness patterns based on the zonation of fern distributions have also been observed in the Andes ( Kessler 2000) and Costa Rica ( Kluge et al. 2008).

We assume here that most Mascarenan Hymenophyllaceae would have a Malagasy origin. We also hypothesize that the ecology and elevational gradient of the Mascarenan taxa would reflect the ecology and elevational distribution of the same taxa in Madagascar. In order to understand the patterns observed on the Mascarenes and the related processes, a comparison with Madagascar and neighboring areas is needed. Further studies and analyses should include Malagasy data and the phylogenetic historical background, including molecular dating, that will allow clarifying the assemblage and distribution of Hymenophyllaceae at the western Indian Ocean level.

Figures

FIGURE 5. Distribution of Hymenophyllaceae species richness in relation to elevations, here on the windward side of La Réunion and on Mauritius (data are reported in Appendices 1 & 2). The range of each taxon is assumed to correspond to the range between the minimum and the maximum elevation at which they have been observed, even if the taxon has not been observed in all intermediate localities between such limits.

FIGURE 20. Elevational distribution on each Hymenophyllaceae taxon on the windward (red plots) and leeward (blue plots) sides of La Réunion, and correspondence with thermal zones or ranges according to Cadet (1980) and provided here for the windward side, with ranges of annual average temperatures. The dotted horizontal lines correspond to the limits of leeward ecosystems. The plots indicate the average elevation. Crepidomanes trinerve is here lacking because it was only observed at a single locality on the leeward side (‘Les Canots’ at ~800 m). The range of each taxon, as illustrated by the vertical lines, is assumed to correspond to the range between the minimum and the maximum elevation at which they have been observed, even if the taxon has not been observed in all the intermediate localities between such limits.