INTRODUCTION

In general terms, the Araceae family is notable for comprising a great diversity of species, as well as for its impressive morphological variation (Grayum, 1990), variation in life forms (geophytes, climbers, epiphytes, helophytes and free-floating aquatic) (Croat, 1988; Mayo et al., 1997). This family represents one of the groups with greater economic importance (Duke, 1968; Croat, 1994a; Mayo et al., 1997), owing to edible tubers and especially to its great ornamental value.

According to Boyce and Croat (2018), the Araceae consist of about 3645 species in 144 genera. In Central America, there are 778 species in 26 native aroid genera (Croat, 2017). In this region, the diversity of species is mainly concentrated in Panama and Costa Rica. In the case of Panama, the aroid flora currently comprises 436 described species in 26 genera, but it is estimated that there are more than 600 species in total, representing the richest country for Araceae in Central America (Ortiz et al., 2018).

Most of the existing knowledge of the aroid flora of Panama has been generated in the last fifty years and has been mainly due to the development of taxonomic studies (Ortiz et al., 2018). Currently, there are very few published floristic studies of the Panamanian Araceae. Among them, those of Standley (1928, 1933, 1944) and Croat (1978), were conducted mainly in the former Canal Zone. So far, there are no published intensive inventories or detailed studies on the Araceae in other areas of the country. Floristic studies generate valuable biodiversity information, especially in poorly explored areas and is a fundamental prerequisite for research in ecology, agronomy, forestry, conservation biology and biogeography (Barkley, 2000; Phillips et al., 2003). For this reason, the main objective of this work is to analyze the floristic composition of the aroids of a botanically important region, such as Cerro Pirre, due to its high degree of endemism (Bermúdez et al., 2000; ANAM, 2010). The taxonomic and ecological aspects of the aroid flora of Cerro Pirre was published in a second and third part (Ortiz et al., 2019a, 2019b). The information generated in this work can be used to better understand this important group of plants and, in turn, the information that could be used to design useful conservation measures within the Darién National Park.

MATERIALS AND METHODS

The study was carried out in the vicinity of Cerro Pirre (near the biological station of Rancho Frío), a mountain located to the north of the Serranía de Pirre (Figure 1), within the geographic coordinates of 8°1'8.80"N, 77°44'5.30"W. The name "Rancho Frío" was originally applied to a camp on the Rancho Frío-Cerro Pirre trail; at present, it refers only to the station of the Ministry of the Environment (formerly ANAM) (Siegel et al., 2008). Cerro Pirre has an elevation gradient from 90 to 1550 m (Robbins et al., 1985). The climate of the area is humid tropical, with the average annual temperature is 20–25°C, and the average annual rainfall is 3000 to 3500 mm. There is a pronounced dry season beginning January and ending in April (Gradstein and Salazar, 1992).

To collect material, photograph and study the species, three field trips were made (April, July-August and December of 2016), with each field work session lasting 8 to 12 days. The collections were made using the methodology proposed by Croat (1985). All the material collected was processed and deposited in the University of Panama herbarium (PMA) and duplicates in the Missouri Botanical Garden (MO) and University of Florence (FT) herbaria. The determination of life forms was made using the classifications proposed by Croat (1988) and Mayo et al. (1997). We use the term “nomadic vine”, which includes all climbing plants that germinate on the ground and may lose the older parts of their stem in the process of ascending (see Zotz, 2013). This term includes many aroid species that were previously described as secondary hemiepiphytes (Croat, 1988).

In order to complement the list of species for the floristic inventory, databases and collections deposited in PMA, MO and Smithsonian Tropical Research Institute (SCZ) herbaria were consulted. The abbreviations (Alpha-2 code) used for the countries are according to ISO 3166 (ISO, 2018). The geographical distribution of the species was obtained from Balick et al. (2000), Brako and Zarucchi (1993), Croat (1983, 1986,1991, 1999, 2004), Croat and Acebey (2014), Croat and Carlsen (2013), Croat and Hannon (2015), Croat and Ortiz (2016), Croat and Stiebel (2001), Correa et al. (2004), Idárraga- Piedrahita et al. (2011), Funk et al. (2007), Dorr and Stergios (2014), Grayum (1996,2003), Madison (1977), TROPICOS (2018), Nelson (2008) and Wong et al. (2016).

RESULTS AND DISCUSSION

Floristic composition

A total of 430 herbarium specimens were reviewed. The oldest collections of Araceae from Cerro Pirre dated from 1962 by the botanists James Duke and Narciso Bristán. The majority of the specimens were collected by the botanist Thomas Croat during the years 1976, 1988 and 1994 (104 specimens), followed by the botanists Ronald Hartman in 1977–1979 (44 specimens) and James Folsom during 1977 and 1980 (40 specimens). Through this study, 134 specimens were collected, representing 31% of the total of the reviewed specimens.

Through intensive sampling and the revision of herbarium specimens, 10 new records were made for the flora of Cerro Pirre (cf. TROPICOS, 2018): Adelonema panamense Croat & Mansell, A. wendlandii (Schott) S.Y. Wong & Croat, Anthurium brownii Mast., A. pentaphyllum var. bombacifolium (Schott) Madison, A. salvinii Hemsl., A. talamancae Engl., A. clavigerum Poepp. & Endl., Heteropsis oblongifolia Kunth, Monstera dubia (Kunth) Engl. & K. Krause and M. spruceana (Schott) Engl. Also, Stenospermation ellipticum Croat & D.C. Bay was registered for the first time for the flora of Panama (for more details see Ortiz et al., 2018). Additionally, nine new species to science were identified, two of which having been recently published (Croat et al., 2017a; 2017b)—all currently only known from Panama.

The Araceae of Cerro Pirre consists of 94 species, distributed in 12 genera (Appendix 1). Comparing the results obtained with other works carried out in Panama and other parts of the Neotropics, the Cerro Pirre area represents one of the sites with the greatest diversity of Araceae species (Table 1). But when comparing the results of this study (Cerro Pirre: 94 spp., 100–1550 m) to others conducted in Neotropical areas with similar elevations such as El Cerrado (64 spp., 300–1300 m), Las Quinchas (52 spp., 380–1500 m), Quincemil (47 spp., 650–1200 m) and Reserva de la Biosfera Los Tuxtlas (34 spp., 100– 1700 m), it is observed that the diversity of Araceae in Cerro Pirre is greatest.

The diversity of species of Araceae varies considerably within the tropical areas, with temperature, humidity, precipitation and elevation being some of the most determining factors. The high species richness has been correlated with high levels of precipitation coupled with the absence of prolonged dry seasons (Croat, 1998). Croat (1994b) mentions that the diversity is greater between the sea level and elevations up to about 1500 m; after 2000 m, the diversity decreases dramatically. Leimbeck et al. (2004) analyzed the diversity patterns of Araceae in Ecuador and concluded a strong correlation with elevation and, to a greater extent, by humidity. This, in part, agrees with that proposed by Gentry (1988), who mentions that the richness of neotropical plant species generally increases with absolute annual precipitation. For example, Bajo Calima and Cabo Corrientes (both sites located in the Colombian Chocó), are characterized by lowland areas (<150 m), high levels of precipitation (6000–8000 mm annual in Bajo Calima and 5000 mm annual in Cabo Corrientes) and extremely high diversity compared to other sites (see Table 1). But, Fortuna Forest Reserve, an area that includes mid-elevation areas (700–2200 m), seems to be one of the most diverse sites in the Neotropics (Table 1). Although Fortuna has lower levels of precipitation (5470 mm annual) than Bajo Calima (6000–8000 mm annual), Fortuna is characterized as the most cloudy place in Central America, where the coverage with clouds is about 70% during the rainy season (STRI, 2014).

In poorly diverse lowland sites (< 145 m), such as Barro Colorado Island and Usina São José (see Table 1), there are relatively low levels of precipitation (2750 mm annual in Barro Colorado Island and 1687 mm annual in Usina São José). In the case of Cerro Pirre, although it include sites from lowlands (100 m) to mid-elevation areas (1550 m) and have moderate precipitation records (3000–3500 mm annual), it has a relatively high aroid diversity. Probably, this is due to the high heterogeneity in the vegetation of Cerro Pirre (see Myers, 1969), which comprises three types of vegetation and four life zones (sensu Holdridge et al., 1971), and the changes in temperature and humidity along the elevation gradient (ANAM, 2010).

Chorological affinities

Almost half of the taxa present in Cerro Pirre has a wide distribution (Figure 2). These species are also distributed in other countries of Central and South America. There are only two strictly Central American species, both of the genus Syngonium (S. schottianum H. Wendl. ex Schott and S. hoffmannii Schott). According to Croat (1981), the center of diversity for this genus is Costa Rica and Panama.

Four species are distributed exclusively from Costa Rica to Colombia (Anthurium cucullispathum Croat, A. talamancae Engl., Monstera pittieri Engl. and Philodendron rayanum Croat & Grayum); two species are distributed only in Panama, Colombia and Ecuador [Chlorospatha mirabilis (Mast.) Madison and Philodendron ichthyoderma Croat & Grayum]. Chlorospatha mirabilis was recently recorded for the flora of Panama and is only known from a few collections made in the Province of Darién (Cerro Tacarcuna and Cerro Pirre) (Croat and Hannon, 2015).

In the case of binational species, there are two distributed in Costa Rica and Panama: Monstera oreophila Madison and Philodendron wilburii Croat & Grayum. Another 11 taxa are exclusive to Panama and Colombia: Anthurium crystallinum Linden & André, A. pendens Croat, A. rotundistigmatum Croat, Dieffenbachia isthmia Croat, Philodendron albisuccus Croat, P. lazorii Croat, P. immixtum Croat, P. ligulatum var. heraclioanum Croat, P. panamense K. Krause, P. pseudauriculatum Croat and Stenospermation ellipticum Croat & D.C. Bay. The Panamanian populations currently known of Anthurium crystallinum, A. rotundistigmatum, Philodendron albisuccus and Stenospermation ellipticum are found only in Darién Province (TROPICOS, 2018).

It is important to note that the Cerro Pirre area contains a significant level of endemism of Araceae (see Figure 2). Twenty-three species (24% of the total aroid flora of Cerro Pirre) are also endemic to Serranía de Pirre (Appendix 1). The Cerro Pirre area is included in the Eastern Panamanian montane forests (Bermúdez et al., 2000; ANAM, 2010), which includes several isolated mountains. Due to the complex heterogeneity of habitats present along the mountainous regions of Darién and the isolation between the mountain ranges, these sites are characterized by high endemism and great biological diversity (WWF, 2018).

Life-forms

The species of Araceae on Cerro Pirre are categorized into four life forms: epiphytes, nomadic vines, terrestrials and epilithics. Of the total species, about 43% are nomadic vines, 33% epiphytes, 23% terrestrials and a single species was epilithic (1%) (Figure 3). These results are similar to those obtained by Mora et al. (2006), Acebey and Krömer (2008), Trujillo et al. (2014) and Díaz et al. (2015), where the hemiepiphytic lifeform is predominant in the aroid flora (referred here as nomadic vines).

Some of the species recorded in this study exhibited two different life forms (Appendix 1). Previous studies have reported that aroid species can develop more than one lifeform, depending on the habitats where they grow (see Croat, 1988; Zotz, 2004; Benavides et al., 2005). For example, in the study area, Anthurium cuspidatum Mast. (sensu Croat and Ortiz, 2016), is commonly terrestrial, but at higher elevations (above 1100 m), epiphytic individuals can be observed. Croat (1988) mentions that in the cloud forests the conditions of soils and trunks of the host trees are very similar, due to the continuous drainage of water and the excessive accumulation of organic matter. On the other hand, terrestrial, epiphytic and epilithic individuals were observed in A. salvinii Hemsl. Given its conglomerate roots and the arrangement of leaves (in rosettes), this species can accumulate detritus and moisture, as litter-trapping plants (see Zona and Christenhusz, 2015). Probably, this mechanism allows the plant to establish itself successfully and grow in dry soils with few nutrients, as well as, provides a wide degree of adaptation to various habitats. Other case was identified in which the life form observed contrast with the original descriptions of the species. Croat (1986) described Anthurium rubrifructum Croat and A. niqueanum Croat as "climbing epiphytes”. However, in Cerro Pirre they were observed as nomadic vines. Individuals of both species usually can climb trees from five to 10 meters without disconnecting from the ground.

CONCLUSIONS

The Cerro Pirre area represents one of the sites with the greatest aroid diversity compared to other areas of the Neotropics. This site has 94 species, which mostly have wide distribution (53%), although relatively high percentages of endemic species were also reported (27%). Most of the recorded species were nomadic vines (43%), followed by epiphytes (33%), terrestrials (23%) and epilithic (1%).

Through the review of herbarium specimens and the field work carried out, 10 new records for the flora of Cerro Pirre, one new record for the flora of Panama and nine new species to science, were identified. The surprisingly high number of new records and new species emphasizes the need to carry out future field work in unexplored areas, as well as the importance of herbarium work through the thorough review of botanical material.

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