The subfamily Spiranthinae is the most diverse lineage of neotropical land orchids in the world. It includes approximately 500 species and 40 genera, of which Dichromanthus Garay comprises four species, one of which is D. cinnabarinus [1 , 2]. The species is widely distributed in America, from southern United States through Mexico to Guatemala. It is distributed generally in xerophilic environments with rocky soils in diverse vegetation types that range from pine-oak forests, tropical deciduous forests and even in some rosetophile scrub and grasslands with juniper [3].

In Mexico, this species has been recorded in a large part of the territory, particularly in northeastern Mexico [3], which includes the states of Coahuila, Nuevo León, and Tamaulipas. These states, from a biogeographic perspective, are in the region of the Chihuahua Desert and transition with the Sierra Madre Oriental. Among the floristic and vegetation studies that have been conducted in this region and have registered D. cinnabarinus is the work of Villareal-Quintanilla et al. [4], who studied pinion pine (Pinus pinceana Gordon) communities and register this species as part of the associated flora. Encina-Domínguez et al. [5 - 7] studied the plant communities of the Sierra de Zapalinamé in Saltillo, Coahuila, and recorded the species as characteristic of the region. Also, Estrada et al. [8] conducted a study focused on identifying species with different traditional uses in the Cumbres de Monterrey National Park, Nuevo León, where they recorded it as a wild species in an oak forest.

It is well-known that Mexico has a broad diversity of natural environments generated by its relief, the mosaic of soils and the variety of climates [9], as well as its geographic location in which its mountains form physio-ecological barriers between two important biogeographic regions: the Nearctic and the Neotropical regions [10 , 11]. This has originated the existence of high levels of biodiversity and endemism [12] through the processes of speciation. There is a synergy created by these barriers in conjunction with climate and habitat factors and has induced species to adapt to the ecological conditions of a given region. In most cases, allopatric species develop in different vegetation types because of reproductive isolation [13].

From this viewpoint, arid and semi-arid zones are considered centers of origin and diversification of species. In Mexico, these regions cover approximately 60% of the territory. The Chihuahua Desert is one of these regions. It is in the north-central portion, bounded by the Sierra Madre Oriental to the east and the Sierra Madre Occidental to the west [14]. Part of this desert comprises the states of Coahuila and Nuevo León as well as the southwestern tip of Tamaulipas; the region is known as Northeastern Mexico [4].

In this region, many scientific studies have been conducted, focused on identifying the flora and vegetation. Some of these studies have registered D. cinnabarinus as a member of the local flora. However, the types of vegetation where it has been recorded are diverse. Studies in the state of Coahuila were conducted by several authors [15 - 19]. In Nuevo León, outstanding work has been done [20 - 23]. In Tamaulipas, studies have been undertaken by several authors [24 - 27]. However, it is necessary to develop studies using parsimony analysis of endemicity, taking into account the species present in each type of vegetation, to establish similarities among the sites based on the flora and to determine whether D. cinnabarinus is truly widely distributed or if we are dealing with different allopatric species over the vegetation types in northeastern Mexico. Hypothesis: If allopatry is a process of speciation that because both geographic and reproductive isolation forms new species, then Dichromanthus found in contrasting plant communities in northeastern Mexico could be different species.

The objectives of this study were to identify the plant communities in which Dichromanthus cinnabarinus has been recorded and to develop a parsimony analysis of endemicity to obtain the degree of similarity that exists among the communities based on the associated flora obtained in northeastern Mexico.

General description of the study area: The northeastern region of Mexico (Coahuila, Nuevo León and Tamaulipas) covers an area of 294,099 km²; it is bounded to the north by the United States, to the east by the Gulf of Mexico, to the south by the state of San Luis Potosí and to the west by the states of Chihuahua, Durango and Zacatecas [28]. Its relief ranges in altitude between 1000 and 3050 m; annual temperatures oscillate from 5.5˚C to 35˚C and annual precipitation is approximately 175 to 400 mm (Figure 1) [4].

Physiography and ecoregions: The study area includes parts of two ecoregions: 1) the Desert of Chihuahua (DCH) and 2) the Sierra Madre Oriental (SMO). The DCH is characterized by the presence of plains and small isolated mountain chains [29], while the SMO includes the transition zone between the Nearctic (in the lower areas of piedmont) and the Neotropics (mountainous regions) [12].

Geology: Approximately 80% of the characteristic soils of the DCH are formed by calcareous components [30], while in the SMO and its foothills toward the high plateau, sedimentary limestones, shales, and sandstones dominate. In some sites, gypsum can be seen. Dominant soils are alluvium type, but it is feasible to find soils containing igneous rock [31 , 32].

Climates: The predominant climate of the region is influenced mainly by the presence of mountain chains. However, arid, and semi-arid climates predominate; mean temperatures are 10˚C to 25˚C and mean annual precipitation is 150 to 2000 mm [31].

Vegetation: In the area of the Desert of Chihuahua (DCH) and the Sierra Madre Oriental (SMO), the plant communities are desert microphyll scrub, desert rosetophile scrub, desert crassicaule scrub, oak stands, grasslands, riparian vegetation, and pinion pine forests [11]. The outstanding plant associations are Larrea tridentata in the desert microphyll scrub type, Agave lechuguilla, Dasylirion spp. desert rosetophile

scrub and Yucca sp. rosetophile forest [33], grassland, chaparral, prickly pear stands, spiney Prosopis spp. forest, dune vegetation and halophyte vegetation [34].

However, in the SMO there are also established plant communities of mainly Juniperus deppeana Steud., Juniperus flaccida Schltdl., and Pinus cembroides Zucc., as well as grasslands [34].

Study sites: Seven sampling sites, in which Dichromanthus cinnabarinus was registered, were selected (Table 1):

1) The Zapalinamé mountains located in the municipality of Saltillo, Coahuila, 25˚23'59"N and 101˚59'17"W. Here, there is clear dominance of alluvium, lithosol and rendzina soil types. Climate is hot to cool, with mean annual temperatures that oscillate between 5˚C and 12˚C and average annual precipitation of 498 mm. Predominant vegetation is rosetophile and microphyll scrub [5].

2) Cerro del Cristo Rey, located in the municipality of Galeana, Nuevo León, 24˚50'00"N and 100˚04'00"W, has a clear dominance of lithosol and fluvisol soils. The climate is cool, sub-humid with summer rains, annual temperature of 15˚C to 28˚C and average annual is precipitation 400 to 600 mm. Predominant vegetation is pine and oak forest as well as rosetophile scrub [28].

3) San Joaquín de Solo is in the municipality of Aramberri, Nuevo León, 24˚24'31"N and 99˚91'61"W; average elevation is 1077 m with a mostly warm climate with summer rains. Precipitation is between 600 and 800 mm and temperatures are between 12˚C and 18˚C. Dominant soils are lithosol, xerosol and regosol. Predominant vegetation is pine forests and oak forests [28].

4) La Peña is in the municipality of Miquihuana, Tamaulipas, 23˚48'33"N and 99˚86'67"W. Soils are lithosols. Predominant climates are extremely dry and mean annual temperature is 18˚C with mean annual precipitation that fluctuates between 350 and 500 mm [33]. The altitudinal gradient is 715 to 3265 m [35].

5) Camino al Estanque de los Walle is in the municipality of Miquihuana, Tamaulipas, 23˚34'1.41"N

and 99˚51'28.66"W. Dominant soils are lithosols. Climate is dry; precipitation oscillates between 350 and 500 mm; mean annual temperature is 18˚C. Predominant vegetation is desert scrub [35].

6) Cañón de la Nopalera, municipality of Miquihuana, Tamaulipas, 23˚45'57.1"N and 99˚48'16.1"W, has lithosol and xerosol soils. Climate is predominantly temperate with summer rains; temperatures range from 4˚C to 41˚C, and vegetation is rosetophile scrub and pine forest [35].

7) Camino a Bustamante is in the municipality of Bustamante, Tamaulipas, 99˚45'25"N and 23˚26'07"W. Soils are leptosol and calcisol; climate is warm and dry with summer rains and annual temperature of 18˚C to 36˚C. Average altitude is 1718 m and precipitation is 500 to 900 mm. Dominant vegetation is rosetophile scrub [36].

Methods: To study the vertical structure of the community, we considered the presence of the vegetation strata (tree, shrub and herbaceous). Specimens were collected during April and August 2017. A total of 12 quadrants measuring 10 × 10 m were established in the study areas where rosetophile scrub was present and nine quadrants in the sites with forest or chaparral, surpassing the minimum sampling area reported by other authors for these vegetation types [24 - 37].

In the tree stratum, we considered plants taller than 2 m, and 10 × 10 m quadrants were established. Everyone was measured for total plant height, largest and smallest crown diameter, and diameter at breast height (1.30 m). For the shrub stratum, plants 1 to 2 m tall were considered, and two 2.5 × 5 m sub-quadrants were established within each 10 × 10 m quadrant. The variables obtained were total plant height and the largest and smallest diameter. For the herbaceous stratum, all plants smaller than 0.5 m tall were considered, and five 1 × 1 m quadrants were established; the variables plant height and largest and smallest cover diameter were measured [37] (Table 2).

The collected botanical material (plants in the reproductive stage) was processed following Lot & Chiang [38]. The material was pressed, and dichotomous taxonomic keys were used for identification. Finally, specimen identification was corroborated using lists of flora of northeastern Mexico, review of specialized literature and a visit to the herbarium of the Universidad Autónoma de Tamaulipas (UAT). In addition, we obtained the Importance Value Index (IVI) with percentage values at a scale of 0 to 100 [37].

Importance value index = relative cover area + relative frequency + relative density

IVI = Cr + Fr + Dr

where: relative cover area was obtained with the formula:

Cr = Absolute cover area of each species Absolute cover area of all species × 100

where

Ca = Cover area of one species Total sampled area

Cover area was estimated using the formula of the area for each species:

CA = π ( dM + dm 4 ) 2

dM = Largest diameter.

dm = Smallest diameter.

Relative frequency was obtained with the formula:

Fr = Absolute frequency of each species ( Fa ) Absolute frequency of all species × 100

where:

Fa = Number of quadrants in which each species is present Total number of sampled quadrants

Relative density was calculated with the formula:

Dr = Absolute density of each species ( Da ) Absolute density of all species × 100

where:

Da = Number of individuals of species Sampled area

with the list of species at each site, a parsimony analysis of endemicity was performed using the software PAST 3.9, which permits classifying a population into a given number of groups based on the similarity that exists among the sites, considering the presence of the species that make up each plant community.

Vegetation by site. Based on the results of the IVI (Table 3), plant communities were determined based on the dominant life form of the plant species and are presented below:

1) Sierra de Zapalinamé (SZP): The plant community is chaparral and comprises three strata: the herbaceous stratum is dominated by the species Chysactinia mexicana A. Gray, Aristida adscensionis L. and Stenaria nigricans Lam., while in the shrub stratum Purshia stansburyana (D. Don) S.L. Welsh, Purshia plicata (D. Don) Henrickson and Nolina cespitifera Trel. dominate. The tree stratum includes very few isolated individuals of Juniperus f. and Quercus mexicana Bonpl (Figure 2).

2) Cerro del Cristo Rey (CCR): The plant community recorded is Agave striata rosetophile scrub composed of two strata, herbaceous and shrub, the first dominated by Euphorbia antisyphilitica, Dalea foliolosa (Aiton) Barneby var. foliolosa and Chrysactinia mexicana A. Gray, while the second is dominated by Agave striata var. striata (Salm-Dick), Opuntia stenopetala Engelm. and Echinocactus platyacanthus Link & Otto (Figure 3).

3) San Joaquín de Soto (SJS): An oak forest comprising three strata was registered. The herbaceous stratum is dominated by Avena fatua L., Tagetes lucida Cav. and Bouteloua sp., the shrub stratum by Crataegus

baroussana Eggl., Rhus virens and Mimosa biuncifera var. lindheimeri (A. Gray) BL Rob., while the tree stratum is dominated by Q. mexicana, Arbutus xalapensis Kunth and Pinus nelsonii (Figure 4).

4) La Peña (LPM): The community has H. hernandez-sandovalii rosetophile scrub vegetation with three strata. A. adscensionis, E. antisyphilitica and D. foliolosa dominate the herbaceous stratum, H. hernandez-sandovalii I. Ramírez, CF Jiménez & J. Treviño and A. striata are outstanding in the shrub stratum, and in the tree stratum only a few isolated individuals of Pinus cembroides Zucc. are found (Figure 5).

5) Camino al Estanque de los Walle (CEW): At this site, vegetation is Hechtia hernandez-sandovalii rosetophile scrub with two strata. The herbaceous stratum is made up of Heliotropium sp., Zinnia juniperifolia (D.C) A. Graya and Dodonanea viscosa Jacq., while in the shrub stratum H. hernandez-sandovalii, Mortonia greggii A. Gray, and P. stansburyana are outstanding (Figure 6).

6) Cañón de la Nopalera (CNM): In this chaparral-type plant community, three strata are present. The dominant species in the herbaceous stratum are A. adscensionis, D. foliolosa and Tradescantia bracteata Small ex Britton, while the herbaceous stratum is made up of R. virens, Quercus greggii and Q. Mexicana. In the tree stratum there are isolated Pinus cembroides individuals (Figure 7).

7) Camino a Bustamante (CBB): This plant community is A. lechuguilla rosetophile scrub and has three strata. The herbaceous stratum is represented by C. mexicana, D. foliolosa and A. adscensionis, while in the shrub stratum A. lechuguilla, O. stenopetala and Berberis trifoliolata Moric. var. trifoliolata are outstanding. In the tree stratum, there are only a few Yucca filifera Chabaud individuals (Figure 8).

Floristic inventory: A total of 33 families, 60 genera and 69 species were recorded (Table 4). The most represented families were Asteraceae, Asparagaceae, Fabaceae, followed by Cactaceae, Poaceae and Rosaceae. The sites with higher species richness are Sierra de Zapalinamé with 23 and Camino a Bustamante with 21, followed by Cerro del Cristo Rey with 19 and Camino al Estanque de los Walle with 18, while the sites with the smallest number of species are Cañón de la Nopalera with 14, and San Joaquín de Soto and La Peña with 13 species each.

Endemism by study site: Eleven endemic species were registered (Table 5), representing five families. Asparagaceae is the best represented family with three species, while the families Bromeliaceae, Cactaceae, Cupressaceae, Krameriaceae and Scrophulariaceae were represented by one species each. The endemic species mentioned are distributed in the seven study sites. A. lechuguilla, D. cedrosanum and N. cespitifera are found in Sierra de Zapalinamé, in Saltillo, Coahuila. A. lechuguilla and O. stenopetala are found in Cerro del Cristo Rey, Galeana, Nuevo León; A. lechuguilla, Agave striata, H. hernandez-sandovalii, L. frutescens and O. stenopetala are endemic to La Peña, Miquihuana, Tamaulipas. A. lechuguilla, D. miquihuanense, H. hernandez-sandovalii, K. cytisoides, L. frutescens and O. stenopetala are found in Camino al Estanque de los Walle, Miquihuana, Tamaulipas. A. striata, D. miquihuanense and O. stenopetala are found in Cañón de la Nopalera as well as in Camino a Bustamante, Bustamante, Tamaulipas. In contrast, no endemic species were registered in San Joaquín de Soto or Aramberri, Nuevo León.

The Parsimony Analysis of Endemicity (Figure 1) found that the plant communities are grouped as follows. CEW belongs to H. hernandez-sandovalii Rosetophile Scrub and CBB is A. lechuguilla Rosetophile Scrub. LPM and CNM have A. striata Roseophile Scrub and R. virens Chaparral, respectively. CCR vegetation is A. lechuguilla Rosetophile Scrub, while in SZP R. virens Chaparral is observed, forming separate group (Figure 9).

Vegetation by site. Six of the sites (the exception was San Joaquín de Soto) converge in SMO and DCH, and thus, according to the vegetation identified, correspond to rosetophile scrub. In each site associations of species, such as L. tridentata, A. lechuguilla, Y. filifera, Z. juniperifolia as well as Cactaceae, are

SZP = Sierra de Zapalinamé, Saltillo, Coahuila, CCR = Cerro de Cristo Rey, Galeana, Nuevo León, SJS = San Joaquín de Soto, Aramberri, Nuevo León, LPM = La Peña, Miquihuana, Tamaulipas, CEW = Camino al Estanque de los Walle, Miquihuana, Tamaulipas, CNM = Cañón de la Nopalera, Miquihuana, Tamaulipas. CBB = Camino a Bustamante, Bustamante, Tamaulipas.

SZP = Sierra de Zapalinamé, Saltillo, Coahuila, CCR = Cerro de Cristo Rey, Galeana, Nuevo León, SJS = San Joaquín de Soto, Aramberri, Nuevo León, LPM = La Peña, Miquihuana, Tamaulipas, CEW = Camino al Estanque de los Walle, Miquihuana, Tamaulipas, CNM = Cañón de la Nopalera, Miquihuana, Tamaulipas. CBB = Camino a Bustamante, Bustamante, Tamaulipas.

found (Figure 10). These results coincide with Granados-Sánchez [38], it has been reported that the presence of these species is influenced mainly by soil type and composition [39].

Estrada-Castillón [40] coincides in that Asteraceae is one of the most representative families for the case of scrub vegetation. Within the outstanding group of families, Asteraceae, Asparagaceae and Fabaceae are notable, the first two of these are reported as dominant [7 , 16 , 18].

The plant communities were grouped as follows: CEW and CBB, with a dominance of the shrub stratum with the Asparagaceae family. For this reason, the presence of D. cinnabarinus is influenced by the dominance of the shrub stratum and of those species with the capacity to store water and nutrients, as is

the case of the genera Agave, Hechtia and Dasylirion, while in LPM and CNM the shrub stratum with R. virens and Quercus greggii, respectively, is dominant. In both sites the shrub stratum dominates, and a similar type of vegetation is present, forming another group in the dendrogram resulting from the parsimony analysis of endemicity. This is due to the difference between the vegetation of the two sites. For CCR, dominance of the shrub stratum with A. lechuguilla is observed, while for SZP the shrub stratum dominates, and Purshia stansburyana is the outstanding species. These two sites form separate groups

despite having similar vegetation type. The rosetophile shrub, however, contains different species. Finally, SJS has totally different vegetation (oak forest); thus, a taxonomic and genetic study should be conducted to determine whether the Dichromanthus species is the same for the rest of the sites.

Floristic analysis. The floristic inventory of northeastern Mexico (Table 4) shows high diversity in the area containing the study sites: a total of 33 families, of which the most diverse families are Asteraceae and Asparagaceae. This coincides with Villaseñor [41]. Moreover, results of studies conducted in related areas coincide with the work of Molina [19], who reports that the outstanding families were Cactaceae and Asteraceae, while Mora-Donjuán [23] reports the families Fabaceae, Cactaceae and Asteraceae [20] and mentions the family Fabaceae as the most representative.

Sierra de Zapalinamé, in Saltillo, Coahuila (DCH), and Camino a Bustamante, Tamaulipas (SMO), contain 23 and 21 species, respectively. These sites were the most diverse, followed by Cerro del Cristo Rey in Galeana, Nuevo León (SMO) with 19 species, coinciding with the results reported by Encina et al. [6]. Camino al Estanque de los Walle, in Miquihuana, Tamaulipas (DCH and SMO), contains 18 species, followed by Cañón de la Nopalera, in Miquihuana, Tamaulipas (DCH and SMO) with 14 species, and San Joaquín de Soto, in Aramberri, Nuevo León (SMO) and La Peña, Miquihuana, Tamaulipas (DCH and SMO) have 13 species. Despite having relatively few species, they are areas with high biological diversity, according to Gonzáles-Elizondo [31], who mention that these eco-regions have a high degree of diversity.

Endemism. The most outstanding families were Asparagaceae, Cactaceae, Cupressaceae, Krameriaceae and Scrophulariaceae, coinciding with the results of Alanís-Flores et al. [42]; Villarreal-Quintanilla et al. [4] mention that the Cactaceae and Asparagaceae families are outstanding for the number of endemics in the northern region of Mexico.

D. cinnabarinus is found in diverse vegetation types and associated with Aristida adscensionis L., Agave lechuguilla Torr. Bouteloua sp., Euphorbia antisyphilitica Zucc., Agave striata var. striata (Salm-Dyck) and Rhus virens Lindh. ex A. Gray. Moreover, Dichromanthus cinnabarinus is observed in some sites with contrasting flora, according to the resulting list of endemic species. However, we consider that a process of speciation has occurred, and therefore, the specimens found may be new species of the genus Dichronmanthus in sites with contrasting vegetation and flora. The results of our study are the basis for development of later studies with DNA to corroborate the hypothesis that there may be new species of the genus Dichromanthus.

It is relevant to mention the importance of establishing priority areas as a strategy of conservation and sustainable management of wild species in northeastern Mexico with the aim of protecting the processes of speciation occurring in this region.

Our thanks to the Universidad Autónoma de Tamaulipas, Facultad de Ingeniería y Ciencias and to the Programa de Maestría en Ciencias Sistemas Agropecuarios y Medio Ambiente for their support. We acknowledge CONACyT for the grant conferred (781879), and Dr. Juan Castillo Landeros for his contributions to the manuscript.

The authors declare no conflicts of interest regarding the publication of this paper.