Study Area

El Cielo Biosphere Reserve is located in the southwest of the state of Tamaulipas, in the foothills of the Sierra Madre Oriental. It is limited by the parallels 23°12'and 23°03'N and the meridian 99°18' W (^{Puig and Bracho, 1987}). According to ^{Conabio (2010)}, El Cielo includes the most northern fragments of BMM distribution in the American continent. The largest portion of BMM in Tamaulipas is located in the reserve (approximately 16 142.86 ha), and it also has a high degree of conservation (^{Sánchez-Ramos et al., 2005}).

The altitudinal limits of the mesophilic mountain forest in El Cielo are 800 and 1 500 m, covering an approximate area of 100 km². The predominant soils are lithosols (black), rendzinas (black) and chromic luvisols (black) (^{Puig and Bracho, 1987}). The semiwarm subhumid climate prevails in most of the mesophilic forest; rain precipitations are greater than 2 000 mm annually, and they are located in heights between 700 and 1 400 masl (^{Casas and Requena, 2005}).

In places without disturbance there is a dense forest in the tree stratum, which in turn differs according to the height in four sub-strata. The canopy has a height between 18 and 30 m; is composed of Liquidambar styraciflua L., Clethra pringlei S. Watson, Quercus germana Schltdl. & Cham. and Quercus sartorii Liebm., in addition to Fagus mexicana Martínez in some areas. Other species are distributed between 12 and 18 m, such as Carya ovata (Mill.) K. Koch, Magnolia tamaulipana A. Vázquez, Ostrya virginiana (Mill.) K. Koch, Podocarpus reichei J. Buchholz & N.E. Gray and Tilia houghi Rose. In a third substratum (8 to 12 m), Meliosma oaxacana Standl. and Turpinia occidentalis (Sw.) G. Don grow; and in the fourth, Eugenia capuli (Schltdl. & Cham.) Hook. & Arn., Rapanea myricoides (Schltdl.) Lundell and Ternstroemia sylvatica Schltdl. & Cham. (^{Puig et al., 1983}).

Selection of sites and sampling

Field work began in April 2011, with monthly trips of one week, in a period of six months. Sites located in the altitudes of 800, 1 000, 1 300 and 1 500 masl were selected, according to the distribution of the mountain mesophylic forest in the Reserve (^{Puig and Bracho, 1987}).

Based on the methodology proposed by ^{Gradstein et al. (2003)}, in each altitudinal elevation five mature trees were chosen, with a minimum distance of 25 m between each individual. The largest ones were chosen, because they are presumed to be the oldest, and therefore, subject to greater colonization by epiphytes (^{Gradstein et al., 1996}).

Around each mature tree a quadrant of 20 × 20 m was delimited, in order to sample the neighboring specimens with the presence of epiphytes. The location of each sampled arboreal individual was recorded by means of a Garmin e-trex GPS and its diameter at breast height (DAP) was measured with a 10 m diameter tape (Forestry suppliers); and its height, with a clinician PM5 / 360PC Suunto.

For the registration of the epiphytic species, the sampled tree (both mature and those of the quadrants) was subdivided into the five zones proposed by ^{Johansson (1974)} (Figure 1). The epiphytes were considered by arboreal individual, from the base of the trunk to the outer branches.

Source: ^{Bøgh, 1992}.

Zona = Zone Zone 1 = 0.22-3 m from the ground; Zone 2 = 3 m-first main branches; Zone 3 = Large branches; Zone 4 = Middle and thin branches; Zone 5 = The thinnest branches

Figure 1 Zoning of the host tree (^{Johansson, 1974}).  

In order to have access to the canopy, climbing equipment, harness, hammers, elevators, stirrups, carabiners and descent device were used (^{Benzing, 1990}). In addition, observations were made from the ground, with the help of binoculars (Eagle Optics Ranger ED). A total of 188 trees of 24 species, 20 genera and 17 families were sampled: 40 at an elevation of 1 500 m, 50 to 1 300 masl, 46 to 1 000 masl and 52 to 800 masl.

Species of vascular epiphytes, both adult and juvenile specimens, were included in the sampling. They were taken as vegetative units: individuals, rosettes and leaves, (^{Hietz and Hietz-Seifert, 1995}). For the first category, the Peperomia genus was considered; for the second, most of the bromeliaceae taxa; and for the third, the ferns and most of the genera of orchids that form large aggregates, in which it is not reliable to distinguish one individual from another. Copies of all the taxa were collected, which were herborized, identified and deposited in the herbarium of the Facultad de Ciencias Forestales de la Universidad Autónoma de Nuevo León (School of Forest Science of the Autonomous University of Nuevo León).

Statistical Analysis

The diversity of the epiphytes was calculated with the Shannon index. Phenograms were elaborated to know the degree of similarity that exists between the zones of the tree, according to the epiphytic species contained. These also provide the value of the Sørensen similarity index. A χ² test was applied to determine the preference of the epiphytes towards the hosts, as well as to know the association between the epiphytes and each one of the zones of the tree proposed by ^{Johansson (1974)}. The phylogenetic distance between the species, measured in millions of years from a common ancestor was detected with Timetree (^{Hedges et al., 2006}), and the differences between phylogenetic distances were analyzed with an ANOVA.

Abundance and species richness

Ten families, 23 genera and 33 species of epiphytes were identified (Table 1). The total area sampled was 0.8 ha (8 000 m²). The Orchidaceae family represented 43 % of the total taxa, followed by Polypodiaceae (15 %) and Bromeliaceae (12 %).

115 069 vegetative units were recorded. Six species have the highest percentage of abundance of individuals: Pleopeltis polylepis (Roem ex Kunze) T. Moore (Polypodiaceae, 40.65 %), Isochilus unilateralis B.L. Rob. (Orchidaceae, 16.52 %); Polypodium polypodioides (L.) Watt (Polypodiaceae, 9.74 %); Asplenium cuspidatum Lam. (Aspleniaceae, 9.22 %), Tillandsia deppeana Steud. (Bromeliaceae, 4.87 %); and Tillandsia bartramii Elliott (Bromeliaceae, 4.81 %). The rest presented a low abundance, lower than 1 % (Figure 2).

Especies epífitas = Epiphyte species; Abundancia relative = Relative abundance; Otras = Others.

Figure 2 Relative abundance (% of individuals) of the epiphyte species.  

Quercus germana was the phorophyte with the highest percentage of epiphytes (38.08 %), followed by Q. sartorii (21.61 %) and Clethra pringlei (13.31 %)

Phorophyte -epiphyte association

Table 2 shows the results of the χ² test. Epiphytes associated with phorophytes were not phylogenetically more distant from phorophytes than non-associated epiphytes (F = 0.51, P = 0.43).

Similarity between phorophytes by epiphytic load

The phenogram to analyze the similarity between tree species, with respect to the epiphytes they harbor (Figure 3) shows that the species with the greatest similarity were: 1) Nectandra sanguinea Rol. ex Rottb., Meliosma oaxacana and Abies vejarii Martínez (Index of Similarity = 1.0); 2) Quercus germana and Quercus sartorii (Similarity Index = 0.945); 3) Podocarpus reichei and Magnolia tamaulipana (Similarity Index = 0.880); and 4) Turpinia occidentalis and Rapanea myricoides (Similarity Index = 0.875).

Coeficiente de Sörensen = Sörensen coefficient

Figure 3 Phenogram of similarity among phorophyte species. 

Proportion of species by families in altitudinal elevations

At 1 500 masl, 15 species and six families were identified, Orchidaceae had the highest proportion of species (33 %), followed by Bromeliaceae (26 %) and Polypodiaceae (20 %). At 1 300 masl, 16 species and seven families occurred, including Polypodiaceae (29 %), Orchidaceae and Bromeliaceae (18 % and 17 %, respectively). Two families are restricted to this altitudinal floor: Crassulaceae and Begoniaceae, although their representation within the quadrants is of few individuals. At the level of 1 000 masl, 12 species and five families were recorded, Orchidaceae was the best represented (34 %), followed by Bromeliaceae and Polypodiaceae, both with 25 %. At 800 masl there were 24 species and eight families, of which Orchidaceae had the highest proportion of species (41.67 % of the total), Polypodiaceae (16.67 %) and Bromeliaceae (12.50 %). The highest number of epiphytes was obtained at 800 masl, which coincides with the Shannon Index by altitudinal elevation (Table 3). The phylogenetic distance between epiphytic species was similar within and between altitudes (F = 1.41, P = 0.18).

Vertical distribution of epiphytes

The vertical distribution of the epiphytes in each of the sections of the phorophyte shows its marked preference towards zones III and IV of the host, according to the Johansson classification (Figure 4; χ2 = 31 700.00, p <0.001), sites that correspond to the basal and middle part of the branches; there are more epiphytes than would be expected in a random distribution.

Zonas del forofito = Zones of the phorophyte; No. de individuos epífitos = Number of epiphytic individuals.

Figure 4 Number of epiphytic individuals in each of the areas of the phorophyte. 

There were differences in the distribution per zones of all species, whose calculated χ² values were higher than those of the tabulated χ² (χ²= 9.48, p= 0.05). This means that the epiphytes, even those that are presente in the five zones of the phorophyte, are more common in one or two of them and it is there where the greatest abundance was recorded.

Abundance and species richness

The predominance of the families Orchidaceae, Polypodiaceae and Bromeliaceae coincides with the classification made by ^{Gentry and Dodson (1987)} of these taxonomic groups, with respect to the total number of epiphytic species. It is worth mentioning that Orchidaceae, although it has the highest number of taxa, it has a low abundance. Isochilus unilateralis (Orchidaceae) had the greatest value in the fragment of cloud forest studied, perhaps because of the small space it occupies. The pattern of distribution of this family agrees with what is cited in the literature, regarding that orchids have a high diversity of species, although the main characteristic of many of them is their scarcity of individuals (^{Nieder et al., 1999}; 2006).

It stands out for its abundance of Pleopeltis polylepis, which is explained by its tendency to form large conglomerates, which sometimes cover almost completely the branches of the phorophytes, without allowing the growth of other species in the vicinity.

Epiphyte-phorophyte association

The main function of the phorophyte is to provide support to the epiphytes. If its function were restricted to it, the distribution of these plants would be general, regardless of the arboreal species that supports it. Some correlate with the tree taxa by the χ² test. That is, there are characteristics of the phorophytes that affect the distribution and abundance of epiphytes (^{Callawell et al., 2002}, ^{Krömer and Gradstein, 2003}). This correlation was determined in the most abundant species, as in the most scarce; examples of the first are: Isochilus unilateralis, Campyloneurum angustifolium (Sw.) Fée, Polypodium polypodioides and Tillandsia bartramii; and of the latter: Sedum sp., Laelia anceps Lindl., Begonia sp., Arpophyllum laxiflorum Pfitzer and Piper sp. However, the rarity of these epiphytic species may also be the factor that affects the values of χ2, so that they suggest a correlation with the phorophyte. Increasing the number of collections would help to expand the last point raised.

Elements such as the size and shape of the tree determine the abundance and richness of epiphytic species. The predominance of Quercus germana as a preferred phorophyte, it is due to its abundance and physiognomy, since they have a very branched shape and a greater basal area, as recorded by ^{Rosa-Manzano et al. (2017)} in a pine-ecino forest in CBR. In addition, the almost horizontal orientation of its branches propitiates the accumulation of organic matter and the prolonged conservation of environmental humidity, which is ideal for the establishment of many epiphytes (^{Puig, 1993}).

Other tree species very common in the mountain mesophilic forest, such as Liquidambar styraciflua, contain few plants of this type, although they have rough bark. However, its shaft is almost straight, its ramifications are thin and its canopy cover is small, so it offers little fixing surface for epiphytes (^{Puig, 1993}).

Vertical distribution

In several works it is mentioned that epiphytes tend to be more abundant in zones III and IV of the phorophytes (^{Nieder et al., 1999}; ^{Krömer et al., 2007}), as observed in the present study. According to ^{Nieder et al. (1999)}, height preferences correspond to adaptations of physiological and mechanical type. Thus, for example, the genus Campyloneurum is more abundant in the first two zones of the phorophyte, which reflects its adaptation to a gloomy environment. In as much, Pleopeltis is distributed in the five areas of the phorophyte, although it is more abundant in zones III and IV. Also in those sites a large part of the orchids is concentrated, with the exception of Isochilus unilateralis, which can be found in zone V, but in smaller numbers, mainly due to the high insolation and the rapid desiccation prevailing in that area.

Comparison with similar studies in neotropical mountain forests

The total of identified taxa is lower than that documented in studies carried out in another mountain mesophilic forest. ^{Martínez et al. (2006)} recorded 44 species in an area of 0.04 ha in Chiapas; ^{Hietz and Hietz-Seifert (1995)} cite 53 at an altitude of 1 439 masl, within an area of 0.06 ha, with a total of 134 taxa for all their sampling sites. At present, 33 species are recorded in 0.6 ha.

In more southern latitudes, such as Ecuador and Bolivia, the number of species increases, notably (^{Krömer and Gradstein, 2003}). The high number and diversity in the equatorial zones is explained by the multiple niches and microclimates derived from orography (^{Küper et al., 2004}). The lower number of taxa can be explained by the fact that as the latitudinal gradient increases, the diversity of epiphytes decreases (^{Gentry and Dodson, 1987}).

Given that the El Cielo Biosphere Reserve is one of the sites that mark the boreal distribution limit of the mesophilic forest in Mexico (^{Conabio, 2010}), it is understood the low number of species in relation to similar forests, but located in more latitudes. to the south.