La comunidad de Furcraea parmentieri, una especie amenazada, en el centro de México

Lucía Almeida-Leñero, Ana María Mondragón, Beatriz Ludlow-Wiechers, y Verónica Aguilar-Zamora

Laboratorio de Ecosistemas de Montaña. Facultad de Ciencias, Departamento de Ecología y Recursos Naturales, Universidad Nacional Autónoma de México, Ciudad Universitaria, CP 04510, DF, México. Correo electrónico: lal1@hp.fciencias.unam.mx; verin_a16@yahoo.com.mx.

Recibido: 2 abril 2012.
Aceptado: 14 mayo 2013.

Abstract

The flora and vegetation of Furcraea parmentieri (Roezl ex Ortigies) Garcia-Mend. (F bedinghausii) community was studied on the Pelado volcano, at the S area of Mexico City. Following Zurich-Montpellier criteria, 25 phytosociological plots were done, and the Jaccard index of similarity was calculated. Furcraea parmentieri is associated with Muhlenbergia macroura to form an azonal community between 3 020 and 3 300 m, on rocky soils and gaps of Pinus-Alnus forest. Three subcommunities can be distinguished: Furcraea parmentieri-Trisetum virletii, Furcraea parmentieri-Aegopogon cenchroides and Furcraea parmentieri-Solanum cervantesii. 87 species were recorded, representing 63 genera and 27 families, of which the most abundant were Asteraceae and Poaceae. The proper environmental conditions for Furcraea parmentieri are: altitudes between 3 020 and 3 100 m, N exposures, acid soils with pH 6.0 and slopes among 36° and 45°. The main problem for species preservation in the area is repeated burning, that depletes soils and destroys vegetative bulbils, causing population diminishment. Conservation of Furcraea parmentieri community is important because, it is an endangered and endemic species, as well as a soil-stabilizing species for severely degraded zones (NOM-059-ECOL-2001).

Key words: mega-rossette, vegetation azonal, endemic, conservation.

Resumen

Se estudió la flora y vegetación de la comunidad de Furcraea parmentieri (Roezl ex Ortigies) Garcia-Mend. (F. bedinghausii) especie amenazada y endémica de México (NOM-059-ECOL-2001), en el volcán Pelado, al S de la Ciudad de México. Se realizaron 25 levantamientos siguiendo la escuela fitosociológica Zurich-Montpellier, a los que se calculó el índice de similitud de Jaccard. Furcraea parmentieri se asocia con Muhlenbergia macroura formando una comunidad azonal, entre los 3 020 y 3 330 m, sobre suelos rocosos y claros del bosque de Pinus-Alnus. Se diferenciaron tres subcomunidades: Furcraea parmentieri-Trisetum virletii, Furcraea parmentieri-Aegopogon cenchroides y Furcraea parmentieri-Solanum cervantesiis. Se registraron en total 87 especies agrupadas en 63 géneros y 27 familias, siendo las más abundantes Asteraceae y Poaceae. Las condiciones ambientales propicias para el establecimiento de Furcraea parmentieri son altitudes entre 3 020 a 3 100 m, en exposición N, suelos ácidos con pH 6.0 y pendientes de 36° a 45°. El mayor problema para la conservación de la especie en el área son las quemas repetidas que empobrecen el suelo y destruyen los bulbilos, causando la disminución de las poblaciones. La conservación de F. parmentieri es importante porque además de ser una especie amenazada y endémica, estabiliza suelos en zonas severamente degradadas.

Palabras clave: megaroseta, vegetación azonal, endémica, conservación.

Introduction and aim

Various authors have studied the coniferous forest in the TMVB and the Valley of Mexico. It is part of the floristic province of the Mesoamerican mountain region, which is a zone of transition between the Holarctic and the Neotropical Kingdoms and consists of mountainous massifs characterized by temperate forests (Challenger, 1998; Rzedowski, 2006; Luna et al., 2007).

The mega-rosette community (Velázquez y Cleef, 1993) dominated by Furcraea parmentieri (Roezl ex Ortigies) Garcia-Mend. (F. bedinghausii) is a mature community in the Pelado volcano. This is a threatened species endemic to Mexico and protected by specific laws (NOM-059-ECOL-2001; SEMARNAT, 2002). The species distribution is restricted to the TMVB, being the Pelado and Ajusco, the volcanos with the major populations of this species. It is located in the igneous-rock slopes between 2 500 and 3 400 m, associated with Pinus and Abies forest in shallow soils (Rivera y Henze, 2007). F. parmentieri is present in the Valley of Mexico, and in the states of México, Guanajuato, Hidalgo, Jalisco, Michoacán, Morelos and probably in Veracruz (Rzedowski et al., 2001). In spite of its wide distribution, its populations are scarce and small and it forms communities only around the Valley of Mexico, on the Ajusco and Pelado volcanoes on volcanic soils, below the summits of the highest mountains and in some high-lying recent lava streams (pedregales). The Ajusco community populations show teratological phenomena such as fused or twisted flowers and flowering spikes. It has a stenoic habitat specificity, small populations and demographic rarity; studies on the gene pool of the plant have demonstrated a very low level of variation among populations, which suggests that most plants may be of clonal origin, via bulbils (García, 2001).

The species occurs in Abies, Quercus and Pinus forests, on rocky sites. Its physiognomy resembles a cluster of palms with a showy panicle in the middle of the forest (Rzedowski et al., 2001). The species is also used for soil stabilization, the dried leaves are used as fibers for bundling cereal crops, maize plants and fodder for livestock, and the whole plants are used as living hedges. It is cultivated as an ornamental in botanical gardens in Europe and the USA, although it is not grown in large numbers. In Mexico, F. parmentieri flowers from April to July, with flowers arranged in white-to-cream panicles visited by hummingbirds. Flowering seems to occur in plants that are 20-50 years old. Few of the capsules reach the fruiting stage (García, 2000, 2001); most inflorescences are replete with bulbils.

It is essential to investigate factors that will aid conservation and management (Galván, 1988). For these reasons, the aim of this study was to analyse the flora and vegetation of the azonal community of Furcraea parmentieri on the Pelado volcano, DF, Mexico based on a physiognomic-floristic analysis.

Study area

Pelado volcano, with a surface area of ~4800 ha, lies between 19°06' and 19°12' N and 99°12' and 99°16' W. It is located in the Valley of Mexico, forming part of the Sierra Chichinautzin of the central region of the TMVB (Mondragón, 2001). Geologically this volcano belongs to the formation Chichinautzin from the Plio-Cuaternary (Demant, 1978, fig. 1).

The topography of the study area is rugged with an altitudinal gradient of 3 020 to 3 550 m and with a series of ridges having regular and moderate gradients. The volcano has a cineritic cone between 3 450 and 3 550 m (González, 1982). There are andesites and basalts derived from the lava flows of the Chichinautzin formation during the Upper Pleistocene (Rzedowski, 1978). The Pelado volcano forms part of the ridgeline of the Basins of Mexico and Balsas, with a radial hydrologic system that allows the formation of intermittent streams during the raing season, that are important for the recharge of the subterranean aquifers of the S of Mexico City (Cervantes, 1980; González, 1982). The soils of the study area are mainly Lithosols and Humic Andosols of medium texture, with a high capacity for the retention of water and nutrients; they are susceptible to erosion and occur on flat or slight slopes zones (Ortiz-Villanueva, 1975).

The nearest meteorological station is 'El Guarda', at 3 000 m. According to García (1988), the climate corresponds to Cb'(w2)(w)ig type, cold temperate, with rains in summer and precipitation/temperature quotient exceeding 55, equivalent to the most humid of the sub-humid categories. The rainfall in winter is less than 5% of the annual precipitation, the summer is long and cool and there is little oscillation in average monthly temperature (between 5 and 7°C). The average annual temperature is 9°C. The highest monthly average temperature occurs in May and August (14 to 15°C) and the lowest in January and February (2 to 5°C). The total annual rainfall is around 1 226.8 mm, of which 91.3% occurs during the rainy season from June to August. The driest months are November and December, with an average rainfall of 9 mm. December has the most days of freezing temperatures. The rest of the months have a precipitation that goes from 11 to 50 mm. The greatest number of days with frost occurs in December, whereas June and September have the highest number of days with fog.

Methods

Twenty-five phytosociological plots were documented in the azonal vegetation (Walter, 1979), following the criteria of the Zurich-Montpellier phytosociological school (Braun-Blanquet, 1979) adapted for tropical mountains (Van der Hammen et al., 1989), both in the dry and the rain season; the standard plot size was 150 m² (10 x 15 m). Sample sites were selected for physiognomic and floristic homogeneity in areas where F. parmentieri forms communities.

The number of plots was established on the basis of the curve of species/cumulative area (Matteucci y Colma, 1982); the inflexion point occurs at a cumulative area of 2 100 m² and 56 species (fig. 2).

For each plot, the locality, altitude, physiognomy and gradient were recorded, and a complete inventory of vascular plants was compiled. The percentage cover was estimated for the vegetation layers and for species in relation to the surface area of the sample site. At ground level, the bryophyte cover was estimated separately. In the herbaceous layer the cover of grasses was estimated separately from other herbs. In the same sites, 250 field individual F. parmentieri plants, the circumference at breast height (CBH) and the plant height were measured, thereby obtaining the composition of height classes and average diameters in the population. For each species, the following data were considered: stratum and life form sensu Raunkiaer (1934). For the species identification the criteria of Rzedowski et al. (2001) were used. Most collected material was deposited in the "Herbario Agustina Batalla" of the Faculty of Sciences (FCME) at the Universidad Nacional Autónoma de México.

A phytosociological table was constructed, classifying the species according to presence or absence and % cover. In addition, the percentage of bare soil, and the percentage of rock and the depth of soil were recorded. Soil samples to an average depth of 10 cm were studied; in total, 20 samples were analysed, recording colour when wet and when dry for comparison on the Munsell scale, pH ratio 1:2.5, KCl, percentage of allophane and organic matter.

Results

Furcraea parmentieri-Muhlenbergia macroura community (3 020-3 330 m)

Typical plot table 1a and 1b N 20.

This is a semi-open community, in which Furcraea parmentieri is the dominant species with an average cover of 50%. height of young plants rangos from 0.75 and 1.50 m, whereas the height of adult plants ranges from 2 and 8 m. Adult plant, have erect trunks and a mean CBH of 98 cm. The leaves, semi-fleshy and narrow, are 30-60 cm long and arranged in the form of a rosette. Senecio cinerarioides and Symphoricarpus microphyllus occur in the shrub layer. The herbaceous layer, strongly developed, reaches a height of 1.50 m, covering an average of 70% of the land, both with grasses and with other herbs. The ground-level vegetation, less than 5 cm in height, has a mean cover of 14%, and the most frequent species is Lachemilla procumbens.

The characteristic life forms of the community, in terms of percentage cover, are as follows: phanerophytes 50% (Furcraea parmentieri), hemicryptophytes 23% (Muhlenbergia macroura), therophytes 16%, chamaephytes 10%, and geophytes 1%. Growth habits are diverse the erect habit is well represented 71%, an example being Salvia elegans; tussocks such as Muhlenbergia macroura are found in 15%; the prostrate form such as Sicyos deppei in 7%; the climbing habit in 2%; and cushions and rosettes in 1%, for example, the moss Bryum argentum in the most humid areas.

The characteristic species is Furcraea parmentieri, with 20-100%, and the co-dominant is Muhlenbergia macroura with 2-70% cover in the herb layer. With a lower incidence and with a high percentage of cover, there are Lachemilla procumbens, Dahlia scapigera and Eryngium columnare. Other important species are Salvia elegans and Villadia batesii, the latter on basaltic rock. Bryophytes occur on rocks or trunks, although Bryum argentum appears on the surface of the soil. In addition, during the rainy season, the following species appear: Commelina alpestris, Oenothera purpusii, Oxalis alpina, Peperomia campylotropa, Piptochaetium fimbriatum, Tagetes coronopifolia, Trifolium amabile, Trisetum virletii, Sicyos deppei and Conyza canadensis. The two last-named are characteristic of disturbed areas, and they may form dense populations.

The greatest floristic diversity occurs between 3 000 and 3 100 m (fig. 3), where Furcraea parmentieri has a cover of 4050%. At high altitude, the number of species decreases, and this also occurs when Furcraea parmentieri accounts for 60% of cover (fig. 4).

Even though the rocky substratum is not an indispensable environmental condition for the establishment of the Furcraea parmentieri community, it is an important factor, since the greater percentage cover of this species occurs when the rock covers 18-30% of the area.

The greatest number of individual Furcraea parmentieri plants is found on the N slope (65%), followed by the E slope (45%) and the S slope (35%). The greatest floristic diversity is found when the soil pH is 6.0, and it decreases as the pH increases to 7.0-7.2.

Outside the Valley of Mexico, in the states of Hidalgo and Mexico, it has been reported communities of Furcraea parmentieri at altitudes of 2 650-3 200 m in association with Quercus, Pinus, Abies and mixed forest (Rzedowski, 1985).

Subcommunities

Furcraea parmentieri-Trisetum virletii sub-community (3 020-3 270 m)

The surface with vegetation amounts to 90100% cover. The herbaceous layer develops preferentially on areas near rocks, whereas the ground-level vegetation covers 2% of the surface. The life forms present are as follows: therophytes 37%, chamaephytes 32%, geophytes 12%, phanerophytes 10%, and hemicryptophytes 7% (fig. 5).

Furcraea parmentieri is the dominant species with 30-100% cover. The co-dominant species is Trisetum virletii (1-15%). Also occurring are Lachemilla procumbens, Eryngium alternatum, Eupatorium glabratum, Salvia fulgens and Verbesina oncophora.

This subcommunity is found on the S, E and N slopes of the volcano, with 1% of the soil exposed, and with 15-60% rock and 3-15% stones. The soil has an average depth of 10 cm, 1-90% leaf litter content, a pH of 5.9 to 7.2 with and organic matter content.

Furcraea parmentieri-Aegopogon cenchroides subcommunity (3 050-3 110 m)

The vegetated surface covers 80%, of which Furcraea parmentieri accounts for a mean cover of 30%. The herbaceous layer develops preferentially on areas protected by the rocks, with a mean cover of 28%. The ground level has a cover of 2%. The coverages of the respective life forms are as follows: therophytes 37%, chamaephytes 34%, geophytes 13% and hemicryptophytes 10%. Furcraea parmentieri represents the phanerophytes with 6%.

Species found in the herbaceous layer are Aegopogon cenchroides, Castilleja arvensis, Eupatorium prunellifolium, Muhlenbergia robusta and Stevia salicifolia; among the ground-level species are found Lachemilla pringlei, Piptochaetiumfimbriatum and Trifolium amabile. As associated species, Arenaria lycopodioides and Penstemon campanulatus appear with great frequency.

This subcommunity is found on the N slope on the lower parts of the volcano, with mean gradients of 40° to 45°, with 13% bare soil and on average 20% rock. There are zones where soil accumulates with fine material, with a depth of 10 cm, a pH of 6 to 6.8 and with organic matter content.

Furcraea parmentieri-Solanum cervantesii subcommunity (3 020-3 060 m)

Vegetation covers 90% of the surface. Furcraea parmentieri and Solanum cervantesii account on average for 50% cover. In the shrub layer there is only Senecio cinerarioides with 10% cover. The herbaceous layer represents 60% cover, comprising Poaceae and other herbs. At ground level there is 10% cover. The life forms are as follows: therophytes 29%, chamaephytes 29%, hemicryptophytes 21%, geophytes 11% and phanerophytes 8%.

The species best represented are Bromus mexicana, Conyza canadensis, Muhlenbergia microsperma, Physalis coztomatl and Trisetum altijugum.

This subcommunity is found principally on the S and E slopes, with gradients of 19-50°. There is 1% bare soil and on average 25% rock. The soil depth is 10 cm, pH is 6.5, with 20% litter, and organic matter is present.

On the basis of field observations and the data on diameters and heights of the Furcraea parmentieri plants, individual plants are considered to be juvenile plants have a stem diameter of 0.15 to 0.20 m and a height of 2 to 3 m (35%). Adults when the diameter of the stem is 0.22-0.32 m and the height are 4 to 5 m (50%). The oldest plants have diameters of 0.35 to 0.40 m and a height of 6 to 8 m (15%).

Discussion

The Furcraea parmentieri-Muhlenbergia macroura community is azonal because it grows in the lava streams (Velázquez and Cleef, 1993). It is distributed between 3 020 and 3 330 m, on Pelado volcano, DF.

Three subcommunities can be differentiated: Furcraea parmentieri-Trisetum virletii, Furcraea parmentieri-Aegopogon cenchroides and Furcraea parmentieiri-Solanum cervantesii. The shallow soil depth and the low organic matter content are conditions that favour the establishment of Furcraea parmentieri, factors that do not offer sufficient support or nutrients for Pinus spp.

The best localities for establishment of this community are open the clearings in the forests, regularly on rocks, with many plants on the N, E and S slopes, at an altitude of3 020-3 100 m, on N-facing slopes, and soil pH 5.9 to 6.2. The greatest floristic diversity occurs when rock covers 6-17% of the terrain, and this diversity decreases with increasing rock cover. The lowest floristic diversity occurs between 3 300 and 3 400 m, and with a neutral to slightly alkaline soil, on the W side there are only isolated individual plants.

The basaltic rock influences the establishment of some species characteristic of zones with a humid temperate climate just as much as those of a hot climate, by retaining heat on being exposed to the solar radiation; species affected include Echeveria mucronata, Eryngium columnare, Peperomia campylotropa and Villadia batesii.

The mega-rosette vegetation type present on Pelado volcano is well conserved compared with the Ajusco community with its teratological anomalies, and within its area of distribution in other states of Mexico there are only isolated plants or infrequent small populations. This community is unique for Mexico and can be compared only with the branched mega-rosettes community of the liliaceous Yucca that is very common to the W of Xalapa, Veracruz.

The fact that the Furcraea community on Pelado volcano grows on lava streams suggests that it lies at the interface between Furcraea and Pinus communities. If soil accumulation continues, this might lead to the establishment of Pinus species and the eventual disappearance of the Furcraea community. To avoid this, monitoring should be in place to control the establishment of the surrounding Pinus community.

Conclusion

In this locality, the vegetation has a healthy status and includes Furcraea parmentieri, which is on the endemic species list (NOM 059-ECOL-2001; SEMARNAT, 2002).

Since the Furcraea community is unique in Mexico, new monitoring and conservation programs should be developed to maintain an open forest. Otherwise, new invasions of underbrush species could reduce the Furcraea community to a small population with small and isolated plants.

Further, plant communities that resemble the present one in structure and ecology are those of Furcraea-Peperomia and other Compositae communities found in the dry intermontane valleys of the Colombian Andes. Physiognomically similar communities of Furcraea-Agave are present in the mountains of Guatemala (Velázquez and Cleef, 1993).

However, the human settlements around the volcano are causing a deterioration in this community, principally as a result of grazing practices, agriculture, tree felling, unauthorized dumping of rubbish, the large number of roads and trails, and the frequent visits of illegal hunters. The greatest problem is repeated burning, with its consequent impoverishment of the soil and the destruction of the vegetative bulbils of F. parmentieri.

The present information on the structure and floristic composition of the F. parmentieri community on the Pelado volcano represents a further contribution towards the conservation of this species, with great importance in the volcanic soils and vegetation communities of the Valley of Mexico, not least because it is used as a soil stabilizing species in the restoration of severely degraded zones.

Acknowledgments

The authors thank Ph. D. Atila Borhidi and Ph.D. Heike Vibrans for their assistance in the revision of manuscript and valuable corrections, and Ann Grant for the translation of the manuscript.

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