Pinaceae of the Distrito Federal
The family Pinaceae is exceptional for its ecological and human importance (Farjon 1990, Richardson and Rundel 1998). Conifer forests and woodlands are dominant elements of terrestrial vegetation, and members of the family cover great extensions throughout their natural range in boreal, subalpine, and temperate zones in North America and Eurasia. Species of various genera, in particular Pinus (pines), have been widely cultivated in the southern hemisphere. They are also minor components of other diverse vegetation types, such as deciduous forests and tropical broadleaf forests. The family has been studied intensively in the fields of ecology, evolution, physiology, genetics, taxonomy, dendrochronology, and forestry science.
Pinaceae is a family of conifers (order Coniferales or Pinales). Eckenwalder (2009) recognizes 11 genera and 195 species, while Farjon (2010) recognizes 11 genera and 231 species. It is the largest of six families making up 30% of the total conifer species diversity. Members of Pinaceae are trees or shrubs with smooth or rough bark, often either scaly or in large, thick plates with vertical fissures. They have needle shaped leaves arranged helically around the branch, attached either singly, or, in Pinus, in groups (fascicles) of two to five, less commonly solitary or in fascicles of up to eight. In most genera, the leaves are persistent for more than one year (evergreen), but in Larix and Pseudolarix they lose their leaves in winter (deciduous). Resin canals are present in wood, bark, leaves, and cones. Seeds and pollen are produced in separate cones in the same individual (monoecious). The pollen (microsporangiate) cones are simple, formed by microsporophylls, each bearing two microsporangia, arranged helically around a central axis. The pollen grains are winged in all genera except Larix and Pseudotsuga. The ovulate (megasporangiate) cones are woody, formed by a bract-scale complex helically arranged around a central axis; they develop in the first year in most species, although in Pinus they develop over two or three years. Each scale has two ovules that are exposed at pollination. The seed has a wing derived from scale tissue, although this can be extremely reduced (for example, in the pinyon pines). The number of cotyledons is from 2 to 15, exceptionally 25 in Pinus maximartinezii.
Diversity and distribution
Only three of the 11 genera of Pinaceae are distributed naturally in the Trans-Mexican Volcanic Belt. Abies (fir) and at least seven species of pine (Pinus) occur in natural areas of the Distrito Federal (D.F.). A third genus, Pseudotsuga, occurs in the mountains in the north of the Basin of Mexico in the state of Hidalgo, outside of the Distrito Federal. The pinaceous species of the Distrito Federal represent 13% of the diversity for the country. All eight are widely distributed in montane habitats of México and Central America; only three, P. leiophylla, P. patula, and P. teocote, are endemic to Mexico, and P. patula is not native to the Distrito Federal, but has been naturalized in many places inside and outside of the country.
Of the 148,549 ha occupied by the Distrito Federal, 17,222 ha were classified as forest in 2005, of which 13,689 were classified as conifer forest by Inegi (2005); in 2010, 417 km2 were classified as forest. These include a minor proportion of juniper forest and more extensive Pinus and Abies forests (Rzedowski et al. 2001). These are predominant vegetation types in the Sierra del Ajusco-Chichinautzin and in the Sierra de las Cruces, located in the south and southwest of the Distrito Federal in the boroughs (Delegaciones) Milpa Alta, Tlalpan, La Magdalena Contreras, Cuajimalpa de Morelos, and Álvaro Obregón. Several species of Pinus also occur in Quercus forests, which reach altitudes greater than 3 100 msnm; for example, in the Pedregal de San Ángel P. leiophylla occurs in oak woodlands of Quercus rugosa and Q. crassipes distributed between 2 500 and 2 900 msnm on volcanic soils deposited by eruptions of the Xitle volcano (Rzedowski 1994). In the Sierra del Ajusco-Chichinautzin and in the Sierra de las Cruces, Pinus forests predominate at altitudes at and above 2 450 msnm. The Pinus forests can be predominantly closed with mixes of P. ayacahuite, P. montezumae, P. hartwegii, and P. patula and include a diversity of herbaceous and shrubby plants. Pinus hartwegii often forms monospecific forests at higher altitudes, often associated with grasslands (Muhlenbergia spp.). Pinus hartwegii reaches treeline in many mountains in Mexico and Central America. It occurs at 3 930 msnm around the summit of the Ajusco volcano, the highest point in the Distrito Federal. In the Mexico Basin, Abies forests are found on less exposed sites with deep soils, at altitudes between 2 700 and 3 500 msnm (Rzedowski 2001). They frequently form mixed forests with Pinus (for example, in the Sierra del Ajusco with P. ayacahuite and P. montezumae).
Many more non-native species of Pinaceae are cultivated in nurseries, parks, and gardens in rural zones of the Mexico Basin, mainly Pinus spp., and to a much lesser degree, Abies religiosa and Cedrus deodara (Martínez González 2008). Some Pinus species very common in gardens are P. radiata var. radiata, originally from the California coast in the United States, and Mexican species P. patula, P. devoniana, and the pinyon pines P. cembroides and P. maximartinezii. The Parque Urbano Bosque de Tlalpan and the Viveros de Coyoacán include an appreciable diversity of Pinaceae species.
Ecological, economic, and cultural importance
As dominant components of many forested regions, members of Pinaceae impact diverse biochemical, hydrological, and climatic processes, has an important effect on the frequency and intensity of forest fires and provides habitat to many other species of animals, plants, fungi, and bacteria (Farjon 1990, Richardson and Rundell 1998). In the Distrito Federal, the Abies and Pinus forests are also referred to as the “Great Water Forest” (“Gran Bosque de Agua”) and are important in the capture of CO2 and of airborne particles, moderation of climate, recharging aquifers, stabilization of soils, as barriers against noise and wind, and for their esthetic and recreational value (Gobierno del Distrito Federal, 2002). Several Natural Protected Areas have been designated for these forests like the Parque Nacional Cumbres del Ajusco (Tlalpan), the Bosques de la Cañada de Contreras (which includes the Parque y Corredor Ecoturístico “Los Dinamos” in La Magdalena Contreras), the Parque Nacional Desierto de los Leones (Cuajimalpa de Morelos and Álvaro Obregón), and the Parque Nacional Insurgente Miguel Hidalgo y Costilla (better known locally as “La Marquesa” of which, 9% is located in Cuajimalpa de Morelos; most of its area is located in three municipios of the state of México).
Pinaceae is the most important tree family due to its extensive use for wood, cellulose, resin, seeds (pinyons) and many other products. Pinus is the principal source of softwood internationally. In Mexico, more than 75% of wood production is based on Pinus, compared to near 2 to 4% for Abies. In the Distrito Federal, Abies exceeds Pinus in production of wood and cellulose (Semarnat 2011). Historically, Abies and Pinus were used as a construction material, cellulose for paper (three large factories in the 20th century: San Rafael, Loreto, and Peña Pobre relied heavily on nearby forests), fuel (firewood, and charcoal, including “ocote”) and for resin (de la Lanza Espino and García Calderón 2002). There are nurseries of Abies and Pinus for reforestation, and for the sale of Christmas trees (mainly P. ayacahuite). Also, leaves of P. montezumae are gathered for making baskets.
Principal threats and opportunities or conservation measures
None of the species of Abies and Pinus in the forests of the Distrito Federal are considered rare or endangered (Semarnat 2010). However, as a vegetation type, these forests face diverse threats provoked by human activity. Any reduction in their coverage represents an important loss in the health and wellbeing of the Mexico Basin and its occupants. Its principal threats are soil conversion, authorized and illegal, for establishing homesteads and for crops and pastureland, illegal logging, forest fires, air pollution, water, and soil contamination, and global warning.
One of the best-documented threats particular to the Pinaceae forests of the Distrito Federal is air pollution. Reduction in growth, chlorotic lesions (spots and bands), and poor retention of leaves and branches were noted for P. hartwegii in Ajusco and then for A. religiosa in Desierto de los Leones in the 1970s and 1980s (Krupa and de Bauer 1976, de Bauer and Hernández-Tejada 1986, Alvarado et al. 1993, de Bauer 2007). Shortly after the first symptoms appeared there was an increase in populations of bark beetles, principally Pseudohylesinus variegatus, followed by a massive die-off, leaving thousand of trees dead en sites called “cemeteries” (Alvarado-Rosales and Hernández-Tejada 2002). Several studies identified elevated levels of ozone as the probable cause; this triggered a complex decline, with further pathological interactions with other air contaminants (for example, peroxyacyl nitrates), the lack of forest management, a reduction in the availability of water, and attack by pathogens, not only bark beetles, but also pathogenic fungi and mistletoe (Alvarado et al. 1993). The decline is not evident in other sites in the Mexico basin that do not occur downwind of the urban part of Mexico City, such as in Abies and Pinus forests in the state of Puebla. The species most sensitive include A. religiosa, P. hartwegii, P. montezumae, and P. leiophylla; P. ayacahuite is more resistant (Miller et al. 1994).
Deforestation and soil conversion for urban use is also recognized as an important longstanding threat. Forests were logged on a large scale throughout the centuries to satisfy the demands for construction and for fuel in the prehispanic era: this process accelerated greatly in the 20th Century (Ezcurra et al. 2002).
Alvarado R., D., L.I. de Bauer, et al. 1993. Decline of sacred fir (Abies religiosa) in a forest park south of Mexico City. Environmental Pollution 80: 115-121.
Alvarado-Rosales, D. and T. Hernández-Tejeda. 2002. Decline of sacred fir in the Desierto de los Leones National Park. Urban air pollution and forests: resources at risk in the Mexico City air basin. M.E. Fenn, L.I. Bauer and T. Hernández-Tejeda. New York, Springer-Verlag: 243-260.
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de Bauer, M. d. L. and T. Hernandez-Tejeda. 2007. A review of ozone-induced effects on the forests of central Mexico. Environmental Pollution 147: 446-453.
de la Lanza Espino, G. and J.L. García-Calderón. 2002. Historical summary of the geological, climate, hydrology, culture, and natural resource utilization in the Basin of Mexico. Urban air pollution and forests: resources at risk in the Mexico City air basin. M. E. Fenn, L. I. de Bauer and T. Hernández-Tejeda. New York, Springer: 3-23.
Eckenwalder, J.E. 2009. Conifers of the world : the complete reference. Portland, Timber Press.
Ezcurra, E., M. Mazari-Hiriart, et al. 2002. Socioeconomic change and its impact on forest resources in the Basin of Mexico. Urban air pollution and forests: resources at risk in the Mexico City air basin. M. E. Fenn, L. I. de Bauer and T. Hernández-Tejeda. New York, Springer: 24-43.
Farjon, A. 1990. Pinaceae: drawings and descriptions of the genera Abies, Cedrus, Pseudolarix, Keteleeria, Nothotsuga, Tsuga, Cathaya, Pseudotsuga, Larix and Picea. Költz Scientific, Königstein.
Farjon, A. 2010. A handbook of the world's conifers. Leiden; Boston, Brill.
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Gernandt, D.S. and J.A. Pérez de la Rosa. 2013. Biodiversidad de las coníferas en México. Revista Mexicana de Biodiversidad. In press.
Krupa, S.V. and L.I. de Bauer. 1976. La ciudad daña los pinos de Ajusco. Panagfa 4: 5-7.
Martínez González, L. 2008. Árboles y áreas verdes urbanas de la Ciudad de México y su zona metropolitana. México D.F., Fundación Xochitla A.C. y Comisión Nacional para el Conocimiento y Uso de la Biodiversidad.
Miller, P.R., L.I. de Bauer, and T. Hernández-Tejeda. 1994. Oxidant exposure and effects on pines in forest in the Mexico City and Los Angeles, California, Air Basins. Urban air pollution and forests: resources at risk in the Mexico City air basin. Ecological Studies Series. M.E. Fenn, L.I. de Bauer and T. Hernández-Tejeda. New York, Springer-Verlag: 225-242.
Richardson, D.M. and P.W. Rundell. 1998. Ecology and biogeography of Pinus: an introduction. Ecology and Biogeography of Pinus. D.M. Richardson. Cambridge, Cambridge University Press: 3-46.
Rzedowski, G.C. de, J. Rzedowski, et al. 2001. Flora fanerogámica del Valle de México. Pátzcuaro, Instituto de Ecología A.C. y Comisión Nacional para el Conocimiento y Uso de la Biodiversidad.
Rzedowski, J. 1994. Vegetación del Pedregal de San Ángel. Reserva Ecológica del Pedregal de San Ángel: ecología, historia natural y manejo. A. C. Rojo. México D.F., Universidad Nacional Autónoma de México: 9-64.
Semarnat. Secretaría de Medio Ambiente y Recursos Naturales. 2010. NORMA Oficial Mexicana NOM-059-SEMARNAT-2010. Diario Oficial de la Federación (DOF), jueves 30 de diciembre de 2010.
Semarnat. Secretaría de Medio Ambiente y Recursos Naturales. 2011. Anuario estadístico de la producción forestal 2009. S. d. M. A. y. R. Naturales, Dirección General de Gestión Forestal y de Suelos y Secretaría de Medio Ambiente y Recursos Naturales.
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