Biodiversity of Conifers in Mexico
David S. Gernandt and Jorge A. Pérez de la Rosa
Spanish version available online at the Revista Mexicana de Biodiversidad
Conifers (Pinophyta) are woody trees or shrubs with simple leaves and reproductive structures arranged in simple pollen cones and (except in Taxaceae) compound seed cones. They are dominant components of several vegetation types. In Mexico conifers grow from sea level to above 4,000 m, with the greatest diversity concentrated in montane forests of the Sierra Madre Occidental and Sierra Madre Oriental. Four families have natural distributions in the country: Pinaceae (four genera and 61 species), Cupressaceae (4 genera and 29 species), Podocarpaceae (1 genus and 3 species), and Taxaceae (1 species). Of the 93 Mexican conifer species, 43 are endemic to the country, 15 with ranges restricted to three of fewer states.
Key words: conifers, cones, distribution, species.
Conifers (Pinophyta o Coniferophyta) are dioecious or monoecious trees or shrubs with rough or smooth bark, in large and thick plates with fissures or in long and thin strips (Figs. 1-4). Lateral branches are well developed. Their leaves are simple, needle- or scale-shaped, linear to lanceolate, or occasionally oblong or falcate. Their leaves are generally persistent for more than one year, but in a few species are deciduous. Their compact xylem is composed principally of tracheids with thick walls and uniseriate or multiseriate pores; in contrast to angiosperms it lacks vessels. Resin canals are often present in wood, bark, leaves, or cones. Their strobili, or cones, are moosporangiate. The microsporangiate or pollen cones are simple, formed of microsporophylls helically arranged around a central axis and with two to many microsporangia located distally on the adaxial face. The pollen grains are saccate in some species. The ovulate, or seed cones are formed by a bract-scale complex arranged helically around a central axis; although in Podocarpaceae and Taxaceae they are usually reduced to one or two scales with a single ovule. The cones are usually woody, but in Juniperus they are fibrous or fleshy, and in Podocarpaceae and Taxaceae, the ovules are often covered by sterile, fleshy structures, often with showy colors, called epimatia or arils, respectively. Cones develop in one year in most species, although in Pinus and in some Juniperus species they require two or three years to develop. The bract-scale complex can be free or fused and have one to many ovules on their adaxial surface, which are exposed at pollination; this latter character is common to all gymnosperms (conifers, cycads, Ginkgo, and gnetophytes). The micropyle in some groups is oriented distally and in others is oriented towards the axis (inverted). Most species of Cupressaceae and Pinaceae have a seed wing that facilitates wind dispersal, or water dispersal in Taxodium; wings are absent in Juniperus, Podocarpaceae, and Taxaceae, and vestigial in pinyon pines, where birds or other animals disperse the seeds. The number of cotyledons is 2 to 15, exceptionally 25 in Pinus maximartinezii.
Living conifers have long been considered a natural group, although Taxaceae has been treated as a separate order (Taxales) by some authors, primarily because its members lack recognizable ovulate cones (Florin, 1954). Phylogenetic analyses have placed Taxaceae with the remaining conifers. Surprisingly, phylogenetic analyses of DNA sequences have recovered a close relationship between the conifers and the gnetophytes, most notably phylogenies in which gnetophytes are derived from within the conifers, for example as the sister group to Pinaceae, often called the gnepine hypothesis (reviewed by Mathews, 2009). The conifers and the gnetophytes share several wood characters and both groups have simple leaves and simple pollen cones; however, a close relationship between the two gymnosperm groups is in conflict with phylogenetic results based on morphological characters, which find gnetophytes as the sister group to the angiosperms (the anthophyte hypothesis); these two groups share vessels en the wood and double fertilization. Molecular phylogenies with more characters have recovered conifers as a monophyletic group, and the gnetophytes as an independent clade (Rai et al., 2008; Lee et al., 2011).
Conifers include some of the largest and longest-lived plants known; the world’s tallest tree is Sequoia sempervirens Endl. (up to 115 m), the most massive is Sequoiadendron giganteum (Lindl.) J. Buchholz (>1,400 m3), and the oldest is Pinus longaeva D. K. Bailey (up to ca. 4,700 years). In Mexico, the national tree, the “ahuehuete” (Taxodium mucronatum), reaches a trunk diameter of over 11 m (the Árbol de Tule in Oaxaca). Mexico also has species with shrub growth forms like Juniperus monticola or Pinus culminicola, which regularly measure less than one meter in height, to trees that reach up to 70 m, like Abies religiosa and Pinus ayacahuite (Farjon & Styles, 1997; Farjon, 2010; Ricker & Hernández, 2010).
Conifers occur in diverse vegetation types in Mexico. According to Rzedowski (1978) they occur in pine forest, pine scrub, Abies forest, Pseudotsuga and Picea forest, Juniperus forest or scrub, and Cupressus (= Callitropsis) forest. They also inhabit other vegetation types like montane broadleaf forest, Quercus forest, and xerophilous scrub. Taxodium mucronatum often occurs in riparian forest.
The sporophyte phase dominates in conifers and can last up to hundreds or thousands of years. The reproductive cycle, which comprehends the time from bud initiation to liberation of mature seeds, lasts about 17 months in Pseudotsuga menziesii (Allen & Owens, 1972), but two or up to three years in other species (Tomlinson & Takaso, 2002). Vegetative bud primordia undergo an increase in activity, often in early spring, with differentiation into pollen and seed cones apparent in late spring or early summer. Sporogenesis lasts from a few months to up to a year depending on the species, with a period of dormancy in the winter. The following spring pollen cones liberate great quantities of pollen, which is dispersed by wind, and some lands on the (typically) upright seed cones and passes between the scales to enter the micropylar canal. In species with saccate pollen, the pollen is drawn in to the micropyle on a pollen drop, but in Cupressaceae, Taxacaee, and other genera like Pseudotsuga, pollen is nonsaccate and ovules lack a pollen drop. The pollen germinates to form a pollen tube, which grows through the nucellus toward the egg cell, eventually penetrating the archegonium and releasing its contents, including two unflagellated gametes of unequal size, the larger of which enters the nucleus of the egg cell and the smaller of which eventually degenerates. Embryogeny initiates upon fertilization, and results in a diploid embryo composed of radicle, hypocotyl, and two or more cotyledons. The seeds mature and eventually are liberated from the cone.
Of the six conifer families recognized worldwide, four are present in Mexico: Cupressaceae, Pinaceae, Podocarpaceae, and Taxaceae (Figs. 1-4; Table 1). The remaining two families are the monotypic Sciadopityaceae (Sciadopitys verticillata (Thunb.) Siebold & Zucc.) from Japan and Araucariaceae represented by three genera distributed in the southern hemisphere. Only 10 of the 71 conifer genera are represented in the country. There is little agreement in the number of conifer species. For example, Eckenwalder (2009) recognizes 546 species worldwide, while Farjon (2010) recognizes 615. These works are based on broad species concepts; however, other generic level works employed narrower concepts and recognized more species, particularly for diverse genera such as Abies (Liu, 1971), Pinus (Martínez, 1948; Perry, 1991; Styles, 1993), Callitropsis (Little, 2006), and Juniperus (Adams, 2011). We estimate that there are 670 species worldwide, of which, 94 (14%) occur naturally in Mexico (Table 2). These figures are greater than the 77 Mexican species recognized by Eckenwalder (2009) or the 74 species recognized by Farjon (2010), but less than the 104 species recognized by Debreczy & Rácz (2011). Several conifer clades probably underwent recent diversification, followed by migration, secondary contact, and hybridization. High levels of morphological and molecular variability and low levels of interspecific differentiation have been observed for Mexican Pinus (Mirov, 1967; Perry, 1991; Delgado et al., 2007; Gernandt et al., 2009; Moreno & Piñero, 2009), Abies (Jaramillo-Correa et al., 2008; Strandby et al., 2009; Aguirre-Planter et al., 2012), and Callitropsis (Rosas Escobar et al., 2009). Recent diversification followed by gene flow, and slow evolutionary rates probably have resulted in poorly defined species limits in these genera.
Table 1. Taxonomic diversity of Mexican conifer families.
Mexico is a secondary center of diversification of the genus Pinus, with 49 of the approximately 120 species (40%), in the world. Other genera with high percentages of species in the country include Callitropsis with seven of the 18 species (39%; here we accept the transfer of the New World cypresses from Cupressus to Callitropsis by Little, 2006), Juniperus with 20 of the 68 species (30%; Adams, 2011), and Abies, with eight of 48 species (17%). Farjon (2010) reports one of two species of Taxodium (50%) as distributed naturally in Mexico, one of four species each of Pseudotsuga and Calocedrus (25%), one of ten species of Taxus (10%;), three of the 38 species of Picea (8%), and three of 97 species of Podocarpus (3%). Despite their abundance in the landscape, conifers represent only 0.4% of the 23,000 species of vascular plants registered for the country (Villaseñor, 2004).
The distribution of conifers has been relatively well documented in generic monographs and catalogs (e.g., Liu, 1971; Farjon, 2010; Adams, 2011) and in local floras and lists (e.g., Carvajal y McVaugh, 1992; Rzedowski et al., 2001; García Arévalo y González Elizondo, 2003; Del Castillo et al., 2004). Conifers have a wide altitudinal and latitudinal distribution in Mexico. Pinus occurs from near sea level along the coast of Baja California (Pinus muricata) to tree line on the tallest volcanoes of the Transverse Neovolcanic Belt in the south-central part of the country (Pinus hartwegii on Pico de Orizaba, Popocatepetl, Iztaccihuatl, Nevado de Toluca, and Nevado de Colima). According to Styles (1993), Pinus oocarpa is the pine with the widest distribution in the tropics, occurring from central Sonora to northern Nicaragua. Other species with a wide distribution in Mexico are Abies religiosa, Pinus devoniana, P. cembroides, P. douglasiana, P. montezumae, P. pseudostrobus, P. teocote, Juniperus deppeana, J. flaccida, and Taxodium mucronatum. Several species that are widespread in western North America, like Abies concolor, Pinus contorta, and Calocedrus decurrens, reach their southern limit in Mexico. Species like Pinus greggii, P. patula, and Callitropsis lusitanica have been cultivated widely in both Mexico and in other countries.
Nine of the ten genera of Mexican conifers are considered part of the north-temperate flora, with representatives in the United States and some in Canada that reach their southern distributional limit in Mexico (Calocedrus, Picea, and Pseudotsuga), or farther south in Central America (Callitropsis, Juniperus, Taxodium, Abies, and Pinus). Only Podocarpus does not belong to the north-temperate flora; three species occur in Mexico and these represent the northernmost distribution of the genus in the Americas.
Northwest Mexico is the most species rich region of the country (Table 3). In Jalisco, the Sierra Madre Occidental meets the Transverse Neovolcanic Belt, and this is the most diverse state, with 34 registered species; other states in northwest Mexico nearly as rich include Durango (32) and Chihuahua (29). The Sierra Madre Oriental in northeastern Mexico contains almost as many species as the Sierra Madre Occidental; states with greatest diversity are Coahuila (28), Nuevo León (24), and Hidalgo (24). In southern Mexico, the most diverse state is Oaxaca (24). Few conifers occur naturally in southeast Mexico. Taxodium mucronatum is registered in Tabasco (Martínez, 1950), and Pinus caribaea var. hondurensis reaches the northern limit of its range in southern Campeche and Quintana Roo (Delgado et al., 2011). No conifers occur naturally in Yucatán (Martínez, 1963). The greatest geographic concentration of conifer genera occurs in Hidalgo, Veracruz, Puebla, and Oaxaca with eight (Abies, Pinus, Pseudotsuga, Podocarpus, Taxus, Callitropsis, Juniperus, and Taxodium).
Half of the Mexican conifer species were described between 1770 and 1905, and 95% were described by 1995 (Fig. 5). New species of Pinus have been discovered based almost solely on morphology (e.g., Debreczy & Rácz, 1995; Pérez de la Rosa, 1998; 2009), but many recent taxonomic treatments have also taken molecular data into account (e.g., Gernandt et al., 2003; 2009; Little, 2006; Adams, 2011; Adams & Schwarzbach, 2011). Pinus and Juniperus are the most species rich conifer genera in the national flora, and for that reason they are the best candidates for harboring new species to science. Further species discovery could be accelerated through exploration of geographically inaccessible areas, revision of herbarium specimens, and more intensive use of molecular markers to study genetic variation in natural populations.
The level of endemism in Mexican conifers is relatively high, with 43 of 93 (46%) endemic species in five genera: Pinus has 22 endemic species, Juniperus has ten, Abies has five, Callitropsis has three, and Picea has three. There are fewer narrow endemics. Only 18 Mexican endemics have ranges restricted to three or fewer states. Four more species occur in Baja California and in a few populations across the United States border in California, and one other species is restricted to Chiapas and two states in Guatemala (Table 4). Pinus caribaea var. hondurensis is restricted to only two states in Mexico but is abundant in Central America. No conifer genera are endemic to Mexico.
Many conifer forests have suffered reduction in area, disappeared completely, or changed in composition, generally resulting in substitution by angiosperms. There are only a few cases of range expansions, such as in the expansion of pinyon-juniper woodlands, or the widespread cultivation of a few species for forestry. Much degradation of conifer forests has occurred in only the current generation. Causes of forest reduction include the constant increase in the frequency of fire, provoked both by natural phenomena and anthropogenic impact on the environment, and logging, whether for direct use of wood, for secondary products, or for the expansion of agricultural activities. Climate change also represents and important threat for many species. This presents us with an unfavorable panorama for the future forest resources of the country. Furthermore, the progressive contraction of the original area or total disappearance of populations provokes what Ledig (1997) calls “secret extinctions” within species; referring to genetic erosion of the taxon. The gradual loss of adaptive variability carries with it the threat of medium or long-term final extinction. Although no conifer species is known with certainty to have completely disappeared in the last decades, Pinus oocarpa var. manzanoi Martínez, is known only from the holotype collected in Hidalgo more than seven decades ago and never seen since.
Thirty-nine conifer taxa are listed as endangered, threatened, or as subject to special protection in the Norma Oficial Mexicana NOM-059 (SEMARNAT, 2010). The list includes Calocedrus decurrens, four species of Callitropsis, two species of Juniperus, five species of Abies, all three species of Picea, 20 species of Pinus, Pseudotsuga menziesii var. glauca, Podocarpus matudae, and Taxus globosa. Seventeen species are listed as endangered, 12 of which are endemic to Mexico, and others are considered rare because they reach their northern or southern limit in the country. One species listed, Pinus martinezii E. Larsen, was treated by Farjon & Styles (1997) as a synonym of Pinus durangensis, a species with a widespread distribution. One taxon omitted from the NOM-059, Pinus radiata var. binata, has an exclusively insular distribution on Guadalupe Island and Cedros Island. The conifers have also been evaluated by the IUCN (2012), and five Mexican species are listed as near threatened, nine as vulnerable, six as endangered, and one (Picea martinezii) as critically endangered. The smaller number of the species listed by the IUCN is partly due to this list considering the global distribution of species, while the NOM-059 often lists species with few or threatened populations in Mexico but wide distributions in other countries. Other differences between the NOM-059 and the IUCN are derived from the different criteria used to evaluate threats, and different classifications. These differences highlight the importance of improving the conceptual framework that is used to recognize species and to evaluate risk, and for further research on the taxonomy, ecology, genetic variation, and conservation management of conifers. Conservation programs should implement an integrated approach that includes the maintenance of habitats for conservation in situ, germplasm conservation in botanic gardens and seed banks, and reforestation.
The authors thank Sol Ortiz García, Luz María González Villarreal, and Aaron Liston for reviewing drafts of this manuscript and José Luis Villaseñor and Maribel Paniagua Ibañez for compiling distributional data by state.
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Naturally distributed conifer taxa in México
4 of 11 genera worldwide
A. concolor (Gordon) Lindl. ex Hildebr., A. durangensis Martínez var. durangensis, A. durangensis Martínez var. coahuilensis (I. M. Johnst.) Martínez, A. flinckii Rushforth, A. guatemalensis Rehd. var. guatemalensis, A. guatemalensis Rehd. var. jaliscana Martínez, A. hickelii Flous & Gaussen var. hickelii, A. hickelii Flous & Gaussen var. oaxacana (Martínez) Farjon & Silba, A. hidalgensis Debreczy, Rácz & Guizar, A. religiosa (Kunth) Schltdl. & Cham., A. vejarii Martínez var. vejarii, A. vejarii Martínez var. macrocarpa Martínez, A. vejarii Martínez var. mexicana (Martínez) T. S. Liu
P. chihuahuana Martínez, P. martinezii T.F. Patt., P. mexicana Martínez
P. arizonica Engelm. var. arizonica, P. arizonica Engelm. var. cooperi (C.E. Blanco) Farjon, P. arizonica Engelm. var. stormiae Martínez, P. attenuata Lemmon, P. ayacahuite Ehrenb. ex Schltdl. var. ayacahuite, P. ayacahuite Ehrenb. ex Schltdl. var. veitchii (Roezl) Shaw, P. californiarum D. K. Bailey, P. caribaea Morelet var. hondurensis (Senecl.) W. H. Barrett & Golfari, P. cembroides Zucc. subsp. cembroides, P. cembroides Zucc. subsp. orizabensis D. K. Bailey, P. chiapensis (Martínez) Andresen, P. chihuahuana Engelm., P. contorta Douglas ex Loudon var. murrayana (Balf.) Engelm., P. coulteri D. Don, P. culminicola Andresen & Beaman, P. devoniana Lindl., P. discolor D. K. Bailey & Hawksw., P. douglasiana Martínez, P. durangensis Martínez, P. engelmannii Carrière, P. georginae Perez de la Rosa, P. greggii Engelm. ex Parl. var. greggii, P. greggii Engelm. ex Parl. var. australis Donahue & Lopez, P. hartwegii Lindl., P. herrerae Martínez, P. jaliscana Perez de la Rosa, P. jeffreyi Balf., P. johannis Rob.-Pass., P. lagunae (Rob.-Pass) Passini, P. lambertiana Douglas, P. lawsonii Roezl ex Gordon, P. leiophylla Schiede ex Schltdl. & Cham., P. lumholtzii B. L. Rob & Fernald, P. luzmariae Perez de la Rosa, P. maximartinezii Rzed., P. maximinoi H. E. Moore, P. montezumae Lamb. var. montezumae, P. montezumae Lamb. var. gordoniana (Hartw. ex Gordon) Silba, P. muricata D. Don, P. nelsonii Shaw, P. oocarpa Schiede ex Schltdl., P. patula Schiede ex Schltdl. & Cham. var. patula, P. patula Schiede ex Schltdl. & Cham. var. longipedunculata Loock ex Martinez, P. pinceana Gordon, P. praetermissa Styles & McVaugh, P. pringlei Shaw, P. pseudostrobus Lindl. var. pseudostrobus, P. pseudostrobus Lindl. var. apulcensis (Lindl.) Shaw, P. quadrifolia Parl. ex Sudw., P. radiata D. Don var. binata (Engelm.) Lemmon, P. remota (Little) D. K. Bailey & Hawksw., P. rzedowskii Madrigal & M. Caball., P. scopulorum (Engelm.) Lemmon, P. strobiformis Engelm., P. tecunumanii Eguilez & J. P. Perry, P. teocote Schiede ex Schltdl. & Cham., P. yecorensis Debreczy & Rácz
P. menziesii (Mirb.) Franco var. glauca (Beissn.) Franco
4 of 31 genera worldwide
C. decurrens (Torr.) Florin
C. arizonica (Greene) D. P. Little, C. benthamii (Endl.) D. P. Little, C. forbesii (Jeps.) D. P. Little, C. guadalupensis (S. Watson) D. P. Little, C. lusitanica (Mill.) D. P. Little, C. montana (Wiggins) D. P. Little, C. stephensonii (C. B. Wolf) D. P. Little
J. angosturana R. P. Adams, J. arizonica (R. P. Adams) R. P. Adams, J. ashei J. T. Buchholz var. ovata R. P. Adams , J. blancoi Martínez var. blancoi, J. blancoi Martínez var. mucronatum R. P. Adams, J. californica Carrière, J. coahuilensis (Martínez) Gaussen ex R. P. Adams, J. comitana Martínez, J. deppeana Steud. var. deppeana, J. deppeana Steud. var. pachyphlaea (Torr.) Martínez, J. deppeana Steud. var. sperryi Correll, J. deppeana Steud. var. zacatecensis Martínez, J. durangensis Martínez, J. flaccida Schltdl. var. flaccida, J. flaccida Schltdl. var. poblana Martínez, J. gamboana Martínez, J. jaliscana Martínez, J. martinezii Pérez de la Rosa, J. monosperma (Engelm.) Sarg., J. monticola Martínez, J. pinchotii Sudw., J. saltillensis M. T. Hall, J. scopulorum Sarg., J. standleyi Steyerm.
1 of 19 genera worldwide
P. guatemalensis Standl., P. matudae Lundell, P. oleifolius D. Don
1 of 5 genera worldwide
T. globosa Schltdl.
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