Taxonomía y sistemática

 

Tree and tree–like species of Mexico: gymnosperms, monocotyledons, and tree ferns

 

Especies arbóreas y arborescentes de México: gimnospermas, monocotiledóneas y helechos arborescentes

 

Martin Ricker^* and Héctor M. Hernández

 

Herbario Nacional de México, Departamento de Botánica, Instituto de Biología, Universidad Nacional Autónoma de México, Apartado postal 70–233, Ciudad Universitaria, 04510 México D. F., Mexico.

 

^*Correspondent:
mricker@ibiologia.unam.mx

 

Recibido: 23 septiembre 2008
Aceptado: 01 junio 2009

 

Abstract

Trees or tree–like plants are defined here broadly as perennial, self–supporting plants with an adult height of at least 5 m (without ascending leaves or inflorescences), and with 1 or several erect stems with a diameter of at least 10 cm. We present an updated list of all Mexican tree species under that definition in the Gymnospermae (86 species, 38% endemic to Mexico), Monocotyledonae (75 species, 55% endemic), and Pteridophyta (9 species, none endemic). The list contains a total of 170 species in 37 genera and 12 families, with 74 species (44%) being endemic to Mexico. The list was compiled consulting recent literature, the National Herbarium of Mexico (MEXU), and taxonomic specialists.

Key words: biodiversity, flora, tree definition.

 

Resumen

Las plantas arbóreas o arborescentes se definen aquí en un sentido amplio como plantas perennes que se pueden sostener por sí solas, con una altura del adulto de al menos 5 m (sin considerar hojas o inflorescencias ascendentes), y con 1 o varios tallos erectos de un diámetro de al menos 10 cm. Presentamos aquí una lista actualizada bajo esta definición de todas las especies arbóreas mexicanas que pertenecen a las Gymnospermae (86 especies, 38% endémicas de México), Monocotyledonae (75 especies, 55% endémicas), y Pteridophyta (9 especies, ninguna endémica). La lista contiene un total de 170 especies en 37 géneros y 12 familias, con 74 especies (44%) endémicas de México. La lista fue compilada al consultar fuentes bibliográficas recientes, el Herbario Nacional de México (MEXU), y taxónomos especialistas.

Palabras clave: biodiversidad, flora, definición de árbol.

 

Introduction

Trees and tree–like plants characterize many landscapes, and create the structure of forest ecosystems. Mexico lacks a taxonomically updated inventory of tree species, despite and because of its high plant diversity. For many years, the classical and most useful reference for botanists interested in the Mexican tree and shrub flora was Standley's book (1920–26). After over 80 years, however, the taxonomy of this work is largely outdated for many plant groups. Taxonomic investigations carried out over the last several decades have much improved our understanding of the diversity and systematics of groups containing tree species (e.g., Farjon, 2001; Henderson et al., 1995; Judziewicz et al., 2000; Mickel and Smith, 2004). In addition, there are several ongoing regional floristic inventories, such as Flora Neotropica, Flora Mesoamericana, Flora de Veracruz, Flora del Bajío, Flora Novo–Galiciana, and Flora del Valle de Tehuacán–Cuicatlán, that updated and systematized knowledge of the Mexican tree flora during the last decades.

There is a great need from scientists, foresters, and conservationists for an updated checklist of tree species for Mexico (e.g., Ricker et al., 2007). To produce such a list is a dynamic enterprise from the taxonomist's viewpoint, as molecular information continuously provides new evidence, resulting in constant taxonomic adjustments, and different specialists frequently express contrasting taxonomic viewpoints. We are nevertherless convinced that a compilation and update of current knowledge is important to inform and guide the above–mentioned user groups. Such lists are also important for carrying out floristic projects and for exploring the possibilities of molecular barcoding as a species identification tool (e.g., Lahaye et al., 2008).

A recent attempt to produce a list of tree species names for the area from Canada to Panama is Elsevier's Dictionary of Trees (Grandtner, 2005). It includes tree species of at least 5 m height, with synonymy, distribution at the country level, common names in English and French, and sometimes Spanish and other languages. Unfortunately, this work does not include specific literature references for each taxonomic group, nor is it based on a thorough revision by specialists, and thus can only be considered an approximation to an updated list.

Before presenting a checklist of tree species, we need to circumscribe what we mean by the term "tree". There is no single definition of what constitutes a tree. For example Hora (1986: 11) states that "ordinarily our image of a tree is that of a perennial plant, capable of attaining at least 6 m, with a single woody self–supporting trunk or stem which is usually unbranched for some distance above ground." For Barwick (2004: vi) "the definition of a tree is based on the loose scientific description of species that have a columnar, woody stem supporting branches, and whose height varies according to the species, the environment and other factors". Raven and Crane (2007) write that "The popular concept of a tree, which encompasses everything from tree–ferns and Joshua trees to coconut palms and oaks, confounds any simple botanical definition. Nevertheless, almost all 'trees' have one (rarely a few) above ground, perennial vegetative shoots, of a size requiring significant non–hydrostatic structural support…" And Mori et al. (1997) define trees as "erect, usually single–stemmed woody plants ≥5 cm DBH", while "some trees have multiple trunks but at least some of them ≥5 cm diameter". Hallé et al. (1978: 1–2) concluded that it is "unwise to offer rigid definitions where they are not needed", and included palms, banana plants, and many other species of diverse growth form in their book on architecture of tropical trees. Finally, in Elsevier's Dictionary of Trees (Grandtner, 2005: xiii) "A tree is defined as a woody plant, with a single, erect and persistent stem of at least 10 cm in diameter, measured at 1.3 m above the mean ground level, and with a total height of at least 5 m… Willows (Salix), birches (Betula), palms (Palmae) and bamboos (Bambusa, Guadua), with several stems branching from the same root system, as well as cacti (Cactaceae) are considered as trees, provided that they meet the diameter and height criteria."

In the present paper, we adapt Grandtner's definition as follows: Trees or tree–like plants are defined broadly as perennial, self–supporting plants with an adult height of at least 5 m (without ascending leaves or inflorescences), and with one or several, erect stems with a diameter of at least 10 cm (measured at 1.3 m above the ground level or measured above buttresses if these are present). A tree species or tree–like species contains individuals with tree characteristics at least somewhere in its geographic range, but not necessarily everywhere. Our definition avoids the often arbitrary distinction of "trees" with a dominant trunk, from large "shrubs" with multiple stems. The term "self–supporting" excludes lianas, but the definition includes large tree–like plants like some massive bamboo species. Thus, our definition includes not only the usually considered conifers and dicotyledoneous tree species, but also tree ferns, some monocotyledons (e.g., palm and Yucca species), and unusual gymnosperms such as a few cycads.

Trees are usually associated with the presence of wood. Wood in a narrow sense is sometimes defined as being the result of secondary diameter growth (e.g., Judd et al., 2002: 82). Tree ferns and monocotyledonous flowering plants such as palms would not contain "true" wood under that definition. In monocotyledons, the acquisition of the tree habit via true stems is either by enhanced primary growth of the axis (e.g., palms) or by a form of anomalous secondary growth (e.g., Dracaena). Other monocotyledons, such as Musa (e.g., banana), acquire a treelike habit by developing pseudostems that consist of embracing sheaths of foliage leaves (Porembski, 2006: 129). Finally, the trunks of tree ferns are formed of a large central area (pith) and an outer region (cortex) of soft, thin–walled parenchyma cells, which often stores starch (Large and Braggins, 2004: 17). The literature does nevertheless refer to non–herbaceous stems that are at least to some extend lignified, as containing wood. For example, Parthasarathy and Klotz (1976) analyzed the anatomy of palm stems, and speak of "palm wood." While not insisting in our definition of trees on the presence of wood, the species presented here all contain wood in a wide sense.

 

Materials and methods

Given the definition of "tree species" in the introduction, the first step consisted of reviewing recent literature that treated the taxonomic groups of interest, including bibliography that described species and world checklists, as well as treatments up to date in the Flora Mesoamericana and Flora Neotropica. Only native species and no hybrids were included in our compilation in Appendix 1. Maybe the most notable example of a naturalized (and thus not included) species in the groups treated here is the coconut palm (Cocos nucifera L.). It is widely found on Mexico's coastal areas, even though it originated in Melanesia (Harries, 1978; Harries, 1992).

Subsequently, we compared our list with the following publications (in chronological order), to detect additional species or resolve taxonomic problems:

1) The compilation of tree species in the tropical dry forest around the Chamela biological research station in Jalisco (Lott, 1993).

2) The compilation of canopy tree species of the Mexican Atlantic slope rain forests by Wendt (1993).

3) The list of tree species for the Yucatán Peninsula by Ibarra–Manríquez et al. (1995).

4) The compilation of tree species in the tropical evergreen forest around the Los Tuxtlas biological research station in Veracruz (Ibarra–Manríquez and Sinaca, 1995; Ibarra–Manríquez and Sinaca, 1996a; Ibarra–Manríquez and Sinaca, 1996b).

5) The already mentioned dictionary of trees from Grandtner (2005).

6) The book on tropical tree species in Mexico from Pennington and Sarukhán (2005).

7) The list of tree species for the State of Michoacán by Cué–Bär et al. (2006).

These lists frequently contain synonyms that were resolved with the literature cited in Appendix 1 or consulting specialists; synonyms are only mentioned here when the cited literature contains different opinions and we had to follow one.

Family names are those recognized by Farjon (2001) for conifers (class Coniferae), by Whitelock (2002) for cycads (class Cycadopsida), Heywood et al. (2007) for monocots (class Monocotyledonae), and Mickel and Smith (2004) for tree ferns (class Filicopsida). Taxonomic literature for the recognized species names is cited in Appendix 1. In order to verify and sometimes update the taxonomic nomenclature, sections of the list were reviewed by several specialists (see acknowledgments). The authors of species names and original publications were double–checked in TROPICOS (http://www.tropicos.org).

When information on maximum height, distribution, and endemism in Mexico was not available from the literature, it was added after revision of specimens in the National Herbarium of Mexico (MEXU). The heights in Appendix 1 refer to the maximum heights reported in the literature and/or on herbarium specimens that are supposedly reached by the species anywhere in Mexico (with exceptional values for Mexico given in parentheses). In the case of species that are not endemic to Mexico, it is difficult to know whether there is a difference between maximum heights in Mexico and elsewhere. In 2 cases we mention such a difference (Pseudotsuga menziesii y Pinus jeffreyi). Endemism refers to the species being restricted to Mexico, as far as is currently known.

 

Results and discussion

The total number of tree species listed in Appendix 1 is 170, of which 74 species (44%) are endemic to Mexico. This high proportion is still below the calculated endemism rate of 46% for Mexican legume trees by Sousa et al. (2001), and the endemism estimate of 52% for the whole Mexican flora by Rzedowski (1993: 129). None of the 37 genera in 12 families in the appendix is endemic to Mexico.

There are 86 tree species of Gymnospermae (of which 33 species or 38% are endemic to Mexico), 75 Monocotyledonae (41 species or 55% endemic), and 9 Pteridophya (none endemic). Summary information for each of the 12 families is given in Table 1. The table mentions also the taxonomic classes, subdivisions, and divisions to which the plant families belong. The highest number of species (56) is found in the Pinaceae, followed by the Arecaceae (36). The genus with the higest number of species is Pinus with 46 (18 or 39% endemic), followed by Yucca with 18 (11 or 61% endemic). Farjon (2001) provides a worldwide list of gymnosperm species. For Pinus, he recognizes 109 species worldwide of which 44 species are found in Mexico. Consequently, according to Farjon's data, 40% of the worldwide species diversity in Pinus is found in Mexico. We recognized in addition Pinus discolor and Pinus johannis at the species rank in Appendix 1.

Many grass species (Poaceae) reach a height of 5 m, but not a diameter of 10 cm. In Mexico there are 40 grass species that reach a height of at least 5 m, found in the genera Arthrostylidium, Aulonemia, Chusquea, Guadua, Gynerium, Olmeca, Olyra, Otatea, Pennisetum, Phragmites, and Rhipidocladum (see http://www.kew.org/data/grasses–db/). Only 4 Guadua species, however, reach a diameter of 10 cm and are included in Appendix 1. Notable grass species that are not included in the appendix are Chusquea simpliciflora Munro with a height of up to 25 m, Olmeca recta Soderstr. (15 m) and Olmeca reflexa Soderstr. (12 m), the only 2 species of an endemic genus, and Rhipidocladum martinezii Davidse et R. W. Pohl (8 m), known only from the type collection in Chiapas.

The family with the highest endemism rate is the Zamiaceae, with all 4 reported Mexican species being endemic. With 92% and 68% endemic species, respectively, the Nolinaceae and Agavaceae are also outstanding in this regard. Finally, the high absolute number of endemic species in the Pinaceae (23 species) and Arecaceae (14 species) is worth mentioning. It is also interesting to note that there are only 9 species of tree ferns, none of which is endemic to Mexico.

Of the species in Appendix 1, the ones observed to reach in Mexico the largest maximum heights are Abies religiosa and Pinus ayacahuite (both 70 m; see Table 1). Notable for their heights are also some species in the family Arecaceae (45 m), Podocarpaceae (30 m), the large bamboos in the family Poaceae (30 m), and 1 tree fern species (20 m).

The native tree species of Mexico in this paper are in 4 taxonomic classes (Coniferae, Cycadopsida, Monocotyledonae, and Filicopsida) that are interesting in terms of their habitat specialization, economic uses, and inclusion of several record–breaking species. Many tree–like monocyledoneous species, such as the Yucca species and several Nolinaceae, are adapted to very dry and hot desert conditions, while most gymnosperm species are dominant elements of high altitude ecosystems in Mexico. The gymnosperms, and in particular the pines, are the most important timber species in terms of processed volume. They include also the oldest–living and tallest–growing trees, while the bamboos include the fastest–growing plants. Trees of Pinus longaeva D.K. Bailey in California attain ages of almost 5 000 years (LaMarche, 1969, there under the name P. aristata), but the distribution of Pinus longaeva does not reach Mexico. Trees of Taxodium mucronatum, with ages up to almost 2 000 years, are the longest–living known tree species in Mexico, forming trees of impressive size. Famous is the "árbol de Tule" near Oaxaca City, with 14 m diameter and 40 m height (Villanueva et al., 2006). One of the world's tallest trees belonged to Pseudotsuga menziesii: in British Columbia (Canada) a specimen was felled in 1895 that was reported to measure 133 m (Mabberley, 2008: 711). The species is also widely distributed in Mexico, though herbarium specimens report a height of up to only 50 m there. Finally, Bambusa bambos (L.) Voss from India is reported to grow up to 91 cm per day in height, reaching eventually up to 37 m (Mabberley, 2008: 90). In Mexico, the closely related Guadua angustifolia reaches a height of 30 m, an amazing size for a grass (Guadua was originally included in the genus Bambusa; Young and Judd, 1992). All in all, the 170 species in this paper are an interesting representation of Mexico's high biodiversity.

 

Acknowledgements

We are grateful to the following taxonomic specialists for carefully reviewing sections of the appendix: David Gernandt (Pinaceae), Abisaí García–Mendoza (Agavaceae and Nolinaceae), Hermilo J. Quero (Arecaceae), J. Gabriel Sánchez–Ken (Poaceae), all of the Instituto de Biología (Universidad Nacional Autónoma de México), as well as Andrew Henderson of the New York Botanical Garden (Arecaceae). Furthermore, we thank Pilar E. Mendoza for helping with the compilation of species lists, and Esteban Martínez and Clara H. Ramos for reading a draft manuscript and commenting on tree sizes that they have seen in the field.

 

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