Mexico is one of the four countries classified as megadiverse and hosts between 10 and 12 % of the planet's known terrestrial diversity (Sarukhán et al., 2009). Practically, all of the great vegetation types of the planet are found in this country (González-Medrano, 2003) and some ecosystems such as the wetlands of Cuatro Ciénegas, Coahuila, are unique in the world (Espinosa, Escalante, Eguiarte, & Soauza, 2005). This diversity of ecosystems is related to the heterogeneity of the physical environment, which in turn is the product of a complex geological and climatic history (Espinosa-Organista et al., 2008). The geographic location where two biogeographic regions converge, the Nearctic region and the Tropical region, has also favored a high degree of endemism in the Mexican flora (Rzedowski, 1991).
Some research keeps record of the ecological value of Mexican ecosystems, whether due to its diversity (Balvanera, Lott, Segura, Siebe, & Islas, 2002; Castillo-Campos, Halffter, & Moreno, 2008), distribution (Gelviz-Gelvez & Pavón-Hernández, 2013; Sánchez-González & López-Mata, 2003), degree of conservation (Encina-Domínguez, Encina-Domínguez, Mata-Rocha, & Valdes-Reyna, 2008; García-Arévalo, 2008; Huerta-Martínez & García-Moya, 2004; Santibañez-Andrade, Castillo-Argüero, & Martínez-Orea, 2015) or structural variables (Cuevas-Guzmán et al., 2011; Mora-Donjuán et al., 2013; Vázquez, López, Montalvo, Méndez, & Castillo, 2010). Moreover, Mexican forest ecosystems are a source of various environmental services (Comisión Nacional Forestal [CONAFOR], 2012), such as the regulation of climatic conditions, water cycle (Bonan, 2008) and carbon sequestration (Ordóñez & Masera, 2001; Torres & Guevara, 2002). The values of importance, indices of wealth, diversity, dominance and abundance have become indicators that qualify the ecological contribution of the species in the forest ecosystems (Clark & Covey, 2012; Duelli & Obrist, 2003).
In spite of the heterogeneity of the Mexican forest ecosystems (González-Medrano, 2003) and the studies that highlight the ecological value of the species present (Estrada, Yen, Delgado, & Villarreal, 2004; Trejo & Dirzo, 2002), there is currently a lack of a database that systematizes the studies; that is to say, there is no systematic, updated and accessible report that contributes to the knowledge of the subject and supports the scientists interested in this area of research.
Therefore, the main aim of this study is to develop and present an exhaustive bibliographical review on importance assessment, composition, dominance and abundance studies of Mexican forest species, in order to contribute to their diagnosis and perspectives.
Materials and Methods
An extensive and systematic literature review was developed on the subject of interest using databases such ISI Web of Science, Science Direct, Google Scholar, Sistema Regional de Información en Línea para Revistas Científicas de América Latina, el Caribe, España y Portugal (Latindex) and Scientific Electronic Library Online (SciELO). For that purpose, different combinations of a predefined list of keywords were used: "importance value index", "dominance", "structure", "composition", "ecological indexes", "terrestrial ecosystems", "tree species" and "Mexico". The search was delimited from January 1980 to December 2015, taking into account only articles in indexed journals. Although valuable information exists in theses, technical reports, and conference abstracts, these documents were not considered due to the lack of a peer review process.
In order to ensure the reliability and comparability of the review, we verified that the articles included the following criteria: 1) scientific name of the species studied, 2) tree species (> 10 cm in diameter) to prevent misunderstanding with herbaceous or shrubs and 3) explicit description of the ecological indexes used to qualify the ecological relevance of the species under study.
Each document was recorded in a database using Microsoft Access 2010® and was classified by species, genus, family and ecosystem, the state where the study was developed, name of the journal, volume, number and query string were also recorded. A frequency analysis was conducted for each of the classification variables to define the most studied ecosystems, families, genera and species.
Results and Discussion
As a result of the review, a database was set up with 161 records of studies related to species of ecological importance in forest ecosystems in Mexico (Table 1) Articles were collected from 44 journals, 23 of these articles are Mexican publications and account for 69 % of the studies. More than 10 papers were collected from each one of five journals. The Journal Revista Chapingo Serie Ciencias Forestales y del Ambiente has the largest number of articles on the subject (Table 2). The oldest study found was Lott, Bullock, and Solis-Magallanes (1987), which corresponds to a study of diversity and structure in an area of tropical deciduous forest in Jalisco. Most of the articles were published in the decade from 2001 to 2010; however, there has been a considerable increase in the last five years (2011 to 2015).
|Revista Chapingo Serie Ciencias Forestales y del Ambiente||Mexico||4||11||15|
|Madera y Bosques||Mexico||1||6||5||12|
|Boletín de la Sociedad Botánica de México||Mexico||9||1||10|
|Forest Ecology and Management||USA||2||7||1||10|
|Revista Mexicana de Biodiversidad||Mexico||2||8||10|
|Acta Botánica Mexicana||Mexico||3||1||4||8|
|Revista Mexicana de Ciencias Forestales||Mexico||1||6||7|
|Biodiversity and Conservation||Internacional||5||5|
|Anales del Instituto de Biología. Serie Botánica||Mexico||4||4|
|Revista Iberoamericana de Ciencias||USA||3||3|
|Universidad y Ciencia||Mexico||3||3|
|Investigación Agraria: Sistemas y Recursos Forestales||Spain||2||2|
|Journal of Vegetation Science||Internacional||2||2|
Of the 161 articles collected, 37 (about 19 %) were related to the study of species in the tropical deciduous forest (Rzedowski, 1978). The second ecosystem by study frequency is the pine-oak forest with 20 articles (Figure 1). The less studied ecosystems were those with smaller surface in the country such as the vegetation of petenes (known locally under that name) and coastal dunes.
The largest number of families, genera and species studied is located in the tropical deciduous forests (Figure 1). This ecosystem has a wide distribution in the Mexican territory and is characterized by the dominance of extended treetops with heights between 7 and 8 m, and by a high density of shrub species (Trejo, 1999); furthermore, it is distinguished by its high diversity of species and endemic species (Rzedowski, 1991).
Figure 2 shows the number of studies per state. Most of the studies have been conducted in the state of Nuevo Leon (35) followed by Jalisco (17); in the first case, the number corresponds to studies in scrub ecosystems, such as Tamaulipan thornscrub (17) and submontane scrub (9), and pine-oak forests (13). In Jalisco, studies have been conducted mainly in the tropical deciduous forests (11).
Main Botanical Families
Figure 3 shows botanic families and their importance value index from the studies collected. The most frequently cited families were Fabaceae, Fagaceae, and Pinaceae; on the other hand, those cited only in one study were: Actinidiaceae, Annonaceae, Asteraceae, Cupressaceae, Meliaceae, Nyctaginaceae, Platanaceae, Polygonaceae, Rhizophoraceae, Rosaceae, Rubiaceae, Salicaceae, and Sapindaceae. The families found with higher value in the index of ecological importance (> 70) were Acanthaceae (Avicennia germinans [L.] Stearn), Rhizophoraceae (Rhizophora mangle L.), belonging to the mangrove ecosystems, and Apocynaceae with the genus Tabernaemontana in the tropical deciduous forests. The families with a higher number of studies (Fagaceae, Fabaceae, and Pinaceae) showed values of average importance from 27 to 34.
Main Genus and Species
According to Figure 4, the most frequently cited genus were Quercus (Fagaceae), Pinus (Pinaceae) and Acacia (Fabaceae), which is consistent with the family analysis. Another important genus was Bursera (Burseraceae) characteristic of the tropical deciduous forests. In relation to the importance value index at the genus level, Sabal has obtained the highest value, although this was only found in two studies carried out in the petenes and the deciduous forest. Haematoxylum, common in the tropical deciduous forest, had also high values of importance. Hymenaea, Avicenia, Pseudotsuga and Enterolobium had values of importance greater than 100, although these values were only recorded in one study.
The families with a higher number of genera recorded were Fabaceae, Euphorbiaceae, and Asteraceae. Although the family Fagaceae was the most frequent in the studies, only present two genera (Quercus and Fagus). Figure 5 includes the most frequent species, among which are: Bursera simaruba (L.) Sarg., Pinus pseudostrobus Lindl. and Havardia pallens (Benth.), as well as some species of the genus Quercus such as Q. rugosa Née and Q. laurina Humb. & Bonpl.
There are a lot of studies that keep record and value the ecological importance of tree species in Mexico. Most of the research has been developed from a perspective of ecosystem structure analysis, with two directions. On the one hand, some describe the current condition that keeps a particular ecosystem and highlights the species of greater abundance and dominance under certain conditions of site or station quality. In contrast, other studies provide ecological indicators that explain the structural condition of forests that have been disturbed by both anthropogenic and natural factors. The two perspectives of analysis have differed in recent years; thus, the first studies conducted in the 1980s, such as those by Lott et al. (1987); Bongers, Popma, Meave-Castillo, and Carabias (1988); Popma, Bongers, and Meave-Castillo (1988); and Arriaga and León-de la Luz (1989) in tropical forests, are distinguished because they describe the structural condition and diversity that these ecosystems preserve. On the other hand, since 2000, and especially in the last five years, the studies deal with aspects of the impact of anthropogenic and natural factors on forest structure, such as those conducted by Alanís-Rodríguez et al. (2008), Alanís-Rodríguez et al. (2008), Alanís-Rodríguez et al. (2010a), Alanís-Rodríguez et al. (2010b) y Alanís-Rodríguez, Jiménez-Pérez, Valdecantos-Dema, Aguirre-Calderón, and Treviño-Garza (2011). These papers analyze the diversity and tree structure of the forests in Nuevo León affected by forest fires.
The dissemination of research has diversified over time. The oldest studies recorded in this review correspond to international journals such as Plant Ecology (formerly known as “Vegetatio”), a trend that was maintained in the 1990s with the exception of three articles published in Acta Botánica Mexicana. It seems that the creation of new journals, the diversification of contents and their inclusion in indexes such as JCR (Journal Citation Reports), such as the case of the journal Revista Chapingo Series Ciencias Forestales y del Ambiente, has allowed research scientists to publish articles in national journals, together with the predominant Spanish language in these journals and perhaps the commitment of the Mexican research scientists to publish in national journals.
With regard to the number of publications, the trend has been increasing over the last five years, which may reflect a growing interest in the subject matter. Although an analysis of the reasons for such a trend goes beyond the scope of this study, it seems that in part this could be attributed to the anthropogenic and climatic vulnerability to which forest ecosystems undergo (Weber, Bugmann, Fonti, Rigling, 2008; Williams et al., 2013), without neglecting their economic importance and government support that motivate to improve the understanding of their structural variables. On the other hand, the consolidation of research groups on these topics (Table 1) could also have caused the growing number of publications. To our knowledge, this is the first effort recording the studies at the national level, where ecological structural variables of some tree species are studied (Figures 1, 3 and 4). This provides a starting point with a synthesis of the research landscape of the structural ecology of plant communities and gives a perspective on the subject to research scientists.
The ecosystem with a higher concentration of studies is the tropical deciduous forest (Table 1). One of the possible reasons why this ecosystem arouses the interest of research scintists is perhaps its high diversity of species (Bullock & Solis-Magallanes, 1990). Most of the studies of the tropical deciduous forest have been carried out in Jalisco (11 studies), usually conducted by the Universidad Nacional Autónoma de México at the Estación Biológica Chamela, although this ecosystem has also been studied in other states of the country. This allows us to assume that, coincidentally, the studies have been spatially distributed according to their proximity to the area of influence of the research institutions. For example, Nuevo León is the entity with the highest number of studies, mostly carried out by research scintists from the Universidad Autónoma de Nuevo León, mainly conducted in the Parque Ecológico Chipinque and the Tamaulipan thornscrub.
The families with higher frequency in the literature are Fabaceae, Fagaceae and Pinaceae, which coincides with the high number of species belonging to these families (19,400, 670 and 220, respectively, according to http://www.naturalista.mx/), and wide distribution (Rzedowski, 1991). The Fabaceae family belongs to the leguminous (Fabales) group, the second most diverse group in Mexico (Sousa & Delgado, 1993), and has an important ecological and socioeconomic role in the different ecosystems (Estrada-Castillón et al., 2010). From this family, A. farnesiana, known in Mexico as huizache, is common in thornscrub forest (Miranda & Hernández-Xolocotzi, 1963); it has also been described as a species that can colonize disturbed areas (Estrada-Castillón & Martínez-Muñoz, 2000), making it the most abundant in the scrubs in the short term, after a disturbance.
The family Fagaceae is present practically all over the world and the genus Quercus is its most numerous genera with 161 species distributed in Mexico (Valencia, 2004). In the same way, the family Pinaceae is one of the most important ecologically and economically in Mexico (Rzedowski, 1991; Styles, 1993). The genus Pinus is the most widely distributed and is often the dominant component in temperate and cold ecosystems, which influence the ecosystem's functional processes, such as biogeochemical, hydrological, and fire regimes (Sánchez-González, 2008).
The family Burseraceae, characteristic of the tropical deciduous forest, is also subject matter in the studies, represented mainly by the genus Bursera (Miranda & Hernández-Xolocotzi, 1963); B. simaruba is the most frequently studied species (Figure 5). Rzedowski, Medina, and Calderón (2007) cite it as a widely distributed taxon in the American continent; however, these authors indicate that it is a plant whose morphological variation makes identification difficult.
It is important to highlight that the studies carried out on species or plant communities with some category of risk in NOM-059-SEMARNAT-2010 (Secretaría de Medio Ambiente y Recursos Naturales [SEMARNAT], 2010) appear rarely in the literature. Some species of temperate ecosystems, such as those of the genera Abies and Picea or species characteristic of mangroves, are scarcely published. This highlights the need to characterize the structure of these populations or communities to design management strategies for proper conservation. We do not discard additional efforts that delve into the description of structural attributes, quantification methods, types of research (descriptive, comparative or experimental research) of each publication, assessing the successional trajectories of plant communities in a temporal and spatial manner.
The species commonly studied belong to the genus Quercus, Pinus, Acacia and Bursera, and most studies focus on the tropical deciduous forest. This review represents the first effort to provide a perspective on the tree species that have been recorded more frequently in the Mexican scientific literature, with quantitative forest parameters. Therefore, the study contributes to the knowledge of plant communities and serves as a base of support for researchers in the subject matter. The increase in publications in recent years is evidence of growing interest in this area; however, it is important to highlight that the studies carried out for species or plant communities with some category of risk are rare in the literature.