Chinese Introduction 中文简介

Yifan Yu

热带干旱森林是墨西哥分布最为广泛的四种生态环境之一，主要位于墨西哥沿太平洋一带，从北到南纵向延生。该生态系统有着明显的旱季雨季之分，主要由低或中等高度的季节性落叶植物组成。自古以来，人类和热带干旱森林有着密切的联系。 人民从中获取生活所需的木材，将其改造成适合种植农作物的农田或适宜放牧的草场，在维持了人们的日常生活的同时，造成了大量的经济效益。然而，这种多年持续的森林获取对墨西哥的热带干旱森林系统造成巨大的负面影响，致使其现覆盖率已远远小于原有森林面积。

Brief Introduction with a Map of the Study Area

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source: Tropical Deciduous Forest and the Sonoran Desert: A Strong Connection(2006) 
Arizona-Sonora Desert Museum. http://www.desertmuseum.org/programs/alamos_index.htm

According to Rzedowski, the tropical dry forest is one of the four most extensive types of vegetation of Mexico.( as cited in Becerra, 2005) It is found “ from a northernmost distribution limit at approximately 28° N, down to the border with Guatemala in the south. On the Pacific coast it forms a nearly continuous strip, with major areas in western Jalisco, and in the Balsas and Santiago river basins. On the Gulf coast it is present in more isolated and discontinuous areas.” (Trejo & Dirzo, 2000) 

Tianyi Peng

 Image of historical tropical dry forest. La Servilleta Canyon. Huasteca Tropical dry forest. Tamaulipas. Mexico. (age fotostock/SuperStock)

Image of deforestation of tropical dry forest (Petz, 2012)

Historical State

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Tropical dry forest mainly consists of deciduous trees with low to medium sizes and has a sharp contrast between its desolateness in dry seasons and exuberance in its rainy seasons (Becerra, 2005) There are little direct information about the origin of Mexican tropical dry forest, yet based on a research on temporal and spatial changes of Bursera, a dominant member of this ecosystem, scientists infer that tropical dry forest in Mexico probably originated between 30 and 20 million years ago and initially mainly appeared in the west part of Mexico and lately gradually expanded to south and central parts. (Becerra, 2005) Besides, according to Rzedowski, historically tropical dry forest may have occupied an area of nearly 270 000km², which means covering about 14% of the Mexican territory(as cited in Trejo& Dirzo, 2000). 

The colored part in the following picture show the distribution of original Mexican tropical dry forest according to Rzedowski’s analysis of potential vegetation. 

The status of conservation (sensu Oropeza et al., 1995) of seasonally tropical dry forest (SDTF) in relation to its potential distribution area (sensu Rzedowski, 1990)

source: Trejo & Dirzo, 2000

Additionally, Castillo, Magana, Pujadas, Martinez and Godinez (2005) point out that the history of tropical dry forests in the Chamela-Cuixmala region, a major area of this ecosystem in Mexico, can roughly be categorized into four distinct periods, which may provide us with a general idea of how the whole Mexican tropical dry forests interact with human through the years. In their view, until around 1800, tropical dry forest remained intact without much human disturbance and in the late 19th century, human began to exploit this ecosystem for cultivation and domestication, while the impacts resulted were relatively small. However, around 1950 with supportive policies from the administration, huge areas of tropical dry forest were conversed into agriculture fields and livestock pastures, resulting in seriously negative impacts on forests. Such large area of  transformation lasted for nearly 30 years and until around 1990, situation started to slightly improve.

current human impacts on the ecosystem and supportive data for assessment

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Overview

The current human impacts on the Mexican tropical dry forest is really bad.The following table showed the estimated conservation status of Mexican tropical dry forest in the early 1990s. 

source : Trejo& Dirzo, 2000 

Intact tropical dry forests refer to those whose native features are largely conserved without much human disturbance. Altered tropical dry forests refer to those where human activities have already caused measurable impacts while relatively large area of this vegetation type still remain. Degraded tropical dry forests refer to those where only few and isolated vegetations remain, showing the previous existence of large area of tropical dry forests. Converted tropical dry forests refer to those which once were tropical dry forests while now have completely been converted into other land types for other uses. (Trejo & Dirzo, 2000)

It is estimated that at that time, around 73% of the Mexican tropical dry forest were more or less altered, degraded or converted under negative impacts from human activities. (Trejo & Dirzo, 2000) 

A Specific Example in Morelos

The following figure shows the distribution change of tropical dry forests in Morelos, a state in the south of Mexico. It is estimated that in its primitive state, about 60% of this area was occupied by tropical dry forests, which was equal to 2842.9 km² in area. While in 1973, nearly one half of these vegetation type disappeared in the state and between 1973 and 1989, the tropical dry forest was lost at a dramatic rate of 1.4% per year, resulting in only 38% of original cover left in 1989. (Trejo & Dirzo, 2000)

 source:  Trejo & Dirzo, 2000

Original estimated area, lost area and deforestation rate of SDTF in the state of Morelos, Mexico (1973–1989)

source: Trejo & Dirzo, 2000

Several ways people make use of tropical dry forests in Mexico

In Mexico, conversion of tropical dry forests into agriculture fields, pastures and timber extraction are the three main human uses of this ecosystem. (Trejo & Dirzo, 2000; Castillo, Magana, Pujadas, Martinez and Godinez, 2005; Stoleson et al. 2005)

The history of Mexican people transforming tropical dry forests into agriculture fields can trace back thousands of years. According to Toledo et al, Maass and Challenger, some crops that we are familiar nowadays such as maize, beans and squashes were initially cultivated in such areas in Mexico (as cited in Maass et al., 2005). In the 20th century, such conversion of tropical dry forests into agriculture fields was greatly accelerated since Mexico government launched several supportive policies to encourage people to clear forests, establish farms, and grow more crops for self-consumption and trades. (Maass et al. 2005) Pastures for livestock ranching are also in great need in Mexico. Perramond claims that land for livestock grazing covers the largest area in northern Mexico (as cited in Stoleson et al. 2005). People there introduce exotic grasses, among which buffelgrass is one common type, conversing tropical dry forests into ranches(Stoleson et al., 2005).Timber resource is also a major benefit people get from tropical dry forests. According to Guerrero et al., many wood products are not only used for domestic production but also exported to other countries, such as the United States(as cited in Stoleson et al., 2005) Besides, Felger at al. states that the exploitation of mineral resources, which is rich in Mexico, indirectly affects tropical dry forests since in some areas, this ecosystem is cleared for building tunnels which are essential to mining ( as cited in Stoleson et al., 2005). In addition, Simon   claims that historically some tropical dry forests in Mexico were also cleared for production of such drugs as marijuana and opium. ( as cited in Stoleson et al., 2005)

In conclusion, tropical dry forests in Mexico are extensively used in many ways to support human life and production, resulting in really terrible conservation status.

Are there any protected areas for that ecosystem? What is the status of that protected ecosystem?

Tianyi Peng

There are several biosphere reserve regions for tropical dry forest in Mexico, yet only 0.2% of total tropical dry forest potential extent is under protection (Portillo-Quintero, 2010). Chamela-Cuixmala Biosphere Reserve is located along the central Pacific coast of Mexico and it has a large variety of ecosystems. Tropical dry forest is the dominant one in the region. Human activities, mainly livestock and tourism, have been developed in the ecosystem (United Nations educational, Scientific, and Cultural Organization[UNESCO],2006). A part of the landscape surrounding the Biosphere Reserve has been converted by human settlements, but “a high percentage of mature and successional forest is still found in fragments and continuous patches, which are owned by the communal land ownership of Ejidos” (Sanchez-Azofeifa, 2013, p. 32).

Site image of Chamela-Cuixmala Biosphere Reserve (Romero-Duque., Jaramillo, & Perez-Jimenez, 2007)

La Sepultura Biosphere Reserve, located in the Sierra Madre mountain range in Chiapas, is also a highly diverse forest reserve. Its main forest type is tropical dry forest. Various traditional forms of local ownership of land have been developed, which promote special management and conservation of the region (UNESCO, 2006).

Consisting mainly of tropical dry forest, the Sierra de Álamos Biosphere Reserve runs parallel to the coast of the Pacific Ocean.  “It hosts a wealth of key species like puma (Puma concolor), jaguar (Panthera onca), ocelot (Leopardus pardalis) and kinkajous (Herpailurus yaguaroundi)" (UNESCO, 2007). 

Sierra La Laguna Biosphere Reserve includes tropical dry forest and semi-desert. It is located in the State of Baja California Sur, close to the southern tip of Baja California Peninsula. The ecosystem is established by President Decree of 6 1994, forming only core area and buffer zone at that time. The ecosystem has rich biodiversity with 974 higher plant species reported of which 23.2% endemic species, including 5 endemic geneva (UNESCO, 2003). 

What is likely to happen in the future to that ecosystem?

Tianyi Peng

The recovery actions of some tropical dry forest in Mexico are already underway. Fast growing of tree biomass occurs during succession following the shifting cultivation of maize in Southern Yucatan Peninsular Region,Mexico (Read & Lawrence, 2003). According to Read and Lawrence’s research, an estimate recovery to precultivation level may take 55-95 yr and recovery to pre-logged state may take 65-120yr. In many other cases, situations are not optimistic. For example, Chamela-Cuixmala Biosphere Reserve faces threats from the Ejidos land, private ranches, and urban development (Sanchez-Azofeifa, 2013). Had the rate of deforestation not been contained, the continuous forest surrounding the Biosphere Reserve would disappear in less than 50 years and then the Biosphere Reserve would become an island (Sanchez-Azofeifa, 2013).

Also, government has found it difficult to balance the relationship between peasants in Ejidos and the ecosystem. Even though many peasants recognize the benefits provided by the tropical dry forest, they are “proud of their pasture fields and economic activities” and thus the conservation of the forest is not valued as necessity (Sanchez-Azofeifa, 2003). Therefore, until effective governmental policies are implemented to balance peasants’ livelihood and environmental protection, many tropical dry forests will still be in danger of deforestation. 

What can be done to maintain balance between people and the ecosystem?

Tianyi Peng

Mexico government has developed strategies to promote the conservation and restoration of tropical dry forest. One of the ways is to stimulate conservation through payment for environmental services (Sanchez-Azofeifa, 2013). According to Sanchez-Azofeifa (2013), the most important environmental services are “(1)pollination of crops by wild pollinator species…, (2) protection of watersheds and aquifers…,(3) carbon sequestration by mature and regenerating forests”. As a result, the government proposes the creation of “Ejidos’ protected areas network" (Sanchez-Azofeifa, A., 2013). In this system, government and reserves work together to provide peasants with technical assistance, training, educational programs and environmental service related employment. Peasants can work for the protection of the forest and get payment. Also, communication between the Ejidos and biosphere reserves should be enforced (Sanchez-Azofeifa, 2003, p.33). While governmental assistance is important for the industrial restructuring and long-term sustainable relationship between peasants and ecosystems, mandatory regulations should also be imposed to prevent forests from further damage. Peasants should “stop man-made fires, hunting, cattle ranching and other free-ranging perturbations” in the adjacent region of dry forest (Wilson, Peter, National Academy of Science & Smithsonian Institute, 1988, p. 134-135). Furthermore, rain forest wildlands should also be preserved “within migratory reach of the dry forest areas that are subject to restoration” (Wilson et al, 1988).

References

Becerra, J.X. (2005). Timing the origin and expansion of the Mexican tropical dry forest. Proceedings of the National Academy of Sciences of the United States of America, 102. doi: 10.1073/pnas.0409127102

Casitllo, A., Magaña, A., Pujadas, A., Martínez, L., Godínez, C. (2005). Understanding the Interaction of Rural People with Ecosystems: A Case Study in a Tropical Dry Forest of Mexico. Ecosysstems, 8, 630-643. doi: 10.1007/s10021-005-0127-1

Maass, J. M., Balvaner, P., Castillo, A., Daily, G. C., & Mooney, H. A.(2005). Ecosystem Services of Tropical Dry Forests: Insights from Long- term Ecological and Social Research on the Pacific Coast of Mexico. Eclology & Society, 10

Petz，B. (2012, June 25). Study Slashes Estimates of Carbon Emissions Due to Deforestation. Ecology Global Network. Retrieved from http://www.ecology.com/

Portillo-Quintero, C. A., & Sánchez-Azofeifa, G. A. Extent and conservation of tropical dry forests in the Americas (2010). Biological Conservation，143，144-155. doi:10.1016/j.biocon.2009.09.020

Read, L., & Lawrence, D.(2003). Recovery of biomass following shifting cultivation in dry tropical forests of the Yucatan. Ecological Applications, 13, 85-97. Doi: 10.1890/1051-0761(2003)013[0085:ROBFSC]2.0.CO;2

Romero-Duque, L. P., Jaramillo, V. J., & Perez-Jimenez, A. (2007). Structure and Diversity of Secondary Tropical Dry Forests in Mexico, Differing in Their Prior Land-use History. Forest Ecology and Management , 253, 38-47. doi: 10.1016/j.foreco.2007.07.002

Sanchez-Azofeifa, A. (2013). Tropical Dry Forest Ecological Succession in Mexico: Synthesis of a Long-Term Study. Hoboken: CRC Press.

Stoleson, S. H., Relger, R. S., Ceballos, G., Raish, C., Wilson, M. F., & Burquez, A. (2005). Recent History of Natural Resource Use and Population Growth in Northern Mexico. In Cartron, J. E., Ceballos, G., Felger, R. C (Eds.), Biodiversity, Ecosystems, and Conservation in Northern Mexico (pp.52-86). Oxford, NY: Oxford University Press.

Trejo I. & Dirzo R. (2000). Deforestation of seasonally dry tropical forest: a national and local analysis in Mexico. Biological Conservation, 94, 133-142. doi: 10.1016/S0006-3207(99)001883

Tropical Deciduous Forest and the Sonoran Desert: A Strong Connection(2006). Arizona-Sonora Desert Museum. http://www.desertmuseum.org/programs/alamos_index.htm

United Nations educational, Scientific, and Cultural Organization. Paris, France. http://www.unesco.org/mabdb/br/brdir/directory/contact.asp?code=MEX

Wilson, E. O., Peter, F. M., National Academy of Science & Smithsonian Institute (1988). Biodiversity. Washington, D.C. : National Academy Press

About Us

Yifan Yu

Tianyi Peng

We are a group of undergraduates who make this project for our Geography 5 class at UCLA. Tropical dry forest is an really endangered ecosystem in the world, while nowadays more studies and most public attention focus on tropical rain forest (Mooney et al., 1995). Therefore, we want to know more about this valuable ecosystem and hope more people can begin to also pay attention to this beautiful vegetation type.