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The Tropical Dry Forest of the Sierra de la Laguna

by Bob Chamlee

      The Cape region of the southern state of Baja California is home to the Sierra de la Laguna dry forest. Tropical dry forests make up the majority of tropical forests on earth. Even though there is more dry tropical habitat throughout the world than rainforest, this habitat is not as well known and tends to be overlooked by conservation efforts.
      In a tropical dry forest, the dry season is far longer than the brief period of rainfall. Most of the trees and shrubs found in this type of habitat are deciduous, losing their leaves at the onset of the extended dry season, and for this reason they are also known as tropical deciduous forests. In the Americas, vast tracts of tropical dry forest once reached along the Pacific coast from the tip of Baja California in the north to northern Argentina. Much of this forest has been lost, but some still remains today in many rugged mountainous areas, national parks, and biosphere reserves.

Geology
     The Cape region of Baja California lies at the southeastern tip of the world's fourth longest peninsula, separated from the mainland coast of Mexico by 250 kilometers of the Sea of Cortez. This peninsula of land was once a part of mainland Mexico between 65 and 100 million years ago when the long chain of the Rocky Mountains was formed.
     The peninsula's divergence from mainland Mexico came about as the result of the shifting of large sections of the earth's crust, called plates. Around 30 million years ago, a large piece of the coast of the mainland was carried towards the northwest. Magma pushed upwards creating the volcanic ranges of south central Baja. This movement tore the crust of the earth, allowing basins to form. The Pacific ocean rushed into these basins, creating the Gulf of California. At the northern end of the Gulf, the San Andreas fault is the boundary between the plates, and the Pacific Plate still slips northwest at the rate of three to five centimeters per year. At the southern end of the peninsula, a remnant of the Rocky Mountain chain remains -- the Sierra de la Laguna. Because of the way the fault blocks tipped to create these sierras, the rugged range runs true north and south for over fifty miles, with the steepest slopes on the west side. The highest peaks reach six to seven thousand feet in elevation. The land to the east side of the sierra is known as the Cape.

Climate
     The Sierra de la Laguna has been called an ''island in the sky'' because of its topography which isolates it from the surrounding deserts. The Tropic of Cancer runs directly across the Cape and the Sierra de la Laguna at 23.5 degree latitude. Temperatures average highs of 74F in January and 88F in August. The area receives an average of almost forty inches of rain per year on the peaks -- more rainfall than any other part of Baja, except the Californian region to the northeast. This moisture gives support to several unique ecosystems, a rare mix of desert, sub-tropical, tropical, and sub-alpine species that may be found together nowhere else in North America. The highest elevations of the Sierra de la Laguna are blanketed in pine-oak forests, merging with tropical dry forest at about 750 meters in elevation. At about 300 meters in the Cape region, the tropical dry forest gives way to the Sonoran Desert. The tropical dry forest here averages only about eleven inches of rain annually, brought from the south in the form of tropical monsoons which form along the west coast of Mexico. As in most tropical dry forests, this precipitation comes only during a short period in the end of the summer. These monsoons, known locally as ''chubascos'', often feed on the warm waters of the Gulf, bringing heavy precipitation in a short time.

Biodiversity
     Tropical dry forests have less overall diversity than rainforests, but an amazing array of biodiversity is present. As many as 224 species of plants, a number of reptiles and the vast majority of the region's mammals are found in the Sierra de la Laguna dry forest As a result of the topography, many of these species have evolved in isolation. As many as 10% of the species are endemic to this habitat.

Flora
     The Sierra de la Laguna dry forest consists of trees, shrubs, and undergrowth of different heights which form a canopy of lush green growth after a rain. Some evergreens do live here, but the majority of plant life shed their leaves to retard water loss in the dry season. Many species also feature thick waxy skins, or store moisture in their fleshy leaves and stems. Columnar cacti, agaves and other succulents mingle with plants and trees normally associated with the thorn forests of mainland Mexico and some tropical species from Central America.
     Like desert plants, arid forest plants develop larger fine root systems to gather moisture as quickly as possible. The Sierra de la Laguna soil quality varies by location, but overall it is quite poor. The extremely coarse nature of the granite found here means that even as it breaks down -- a slow process due to little weathering from rain -- the individual grains of sand remain quite large. The porous nature and low organic content of this soil does little to stop the rapid drainage of any available moisture.
     Many of the arroyos run year-round with the water filtering down from the peaks above. But, with the exception of these riparian areas, most plants and animal species have had to find ways to adapt to the uncertain rainfall. Within hours of a rain, some species of plants have germinated from their dormancy and start to push up to the surface from the cracks. Some have large scented flowers, to more easily attract insect species as they must be pollinated quickly. Some species complete the whole cycle of growth and reproduction in as short as two weeks.
      Many of the perennial plant species have small leathery leaves to slow water loss, and spines to thwart grazing. Legumes are the predominant tree and shrub members of this arid forest. Many of the 42 species found in the Cape dry forest, such as lotus, ironwood, and acacias fix nitrogen in nodules at their roots, or have low nitrogen requirements as an adaptation to the poor soil conditions. Another mechanism adapted by some of the dry forest plant genera, such as cryptocarpa, and bursera, is a swollen trunk, used as an organ to store water. The wood water content in these trunks ranges from 60% to 80%.
     The columnar cacti have also adapted well to the dry conditions. The cardon, for instance, is the largest cactus in the world, reaching 15-20 meters in height, with stems up to 1.5 meters in diameter. The trunk and stems are nearly vertical, giving minimal exposure to the hot midday sun, yet maximum surface area to catch the cooler morning and evening sunlight. Cardon cacti and many other plants of the tropical dry forest have adapted a different method of minimizing water loss during photosynthesis. The stomata on these cacti open only after dark, allowing the cactus to absorb carbon dioxide during the cooler night hours, storing it in the leaf tissues. This ability to store carbon dioxide makes these plants very water efficient and they can live in very arid conditions.

Fauna
    The tropical dry forest supports a great diversity of fauna. Many of the animals have adapted to cope with the hot, arid conditions. The Merriam's Kangaroo rat found in the Cape dry forest is a good example. These small rats are nocturnal so they can gather their food in the cool of the night, helping to conserve water. Large ears help to dissipate heat, and the Kangaroo rat like many members of this dry forest spends its days in burrows underground to escape the sun's hot rays. A common trait shared by the Kangaroo rat and some other desert mammals is enlarged nasal passages, which allow the moisture lost in respiration to be reclaimed by condensation on the cooler nasal membranes.
      In this arid ecosystem, ants thrive and are very important organisms, providing a ready source of food to many of the numerous lizards, which comprise 40% of the vertebrates in the Cape dry forest. Shrews and bats also eat many insects. The carnivores in this ecosystem include coyotes, kit and gray foxes, ringtails, raccoons, skunks, badger, and bobcat. The mountain lion is the largest predator in the Sierra de la Laguna.

The Delicate Balance of the Ecosystem
    The light energy from the sun is assimilated by the plant communities through photosynthesis and used by plants to sustain themselves and reproduce. Some of the excess energy is available as biomass, providing the herbivores with the first trophic level, or the base of the food chain. In this ecosystem, herbivores include desert, deer, cactus and pocket mice, wood and kangaroo rats, gophers, squirrels, cotton-tail and jack rabbits. The desert mule deer is the largest herbivore in the ecosystem. The herbivore uses the energy ingested to maintain itself and grow, losing some of the energy to waste excretion and respiration. The energy now tied up in the herbivore, is available to the carnivore as the next trophic level, or step-up the food chain.
     Predators, like the herbivores, produce waste that is available back to the plant community after being broken down by the scavenger and decomposer communities. Decomposers are usually molds, fungi, and microbes which work in the brief wet season to break down waste into nutrients for themselves, with the leftovers going back to the plants to convert once again into the biomass. This complex food cycle is actually a web, with each plant, bird, insect, slime-mold, and bacteria occupying it's own niche. Keeping the ecosystem working is a delicate balancing act, with adaptation playing a vital role by helping organisms get the most of the energies available in the ecosystem.
     The dry forest of the Sierra de la Laguna represents a unique example of one of the most endangered types of ecosystem. The rugged terrain and relatively low human population have somewhat spared this tropical dry forest from the habitat loss experienced in other parts of the world. The creation of the Sierra de la Laguna biosphere reserve by the Mexican government has helped to further protect this area and it should be able to remain intact and functioning. Study of this relatively undisturbed habitat could help to better understand this fascinating and endangered ecosystem and its inhabitants.



If you enjoyed this, check out the rest of our Los Cabos Information pages:

  • Sonoran Desert of Baja California
  • Cardon cactus (pachycereus pringlei)
  • San Jose Estuary
  • Golf in Los Cabos
  • Los Cabos Shopping Guide



    Sources:

    Coyle, Jeanette and Roberts, Norman C. 1975. A Field Guide to the Common and Interesting Plants of Baja California. La Jolla, CA. Natural History Publishing Company.

    Cummings, Joe. 1995. Cabo Handbook: La Paz to Cabo San Lucas. Chico, CA. Moon Publications, Inc.

    Friesen, Larry Jon. 2001. Biology 122 Ecology. http://www.saturdaze.net/eco/

    Olsen, Mark. 1999. Images of dry tropical habitat: Mexico. http://www.mobot.org/gradstudents/olson/mexico.html

    Ricklefs, R. E. 2001. The Economy of nature, fifth edition. New York: W.H. Freeman and Company.

    Taggart, Ralph E. 2001. Tropical Biomes. http://taggart.glg.msu.edu/bs110/tropical.htm

    Turner, Raymond M, et al. 1995. Sonoran Desert Plants: An Ecological Atlas. Tucson. The University of Arizona Press.

    Wilson, John, et al. 1999. Global Forest Restoration: A Review. Corvallis, OR. Conservation Biology Institute. http://www.consbio.org/cbi/what/globalforest_pdf.htm

    World Wildlife Fund. 2001. Terrestrial Ecoregions - Sierra de la Laguna dry forests (NT0227). http://www.nationalgeographic.com/wildworld/profiles/terrestrial/nt/nt0227.html

    Zwinger, Ann. 1983. A Desert Country Near the Sea. New York. Harper & Row, Publishers.

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