Environment and vegetation

Environment and Vegetation of the Rio Bravo Conservation and Management Area, Belize

(For more details than in this summary go to:  Vegetation of Rio Bravo)

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Spider monkeys near La Milpa Ecolodge and Research Center, RBCMA (photo by N. Brokaw)

The Rio Bravo Conservation and Management Area (RBCMA), in northwest Belize, is perhaps the most accessible, largely intact tropical landscape in the world.  In one day’s travel from the U.S. you can be in old-growth tropical forest containing big animals, such as the great curassow, jaguar, spider monkey, and white-lipped peccary, that are absent at other accessible sites in the tropics.  The RBCMA covers c. 100,000 hectares (c. 240,000 acres), including forest, savanna, swamp, marsh, river, and lake habitats.  Beneath the forest are Maya ruins.  The RBCMA is managed by The Programme for Belize, a Belizean non-profit organization dedicated to sustainable resource use, conservation of biodiversity, and environmental education.

You can find these ecosystems near La Milpa and Hill Bank field stations in the RBCMA.    Near La Milpa station you can learn some common trees in upland forest by studying name-tagged trees on the Mahogany Trail (“Tree trail” page in Contents) and referring to the guide Trees of La Milpa ( “Field guide to trees” page in Contents).

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Monthly rainfall and maximum and minimum temperatures at Chan Chich Lodge in northwest Belize (averages of ~5 yrs data; O’Hara 1999, data from Tom Harding)

Environment.   The RBCMA is in the subtropical moist life zone.  It receives about 1500 mm (60 in) of rain per year.  January to May are usually dry months and the rest of the year is comparatively wet.

Soils are derived from limestone.  Topography includes level and gently rolling areas, hills, and escarpments cut by deep ravines.  Topography and soil influence the level of soil moisture and for that reason largely control forest type.  Contrasts are evident where topography is extreme, while, the gently varying topography over much of the RBCMA produces long, shallow gradients of soil conditions and correspondingly long continua of subtly changing forest types.

History.   Tropical forest has existed in the area since the Pleistocene.  The ancient Maya cut much of the forest in what is now the RBCMA, but the forest has regrown in the 1100 years since the decline of the Maya.  From the middle of the nineteenth century the area has been exploited for mahogany and later for chicle.  There has also been a small amount of milpa agriculture and natural disturbances such as treefalls and hurricanes.  However, much of the forest is old growth.

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Profile of a 40 x 10 m plot of upland forest.  Trees = or > 10 cm diameter are shown; understory not shown.  Axes in meters  (drawing by Jennifer O’Hara)

Vegetation.   Upland forest occurs on well-drained soils.  Its canopy is 15-20 m high with some taller trees.  In a one-hectare plot of dry upland forest we found 700 trees ≥10 cm dbh, of 56 species.  In a hectare of mesic upland forest there were 450 trees of 48 species.  Pouteria reticulata, Pouteria amygdalina, Drypetes brownii, Manilkara zapota, Pseudolmedia sp., Brosimum alicastrum, Sabal mauritiiformis, Hirtella americana, and Ampelocera hottlei are common tree species in the upland forest.  The subcanopy palm Cryosophila stauracantha and the shrub Piper psilorhachis are abundant.  From place to place there is much variation in the species composition and structure of upland forest.

bajo-profile
Profile of a 25 x 5 m plot of bajo swamp forest.  Trees = or > 2.5 cm diameter are shown.  Axes in meters (drawing by Jennifer O’Hara)

Bajo swamp forest is found on clay soils that are seasonally waterlogged and edaphically dry.  It is a dense forest of small stems mostly three to five m tall.  Many tree species are restricted to scrub swamp forest, while a few typical of dry upland forest are also found there.  Bajo swamp forest, usually has a sedge groundlayer and sometimes dense “sawgrass”.

 

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Bajo swamp forest (fine-grain canopy) and upland and transition forests (coarse-grain), RBCMA (photo by N. Brokaw)

Transition forest occurs in areas intermediate between upland forest and bajo swamp forest.  It largely resembles dry upland forest in structure, but is somewhat shorter, and has some features of bajo swamp forest.  It shares tree species with both those forest types.  Some typical tree species of transition forest are Calophyllum brasil­iense, Gymnanthes lucida, Manilkara zapota, Matayba oppositifolia, and Metopium brownei.

Cohune palm forest occurs on rich, well-drained soils.  The canopy is 15-20 m high, with some taller trees.  In a one-hectare plot of cohune palm forest we found 374 trees ≥10 cm dbh, of 46 species.  Attalea cohune, the cohune palm, is a canopy dominant, but there are many other tree species, including most that are common in upland forest.  Cohune palm forest soils are good for agriculture and the abundance of long-lived successional tree species in some stands are evidence of past clearing.  These include such species as Swietenia macro­phylla, Cedrela odorata, Spondias mombin, and Ficus spp.

rb-cohune-riparian-profile
Profile of a 40 x 10 m plot of riparian forest.  Trees = or > 10 cm diameter shown; understory not shown. Axes in meters (drawing by Jennifer O’Hara)

Riparian forest occurs along the temporarily flooded margins of the Rio Bravo.  It tends to have a broken canopy, with much liana cover and occasional large emergent trees.  In a one-hectare plot of cohune palm riparian forest we found 394 trees ≥10 cm dbh, of 59 species, the highest species tally among our plots.  Attalea cohune was a dominant in this plot, as in some other areas of riparian forest, but it can also be uncommon in this forest type.  Other species include some of those common in upland forest and such characteristic species as Inga edulis, Bucida buceras, Pachira aquatica, Pterocarpus hayesii, Zygia sp., and Vachellia [formerly Acacia] spp.