Environment and Vegetation of the Rio Bravo Conservation and Management Area
(For more details than in this summary go to: Vegetation of Rio Bravo, which document needs revision but is still useful.)
The Rio Bravo Conservation and Management Area (RBCMA), in northwest Belize, covers c. 100,000 hectares (240,000 acres), including forest, savanna, swamp, marsh, river, and lake habitats (Fig. 1). Much of the forest is old growth. Beneath the forest are Maya ruins. Large animals that are rare elswwhere are present, such as great curassow, jaguar, spider monkey, and white-lipped peccary. 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 explore the varied ecosystems of the RBCMA from La Milpa Lodge and Hill Bank field station. At La Milpa you can learn the most common trees in upland forest by studying name-tagged trees on the Mahogany Trail and referring to the guide Trees of La Milpa (see pages in this website: Trees on Mahogany trail and Field guide to trees).
Vegetation history: Tropical forest has existed in the area since the end of the Pleistocene (~11,700 years ago). 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 Maya decline. From the mid-nineteenth century the area has been exploited for mahogany (Swietenia macrophylla) and the sap of the chicle tree (Manilkara zapota). There have also been some small milpas (farm plots) and natural disturbances such as treefalls and hurricanes. But much of the forest is old growth.
Climate: The RBCMA is in the subtropical moist life zone. It receives about 1500 mm (60 in) of rain per ear. December or January to May are usually dry months and the rest of the year is comparatively wet (Fig. 2). Annual rainfall and length of dry and wet seasons can vary greatly.
Soil and topography: Soils are derived from limestone. Topography includes level and gently rolling areas, hills, valleys, and escarpments cut by deep ravines. Topography influences soil type and soil moisture, which influence the local composition of tree species, creating different forest types on different topographic settings (Fig. 3). Where hill and valley are closely juxtaposed, contrasts between forest types are obvious. Elsewhere, the gently varying topography over much of the RBCMA produces long, shallow gradients of soil conditions, along which forest types, with their component species, are recognizable but blend into each other.
Forest types: 1. Upland forest occurs on well-drained soils. Its upper canopy is 15-20 m high, with some taller trees (Figs. 1, 4). In a 1-hectare (10,000 m2) plot of dry upland forest (upper slopes) there were 700 trees ≥10 cm diameter, of about 56 species (73 species cumulative over two censuses 25 years apart). In a hectare of moist upland forest (lower slopes) there were 450 trees of 48 species (59 cumulative). Common tree species in the upland forest are Pouteria reticulata (zapotillo), Pouteria amygdalina (silión), Drypetes brownii (male bullhoof), Manilkara zapota (chicle, sapodilla), Pseudolmedia spuria (wild cherry), Brosimum alicastrum (ramón, breadnut), Sabal mauritiiformis (botán, thatch palm), Hirtella americana (pigeon plum), and (locally) Ampelocera hottlei (female bullhoof). The subcanopy palm Cryosophila stauracantha (give-and-take, escoba) and the shrub Piper psilorhachis (Spanish elder) are abundant. From place to place there is much variation in the species composition and structure of upland forest. The Mahogany Trail, near the field station, starts in cohune palm forest (see below) but mainly passes through upland forest.
2. Bajo forest (or “bajo”) is found on clay soils that are waterlogged in the wet season and edaphically dry (holding moisture too tightly for plants to extract it easily) in the dry season. It is a dense forest of small stems mostly 3-8 m tall, with some taller (Fig. 5). Tree species typical of bajos are: Byrsonima bucidaefolia (craboo), Coccoloba reflexiflora, Gymnopodium cf. ovatifolium (bastard logwood), Croton spp. and others, while a few from dry upland forest also occur there. Bajos, usually have a sedge groundlayer and sometimes dense “sawgrass”. As with upland forest, there is great variation among bajos. Visit the “Bajo Trail” near the La Milpa &&& field station to see bajo forest.
3. Transition forest occurs in areas intermediate between upland forest and bajo (Fig. 6). It largely resembles dry upland forest in structure but is somewhat shorter, and has some features of bajos. It shares tree species with both those forest types. Some typical tree species of transition forest are Calophyllum brasiliense (santa maría), Gymnanthes lucida (pi), Manilkara zapota (chicle), Matayba oppositifolia (boyjob), and Metopium brownei (poisonwood).
4. Riparian forest occurs along the seasonally flooded margins of the Rio Bravo. It tends to have a broken canopy (due to poor tree anchorage and resultant treefalls), with much liana (large woody vine) cover and occasional large emergent trees. In a 1-hectare plot of cohune palm riparian forest there were 394 trees ≥10 cm dbh, of 59 species (81 species cumulative over two censuses 25 years apart). Attalea cohune (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 sapindoides (bribri), Bucida buceras (bullet tree), Pachira aquatica (provision tree), Pterocarpus rohrii (mountain kaway), Zygia sp., Vachellia gentlei (ant acacia), and Ficus spp. (fig). Walk to the end of the road passing the Dos Barbaras Maya site to see riparian forest along the Rio Bravo.
5. Cohune palm forest occurs on deep, well-drained soils. The canopy is 15-20 m high, with some taller trees. In a 1-hectare plot of cohune palm forest there were 374 trees ≥10 cm dbh, of 46 species (58 cumulative). Attalea cohune, the cohune palm, is a canopy dominant, but there are many other tree species, including most that are common in moist upland forest. Soils are good for agriculture, and the abundance of long-lived successional tree species in some stands of cohune forest are evidence of past forest clearing. These successional species include Swietenia macrophylla (mahogany), Cedrela mexicana (Spanish cedar), Spondias radlkoferi (jobo), and Ficus spp. (fig). The first 50 m of the Mahogany Trail passes through a cohune palm forest. This forest type covers little area in the RBCMA.
Forest ecology: 1. Old-growth and secondary forest. Much of the RBCMA is covered by old-growth forest probably established after the decline of the ancient Maya. Present evidence for the forest’s old-growth status includes: relatively tall and large-diameter stems of trees for this climate zone, complex three-dimensional forest structure, large lianas (woody vines), and abundant seedlings and saplings of the tree species that dominate the canopy, indicating a degree of stability in species composition. Human activity has produced secondary forest in local areas. Logging has been widespread but selective for a few, low-density species. Hurricane Janet in 1955 probably felled and delimbed many trees and promoted recruitment of secondary species, but it probably did not alter species composition greatly (based on studies of hurricane impacts elsewhere).
2. Species dominance, spatial distribution, species richness. In each forest type a different group of tree species dominates. In each of the four 1-hectare plots mentioned above, five to six species accounted for 50% of the stems ≥10 cm dbh, while the remaining 50% of stems included 53 to 41 species. There were many uncommon and rare species; in each plot 15 to 19 species were represented by only one individual. About 33 species, of 118 total (one census, not cumulative) in the four 1-hectare plots, were represented by only one individual over all the four plots.
The spatial distributions of most tree species are patchy, within and among forest types. Only 13 species, of 118 total species, were found in all four 1-hectare plots (each plot in a different forest type); 65 were found in only one plot. There are many different patterns of distribution, from widespread and common species to local and rare species.
Species richness (number of species) in any one forest type (alpha diversity) is not exceptionally high (compared to forests nearer the equator), but cumulative richness across forest types (beta diversity) is high.
3. Forest dynamics. In the four 1-hectare plots mortality of stems ≥10 cm dbh ranged from 25 to 54% over a 25-year interval between censuses, or about 1-2% of stems per year. One percent per year is typical of mature forests. Recruitment (stems growing into the ≥10 cm dbh class) ranged from 13 to 52%, about 0.5-2% per year. The plot with 2% mortality and recruitment is the riparian plot, where flooding probably loosens tree rooting, evidently leading to high mortality, recruitment, and species richness (see above). Also, high winds from a nearby hurricane killed trees in the plots during the census interval. A significant number of species were both eliminated from and added to each plot during this interval.
4. Seasonality of the vegetation, degree of deciduousness. Flowering, fruiting, and leaf replacement are all highly seasonal in the RBCMA. Each tree species has a different seasonal schedule, but many are similar enough that community patterns are evident.
Flowering by trees, in terms of numbers of species and individual, peaks in the mid to late dry season (March and April) and is least in the late wet season (December). Fruiting patterns are less clear, but fruit abundance appears to be relatively higher in the late dry season and early wet season (June). Leaf drop is highest in the dry season. New leaves appear mostly during the dry season and early wet season.
The great majority of trees in the RBCMA are evergreen. The terms “deciduous” or “semi-deciduous”, which have been applied to RBCMA forests, give an exaggerated impression of seasonal leaflessness in the forest. Only a small proportion (6-7%) of the species or of total tree individuals are leafless at any one time, except in local situations, such as where there is thin soil.