VEGETATION CLASSIFICATION AND FLORISTICS OF THE SAVANNAS AND ASSOCIATED WETLANDS OF THE RIO BRAVO CONSERVATION AND MANAGEMENT AREA, BELIZE

A ﬂoristic inventory and preliminary vegetation classiﬁcation were made for the tracts of savanna and associated wetland vegetation in the Rio Bravo Conservation and Management Area (RBCMA) in Belize. A total of 258 species were recorded, representing c.7 % of the Belizean ﬂora. Of these, 148 species are characteristic of the drier savanna systems, while 47 show a preference for hydrologic savanna and wetland areas. Only 57 species (22 % of RBCMA savanna ﬂora) are woody, with the savanna tree ﬂora comprising 15 species. The ﬂora of the RBCMA was found to be fairly typical of the savannas of the Central American and Caribbean region. Savanna systems are generally poorly represented in conservation areas in Central America, and due to the diverse range of structural and ecological formations of this vegetation type found within the RBCMA, and its relatively diverse ﬂora, this reserve constitutes an important protected area.


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Neotropical savanna covers over 2 million km2, and represents the second largest vegetation formation in the American tropics (Mistry, 2000).Several studies have assessed the phytogeographic affinities of this widely distributed vegetation type (e.g.Sarmiento, 1983;Lenthal et al., 1999), although a lack of detailed floristic information for many savanna systems prevents the undertaking of rigorous, analytical studies.The savannas of Belize represent the most northerly distribution of the Central American savannas.They lie on the margin of lowland humid tropical formations and as such are of phytogeographic and ecological importance.At present there is only a poor account of the Belizean savanna flora, and there is a need for detailed floristic inventories to assist local and regional conservation planning.
In Central America and on the Caribbean islands, major savanna areas occur in southern Mexico, Belize, Honduras, Nicaragua, southwest Panama and central and eastern Cuba (Huber, 1987).These widespread savannas are ecologically varied; the lowland savannas tend to be hyperseasonal in nature (sensu Sarmiento, 1983), i.e. with flooding and drought conditions occurring during each annual cycle, while the upland savannas are usually better drained, with reduced ground water effects.
The study reported here focused on the hydrologic and well-drained savannas found in two areas in Belize, both lying within the Rio Bravo Conservation and Management Area (RBCMA) managed by the NGO (non-governmental organization) Programme for Belize (PfB) (Fig. 1).It seeks to refine the savanna and wetland vegetation classifications of Brokaw et al. (1990) for the RBCMA, and to provide comprehensive species lists for the savanna and related wetland flora for use in environmental planning.
The vegetation of Belize was first mapped in detail by the Land Use Survey Team ( Wright et al., 1959), and their classification of savanna and related vegetation (including pine forest) attributed c.262,467 hectares to these communities.This constitutes approximately 12% of the surface area of Belize, which covers a total of 22,963km2.The report classified Belizean savannas and associated forest-type communities into seven broad categories.These include pine and oak forests, orchard and open grass communities, and areas of savanna with scattered pine.Wright attributes a further 62,157 hectares (2.8% of land area) to herbaceous marshes dominated by rush and sedge vegetation.A more recent study, by Rejma ´nkova ´et al. (1996), characterized all Belizean wetland vegetation types into three basic types defined by species of Eleocharis, Cladium and Typha.
A later study, by King et al. (1992), groups all Belizean savannas and pine forests into one group -pine forest and orchard savanna.The stage at which 'pine savanna' ('savanna with scattered pines' or its denser form 'pine ridge'1) can be differentiated from 'pine forest' is a moot point, as the species composition of the two can be extremely similar and there is a continuous range of physiognomy uniting them.Most authors agree, therefore, that they should be classified together.Brokaw et al. (1990) conducted the first detailed study of the vegetation of the RBCMA, distinguishing upland forest, cohune ridge and palm forest, swamp forest, marsh, pine ridge and savanna.This study estimated that 2.8% of the RBCMA area was covered by savanna.This includes pine ridge (pine woodland) which occurs mainly on the sandy soils in the northeast of the area.The vegetation descriptions resulting from this preliminary vegetation classification are valuable, but do not give any detailed floristic information on the savannas, nor do they make any floristic comparisons with other savanna areas.Owing to the reconnaissance nature of this initial study, the herbaceous component was also ignored.Iremonger & Brokaw (1995)  types of non-saline seasonally waterlogged scrubs, three types of non-saline scrub which are not influenced by high water tables, and four non-saline herbaceous communities, including freshwater wetlands.

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The RBCMA is situated in Orange Walk district, northwest Belize, and is considered geologically part of the Yucata ´n Peninsula.It covers an area of 82,000 hectares and is almost entirely covered with natural forest, savanna and marsh (Brokaw et al., 1990).Figure 1 shows the distribution of savanna within the RBCMA, as well as the principal savanna and pine forest areas within Belize.The climate is subtropical, with only minor seasonal variations in temperature and a distinct dry season.Annual daytime temperatures vary between 26°C and 32°C, with the highest temperatures occurring in April and May, towards the end of the dry season.Total annual rainfall averages about 1500mm, with a marked dry season between February and May ( Wright et al., 1959).Following Koeppen's classification the climate is tropical wetdry.The RBCMA occurs on a geological formation known as the Yucata ´n Platform.This consists of a limestone plain covering the northern half of Belize with geology and vegetation continuous with the southern half of the Yucata ´n of Mexico and the northern Pete ´n of Guatemala (Standley & Record, 1936).
The savanna and wetland vegetation surveys were focused on the areas lying between East Gate and Hill Bank, and those around Booth River (Fig. 1).The study areas were initially identified through remotely sensed images using a 1993 Landsat TM image covering the full extent of the RBCMA, with additional information coming from aerial photography and a SPOT panchromatic image (Furley et al., 2001).Areas identified as being savanna, wetland or savanna/forest transition communities were subsequently ground-truthed by wide-patrolling, general observation and collecting.The surveys loosely followed a series of transects cutting across the savanna area.Quantitative data were collected for 80 trees (20 points) for one area of dense savanna pine ridge close to East Gate using the point-centre quarter technique (Mueller-Dombois & Ellenberg, 1974).Field work was conducted in three phases: a two-month period from mid-July to mid-September 1996, a two-week period in March 2000, and a four-day period during March 2001.
Broad descriptions were made for all vegetation types encountered and an attempt was made to classify the vegetation subtypes following Brokaw & Mallory (1993).Where this was not possible new classifications were devised using terminology that complemented those already used locally.The dominant defining species and general physiognomy of all subtypes were noted.Duplicate collections were made of all higher plants found in the area and are currently held at the Royal Botanic Garden Edinburgh (E) and the Forestry Department, Ministry of Natural Resources, Belmopan, Belize (BRH ).

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A total of 400 collections were made, representing 258 species.A full species list by family is given in Appendix 1, together with notes on growth forms.Appendix 2 provides species lists by habitat.Table 1 presents the phytosociological parameters for those tree species recorded in an area of broken pine ridge (pine woodland), and Table 2 lists the primary savanna and wetland vegetation subtypes found within the RBCMA.The text below gives general descriptions of the savanna and associated vegetation types encountered within the RBCMA.

Savanna and wetland vegetation classification
The vegetation encountered during the field surveys has been broadly grouped into three main categories: savanna, wetland and savanna/forest transition, according to overall physiognomy, water regime and species composition.Within each of the wetland and savanna categories a number of subtypes have been identified.The subtypes give an idea of the range and variety of savanna vegetation and provide a useful terminology, but most form part of an ecological succession, part of an ecotone or reflect anthropomorphic modifications.As such they should not necessarily be considered as stable communities.Although isolated forest patches occur on better soils within the savanna region and narrow fringes of gallery forest can be found crossing it in association with watercourses, these were not surveyed, and Brokaw & Mallory's forest classification (1993) should be consulted.The transition zone from savanna to forest is often marked by the presence of oak thickets.Ecological descriptions and species lists have been compiled for these.

Savanna
Following Wright et al. (1959), the term savanna is used here in the broadest sense, and includes grasslands, dense woody thickets, orchards, woodlands and broken pine ridge.Where there is a conspicuous woody component of the vegetation, the defining species are Pinus caribaea Morelet ( pine Within the broad definition of savanna, a number of savanna vegetation subtypes can be identified.These occur as a mosaic across the area.Although each subtype varies in its exact structure and species composition, they can usually be easily recognized.

Wetland
Wetlands are defined here as permanently or seasonally inundated open areas, usually characterized by the dominance of Cyperaceae (sedges) in the herbaceous layer, rather than Gramineae (grasses).The water table is very close to the surface and standing water creating anaerobic waterlogged conditions is frequently present during the wet season.Trees and shrubs are generally absent, or if present are species characteristic of wetter areas such as Dalbergia glabra, Bucida buceras, Crescentia cujete, Mimosa asperata L. and occasionally Rhizophora mangle L. with its distinctive stilt roots.

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1 Savanna

Grassland and scrub grassland
This subtype is characterized by the complete absence of trees, the only woody vegetation being occasional dwarf shrubs of Byrsonima crassifolia, Semialarium mexicanum, Crescentia cujete, Curatella americana, Calliandra houstoniana (Mill.)Standl., Coccoloba cf.reflexifolia Standl., Zamia loddigesii, Eugenia winzerlingii and Gliricidia sepium.Randia spp.and Myrica cerifera L. occur in transitional wetter areas.There is an almost complete ground cover of grasses and sedges and a rich herbaceous flora, the most conspicuous species being Ageratum radicans, Diodia teres, Agalinis hispidula, Sauvagesia erecta, Xyris ambigua Beyr.ex Kunth, Turnera diffusa, Polygala variabilis, Melochia spicata (L.) Fryxell, Oxalis frutescens L., Spermacoce verticillata L., Drosera capillaris Poir, Hyptis conferta Pohl ex Benth.and Lycopodiella caroliniana (L.) Pic.Serm.The herbaceous vegetation is frequently covered by the dodder Cassytha filiformis.This vegetation type is probably transitional in nature, and is related to frequent occurrences of fire.The majority of the shrub species can attain sizeable tree stature and in the absence of fire could be expected to reach significant heights.However, edaphic determinants -the presence of infertile sandy soils and the seasonally high water table -may also be responsible for checking woody growth.

Pine-palmetto savanna
A very open vegetation, often on sandy soils, in which the ground layer is dominated by tussock-forming grasses, with occasional scattered pines to 20m (although the majority are smaller).With the exception of Pinus caribaea, few other woody species reach tree stature and are usually present as shrubs, although Crescentia cujete, Byrsonima crassifolia, Curatella americana, Cameraria latifolia L. and Quercus oleoides can reach 10m, with Quercus oleoides and Byrsonima crassifolia even reaching 15m at forest margins.Eugenia winzerlingii, Semialarium mexicanum, Pithecellobium insigne Micheli, Roupala montana Aubl.and Clidemia sericea are other common woody associates, usually found as shrubs or saplings.Many of the areas have the appearance of having been burnt regularly and there is evidence of logging.The burning might explain the dwarf nature of many of the woody species which would usually form larger shrubs or trees.The ground layer is made up of an almost continuous cover of tussock-forming grasses to 40cm; species of Paspalum are especially abundant.Pine-palmetto savanna has a rich herbaceous flora similar in composition to the savanna grasslands (1.1).

Savanna orchard
This vegetation has the appearance of an 'orchard' with the trees evenly spaced, the majority rarely exceeding 8m, although some mature individuals of Bucida buceras are emergent and can attain 15m.It has a greater density of woody shrubs and small trees than other savanna subtypes, but its relatively open canopy and small stature easily distinguish savanna orchard from forest and pine ridge (savanna woodland).This vegetation type has been seen only in hydrologic transitional areas from wetland to more typical savanna.Savanna orchard is frequently seasonally waterlogged.The species composition of the savanna orchards varies greatly, but they tend to be dominated by Bucida buceras, Haematoxylon campechianum L. and Cameraria latifolia.Crescentia cujete is a common associate in wetter areas, sometimes even to the extent of dominating the vegetation (e.g.close to Booth River), others being Malpighia glabra L., Jacquinia macrocarpa Cav., Coccoloba sp., Semialarium mexicanum, Byrsonima crassifolia, Chrysobalanus icaco L., Myrica cerifera and occasional clumps of Acoelorraphe wrightii.Pinus caribaea and Quercus oleoides are conspicuously absent.The ground layer is open, dominated by grasses and sedges, with abundant Cassytha filiformis, and the herbaceous flora is more depauperate than in other drier savanna areas.The strong influence of water is often reflected in the ground flora, which can be dominated by a single species of sedge (Eleocharis interstincta R. Br.), with Nymphoides humboldtianum Kunth and Mimosa asperata sometimes present.The trees and shrubs can support an abundant epiphytic flora of Tillandsia sp., other bromeliads, orchids and parasitic mistletoes' Phthirusa spp.

Woodland and pine ridge
These tend to be Pinus caribaea-dominated areas (although oak is sometimes a conspicuous component), with the larger trees attaining 15m (and c.50cm dbh), forming a broken canopy.The woodland is conspicuously denser and has a far greater abundance of pine than pine-palmetto savanna, but is still considered a savanna subtype because of its typically savanna floristic composition.These denser pine areas usually show signs of burning, and cut stumps are common, evidence that these areas were managed or exploited in the past.Quercus oleoides, Curatella americana and occasionally Guazuma ulmifolia are associates which, together with sparse shrubby Byrsonima crassifolia, Semialarium mexicanum, Calliandra houstoniana, Clidemia sericea, Chomelia protracta (Bart.ex DC.

Sedge marshland
This consists of waterlogged areas dominated by Cyperus spp. to 1m, with occasional Mimosa asperata and Sagittaria lancifolia.

Eleocharis-calabash marsh
This is marshland where the ground layer is completely dominated by Eleocharis interstincta.The area is probably inundated for most of the year.Occasionally scattered trees of Crescentia cujete form a conspicuous component of this formation.Associate ground species include Centella asiatica, Pluchea foetida L. and Sagittaria lancifolia.

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The 258 species found in the pine savanna and related vegetation of the RBCMA reserve represent c.7.5% of the total flora of Belize (3408 species as recorded by Balick et al., 2000).None of the 41 species recognized as endemic by Balick (1.2% of the flora) was recorded.Of those species observed during the present study, 148 have an apparent preference for drier savanna communities, with 44 preferentially found in the more humid wetland areas and 74 associated with forest margins.Brokaw et al. (1990) cite a total forest tree list of 167 species for the RBCMA.Of the 'true' savanna species found during the present study, 57 (22%) are woody, of which 15 species (c.8% of the total savanna flora) can be classified as trees.
The characteristic savanna formations of RBCMA occur on soils of low fertility, mild acidity and coarse texture (Furley et al., 2001).The distinctive palmetto associations have a strong tendency to increase in areas where the water table rises to the surface, their root systems being adapted to seasonal changes in water availability (Milne, 1997).Iremonger & Brokaw (1995) identify one of their forest vegetation types as 'palmetto/coco plum variant' forest, and to some extent the RBCMA palmetto thickets equate to these, although those found in this study can be considered only thickets as they are of limited stature and extent.The wetland communities occur in areas of seasonal or permanent inundation on soils with an increased amount of clay and silt.Vegetation types typical of the transition zone between the savanna and wetland habitats (e.g.savanna orchard) are heavily influenced by the height of the water table during the wet and dry seasons, with the savannas on the boundary of the wetlands often being hydrologic in nature.
The woody flora of the study site, although depauperate when compared with the rich tree savannas of the Brazilian cerrado biome (Ratter et al., 1997), is fairly typical of the general savannas of the Central American and Caribbean region.In the Caribbean, the floristically most diverse savannas are found in Cuba, characterized by a high number of endemic palm species (Borhidi & Herrera, 1977).The dominant tree defining the RBCMA savannas is Pinus caribaea var.hondurensis, and this species occurs widely across the Central American region from Mexico to Honduras, in savanna, woodland and forest formations, up to an altitude of 600m.The other two classic widespread and abundant species found within RBCMA -Byrsonima crassifolia and Curatella americana -are the most widely distributed woody savanna species of the Neotropics.Both are identified as indicator species for Neotropical savannas and are considered to form a so-called 'basic floristic matrix' present in practically all Neotropical savanna lowlands (Huber, 1987).Casearia sylvestris, another species cited by Huber as belonging to this matrix, was not found in the RBCMA savannas, although it is present and common in Belize.Of the other woody savanna species recorded, most occur throughout Central America, and, as one might expect, the phytogeographic affinities of the RBCMA savannas lie very clearly with this region.A phytogeographic analysis of the floristic woody component of Neotropical savannas conducted by Lenthal et al. (1999) grouped together all the savannas of Central America and the Caribbean as a single phytogeographic province, with those from southern Mexico, the Pete ´n and Belize not surprisingly most closely linked.
A comparison of the RBCMA savannas with other savanna areas within Belize reveals it to be floristically similar.The woody species list is nearly identical to those compiled by Furley & Ratter (1986) for savanna areas from Spanish Lookout and Belize Zoo (both to the south of the present study site in central Belize) and by Johnson & Chaffey (1973) for Mountain Pine Ridge (also in central Belize), the main exception being Clethra occidentalis which is recorded from the savannas and pine forest of Mountain Pine Ridge, but is apparently absent from the RBCMA.Clethra mexicana and Leucothoe mexicana are also recorded as trees from the Mountain Pine Ridge savannas, with C. mexicana associated with savanna/gallery forest transition boundaries.Neither of these species was observed within the RBCMA.Another plant typical of the pine forests and savannas of Mountain Pine Ridge region and recognized as a Belizean endemic (Balick et al., 2000) but noticeably absent from the RBCMA is Schippia concolor.The characteristic savanna orchard, dominated by Crescentia cujete, Bucida buceras, Haematoxylon campechianum and Cameraria latifolia, appears to be related to the 'jicaro savannas' described by Taylor (1963) from Nicaragua, which are characterized by Crescentia alata and Haematoxylon brasiletto.In Belize, Haematoxylon campechianum is noted by Balick et al. (2000) as being common in the Orange Walk and Corozal districts on periodically flooded limestone depressions.Savanna orchard appears to equate well with the 'bullet tree-logwood variant' forest of Iremonger & Brokaw (1995), although some of the species they identify as characteristic, such as Calophyllum brasiliense and Manilkara zapota, were not recorded in our studies.
The areas of broken 'pine ridge' are similar in structure to other larger tracts of pine forest areas within Belize.Johnson & Chaffey (1974) calculated a mean of 26 stems per hectare for pine forests at Sittee River and Deep River, for trees with a diameter greater than 15.2cm.A similar forest inventory of the Mountain Pine Ridge by the same authors (Johnson & Chaffey, 1973) recorded 61 stems per hectare for those over 15.2cm dbh.Although these surveys are not directly comparable to the phytosociological data of the present study, which recorded 93 stems per hectare for all trees with a dbh Á10cm, the results suggest a fair degree of structural similarity.Fire is the predominant factor determining the density of pine stocking.It is clear that these savannas, as elsewhere in Central America, are a constantly changing mosaic of intergrading vegetation types representing stages on varying ecological continua, and as such none of the vegetation types can be considered as stable climaxes.
The factors maintaining pine savannas have been widely discussed (e.g.Standley & Record, 1936;Beard, 1953;Wright et al., 1959;Taylor, 1963;Kellman & Meave, 1997;Negro ´n-Ortiz & Gorchov, 2000) and are beyond the scope of this study.Suffice it to say that current research suggests that in lowland tropical areas in the absence of fire, there may be a gradual replacement of pine savanna by broadleaf forest, even on nutrient-poor soils.This successional model has been proposed by Taylor (1963) for the pine savannas and broadleaf forests of Nicaragua where both are recorded as occurring on similarly nutrient-poor soils, with an observed invasion of broadleaf species into the savanna in the absence of fire.A similar invasion of pine forests by hardwood species has been noted in the Everglades National Park, where burning prescriptions were introduced to limit broadleaf invasion (Negro ´n-Ortiz & Gorchov, 2000).In a study on Mountain Pine Ridge, Kellman (1985) has shown that woody savanna plants can facilitate invasion by forest species through local addition of nutrients to the soil.
The present study shows that the savannas and wetlands of the RBCMA harbour a diverse flora, and are structurally varied, comprising both open and closed formations, with both well-drained and hydrologic formations represented.Considering the poor general conservation status of savanna systems, the RBCMA represents an important savanna conservation area.

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List of all species recorded, and growth form classification devised a vegetation classification for Belize, and in this a number of savannas and wetland vegetation types were identified, with general pine savanna classified as 'lowland needle-leaf moist open forests over poor soils'.They also identify three F. 1. Map of main savanna tracts within Belize, and savanna distribution within the RBCMA.
This is a dense subtype often forming discrete clumps or corridors within the other savanna vegetation types, and is dominated by the palm Acoelorraphe wrightii with other trees almost entirely absent, except for very occasional oaks and pines.Palmetto thickets are associated with wetter, poorly drained areas, and range in height from 2 to 6m.Common woody associates of this vegetation type are Byrsonima crassifolia, Crescentia cujete, Curatella americana, Calliandra houstoniana, Parathesis cubana (A.DC.) Molinet & Maza, Clidemia sericea, Eugenia winzerlingii and Gliricidia sepium.The density of the ground flora varies from open to closed, and it is usually composed of tussock-forming grasses and sedges.Blechnum serrulatum L., Lycopodiella caroliniana, Eleocharis spp.and Calea peckii B.L. Rob. are among the most characteristic ground layer species.
) Standl. and clumps of Acoelorraphe wrightii, form an open understorey.The dry and very sandy soils support an open herbaceous layer.Together with the savanna generalists such as Ageratum radicans and Sauvagesia erecta, Polypremum procumbens L. and Hypericum pratense Cham.& Schltdl.are present.Occasionally the woody component is partially or completely dominated by oak (oak woodland), with only a few scattered pines.