Plant Formations in the Llanos BioProvince
Peter Martin Rhind
Llanos Forest
The most extensive forests on the Llanos extend along the eastern piedmont of the
Andean Cordillera, which in places is up to 80 km wide. A feature here is the continuous
input of groundwater and nutrients from the adjacent slopes and a number of areas are
subject to seasonal flooding. These forests can be subdivided into several different types.
The most prevalent type, found on the drier alluvial plains, is a medium tall (25-30 m), semi
deciduous forest dominated by Banara guianensis, Brosimum alicastrum, Brownea
macrophylla, Chrysophyllum argenteum subsp. auratum, Croton gossypiifolius,
Lonchocarpus hedyosmus and Pachira quinata. But possibly one of the most interesting is
the largely evergreen forest of the San Camilo Reserve in western Apure State. This forms
part of the so-called San Camillo forest refuge centre (of Steyermark) for endemic and/or
relict species. Endemic species including Aegiphila scandens (Verbenaceae), Forsteronia
apurensis (Apocynaceae), Inga thibaudiana (Fabaceae), Machaerium paraense
(Fabaceae), Miconia matthaei (Melastomataceae), Odontocarpa steyermarkii (family?),
Ormosia nobilis (Fabaceae), Ouratea apurensis, O. pseudomarahuacensis (Ochnaceae),
Pterocarpus santalinoides (Fabaceae) and Simaba paraensis (Simarubaceae). Also
present are various Amazonian and Guayanan floristic elements reflecting the former
close link with these biomes in the recent past. These include Caraipa punctata, Guatteria
cardoniana, Licania latifolia, Pouteria bangii and Roucheria columbiana. Another
interesting, disjunct element found here is Myrocarpus venezuelanum (Fabaceae). Other
members of this genus are found in Argentina, Paraguay and southern Brazil. This is
thought to be evidence of a former connection between the Brazilian and Guayanan
shields which was disrupted during an arid phases of the Pleistocene.
Llanos Semi-Deciduous Gallery Forest
Many of these linear stretches of forest are thought to be relicts of a once more extensive
forest but some have now been modified by human intervention. The forests associated
with Delgadito Creek in Venezuela’s Portuguese State, however, are thought to be largely
undisturbed. They extend for some 4000 ha and characteristically have three tree stories.
Most of the tree species are evergreen and the upper canopy can reach heights of up to
about 22 m. The dominant trees of the upper canopy include Astronium graveolens and
Samanea saman together with Albizzia caribensis, Hura crepitans, Lecythis ollearia and
Trichilia unifoliata. The middle stratum ranges from 10-20 m and typically includes
Calycophyllum candidissimum, Cecropia peltata, Guazuma ulmifolia and Guettardia
divaricata. Between 8-10 m, palms such as Attalea butyracea and Bactris major largely
dominate the lower stratum. Of particular interest for its timber value is Astronium
graveolens and so its presence is usually a good indicator that forest has not been
disturbed. Among the endemic species associated with these forests is Gustavia acuta
(Lecythidaceae).
Llanos Dry Savanna
In the Colombian Llanos these are largely characterised by bunchgrass formations and
can be broadly divided into three types – Trachypogon vestitus savanna, Paspalum
pectinatum savanna and Trachypogon vestitus-Axonopus purpusii savanna. The first type
is found in the driest areas, mainly on well-drained high plains. Other common grasses are
Leptocoryphium lanatum and Paspalum pectinatus, while others include Andropogon
selloanus, Axonopus pulcher and Thrasya petrosa. Certain sedges such as Bulbostylis
junciformis, Dichronema ciliata and Rhyncospora barbata are also common, together with
Copyright © 2010 Peter Martin Rhind
the forbs Curculigo scorzoneraefolia and Ichthyothera terminalis. Trees and shrubs occur
sporadically mainly Curatella americana and Palicourea rigida, but in slightly more humid
areas Byrsonima crassifolia and B. verbascifolia also make an appearance. Paspalum
pectinatum savanna is characteristic or shallow soils with high gravel content in level
areas. However, it is similar to the previous savanna in that both Leptocoryphium lanatum
and Paspalum pectinatus are common components and they share a number of other
common species. Among the scattered tree are Curatella americana, Jacaranda lasiogyne
and Xylopia aromatica. Savanna dominated by Trachypogon vestitus and Axonopus
purpusii are found on more recently developed soils where leaching is less pronounced
such as in the piedmont regions and on alluvial fans. They typically have a two-layered
structure with an upper layer of Trachypogon vestitus and a lower one dominated by
decumbent Axonopus purpusii. Other common grasses are Andropogon bicornis,
Leptocaryphium lanatum, Panicum versicolor, Setaria genigulata and Trachypogon
plumosus, while common sedges included Bulbophyllis junciformis, Dichronema ciliata and
Sclera hirtella. Arboreal vegetation includes both scattered low shrubs with species such
as Cassia flexuosa, Melochia polystachya, Pavonia speciosa and Psidium eugenii,
together with occasional tree groves.
Llanos Humid Savanna
These, like dry savanna, are also dominated by bunchgrass formations, and occur where
there is intermittent water inundation. In the Colombian Llanos two types have been
recognized – Trachypogon ligularis savanna and Leptocoryphium lanatum savanna. The
first occurs at the dry end of the spectrum and occupies an intermediate position between
dry and wet savanna. Other dominant grasses in this savanna are Andropogon selloanus,
Leptocoryphium lanatum and Paspalum pectinatum, while other common grasses include
Axonopus pulcher, Setaria geniculata and Trachypogon plumosa, while Bulbostylis
junciformis is the common sedge. Besides a scattering of low shrubs and trees such as
Byrsonima crassifolia, Curatella americana, Pavonia speciosa and Psidium guineense the
two suffrutescent shrubs Hyptis brachiata and H. confertai also occur. With increased
grazing pressure these latter two species become more abundant. Savanna dominated by
Leptocoryphium lanatum is often very wet with much standing water in the rainy season.
They have a two-layered structure with Leptocoryphium lanatum conspicuous in the upper
layer while a variety of small, annual grasses dominate the lower layer. Other common
upper layer species include grasses such as Andropogon hypogynus, Axonopus purpusii,
Eriochrysis halcoides, Panicum rudgei, Setaria geniculata, Sorghastrum parviflorum, the
sedges Bulbostylis junciformis and Rhyncospora globosa and the forb Heliconia
psittacorum. Common lower layer grasses are Andropogon brevifolius, Aristida capillacea,
Panicum stenodes and Paspalum minus.
Llanos Wet Savanna
With these savannas periodic inundation is frequent during the wet season often to a
depth of up to 0.3 m and they may remain flooded for several months at a time. Two wet
savanna types have been recognized on the Colombian Llanos. These include
Mesosetum Savanna and Andropogon Savanna. The first is largely confined to the aeolian
plain, which although flooded during the wet season, the water table can drop to a
subsurface depth of up to 2 m in the dry season. Here the dominant grasses are
Mesosetum chaseae, M. rottboellioides and Eriochloa holcoides, while other common
grasses are Andropogon virgatus, Axonopus anceps, Leptocoryphium lanatum and
Trachypogon ligularis. Sedges like Rhychospora barbata and R. globosa are also common
together with the shrubs Buettneria jaculifolia and Jassiaea lithospermifolia. This formation
has much in common with wet savanna found the Llanos of central Venezuela. In places
where the depth of floodwater increases other species may become conspicuous such as
Copyright © 2010 Peter Martin Rhind
the shrub Rhynchanthera grandiflora, the tree Caraipa llanorum or the palm Maurita minor.
Andropogon Savanna occurs mainly in the low basins of alluvial overflow plains on heavy
clay soil with poor drainage where even during the dry season the water table is close to
the surface. The vegetation has a fairly complex structure and often includes several
layers. The dominant grasses are Andropogon bicornis, A. hypogynus, A. virgatus and
Sorghastrum parviflorum. Common upper layer species include Axonopus anceps and the
shrubs Buettnera jaculifolia and Jussiaea lithospermifolia. At medium heights the grasses
Eriochrysis holcoides and Setaria geniculata are usually conspicuous together with the
sedges Cyperus haspan and Rhyncospora barbata. At the lowest level Panicum laxum, P.
parviflorum and Paratheria prostrata are common.
Llanos Savannas with Relict Forest
On the Colombian Llanos these are found in scattered locations such as dunes, dissected
high plains and at the foot of mountains. Three types have been identified but one of them
the Melinis minutiflora Savanna is regarded as a highly modified as a result of human
intervention. The other two are described as Trachypogon ligularis-Paspalum carinatum
Savanna and Paspalum carinatum Savanna. The former, dominated by the twobunchgrass species Trachypogon ligularis and Paspalum carinatum occur on dunes on the
aeolian plain. Other commonly associated grasses included Axonopus purpusii,
Gymnopogon foliosus, Leptocoryphium lanatum and Paspalum pectinatum while other
frequent components are the sedges Bulbostylis junciformis and Rhynchospora barbata
and the endemic forb Schieckia orinomensis (Haemodoraceae). The presence of shrubs,
such as species of Miconia, show that these formations would probably develop into
woodland if left undisturbed, which is further supported by the presence of relict forest in
some of the more inaccessible places. These typically form stands up to 10 m high with
occasional emergents and support an under storey of sedges and bromeliads. Open
stands of the tree Bowdichia virgilioides are also present in places. Paspalum carinatum
Savanna is mainly confined to sloping terrain in the dissected high plain. The bunchgrass
Paspalum carinatum dominates but species diversity is general low. On the other hand,
several characteristic species occur such as the grasses Aristida implexa, Mesosetum
loliiforme and Trachypogon montufari and the sedge Bulbostylis paradoxa. In the less
disturbed areas such as south of the Guaviare River, relict evergreen forests are present,
but these are mainly confined to isolated ravines and south facing slopes.
Further information required.
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Copyright © 2010 Peter Martin Rhind
Copyright © 2010 Peter Martin Rhind