Llanos Vegetation

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 two- bunchgrass 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,

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