Vegetation Types

Clearwater Creeks
Typically draining lateritic or granitic uplands, these fast-flowing waterways are commonly inhabited by Rapatea paludosa (Rapateaceae) along less inclined sections. Steep waterfalls and spillways are often inhabited by small plants specializing on low soil and high moisture locations, such as Spigelia multispicata, Spathiphyllum cuspidatum and Gonzalagunia dicocca (Clarke, Funk et al. 2001).

Falls and Rocky River Islands
Podostemaceae are particularly abundant within the vicinity of the dolerite dikes running across the eroded river channels. Apinagia spp. and Rhyncholacis spp., also known as Paku-weed, are dominant components of the large falls vegetation.

Rocky river islands are commonly found exposed during the dry season and are variously occupied by species capable of surviving complete submersion during several months of the year, including trees such as Genipa spruceana, Bokobokotokon (Hirtella racemosa), Couepia comosa and Psidium guianense. Typical shrubs and herbs found on these islands include Ouratea maassorum, Oxalis frutescens, Sida sp., and Turnera aurantica.

River Levees
These localized ridges separating the main dry season river channel from the surrounding wider floodplain are well developed along the Essequibo and lower Burro-Burro Rivers. Since they are rarely flooded entirely, they are typically less dominated by Mora (Mora excelsa) than the surrounding floodplain areas or along the river margin where levees are not well developed. Species commonly found along these ridges include Monkey Pot (Lecythis zabacujo), Bat seed (Dipteryx odorata) and other species often associated with upland terra firme forest.

Flooded River Margins and Floodplains
These extensive forest areas are seasonally flooded and dominated by Mora, but are also characterized by a distinct set of other trees and palms. Almost all of these species have fruits that are dispersed by water and/or fish.

Larger trees in the flood plains of the Essequibo, Burro-Burro and Siparuni rivers include Wadara (Couratari riparia), Sarabebe (Macrolobium acaciifolium), Clammy cherry (Cordia tetrandra), Common baromalli (Catostemma commune), Crabwood (Carapa guianensis), Swamp dalli (Virola surinamensis), Arapipi (Astrocaryum jauari), Supple jack (Combretum laxum), Couepia spp., Campsiandra comosa, Ituri Wallaba (Eperua schomburgkhiana), Silk cotton (Ceiba pentandra), Manicole (Euterpe oleracea, E. precatoria), Hog plum (Spondias mombin), Fukadi (Terminalia amazonica), and Congo pump (Cecropia obtusa).

Smaller trees and shrubs and climbers include Lana (Genipa americana), Hiraea faginea, Hippocratea volubilis, Bellbird’s Heart (Gnetum nodiflorum), Machaerium lunatum, Barakaro (Ormosia paraensis), Moco-Moco (Montrichardia arborescens), Maipa (Inga laurina), Long John (Triplaris surinamensis), Trysil (Pentaclethra macroloba), Mukru (Ichnosiphon spp.) and others.

Swamps

Areas of permanently impeded drainage occur where:

• Inclined intrusive dolerite sheets occlude downstream flow of water from sandy plateaux
• Localized laterite formation creates a plinthite hardpan with localized depressions
• Dynamic river meandering creates and then pinches off river channels to form ox-bow lakes that then begin to fill up with sediment and organic matter (eutrophication)
• Road-building, mining and other activities have impeded or re-channeled downstream water flow

These localized vegetation types rarely exceed several hectares in size, but are characterized by a group of plants specializing in growth under permanently inundated, lotic conditions. The mix is strongly influenced by the depth of standing water. In relatively deep water these might include Ité (Mauritia flexuosa), Manicole (Euterpe precatoria/oleracaea) and Moco-moco (Montrichardia arborescens). In shallow water swamps one can find Mukru (Ichnosiphon spp.), Manni (Symphonia globulifera), White Cedar (Tabebuia insignis), Kirakaua (Iryanthera spp.), Corkwood (Pterocarpus officinalis), Clusia spp., among others.

Sandy Plains and Terraces
These forests include a number of vegetation types including Wallaba Forest, Dakama Forest, Muri scrub, Greenheart mixed forest and others. They are found exclusively on soils composed principally of quartzite sand, but with various amounts of clay and loam.

The species mix in Wallaba Forest depending on edaphic and historic conditions affecting the size, number and stature of the main dominant species. Trees typically found in Wallaba Forests of all types in the reserve include dominants Soft Wallaba (Eperua falcata), Korokororo (Ormosia coutinhoi), Sclerolobium sp., Komaramara (Duroia eriopila), Sand baromalli (Catostemma fragrans), Kauta (Licania laxiflora, L. persaudii), Moroballi (Talisia squarrosa), Awasokule (Tovomita spp.) and Barataballi (Chrysophyllum sanguinolentum), Kamakuti (Rhodognaphalopsis flaviflora) Manobodin (Emmotum fagifolium), Turu (Oenocarpus bataua), Charcoal Barabara (Lissocarpa guianensis), Hachiballi (Pera bicolor) among others.

Woody plants that typify the fire-prone Dakama Forest include the dominant Dakama (Dimorphandra conjugata) and sub-dominants Sand Baromalli, Korokororo, Kakarua (Pradosia schomburgkiana), Yaruru (Aspidosperma excelsum), Kautaballi (Licania alba) and Huruasa (Abarema jupunba).

Muri scrub (Xeromorphic scrub of Fanshawe [1952]) is described after one of the dominant shrubs and treelets that can be found on these flat white sand plateau, seasonally marshy in places due to occluded drainage and subject to fire during the driest months and years. Muri (Humiria balsamifera var. guianensis), Yeshikushi (Ternstroemia punctata), Kaiakaiadan (Pagamea capitata), Hicha (Byrsonima spicata), Madaburi (Clusia fockeana/nemorosa), Retiniphyllum schomburgkii, Tauroniro (xerophytic habit – Humiria balsamifera var. floribunda), Awati (Maprounea guianensis), and Hikuribianda (Quassia cedron).

Some of the most extensive, well-statured and species-rich forests in the Iwokrama Forest (and central Guyana) occur on the loam-enriched ‘brown sands’ classified as Ferrasols (FAO)/Ultisols (USDA) and found generally north and east of the Iwokrama Mountains. These forests consist of a number of well known Guyanese timber tree species, including Greenheart (Chlorocardium rodiei), Wamara (Swartzia leiocalycina), Crabwood (Carapa procera), Sarabebeballi (Vouacapoua macropetala), Soft Wallaba, Itikiboraballi (Swartzia spp.), Kabukalli (Goupia glabra), Rough-leaf Maho (Sterculia rugosa), Aromata (Clathrotropis brachypetala), Futui (Jacaranda copaia), Purpleheart (Peltogyne venosa), Locust (Hymenaea courbaril), and Suya (Pouteria speciosa), among many others. Unique to this association in Guyana is the high density of the canopy tree Waramadan (Dicorynia guianensis), only previously known from interior locations in Suriname and French Guiana.

Turtle Mt. and Pakatau Hills dolerite uplands

Vegetation associated with these mesa-like formations depends to great extent upon the location along a base to summit slope. At the base and the lower laterite-covered slopes, common elements of mixed forest on laterite are found, including Greenheart, Crabwood, Wirimiri Kakaralli (Lecythis confertiflora), Morabukea (Mora gonggrijpii), Pouteria spp., Bulletwood, Eschweilera spp., and others. On the upper slopes and flat-topped summits, the vegetation becomes progressively smaller and more xerophytic, composed mainly of Clusia spp., Erythroxylon, Psychotria spp., Bonafousia undulata, Palicourea spp., numerous orchids and various Myrtaceae.

Sandstone Hills, Maipa Mountains and Iwokrama Foothills Clarke et al. (2001) concluded from their botanical studies that the vegetation on these geologically different rolling hill and valley formations was generally very similar. Well-formed mixed forests on clayey soils found in these areas are generally composed of a wide-range of species, including Bat-seed (Andira surinamensis), Hububalli (Loxopterygium sagotii), Red Cedar (Cedrela fissilis), Haiawaballi (Protium sp.), Karohoro (Schefflera morototoni), Jack-in-the-box (Hernandia guianensis), Futi, Vataireopsis surinamensis, Freijurs (Cordia alliodora), Brazil Nut, Brasilian Cedar (Cedrelinga cataeniformis), Kokerite (Attalea maripa), Bulletwood (Manilkara bidentata), Licania densiflora, Phenakospermum, Lecythis, Kumakaballi (Coussapoa spp.), Glasswood, Wamara and Kabukalli, among others.

Iwokrama Granite Massif

The granitic substrate, steep slopes and relatively long altitudinal gradient compared to other forestlands in the area expand the number and variety of plants found within the massif of various peaks and narrow valleys. Forests at base and along slopes are characterized by Karia (Astrocaryum sciophilum), Trysil, Burada, Smooth-leaf Maho (Sterculia pruriens), Aruadan (Sloanea spp.), Rose-of-the-Mountain (Brownea sp.), Paripiballi (Chrysophyllum argenteum), Licania spp., Aromata, Itikiboraballi, Eschweilera spp., Asashi (Rheedia macrophylla), Lanaballi (Gustavia augusta), Ocotea spp. and Monkey Syrup (Cassia cowanii).

Vegetation on the top of the peaks that have been botanically explored are reduced in stature and contain many more shrubs and small trees in the Annonaceae, Myrtaceae, Melastomataceae as well as in the genera Psychotria spp., Solanum spp., and Palicourea spp.. Other small-statured plants commonly found in these areas include: Faramea quadricostata, Bonafousia undulata, Ficus maxima, Rinorea pubiflora, Geonoma maxima, Rudgea hostmaniana and Justicia calycina, among many others.

Steep-walled valleys conserve moisture and these areas contain many herbs, shrubs and epiphytes not commonly seen elsewhere, most notably in the Rubiaceae, Gesneriaceae, Araceae, Orchidaceae, Pteridophyta (ferns) and Lycophyta (club and spike mosses).