Étude floristique des îlots forestiers naturels de la Patte d’Oie de Brazzaville, Congo

Abstract

Set as a reserve in 1938 by the general governor of French Equatorial Africa (AEF), the Patte d’Oie forest covered originally 240 ha of which deforestation now isolates three statements for a total area of 95 ha, thus less than 39% of the original area. Originally a perish-urban forest, it has now become intra-urban, and shelters three statements of natural forests, administrative buildings, plantations of Pinus sp. and Eucalyptus sp., an arboretum dedicated to the wet dense forests of the Congo Basin, the sporting and university installations and a zoological park. The Patte d’Oie forest and the four others which completely disappeared (the Tsiémé, the Corniche, the Glacière and the Tchad) from urban space, these four forest statements have not been the object of an in-depth study. Before this study, the phytoecological data on the three forest statements were partial and generalized from studies carried on less than 1000 m2. Moreover, they do not make any difference between the natural forest and the plantations. The floristic inventory of the forest statements reveals pauciflorous ecosystems (with 79.5 ± 5.5 species per statement) compared to the wet dense forests of the Congo Basin. The floristic composition is sub-identic in the three statements forest and it is dominated by the Guineo-Congolese species (30 to 37.8%), in particular those belonging to the endemism sub-centre of the Low-Guinean (22 to 24%). The predominance of the sarcochores indicates that the principal mode of dissemination is the sarcochory (zoochory). However, the proportion of the anemochores dominated by pterochores (6 to 19%) and the good representativeness of heliophilous species are obvious signals of the disturbance of these ecosystems. The relative diversity of the families varies from 0.5 to 19.4% in the forest statements; the most represented are Rubiaceae, Fabaceae, Euphorbiaceae and Dioscoreaceae. The relative frequency of taxa varies between 0.5 and 2.7%, whereas the relative density varies from 0.03 to 37.6%. The most abundant species are Rothmannia octomera, Rhabdophyllum arnoldianum, Markhamia tomentosa, Caloncoba welwitschii and Chaetocarpus africanus. Within this flora, the phanerophytes are dominating over the geophytes and the chamephytes. Finally, from a phytoecological standpoint, the three stands fall within the same ecological group. The analysis of the underwood flora reveals that only introduced species are represented by large trees, whereas the indigenous woody flora is represented by the shrubby species (Bosqueiopsis gilletii, Dracaena arborea, Voacanga chalotiana, Strychnos variabilis, Millettia eetveldeana and Markhamia tomentosa). The relative frequencies and the densities indicate that, except for Markhamia tomentosa, all the ligneous species characterising these ecosystems are disseminated. The consequence, induced by this space distribution of the ligneous species, is an insufficient installation of sowings, thus a poor natural regeneration. Moreover, the immigrant taxa have a good representativeness, in general, in the underwood forest stands. By associating remarkably density and interspecific competition, these species should characterise in the future the three stands of the Patte d’Oie forest.