Hagenia abyssinica is a characteristic species of mountain forests in the Albertine Rift. In Kahuzi Biega National Park, the species is found between 2000 and 2600 m altitude. This study intended to characterize Hagenia monospecific forest by providing an account for its floristic European Scientific Journal May 2016 edition vol.12, No.15 ISSN: 1857 -7881 (Print) e -ISSN 1857 190 composition, structure and biomass. Data were collected from 9 different plots of 1 hectare each located in 3 different sites (Tshivanga 2000-2250m asl; Mugabe and Kasirusiru 2400-2600m asl). Parameters including tree diameter ≥ 10cm, height, dead individuals and young shoots of Hagenia abyssinica were wetting the plot. The results showed that species richness was significantly low, varying between 8-24 species (p < 0.05, Df = 35); while the specific density was 565 ± 121 individuals. The Richness decreased with the increment of the species density of Hagenia. The similarity analysis revealed the influence/impact of the site/altitude on the species composition. The diametric structure showed a regressive dynamism within the population found between 2000 and 2200 m altitude, in which Hagenia specimens were replaced by the specimens of secondary forest species such as Sapium ellipticum, Macaranga neomilbraediana, Neoboutonia macrocalyx. Whereas, between 2400-2600 m altitude, there was a progressive dynamism with several Hagenia young shoots, with no mortality (χ 2 = 487, p < 0.001). The average woody biomass was estimated at 108t / ha. This increased as Hagenia density decreased and vegetation became heterogeneous. Human disturbance and climate change effects in the region, could be the causes of variability observed in this population in the Kahuzi-Biega National Park.Keywords: Kahuzi-Biega National Park, structure, diversity, biomass, Hagenia abyssinica RésuméHagenia abyssinica est une espèce caractéristique des forêts de montagne dans le Rift albertin. Au Parc National de Kahuzi Biega, l'espèce est présente entre 2000 et 2600m d'altitude. Cette étude s'intéresse à la caractérisation du peuplement pour analyser sa composition floristique, sa structure et en estimer la biomasse selon le caractère du peuplement. Les données ont été collectées dans 9 parcelles de 1ha chacune situées dans trois sites différents (Tshivanga 2000-2250m d'altitude ; Mugaba et Kasirusiru 2400-2600m d'altitude). Il s'agit des données sur le diamètre des arbres ≥ 10cm, sur leur hauteur, les individus morts et les jeunes pousses de Hagenia abyssinica au sein de la parcelle. Les résultats montrent que la richesse spécifique est faible et varie entre 8 à 24 espèces (p value=0,03 ; Df=35) pendant que la densité spécifique est de 565± 121 individus. La richesse spécifique diminue lorsque la densité de Hagenia abyssinica devient importante. L'analyse de similarité indique que le site influence la ressemblance entre les communautés. La structure diamétrique montre que le peuplement situé

This work aims to determine the change in the diametric structure, density, basal area, height and dominance of trees dbh ≥ 10 cm depending on altitude range or forest types. The study was conducted in 30 ha in Kahuzi Biega National Park and surrounding areas in DR Congo. In total, 16,797 individual trees were surveyed. The number of family, genus and species regress following the altitudinal gradient as the density increases. Mountain ecosystems in Kahuzi-Biega and its surroundings are dominated by Meliaceae, Euphorbiaceae, Moraceae, Fabaceae and Rubiaceae families. The Chi square analysis showed that the diametric structure of vegetation European Scientific Journal August 2016 edition vol.12, No.23 ISSN: 1857 -7881 (Print) e -ISSN 1857 89 succession following a progressive dynamic and varies according to altitude gradient. The average basal area was 35 m²/ha. The analysis of variance (ANOVA) and post hoc Tukey test does not prove the impact of altitude on the basal area. But it becomes low in elevation due to the reduced diameter of the trees. The vertical structure was analyzed according to the classification IUFRO (International Union of Forest Research Organizations). The maximum height was 48 m and becomes lower in altitude. The tree height diameter relationship varies according altitude range so that the models H/D in this area should be developed for each forest type. We determine that the dominance of woody species following the altitude effect; only Strombosia scheffleri and Xymalos monospora dominated by its basal area, abundance and frequency of the various altitude horizons.