Are the floristic composition of montane forest changing within their woody layers? A study in the Kahuzi-Biega National Park (Democratic Republic of the Congo)

The comparison between overstorey and understorey (woody layers) composition had been investigated in many studies without taking into account the altitude. Therefore, the impact of altitude on the variation of the floristic composition of the understorey vs overstorey remains poorly understood. We postulate here that the rate of overstorey species present in the understorey tends to increase with the altitude. The objective is to assess the dynamic trends of a forest in a case of altitude variation. To investigate the variation of the floristic composition of the understorey and the overstorey of montane forests, we inventoried the highland forests (1800 to 3315m asl) of the Kahuzi-Biega National Park in the Democratic Republic of the Congo. Trees greater than 10 cm of diameter at breast height (dbh) were numbered and identified in 10 x 1ha plots (overstorey), in each of which we nested a 0.1ha (understorey, woody plant with dbh<10cm). We measured the altitude for each plot. Fisher alpha and rarefaction r were calculated. For each couple of plot, we calculated the percent of overstorey species which are present in the understorey to see if the overstorey species are well represented in the understorey. In the two layers alpha and r decrease when the altitude increases. The overstorey species are well represented in the understorey varying from 47.37% to 76.92% and tend to increase with the altitude (coefficient of correlation = 0.34, p=0.34). At least, 23.08% of the overstorey species are absent in the understorey. Differently from the rate of overstorey species present in the understorey, some of the most abundant species in the overstorey are present but rare in the understorey albeit the most common species in the understorey are those which will likely incorporate later on the overstorey. This means that these species don't meet suitable conditions to regenerate or seldom regenerate beneath themselves. We found a likely future change in the floristic composition of the overstorey in these forests. As already found in other studies, woody plant diversity decreases with increasing altitude. We found also that floristic elements of the understory that will likely integrate the canopy are often different from that of the current overstorey. This suggests future changes in the floristic composition of these montane forests if there are no major disturbance in the forest or if the mortality rate will not be greater among the abundant understorey species. (Texte intégral)