The search for accuracy on biomass estimation is growing significantly, however density measurements are still essential to reduce bias in biomass estimations. The objective of this research was to: (1) test longitudinal and radial variations in wood density of Distemonanthus benthamianus, Musanga cecropioides and Trema orientalis. (2) Fitting mixed models of secondary forests pioneer species. Data for density determinations and fitting allometric equations were obtained by destructive method, on a sample of 115 trees. Archimedes' principle applied to the biomass data yielded the average densities of the three respective species (0.726; 0.214 and 0.35 in g.cm-3). This variable associated with tree height, crown diameter and diameter at breast height were used to explain tree biomass through ten fitted mixed models. The model, Aboveground biomass = Exp (-0.85 + 2.19 x ln (DBH) + 1.1 x ln (φ)), with a low Akaike Information Criterion (AIC = 78.76), the high correlation coefficient (Adjusted.R2 = 96.4%), the low rate of residual standard error (RSE = 0.33) and the Relative Root Mean Square Error (RRMSE = 0.39), was selected as the best mixed model. The full model (Aboveground biomass = Exp (-0.84 + 0.63 x ln (DBH2 x H) + 0.85 x ln (φ) + 0.54 x ln (C)) under the validation criteria was found to be efficient (adjusted R2 = 0.96; RRMSE = 0.41; average error = 15,95). However, density variations must be considered to reduce bias in the estimation. In addition, increased collection of large amounts of secondary forest data remains essential for fitting more robust mixed models.

Chad's tree-based parks undergo increasing human pressure, leading to the loss of biodiversity and the modification of the ecological landscape. This study was carried out in the Nya division, Far South of Chad. It aimed to analyze the spatial and temporal dynamics of land cover between 1984 and 2016 and to determine the drivers of vegetation degradation. The databases used were made up of 3 Landsat satellite images (MSS de 1984, TM de 2002 et ORLI_TIRS de 2016) and supplemented by field studies. Satellite images were processed using ENVI and ArcGIS software. Interview was used to identify the activities carried out by the local population. In 1984, Nya division consisted of seven types of land cover, the most important of which was tree-based parks (27.17%), followed by gallery forest (23.59%), shrub savannah (16.68%) and herb savannah (14.59%). Gradually tree-based parks and gallery forest disappeared. In 2016, the main types of land cover were herb savannah (35.74%), house and bare soil (33.32%) and shrub savannah (10.04%). Petroleum project, agriculture, overgrazing, population growth, wood collection and bushfires are perceived by local population as main drivers of forest cover change. These results suggest the establishment of an effective sustainable management policy of tree-base parks of Nya division to avoid its total destruction and its biodiversity loss.