Allometric equations for the evaluation of leaf biomass of three forage woody species. Case of Afzelia africana, Ficus gnaphalocarpa and Pterocarpus erinaceus, from natural rangelands in the Kéniéba circle in southwest Mali.

Abstract

Since the droughts of the 1970s and 1980s, profound ecological and socio-economic changes have marked the Sahel. Among the consequences of this phenomenon we can cite: decline in the production of agropastoral lands; multitude of social conflicts between the various users of these natural resources (FAO, 2012: 41). In the circle of Kéniéba one of the repercussions has been the massive arrival each year of transhumants coming from Senegal, Mauritania and the north of the Kayes/Mali region (Konaré, 2016). Thus, the present study aimed to evaluate the impact of this transhumance on the production of forage biomass of three woody forage species: Afzelia africana Sm. ex Pers, Ficus gnaphalocarpa (Miq.) C. C. Berg and Pterocarpus erinaceus Poir. Del.., by the allometric method. The study was carried out in two sites with different rainfall: The Dialafara terroir located in the isohyets between 700 mm in the North and 1100 mm and that of Faraba in the south with an average rainfall of 1250 mm, all belonging to the Kéniéba circle. The sampling concerned 03 species of trees. Thus, on each tree species, ten to twenty feet were retained, resulting in a total of 40 to 50 samples per site for the three species. The harvest was done by hand using a cutting-cutting machine. Before cutting the branches, the height of the tree, the circumference of the trunk at 1.30 m, the length and width of the tree were measured. Immediately after harvesting, the circumference, length and total weight of the branches were measured. After defoliation, the fresh weight of the different consumable parts (leaves, fruits, flowers, branches) was weighed using a precision balance. Then, a composite sample of 500 grams was taken per species and oven-dried at 105°C for the determination of dry matter (DM). We then used the allometric relationships between dendrometric data and leaf biomass to estimate forage production. Among the three parameters studied, the average crown diameter shows a better correlation with the phytomass of Afzelia africana (R2= 0.94) and Ficus gnaphalocarpa (R2= 0.91), while the trunk circumference gives a better correlation with the phytomass of Pterocarpus erinaceus (R2= 0.89). Based on these correlations, the following allometric equations were established:

Pterocarpus erinaceus: Y= 0.088CT + 0.0094

Fzelia africana, Y= 0.060DmH - 0.238

Ficus gnaphalocarpa, Y= 0.0628DmH - 0.283

The estimated biomass of woody forage species was 1.99 tonnes of dry matter (DM)/ha for the Dialafara pastoral zone (site 1) and 4.55 tonnes of DM/ha for Faraba (site 2). Carrying capacities are low and range from 1.51 to 3.46 TLU/ha/7 months. The developed linear regression model made it possible to estimate the aboveground forage biomass productivity of the three woody species without felling them. This method can be easily used by agents from different departments to carry out rapid assessments to draw up a fodder balance sheet.