�valuation de la biomasse arbor�e et arbustive dans une s�quence de d�gradation de la suberaie � Cytise de Kroumirie (Tunisie)

Abstract

Abstract Evaluation of the overstorey and understorey biomasses in a damaging sequence of cytisus cork oak forest in Kroumirie (Tunisia). The aerial and underground biomasses of the overstorey and understorey have been studied in a damaging sequence of cytisus cork oak stands in Kroumirie region. Measurements of biomass and dendrometric parameters on twelve sampled trees have been realized in different plots, with variable damage stages. The allometric relationships obtained allowed to detect the best predictive variables of partial biomasses at individual and stand levels. At individual level, branch girth at stem insertion and diameter at breast height (DBH) are highly predictive parameters. The aerial biomass of a tree cork oak of medium size (DBH = 20 cm) represents 81.5% of the total tree biomass against 18.5% for the roots. The bole contains 57% of the aboveground biomass with 44% in the wood and 13% in the cork. 43% within the crown are subdivided into branch wood (23% ), branch cork (12% ), twigs and leaves (4% each). Moreover, the percentage of cork in the trunk biomass decreases with increasing DBH. At stand level, the basal area proved the best stand parameter to predict partial biomasses and also a criterion of stand state. Damaging was characterized by a continuous decrease of stand density (from 723 to 223 stems ha-1), of the mean stand DBH (20.21 to 2.97 cm), of stand basal area (32.928 to 0.985 m2 ha-1) and of cork oaks biomass (139 to 4 t ha-1). The opening of the canopy which goes along with damaging involves a quasi-linear decrease of cork oaks biomass and appears as a second quantitative criterion of stand state. However, up to 70% , the canopy opening has contributed to the development of shrubs (9 to 54 t ha-1); beyond this opening level shrubs biomass has decreased in spite of a better cover.