Assessment of species composition change in savanna vegetation by means of woody plants' size class distributions and local information.

摘要

A new method is presented for assessment of compositional changes in disturbed savanna systems and for identification of species susceptible to local extinction. Size class distributions (SCDs) of trees and shrubs from a fire-disturbed savanna in Fathala Forest, Senegal, were analysed and compared with observations provided by local people on declining species. Many species, particularly large trees, had few young individuals and therefore flat SCDs that deviate from the reverse-J shape SCD characterizing species with abundant and constant rejuvenation. Based on comparisons of SCDs with local information and historical records, it is concluded that most of the flat distributions analysed were effects of declining population sizes caused by human impact, particularly controlled burning. Results from the analyses showed that a change in species composition from a vegetation characterized by large trees to one dominated by shrubs takes place. Whereas most of the declining species are locally appreciated, this is not the case for the few species with good rejuvenation. Therefore, if the observed trend continues, it will have severe implications for the local communities.