Population ecology of some keystone tree species in the Jebel Marra region of Darfur

Abstract

Populations of Albizia amara, Acacia senegal, Anogeissus leiocarpus, and Balanites aegyptiaca were monitored in five habitats in the Jebel Marra region: clay plains, sand plains, wadis, northern slopes, and southern slopes of the Jebel Marra (mountain) region. The plains and wadi habitat types had been regularly disturbed because of cycles of shifting cultivation. When land was prepared for cultivation, seedlings and juvenile plants were removed, leaving only larger adult plants, and, therefore, unstable tree populations, lacking juveniles, and growing in fallow lands. A. amara and A. senegal were more abundant than A. leiocarpus and B. aegyptiaca. The healthiest populations, with more juvenile recruits, were found in the grazing lands at higher elevations. However, A. amara and A. senegal showed stable size structure in both grazing and fallow lands. In general, in the four study species, we see a decline in growth and reproduction (with growth measured as tree height and number of branches, and reproduction assessed in terms of number of fruits per tree and dry weight of seeds) along a progression from fallow lands to the grazing lands, and, separately, from lower to higher altitudes. The resilience potential of each species in the fallow lands indicated two distinct population resilience strategies: (1) we saw faster rebound in the case of A. amara and A. senegal and (2) slow recovery from disturbance in the case of A. leiocarpus and B. aegyptiaca which showed low juvenile recruitment in fallow lands.