Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide

Abstract

The potential of natural succession to restore degraded areas of a Ugandan rainforest dominated by the exotic paper mulberry Broussonetia papyrifera (L.) l'HĂ©r. ex Vent.

Abstract

There are many studies on the applicability of natural regeneration for recovery of degraded forests. Studies of the potential of natural regeneration and succession to restore degraded forest areas under invasive exotic plants are few in comparison. This study examined the potential of these processes to bring about the recovery of large areas under the exotic paper mulberry Broussonetia papyrifera in Mabira Forest, Uganda. The areas of focus were degraded by farming but abandoned and invaded by mulberry 30 years ago. Sampling was conducted in three mulberry-dominant stands (Int 1, Int 2, and Hv) that were differentiated by the degree of mulberry cutting by local communities and other anthropogenic disturbances. Primary forest (Nt) was a fourth stand used as a reference. Data were collected using Modified-Whittaker Plots, transects and other plot types adapted to record impact of disturbance. Analyses of stem density showed that both mulberry and native trees occurred in all the stands in varying densities and age classes from seedling to tree class size. Non-metric multidimensional scaling showed Int 1 (a relatively undisturbed stand with fully grown mulberry) to be closest to Nt floristically, followed by Int 2 (a disturbed stand with fully grown mulberry). Hv (a disturbed stand with low stature mulberry) was least similar to Nt. Overall, Int 1 was closer to Nt than to any of the other stands; while Int 2 was closer to Hv than to Int 1 or to Nt. Most analyses of seedling and sapling densities and species diversity of native trees showed a pattern whereby Hv < Int 2 < Int 1 < Nt. Numbers of forest-typical species varied between stands but not significantly. Inter-stand differences in the levels of disturbance were pronounced with Hv and Int 2 having significantly higher incidences of illegal indigenous tree cutting and charcoal kilning than Int 1. Regressions of percent cover values on seedling densities, sapling densities and species richness showed negative relationships between mulberry cover and native tree seedling density and species richness but not sapling density. Indigenous tree sapling densities were positively correlated with indigenous tree species cover. These findings suggest that anthropogenic disturbance and to a smaller extent mulberry factors; and not differences in rates of species colonization and degrees of species performance explain the current state of recovery of the stands. Natural regeneration and succession therefore have potential to recover the degraded sites and reverse the dominance of mulberry if the ongoing human disturbance is stopped.