Disturbance, regeneration and invasion: implications for recovery of degraded montane forests in Kenya.

Abstract

Tropical montane forest loss and degradation, as well as degradation of lands which formerly supported forests, are proceeding at unprecedented rates, eroding biological diversity. Inadequate understanding of disturbance-related succession of these forests severely limits attempts to anticipate the recovery of species diversity and composition following such disturbances. Abandoned fallows and degraded secondary forest were used to study the natural regeneration of community species richness and composition, to identify potential restoration tree species and to examine the community invasibility and the impact of the invasive tree Fraxinus pennsylvanica on plant species recovery in Mau forest, Kenya. The secondary forest had higher species richness and abundance of all functional groups, except herbs and shrubs that were more abundant in the fallows. Similarly, species composition of trees differed between transition zones and secondary forest, indicating different abilities of individual species to establish and grow under habitat changes during recovery. There was a high presence of some generalist tree species in the abandoned fallows and in the secondary forest indicating that these could be potential restoration species. Two of these species, namely Albizia gummifera and Neoboutonia macrocalyx, showed positive relationships with the establishment and growth of other species, suggesting that these could promote species richness during forest recovery. There were contrasting relationships between native community diversity and the density of the invasive Fraxinus pennsylvanica in abandoned fallows and secondary forest, suggesting that community diversity may facilitate F. pennsylvanica invasion in the abandoned fallows but inhibit the invading exotic in the secondary forest. In the study site, Fraxinus appears to impact the species diversity of understory plant communities in the secondary forest, suggesting threats to forest diversity. Thus, specific tools for management and control of Fraxinus in Mau should include (1) complete elimination of livestock grazing in the forest to reduce disturbance, (2) cutting of adult Fraxinus trees in the secondary forest to avoid further recruitment, (3) planting trees in all abandoned fallows in the forest block, and (4) establish monitoring plans for alien species in Mau forest and other forests in the country.