Coupled human-natural regeneration of indigenous coastal dry forest in Kenya

• Coupled human-natural forest regeneration processes were studied over 20 years.
• Forest recovery was accelerated by planting within existing natural regeneration.
• Natural processes dominated, but species-dense plantings elevated biodiversity.
• Invasive Azadirachta indica reduced tree diversity and altered arthropod communities.
• Illegal harvesting highlighted preferences for endemic tree species for eco-services.

Remaining fragments of East African coastal dry forests contain very high levels of endemic species and are in critical need of conservation and restoration. Little is known about natural regeneration dynamics of these forests, or the potential for human action to aid recovery of lost structures and functions after deforestation/degradation. Here, data and analyses are presented from long-term monitoring plots in a 20 year-old forest restoration project in Gede, Kenya, in a fragment of Zanzibar-Inhambane (ZI) regional forest mosaic. Study results provided previously unavailable indigenous tree species growth rates and human-assisted forest regeneration rates for ZI forests and highlighted issues relevant to conserving and regenerating remnants of coastal dry forest throughout East Africa. Enrichment plantings accelerated recovery of indigenous tree species diversity and increased species density above natural levels. A strategy of inter-planting within existing natural regeneration, including leaving large relic trees, accelerated regrowth of the forest, but the main beneficiary of the strategy was exotic Azadirachta indica, which came to dominate significant areas. Analyses indicated that A. indica, which produces insecticidal compounds, was significantly altering the structure of arthropod communities; flying to ground-dwelling arthropod ratios were higher where A. indica made up a higher proportion of above-ground woody biomass. Management strategies appear to be mostly restoring indigenous forest structures, despite continued casual illegal tree cutting and invasion by A. indica. Analysis of illegally harvested trees highlighted the important role of indigenous tree species as a source of ecosystems services to local people; an important consideration for forest conservation planning worldwide.