Recruitment credit cannot compensate for extinction debt in a degraded dry Afromontane forest

Abstract

Question

Anthropogenic disturbances severely affect dryland forests worldwide, but their effects on woody species population structure and long‐term persistence remain poorly understood.

Location

The research was conducted in Desa'a, a dry Afromontane forest in northern Ethiopia, Tigray region.

Aims

Woody species diversity, extinction debt, recruitment credit and colonisation credit were quantified and mapped based on an inventory of mature and regenerating individuals in 303 systematically sampled plots.

Methods

To assess tree species population dynamics at the neighbourhood scale, each plot was pooled with 16 neighbouring plots, in which species composition in the mature and regeneration layers was compared to determine the number of regenerating species, extinction debt, recruitment credit and colonisation credit. At the regional scale (150,000 ha), all plots were pooled to obtain the number of regenerating species, extinction debt and recruitment credit.

Results

The average density of the mature layer was 1,482 individuals per ha, representing 84 species and 44 families, with Juniperus procera and Olea europaea dominating. The regeneration layer had an average density of 14,909 individuals per hectare, with Dodonaea angustifolia and Cadia purpurea dominating. At the neighbourhood scale, the average number of regenerating species, extinction debt, recruitment credit and colonisation credit amounted to 12, 13, 5 and 68 species. Regionally, there were 58 regenerating species, an extinction debt of 26 species, and a recruitment credit of 12 species. Extinction debt was more than twice as high as recruitment credit at both the neighbourhood and regional scale.

Conclusion

Although regeneration density seemed sufficient to maintain forest density, it consisted of only a few species and is inadequate to maintain species diversity. It is urgent that forest managers recognise this and step‐up conservation and restoration efforts in this important forest relic. Assisted restoration is needed for those species potentially at risk of extinction.