Questions: Is plant species richness, diversity and above-ground standing biomass enhanced after establishing exclosures on communal grazing lands? What factors influence the effectiveness of exclosures to restore degraded native vegetation in Tigray, Ethiopia?
Location: Northern Ethiopia.
Methods: We used a space-for-time substitution approach to detect changes in plant species richness, diversity and above-ground standing biomass after conversion of communal grazing lands to exclosures. We selected replicated (n=3) 5-, 10-, 15- and 20-year-old exclosures and paired each exclosure with an adjacent communal grazing land to ensure that soil and terrain conditions were as similar as possible among each pair.
Results: All exclosures displayed higher plant species richness, diversity and biomass than the communal grazing lands. Differences in plant species richness and biomass between an exclosure age and adjacent communal grazing land were higher in oldest than in youngest exclosures. In exclosures, much of the variability in plant species composition and biomass was explained by a combination of edaphic (total nitrogen, phosphorus, texture and soil pH) and site (precipitation and altitude) variables (R2=0.72–0.82). Edaphic and site variables also explained much of the variability in plant species composition in communal grazing lands (R2=0.76–0.82). Our study shows that all exclosures are at an early stage of succession. The increase in economically important indigenous shrub and tree species with exclosure age suggests that, with time, a valuable afromontane forest may develop.
Conclusions: Establishment of exclosures on communal grazing lands is a viable option to restore degraded native vegetation. However, before expanding exclosures, the ecological consequences of additional exclosures should be investigated as further expansion of exclosures could increase grazing pressure on remaining grazing areas. Furthermore, consideration of edaphic and site variables will help optimize selection of areas for establishment of exclosures and enhance natural regeneration in exclosures in the future.