Effectiveness of Exclosures to Restore Ecosystem Carbon Stock and Vegetation in the Highlands of Tigray, Northern Ethiopia

Beschreibung

Converting degraded communal grazing lands into exclosures is one option to promote natural regeneration of plants and to restore degraded ecosystems in Ethiopia. The present study investigated the change in ecosystem carbon stocks (ECS) and the enhancement in native plant species richness and diversity following the establishment of exclosures on communal grazing lands in the highlands of Tigray, Ethiopia. The study also estimated the economic value of exclosure’s ecosystem services mainly the ecosystem carbon and soil nutrient stocks, and grass production obtained after the establishment of exclosures. Moreover, the perception of local communities concerning the contribution of exclosures in counteracting land degradation was assessed. Replicated (n = 3) exclosures of 5, 10, 15, and 20 years old were selected and each exclosure was paired with an adjacent communal grazing land. Also, replicated (n = 3) church forests were selected to detect whether exclosures had reached the ECS, and plant species richness, diversity and aboveground standing biomass levels of the church forests. Soil, vegetation, and exclosure management-related data were collected from November 2007 to October 2008. The differences in carbon stocks and vegetation composition between an exclosure age and adjacent communal grazing land were assessed using a pair t-test. Correlation tests and multiple linear regression analyses were also conducted to establish predictive relationships between carbon stocks, vegetation composition, and easily measurable biophysical and management-related variables. Moreover, a financial analysis was conducted to determine the economic feasibility of exclosures. All exclosures displayed higher (p < 0.05) ECS, and plant species richness, diversity and aboveground standing biomass than the adjacent communal grazing lands. Differences in ECS between exclosures and communal grazing lands varied between 29 (± 4.9) and 61 (± 6.7) Mg C ha and increased with exclosure age. Over a period of 30 years, the carbon dioxide sequestered in the investigated exclosures was 246 Mg CO2 ha-1, total soil nitrogen increased by 7.9 Mg ha-1 and additional available phosphorus stocks amounted to 40 Kg ha-1. The Net Present Value of exclosure’s ecosystem services under consideration was higher than the Net Present Value of alternative wheat production by about 28 % (837 US $). Carbon market revenues alone added up to only about 44 % of the net revenues of alternative wheat production. In exclosures, much of the variability in ECS was explained by a combination of precipitation, clay content, vegetation canopy cover, woody biomass and exclosure duration (R² = 0.77 to 0.90). Precipitation and vegetation canopy cover also explained much of the variability of ECS in the communal grazing lands (R² = 0.48 to 0.55). Differences in plant species richness and aboveground standing biomass between exclosures and communal grazing lands also increased with exclosure age. Moreover, edaphic (e.g. soil total nitrogen and texture) and site (e.g. precipitation and altitude) variables were significant predictors (R² = 0.72 to 0.82; p < 0.05) for plant species richness, diversity and aboveground biomass in exclosures and communal grazing lands. Furthermore, the majority (75 %) of the respondents had positive views on the effectiveness of exclosures to restore degraded soils and vegetation. However, the respondents were concerned about the negative consequences of establishing exclosures such as reduction in the availability of fuel wood and shortage of grazing areas for their livestock. The findings confirm that establishment of exclosures on degraded communal grazing lands can provide positive contribution in restoring degraded ecosystems and provide economic benefits to the local communities. Concerning the carbon market revenues, consideration of carbon market revenues alone would not generate sufficient incentives to establish additional exclosures. However, if all benefits are taken into account and financially rewarded, exclosures can be competitive to alternative land uses. The field-based regression models developed in this study using soil, climate, management-related and vegetation predictors open up possibilities for: (1) establishing baseline information for carbon sequestration projects, (2) evaluating whether exclosure establishment should be expanded, (3) optimizing the selection of areas for the establishment of exclosures in the future, and (4) taking into account the ecological and economical values of exclosure by policy makers in their management decisions. Also the findings concerning the economic feasibility of exclosures can be used as entry points to register exclosures as Clean Development Mechanism (CDM) projects. The positive views of the majority of the respondents on exclosures imply that there is a substantial opportunity to mobilize the local communities in the efforts to manage the existing exclosures in a sustainable manner and to establish exclosures in the future. Yet, before expanding the area covered by exclosures, it is crucial to consider forest management options such as enrichment plantation of indigenous woody species that grow fast in exclosures to address the shortage of fuel wood and to lessen the shortage of grazing area.

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Wolde Mekuria Bori

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Carbon Stock Ecosystem Ethiopia Highlands Vegetation Äthiopien

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