Historical C₃-C₄ vegetation pattern on forested mountain slopes: its implication for ecological rehabilitation of degraded highlands of Ethiopia by afforestation

Abstract

In Ethiopia, plantation forestry for soil conservation and wood supply is mainly based on exotic tree species harvested at 12–25-y rotation age. To evaluate if these forests truly represent ecological rehabilitation of degraded areas through the build-up of soil organic matter before harvest, relative abundances of C₃ vs. C₄ carbon in soils under a 25-y-old forest on Mt. Yegof have been studied based on changes in soil δ¹³C values due to vegetation cover changes by afforestation. At Yegof, shrub and tree leaves had δ¹³C value of −28.7 ± 0.4‰. The grasses showed a value of −30.7 ± 0.6‰ at 2700–3000 m asl (typical C₃) and of −13.6 ± 0.3‰ at 2520 m asl (typical C₄). Soil δ¹³C values were −21.7 ± 0.9‰ in soil at 0–5 cm and −20.7 ± 0.6‰ in soil at 30–50 cm indicating a long history of C₄-dominated grass or cropland over the elevations sampled. The shifts towards lower δ¹³C values in soil at 0–5 cm coincide with 25 y of C₃ vegetation. Carbon content in topsoil was 3.4–9.2% and in tree leaves was 45–56%. However, an estimate of new C₃ carbon (54%) in 0–5 cm soil was low as compared with tropical rain-forest and savanna ecosystems suggesting a slow rate of carbon turnover at Yegof. The results suggest that degraded sites at Yegof may need further duration of forest cover longer than current rotation times of the forests to improve soil conditions and restore soil organic matter and carbon, which have been lost during the past land use. It is encouraging from the perspectives of ecological rehabilitation and soil conservation that a shift in the dominance from C₄ vegetation types of agricultural and grassland ecosystems to C₃ forest ecosystems could be established in less than 25 y on these highly degraded slopes of Mt. Yegof.