The Carbon Dynamics of Dry Tropical Afromontane Forest Ecosystems in the Amhara Region of Ethiopia

Abstract

Forest degradation due to land use change is a severe problem in Ethiopian Afromontane Forests. We investigated such degradation effects by comparing degraded agricultural land (previously covered with forest) with neighboring natural forests, 40 to 50 years after conversion. We selected four different study areas to cover the eco-climatic conditions of the Amhara region in Northwestern Ethiopia. For a paired-stand comparison we collected soil data on both land use types. We calculated forest biomass to evaluate the biogeochemical-mechanistic ecosystem model Biome-BGC, which is used as a diagnostic tool to assess the site and management impacts on productivity as well as ecosystem carbon and nitrogen accumulation. We applied Biome-BGC to assess rehabilitation options on such degraded land. Afromontane forests in the highlands of Ethiopia showed high soil C stocks, resulting from long lasting biomass accumulation. Removing the tree cover and converting forest areas to crop-or grassland, has led to a loss of 40-85% of the soil C stocks and thus a loss in soil fertility within only 40 to 50 years. Rehabilitation efforts by replanting trees will improve soil fertility, but may require over a thousand years to achieve a similar level in biomass and soil fertility versus the situation prior to the land use change.

The Carbon Sequestration Potential of Degraded Agricultural Land in the Amhara Region of Ethiopia

“…Within Biome-BGC, a typical ecosystem or biome type is defined by 35 eco-physiological parameters, which can be obtained from the literature [43], or intensive monitoring sites [44]. For this study, we selected the eco-physiological parameters for the biome type 'tropical evergreen broadleaved forest', suggested by Ichii et al [47] for several reasons: (1) We had already successfully used and evaluated this parameter set for its applicability on diverse Ethiopian dry Afromontane forest ecosystems [10]; (2) We assumed a reforestation with a natural species mix as opposed to frequent plantations with non-natives such as Eucalyptus sp. ; (3) There is no other suitable parameter setting available for Ethiopian Afromontane forests.…”

“…The advantage of the model is that it simulates pools and, compared to other ecosystem models, it does not require tree populations to initialize a simulation run. Furthermore, in a previous study we evaluated Biome-BGC for different forest and agro-ecological zones (across elevation, temperature and precipitation gradients) in the Amhara region, using measured vegetation carbon, mineral soil carbon, and net primary production data [10].…”

“…Additionally, we see heavy disturbances in the remaining natural forests, such as cattle grazing or logging that result in severe soil degradation [8][9][10]. A study in the north-western highlands of Ethiopia revealed a decrease in natural forest cover from 27% in 1957 to 2% in 1982 and to 0.3% in 1995 [11].…”

“…Since forests are an important source of fuel and construction material for the local population, we include (a) three different thinning scenarios and (b) five different rotation lengths (10,30,50, 100, and 150 years), assuming a so called 'Normal-forest' system (equal representation of every age class). No climate change scenarios are considered, though, because of the high additional uncertainty different climate change trajectories would cause over such long simulation periods, when one purpose of this study is to compare the impact of different rotation lengths.…”

“…The model was chosen because it simulates forest growth and soil development by simulating major carbon, nitrogen, and water pools in fluxes in response to climatic and soil variables and different forest management options. In a previous study, the model had been tested and evaluated with measured vegetation carbon, soil carbon, and net primary production (NPP) data across different agro-ecological zones (across elevation, temperature, and precipitation gradients) within the Amhara region [10].…”