Tree line dynamics in the tropical African highlands: identifying drivers and dynamics

Questions: What are the potential drivers of tree line change in the tropical African highlands? Are the temperature-sensitive tree lines in these highlands shifting as a result of climate change? Significance: The high-altitude forests provide important ecosystem services for the vulnerable environment of the tropical highlands. Climate change is expected to have pronounced effects on the tree line limit of these forests. Afro-alpine tropical tree lines are therefore potentially valuable as a proxy of climate change and the related response of ecosystems in the tropical highlands. Location: Tropical African highlands. Results: The influence of climatic factors in the African tropical highlands is significantly different compared to other regions. The potentially determining factors for tree line distribution in tropical Africa are temperature, precipitation and cloudiness, carbon balance, fire and anthropo-zoogenic impacts. Despite recent temperature increase, tree lines have not risen to higher altitudes in the tropical African highlands. Instead, high human pressure has caused stabilization and even recession of the tree lines below their natural climatic limit, particularly through livestock herding. But, even neglecting human pressure, there might be a lag in response time between temperature and tree line change. Conclusions: The actual drivers of tree line change in the African tropical highlands are mainly fire and anthropogenic pressure rather than climate change. But long-term drought periods can be a trigger for fire-induced deforestation of the tree line vegetation. Additionally, in volcanic active mountains, volcanic activity is also a potentially limiting factor for the tree line distribution. Tree line dynamics can thus not be used as a proxy of climate change for the African tropical highlands

Quaternary glacial and periglacial processes in the Ethiopian Highlands in relation to the current afro-alpine vegetation

The highlands of Ethiopia show a great variety in present and past climate. The environments differ in altitude, latitude and local conditions. This has an influence on vegetation and geomorphologic processes. Present knowledge of past glacial and periglacial landforms concentrates around the highest mountain ranges of Ethiopia, the Semien Mountains, the Bale Mountains and the Arsi Mountains. Many intermediate mountains stay unexplored or just briefly discussed. No present glaciated mountains exist in Ethiopia but current periglacial processes occur on the highest peaks. The climate change sensitivity of the mountain environment can be assessed which can contribute to the study of the temperature sensitive treeline and land cover changes. Afro-alpine vegetation can be influenced by the presence of relict (peri)glacial landforms, which change the growing conditions by different soil properties. Further research will need to complement the existing observations by unexplored mountains and to establish altitudinal north-south trajects in Ethiopia regarding vegetation, geomorphological processes and landforms