Seasonal Tea Bag Index data on decomposition (2014-2015) from natural and agricultural sites at Mt. Kilimanjaro

Decomposition is one of the most important processes in ecosystem carbon (C) and nutrient cycles, and is a major factor controlling ecosystem functions. The functioning of Afromontane ecosystems and their ability to provide ecosystem services are particularly threatened by climate and land‐use change. Our objectives were to assess the effects of climatic conditions (elevation and seasonality) and land‐use intensity on litter decomposition and C stabilization in ten ecosystems along the unique 3000 m elevation gradient of Mt. Kilimanjaro. Tea‐Bag Index parameters (decomposition‐rate‐constant k and stabilization‐factor S) were used to quantify decomposition of standardized litter substrate. Nine pairs of tea bags (green and rooibos tea) were exposed in each ecosystem during the short‐wet, warm‐dry, long‐wet and cold‐dry season. Decomposition rate increased from k=0.007 in savanna (950 m elevation), up to a maximum of k=0.022 in montane cloud forest (2100 m). This was followed by a 50% decrease in (sub‐)alpine ecosystems (>4000 m). Savanna experienced the strongest seasonal variation, with 23 times higher S‐values in dry season compared to wet season. The conversion of savanna to maize monocultures (~1000 m), and traditional agroforestry to large‐scale coffee plantations (~1300 m) increased mean k‐values, and stabilization factors were about one third lower. Forests between 1900 and 2100 m represent the zone of sufficient moisture and optimal temperature conditions. Seasonal moisture (lower slope) and temperature limitation (alpine zone) decreases litter decomposition. Mt. Kilimanjaro ecosystems are highly sensitive to land‐use change, which accelerates ecosystem cycles and decreases C stabilization.