Teatime on Mount Kilimanjaro: Assessing climate and land-use effects on litter decomposition and stabilization using the Tea Bag Index

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 10 ecosystems along the unique 3,000-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 (SAV; 950-m elevation), up to a maximum of k=0.022 in montane cloud forest (2,100m). This was followed by a 50% decrease in (sub-)alpine ecosystems (>4,000m). SAV experienced the strongest seasonal variation, with 23-times higher S values in dry season compared with wet season. The conversion of SAV to maize monocultures (similar to 1,000m) and traditional agroforestry to large-scale coffee plantations (similar to 1,300m) increased mean k values, and stabilization factors were about one-third lower. Forests between 1,900 and 2,100m 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.