Investigating diversity dependence of tropical forest litter decomposition: experiments and observations from Central Africa

Questions: Mixed litter may decompose at different rates to single-species litter, leading to differences in ecosystem functioning and decomposition. Studies of the effects of different litter species and combinations are rare in tropical forests and absent from African forests. Therefore we investigated: (1) Are there differences in litter decomposition in two forest types differing in tree diversity; and (2) is litter decomposition diversity-dependent? Location: Old-growth moist evergreen tropical forest Dja Faunal Reserve, southeast Cameroon. Methods: We calculate decomposition rates (leaf litter fall/leaf litter standing crop) along a tree diversity gradient in two forest types (naturally occurring low-diversity monodominant and adjacent higher-diversity mixed forest). Both forests experience the same climate on the same soil type; the former is dominated by a single species, Gilbertiodendron dewevrei (De Wild.) J. Léonard, probably due to lack of a long-term disturbance and has similar edaphic factors. Decomposition experiments were conducted in both forest types using single and mixed species litter bags of standard high-quality (bay leaves; Laurus nobilis L.) and low-quality (G. dewevrei) litter over 9 months. Results: The estimated decomposition rate in mixed forest was four times faster than in monodominant forest, and not significantly correlated with local quadrat-scale tree species diversity. The litter bag experiment showed that decomposition of high-quality leaves was faster than low-quality leaves (k values: 2.0 yr−1 vs 0.6 yr−1). Decay rates for each single species litter type were not significantly different in both forest types. However, G. dewevrei litter in mixed bags decomposed faster than in single-species bags in mixed forest, suggesting an impact of litter mixing on decomposition. In addition, bay litter in mixed bags decomposed faster in mixed than in monodominant forests across the three study sites. Conclusion: The observed difference in litter decomposition rate between low-diversity monodominant and adjacent high-diversity forest is more likely due to dominance of low-quality G. dewevrei litter, rather than low-diversity of the litter itself