Tropical forests represent the largest store of terrestrial biomass carbon (C) on earth and contribute over-proportionally to global terrestrial net primary productivity (NPP). How climate change is affecting NPP and C allocation to tree components in forests is not well understood. This is true for tropical forests, but particularly for African tropical forests. Studying forest ecosystems along elevation and related temperature and moisture gradients is one possible approach to address this question. However, the inclusion of belowground productivity data in such studies is scarce. On Mt. Kilimanjaro (Tanzania), we studied aboveground (wood increment, litter fall) and belowground (fine and coarse root) NPP along three elevation transects (c. 1800–3900 m a.s.l.) across four tropical montane forest types to derive C allocation to the major tree components. Total NPP declined continuously with elevation from 8.5 to 2.8 Mg C ha⁻¹ year⁻¹ due to significant decline in aboveground NPP, while fine root productivity (sequential coring approach) remained unvaried with around 2 Mg C ha⁻¹ year⁻¹, indicating a marked shift in C allocation to belowground components with elevation. The C and N fluxes to the soil via root litter were far more important than leaf litter inputs in the subalpine Erica forest. Thus, the shift of C allocation to belowground organs with elevation at Mt. Kilimanjaro and other tropical forests suggests increasing nitrogen limitation of aboveground tree growth at higher elevations. Our results show that studying fine root productivity is crucial to understand climate effects on the carbon cycle in tropical forests.

热带森林是地球上最大的陆地生物量碳 (C) 储存库，对全球陆地净初级生产力 (NPP) 的贡献超比例。气候变化如何影响森林中树木成分的 NPP 和碳分配尚不清楚。这对于热带森林是正确的，但对于非洲热带森林尤其如此。沿着海拔高度和相关的温度和湿度梯度研究森林生态系统是解决这个问题的一种可能方法。然而，在此类研究中纳入地下生产力数据的情况很少。在山上 乞力马扎罗山（坦桑尼亚），我们沿三个海拔断面（约 1800-3900 米）研究了地上（木材增量、凋落物下降）和地下（细根和粗根）NPP ) 跨越四种热带山地森林类型，得出主要树木成分的碳分配。总 NPP 从 8.5 升高到 2.8 Mg C ha 持续下降^-1 年^-1由于地上 NPP 显着下降，而细根生产力（连续取芯方法）保持不变，约为 2 Mg C ha ^-1 年^-1，表明碳分配显着转移到地下成分随海拔升高。在亚高山Erica森林中，通过根系凋落物进入土壤的 C 和 N 通量远比叶片凋落物输入重要得多。因此，将 C 分配转移到 Mt. 海拔的地下器官。乞力马扎罗山和其他热带森林表明，高海拔地区地上树木生长的氮限制越来越大。我们的结果表明，研究细根生产力对于了解气候对热带森林碳循环的影响至关重要。