Soil and Biomass Carbon Storage is Much Higher in Central American than Andean Montane Forests

Abstract. Tropical montane forests (TMF) play a key role in the global carbon (C) cycle and in climate regulation by sequestering large amounts of above and belowground carbon. Elevation gradients in TMF have helped reveal the influence of environmental factors on C stocks. However, the influence of elevation and soil nutrient availability on C stocks has not been evaluated for mixed arbuscular and ectomycorrhizal (EM) associated forests in the Neotropics. We estimated aboveground biomass (AGB), coarse wood debris (CWD), and soil C based on field inventories in ten 1-ha plots along an elevational gradient from 880 to 2920 m a.s.l varying in relative abundance of EM-trees in western Panama. Trees ≥10 cm diameter at breast height (DBH) and CWD ≥10 cm diameter were measured to calculate biomass and necromass. Soil C to 1 m depth was estimated. Furthermore, climate and edaphic characteristics were described for each plot to evaluate the influence on these variables on each C pool. AGB, downed CWD and soil C were strongly positively correlated with elevation. We found exceptionally high AGB, up to 574.3 Mg ha⁻¹, and soil C, up to 577.9 Mg ha⁻¹ at higher elevations. Variation in total CWD within and among plots was high ranging from 14.75 to 326.5 Mg ha⁻¹. After controlling for elevation, neither nutrient availability nor EM-dominance had an effect on AGB or soil C. Nonetheless, high AGB at high elevations was attributed to the presence of Quercus species. Remarkably high soil C at high elevations might be a consequence of reduced decomposition rates associated with lower temperature, or geological history, where repeated volcanic eruptions buried surface soil organic layers. Our results highlight large regional uncertainty in C pool estimates in Neotropical montane forests, with potentially large underestimates for Central American C stocks.