Soil and biomass carbon storage is much higher in Central American than Andean montane forests

Abstract. Tropical montane forests (TMFs) play a key role in the global carbon (C) cycle and in climate regulation by sequestering large amounts of aboveground and belowground carbon. Elevation gradients in TMFs have helped reveal the influence of environmental factors on C stocks. Few studies have evaluated the influence of elevation and soil nutrient availability on C stocks in the context of mixed ectomycorrhizal (EM) and non-EM-associated forests in the neotropics, especially in the mountains. 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 ma.s.l. varying in relative abundance of EM trees in western Panama. Trees with ≥ 10 cm diameter at breast height (DBH) and CWD with ≥ 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. 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. We found previous lidar-derived estimates for our site substantially underestimated the aboveground carbon (AGC) present in the plots, being between 2 and 6 times lower than our field-based estimates. 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.