Temporal dynamics of carbon fluxes in a tropical montane forest ecosystem in Southern Ecuador

Tropical rainforests represent major carbon reservoirs and play a critical role in regulating carbon dynamics and climate. Quantifying their carbon fluxes is essential for understanding ecosystem responses to climate variability and improving future climate change projections. In this study, we investigated interannual and seasonal variability in gross primary productivity (GPP), ecosystem respiration (Reco), and net ecosystem productivity (NEP) in a montane tropical rainforest in Southern Ecuador using eddy-covariance measurements from 2019 to 2024. GPP showed rather low interannual variability with a gradual increase from approximately 7.79–8.92 (Formula presented) (Formula presented) over the study period. Seasonal differences were generally weak, although a more pronounced contrast was observed in 2024 with higher GPP during the wet phase (10.26 (Formula presented) (Formula presented)) and compared to the dry phase (7.38 (Formula presented) (Formula presented)). The ecosystem consistently functioned as a net carbon sink, with maximum carbon uptake occurring in 2024 (NEP approximately −3.97 (Formula presented) (Formula presented)). Despite the general stability of GPP, variability in net carbon exchange was primarily driven by changes in Reco, indicating a stronger sensitivity of respiration to environmental fluctuations. Principal component analysis (PCA) revealed that carbon flux variability reflects a combination of radiation and moisture controls. Incoming solar radiation (Rg) showed the strongest association with GPP, whereas soil temperature (Ts) and soil moisture (SM) appeared to influence variations in Reco. Furthermore, we assessed the influence of ENSO phases on carbon fluxes and found reduced productivity during El Niño conditions, associated with elevated Ts and slight reduced SM and Rg. These results highlight the sensitivity of tropical montane forests to both local environmental drivers and large-scale climate variability. Overall, our findings demonstrate that while short-term photosynthetic processes remain generally stable, respiration processes were identified as important correlates of interannual variability in carbon balance, highlighting their relevance to tropical carbon dynamics.