## 4 Discussion  
The underrepresentation of Southern Hemisphere ecosystems in global flux databases limits the accuracy of Earth system models and hampers a full understanding of biosphere-atmosphere interactions [62; 63]. This study contributes unique flux data from the Araucaria forest, a structurally complex and endangered subtropical forest, improving the global representation of this biome in flux synthesis studies. Our findings demonstrate the feasibility and value of partitioning fluxes between forest strata in such ecosystems despite methodological challenges. The application of eddy covariance at both the overstory and understory levels required rigorous quality control due to lower wind speeds and more stable atmospheric conditions below the canopy, which can reduce turbulence and compromise data quality [64; 65]. Still, with appropriate corrections and validation, the measurements captured meaningful patterns in vertical flux contributions. We also acknowledge the smaller footprint and higher heterogeneity of the understory fluxes, which are influenced by local vegetation structure and soil characteristics. These factors introduce variability that must be carefully considered when interpreting the energy and carbon dynamics of multi-layered forests such as this one. By quantifying the relative contributions and seasonality of each stratum, this work offers critical insights for improving ecosystem modeling and informing conservation strategies in the Atlantic Forest biome. Despite the methodological constraints, it was clearly demonstrated that the forest exhibits distinct seasonality in its energy and CO\({}_{2}\) fluxes, which is typical of subtropical climates, with higher values observed during the spring-summer (SS) than in autumn-winter (AW). The overstory, comprising nearly 90% of the ecosystem's biomass, showed the most seasonal variations. This dominance is reflected in its contribution of 63% to the GPP  
Figure 7: Relationship between water-use efficiency (WUE) and vapor pressure deficit (VPD) in monthly averages with \(p\)-value < 0.01. For the understory, \(p\)-value > 0.1 and \(r^{2}\) = 0.08 are not presented. and 75% to the LE of the entire ecosystem. The parallelism in GPP and LE contributions is anticipated, as the process of CO\({}_{2}\) uptake via stomatal photosynthesis inherently involves water release through transpiration [66]. However, the disproportionately higher LE relative to GPP suggests that significant energy consumption in the overstory is channeled into evaporation of intercepted precipitation rather than directly into biomass production. While similar processes are expected to occur in the understory, which accounts for 25% of the Ecosystem's LE, a lack of detailed data limits our ability to fully characterize these contributions at the lower strata. In the Araucaria forest site, the highest GPP values were observed in the overstory, where the majority of available energy for photosynthesis is intercepted by the tall trees that dominate the canopy. This significant capture of solar radiation by the canopy explains why the overstory is primarily responsible for the bulk of biomass productivity at this site. Previous studies by [PERSON] et al. [67] and [PERSON] et al. [68] have highlighted that in forest ecosystems where water stress is not a limiting factor, GPP is predominantly influenced by the leaf area index (LAI) and global radiation. At our site, however, despite the lack of marked seasonal variations, GPP is primarily driven by air temperature, as indicated in both the overstory and understory (see Supplementary Material Table S2). This suggests a unique adaptation of the forest's photosynthetic processes to the local climate, underlining the importance of air temperature in modulating biological activity across different strata. In the Araucaria forest analyzed in this study, the distribution of respiration between the overstory and understory is notably disproportionate, illuminating unique aspects of forest ecosystem dynamics.