## 1 Introduction  
The Amazon rainforest, located in the tropical zone, plays a fundamental role in the world's climate due to its vast dimensions and diversity and is predominantly located within Brazilian territory [1, 2]. Since the 1980s, numerous studies have been conducted in this region to describe the exchange processes between different Amazonian forest ecosystems and the atmosphere [3, 4, 5]. However, given Brazil's continental dimensions, there are other ecosystems that remain under-investigated, such as the Atlantic rainforest. This rainforest originally covered around 150 million ha, extending through tropical and subtropical regions from 5\({}^{\circ}\) S to 30\({}^{\circ}\) S [6]. Currently, only about 16% of the Atlantic rainforest's natural vegetation remains, largely in small fragments of around 100 hectares each [6, 7]. The highly heterogeneous environmental conditions of these forests have promoted great biodiversity and high levels of endemism. One example is the Mixed Ombrophilous Forests, among the most threatened forest formations in Brazil, with only 12.6% of its original extent still intact [6, 8, 9, 10]. These forests are primarily found at elevations above 500 m and between latitudes 20\({}^{\circ}\) S and 29\({}^{\circ}\) S, currently covering an area of 25 million hectares. The dominant species is the Brazilian pine, or Parana pine (_Araucaria angustiglia_, referred to here as the Araucaria forest). The deforestation of these areas is a subject of ongoing discussion within Brazilian legislative frameworks, as current laws regulate its commercial use through management and planting but do not prohibit clear-cutting [11]. Adult Araucaria trees are easily distinguished by their umbel-shaped canopies and straight, woody stems, which are free of branches up to two-thirds of their height and can reach up to 30 m [12]. Therefore, the overall forest-atmosphere interactions in the Araucaria forest can be significantly influenced by the partitioning of energy and carbon fluxes between the overstory and understory, underscoring the importance of these vertical structures in maintaining ecosystem stability and functionality. The overstory contribution to the fluxes is often more significant, especially in relation to CO\({}_{2}\) and evapotranspiration [13, 14]. The contribution of understory fluxes is highly dependent on canopy structure, which limits the available energy for flux processes at this level [15, 16, 17, 18, 19, 20]. [PERSON] et al. [13] observed that understory respiration varied between 32% and 79% of the total respiration in different types of forests, including evergreen and deciduous, across various climate types, such as temperate, boreal, arid, and semiarid. [PERSON] and [PERSON] [21] found that the canopy contribution to ecosystem evapotranspiration was around 95% for a temperate deciduous forest. However, [PERSON] et al. [22] found values of 50%-40% in summer, 30%-15% in winter, and 45% on an annual average for a pine flatwood forest. [PERSON] et al. [23] and [PERSON] et al. [24] showed that the greater the canopy cover, the more substantial the contribution of the upper stratum to the total evapotranspiration of the ecosystem. Understanding the partitioning of these vertical fluxes is crucial for various scientific domains, such as biogeochemistry, plant physiology, and ecohydrology, as well as for advancing climate models [13, 25]. In this regard, grasping the factors that control the flux partitioning between the overstory and understory across different climates and forest types remains a challenge. Despite the increasing availability of flux data from tropical and subtropical forests, the dynamics of energy and carbon exchange in pronounced dual-canopy strata, such as those in Araucaria forests, remain poorly understood. This structural configuration suggests significant stratification in energy absorption, evapotranspiration, and carbon assimilation.