Seasonal and intracanopy shifts in the fates of absorbed photons in central Amazonian forests: implications for leaf fluorescence and photosynthesis.

Recent studies have shown a linear relationship between solar-induced Chl fluorescence (SIF) and gross primary productivity (GPP) at large scales. However, this relationship diverges at finer leaf scales, particularly in tropical forests with complex canopy structures. To address this issue, we assessed seasonal and intracanopy variations in leaf energy partitioning in central Amazonian forests with extensive in-canopy sampling and pulse-amplitude-modulated Chl fluorescence measurements. We explored the pathways of photon utilization for photochemistry (ΦPSII), heat dissipation (ΦNPQ), and nonregulated quenching (ΦNO) of fluorescence. We found consistent increases in ΦNPQ and decreases in ΦPSII and ΦNO with increasing canopy height, primarily driven by changes in photosynthetically active radiation. During the dry season, a triphasic relationship between ΦNO and ΦPSII was detected, alternating between positive and negative relationships across leaf irradiance levels, highlighting stress-induced physiological responses. Interspecific variation and vapor pressure deficit also played significant roles in modulating ΦNO, emphasizing the complex interaction between environmental factors, species composition, and energy dissipation across canopy strata. These insights into leaf-level fluorescence and energy dynamics show the complex mediation of ΦNPQ-ΦNO-ΦPSII relationships, offering implications for enhancing SIF-GPP relationships and understanding tropical forest responses to climate change.