Simulation of the Unexpected Photosynthetic Seasonality in Amazonian Evergreen Forests by Using an Improved Diffuse Fraction‐Based Light Use Efficiency Model

Abstract

Understanding the mechanism of photosynthetic seasonality in Amazonian evergreen forests is critical for its formulation in global climate and carbon cycle models. However, the control of the unexpected photosynthetic seasonality is highly uncertain. Here we use eddy‐covariance data across a network of Amazonian research sites and a novel evapotranspiration (E) and two‐leaf‐photosynthesis‐coupled model to investigate links between photosynthetic seasonality and climate factors on monthly scales. It reproduces the GPP seasonality (R2 = 0.45–0.69) with a root‐mean‐square error (RMSE) of 0.67–1.25 g C m−2 d−1 and a Bias of −0.03–1.04 g C m−2 d−1 for four evergreen forest sites. We find that the proportion of diffuse and direct sunlight governs the photosynthetic seasonality via their interaction with sunlit and shaded leaves, supported by a proof that canopy light use efficiency (LUE) has a strong linear relationship with the fraction of diffuse sunlight for Amazonian evergreen forests. In the transition from dry season to rainy season, incident total radiation (Q) decreased while LUE and diffuse fraction increased, which produced the large seasonal increase (~34%) in GPP of evergreen forests. We conclude that diffuse radiation is an important environmental driver of the photosynthetic seasonality in tropical Amazon forests yet depending on light utilization by sunlit and shaded leaves. Besides, the GPP model simulates the precipitation‐dominated GPP seasonality (R2 = 0.40–0.69) at pasture and savanna sites. These findings present an improved physiological method to relate light components with GPP in tropical Amazon.

Effects of Light Component and Water Stress on Photosynthesis of Amazon Rainforests During the 2015/2016 El Niño Drought

“…The water stress factor EF is calculated: where PET PT is the potential evaporation (Priestley & Taylor, ) and E is actual evapotranspiration calculated from the Air Relative Humidity‐Based Two‐Source E model (Yan et al, ) that simulates the surface energy balance, soil water balance, and environmental constraints on E with inputs of remotely sensed LAI, surface meteorological data, and maximum soil available water content. Maximum soil available water content was set to 2,000 mm for Amazonian rainforest (Yan et al, ).…”

“…According to the DTEC GPP model (equation ), PAR, EF, and LAI all could contribute to the decrease in GPP of Amazon rainforests during the 2015/2016 El Niño event relative to 2011–2014. As PAR is absorbed by plant through interaction of diffuse and direct PAR with sunlit and shaded leaves, its impact on GPP depends on LAI (Yan et al, ). We assumed potential GPP (GPP*) represented contributions of PAR and LAI to GPP, where ε msu and ε msh are maximum LUE (g C/MJ) for sunlit and shaded leaves and APAR Sun and APAR shd are incident PAR absorbed by sunlit and shaded leaves, respectively (see equations and ).…”

“…This would seem to imply that light is more limiting than water in these forests (Huete et al, ; Nemani et al, ), presumably because of deep roots allowing trees to avoid water stress during the dry season (Bi et al, ; Goulden et al, ; Huete et al, ). Numerous hypotheses, notably the phenological hypothesis (Lopes et al, ; Wu et al, ), the hydroclimatic hypothesis (Guan et al, ), the forest structure hypothesis (Morton et al, ), the vertical canopy structure hypothesis (Tang & Dubayah, ), and the light component hypothesis (Yan et al, ), have been proposed to explain this photosynthetic seasonality. There is no consensus to date.…”

“…This is because shaded leaves are not light saturated, while sunlit leaves can be light saturated (Alton et al, ; Cirino et al, ; Doughty et al, ; Gu et al, ; Mercado et al, ; Morton et al, ; Rap et al, ). Some physiological models have been developed that include a separate treatment of diffuse and direct sunlight (Alton et al, ; Chen et al, ; Dai et al, ; He et al, ; Ryu et al, ; Yan et al, ). However, most models including dynamic global vegetation models poorly reproduce the GPP seasonality at Brazil flux tower sites (Restrepo‐Coupe et al, ).…”

“…However, most models including dynamic global vegetation models poorly reproduce the GPP seasonality at Brazil flux tower sites (Restrepo‐Coupe et al, ). A diffuse fraction‐based two‐leaf terrestrial ecosystem carbon (DTEC) model simulated the GPP seasonality of rainforests at Brazil flux tower sites and revealed that the proportion of diffuse and direct sunlight governs the photosynthetic seasonality including “dry‐season greening” (Yan et al, ).…”