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1. Stand density and local climate drive allocation of GPP to aboveground woody biomass

   https://doi.org/10.1111/nph.20414  

   Kannenberg, Steven A; Babst, Flurin; Barnes, Mallory L; Cabon, Antoine; Dannenberg, Matthew P; Johnston, Miriam R; Anderegg, William_R L (April 2025, New Phytologist)

   Summary The partitioning of photosynthate among various forest carbon pools is a key process regulating long‐term carbon sequestration, with allocation to aboveground woody biomass carbon (AGBC) in particular playing an outsized role in the global carbon cycle due to its slow residence time. However, directly estimating the fraction of gross primary productivity (GPP) that goes to AGBC has historically been difficult and time‐consuming, leaving us with persistent uncertainties.We used an extensive dataset of tree‐ring chronologies co‐located at flux towers to assess the coupling between AGBC and GPP, calculate the fraction of fixed carbon that is allocated to AGBC, and understand the drivers of variability in this fraction.We found that annual AGBC and GPP were rarely correlated, and that annual AGBC represented only a small fraction (c. 9%) of fixed carbon. This fraction varied considerably across sites and was driven by differences in stand density and site climate. Annual AGBC was suppressed byc. 30% during drought and remained below average for years afterward.These results imply that assumptions of relatively stationary allocation of GPP to woody biomass and other plant tissues could lead to systematic biases in modeled carbon accumulation in different plant pools and thus in carbon residence time. 

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2. A physiological crisis drives the coral recruitment bottleneck

   https://doi.org/10.1098/rsbl.2025.0103  

   Edmunds, Peter; Cheh, Adrian; Burgess, Scott (June 2025, Biology Letters)

   Recruitment failure is an important factor contributing to population declines of tropical corals. Because the causes of death for juvenile corals are unclear, it is challenging to predict how recruitment bottlenecks will change in the future. We tested the hypothesis that depletion of metabolic reserves increases mortality of juvenile corals under thermal stress. Metabolic reserves of juvenile colonies (<30 mm diameter) of broadcast spawningPocilloporafrom Moorea, French Polynesia, were manipulated using elevated temperature to increase respiration, and reduced day length to decrease photosynthesis, and estimated as biomass. Corals with high or low biomass were incubated at 28°C and 31°C for 15 days. JuvenileP. meandrinawith high biomass were six times more likely to die at 31°C versus 28°C, but corals with low biomass were 48 times more likely to die at 31°C versus 28°C. When juvenilePocilloporawere grown in seawater augmented with bicarbonate to reduce the cost of skeletogenesis in support of growth, growth was not affected, but energy expenditure was reduced by 20% to reduce reliance on metabolic reserves. Resource limitation of juvenile corals can affect their response to elevated temperatures, supporting the hypothesis that a physiological crisis initiated by resource limitation mediates the stringency of recruitment bottlenecks. 

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3. Constraining long‐term model predictions for woody growth using tropical tree rings

   https://doi.org/10.1111/gcb.17075  

   Xu, Xiangtao; van_der_Sleen, Peter; Groenendijk, Peter; Vlam, Mart; Medvigy, David; Moorcroft, Paul; Petticord, Daniel; Ma, Yixin; Zuidema, Pieter A (January 2024, Global Change Biology)

   Abstract The strength and persistence of the tropical carbon sink hinges on the long‐term responses of woody growth to climatic variations and increasing CO₂. However, the sensitivity of tropical woody growth to these environmental changes is poorly understood, leading to large uncertainties in growth predictions. Here, we used tree ring records from a Southeast Asian tropical forest to constrain ED2.2‐hydro, a terrestrial biosphere model with explicit vegetation demography. Specifically, we assessed individual‐level woody growth responses to historical climate variability and increases in atmospheric CO₂(C_a). When forced with historical C_a, ED2.2‐hydro reproduced the magnitude of increases in intercellular CO₂concentration (a major determinant of photosynthesis) estimated from tree ring carbon isotope records. In contrast, simulated growth trends were considerably larger than those obtained from tree rings, suggesting that woody biomass production efficiency (WBPE = woody biomass production:gross primary productivity) was overestimated by the model. The estimated WBPE decline under increasing C_abased on model‐data discrepancy was comparable to or stronger than (depending on tree species and size) the observed WBPE changes from a multi‐year mature‐forest CO₂fertilization experiment. In addition, we found that ED2.2‐hydro generally overestimated climatic sensitivity of woody growth, especially for late‐successional plant functional types. The model‐data discrepancy in growth sensitivity to climate was likely caused by underestimating WBPE in hot and dry years due to commonly used model assumptions on carbon use efficiency and allocation. To our knowledge, this is the first study to constrain model predictions of individual tree‐level growth sensitivity to C_aand climate against tropical tree‐ring data. Our results suggest that improving model processes related to WBPE is crucial to obtain better predictions of tropical forest responses to droughts and increasing C_a. More accurate parameterization of WBPE will likely reduce the stimulation of woody growth by C_arise predicted by biosphere models. 

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4. Midday Depression of Photosynthesis in Spartina alterniflora in a Virginia Salt Marsh

   https://doi.org/10.1029/2024JG008338  

   Mast, H M; Yang, X (September 2025, Journal of Geophysical Research: Biogeosciences)

   Abstract Salt marshes sequester a disproportionately large amount of carbon dioxide (CO₂) from the atmosphere through high rates of photosynthesis and carbon burial. Climate change could potentially alter this carbon sink, particularly the response of vegetation to environmental stressors that can decrease photosynthesis. Midday depression of gross primary production (GPP), characterized by a decline in photosynthesis during midday, has been documented in multiple ecosystems as a response to drought, high temperatures, and other stressors linked to climate change. Yet, midday depression has not been thoroughly investigated in salt marsh ecosystems. Here, we show that the midday depression of GPP in aSpartina alterniflorasalt marsh on the Eastern Shore of Virginia was ubiquitous and occurred on 76% of the 283 days studied during the 2019–2022 growing seasons. GPP was estimated from eddy covariance measurements with flux partitioning. Using random forest, we found that the daily maximum tidal height and air temperature were the strongest predictors of midday depression of GPP, with lower high tides and warmer temperatures associated with more severe depression. This result suggests midday depression occurs when GPP decreases in the afternoon in response to salinity and water stress. To our knowledge, this is the first examination of midday depression of photosynthesis inS.alternifloraat the ecosystem scale. Our results highlight the potential of climate change to increase midday depression of photosynthesis and ultimately weaken the salt marsh carbon sink. 

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5. CEDAR-GPP: spatiotemporally upscaled estimates of gross primary productivity incorporating CO ₂ fertilization

   https://doi.org/10.5194/essd-17-3009-2025  

   Kang, Yanghui; Bassiouni, Maoya; Gaber, Max; Lu, Xinchen; Keenan, Trevor F (January 2025, Earth System Science Data)

   Abstract. Gross primary productivity (GPP) is the largest carbon flux in the Earth system, playing a crucial role in removing atmospheric carbon dioxide and providing carbohydrates needed for ecosystem metabolism. Despite the importance of GPP, however, existing estimates present significant uncertainties and discrepancies. A key issue is the underrepresentation of the CO2 fertilization effect, a major factor contributing to the increased terrestrial carbon sink over recent decades. This omission could potentially bias our understanding of ecosystem responses to climate change. Here, we introduce CEDAR-GPP, the first global machine-learning-upscaled GPP product that incorporates the direct CO2 fertilization effect on photosynthesis. Our product is comprised of monthly GPP estimates and their uncertainty at 0.05° resolution from 1982 to 2020, generated using a comprehensive set of eddy covariance measurements, multi-source satellite observations, climate variables, and machine learning models. Importantly, we used both theoretical and data-driven approaches to incorporate the direct CO2 effects. Our machine learning models effectively predict monthly GPP (R2 ∼ 0.72), the mean seasonal cycles (R2 ∼ 0.77), and spatial variabilities (R2 ∼ 0.63) based on cross-validation at flux sites. After incorporating the direct CO2 effects, the predicted long-term GPP trend across global flux towers substantially increases from 3.1 to 4.5–5.4 gC m−2 yr−1, which aligns more closely with the 7.7 gC m−2 yr−1 trend detected from eddy covariance data. While the global patterns of annual mean GPP, seasonality, and interannual variability generally align with existing satellite-based products, CEDAR-GPP demonstrates higher long-term trends globally after incorporating CO2 fertilization and reflected a strong temperature control on direct CO2 effects. The estimated global GPP trend is 0.57–0.76 PgC yr−1 from 2001 to 2018 and 0.32–0.34 PgC yr−1 from 1982 to 2018. Estimating and validating GPP trends in data-scarce regions, such as the tropics, remains challenging, underscoring the importance of ongoing ground-based monitoring and advancements in modeling techniques. CEDAR-GPP offers a comprehensive representation of GPP temporal and spatial dynamics, providing valuable insights into ecosystem–climate interactions. The CEDAR-GPP product is available at https://doi.org/10.5281/zenodo.8212706 (Kang et al., 2024). 

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