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1. Seasonal timing of fluorescence and photosynthetic yields at needle and canopy scales in evergreen needleleaf forests

   https://doi.org/10.1002/ecy.4402  

   Pierrat, Zoe Amie; Magney, Troy; Maguire, Andrew; Brissette, Logan; Doughty, Russell; Bowling, David R; Logan, Barry; Parazoo, Nicholas; Frankenberg, Christian; Stutz, Jochen (October 2024, Ecology)

   Abstract The seasonal timing and magnitude of photosynthesis in evergreen needleleaf forests (ENFs) has major implications for the carbon cycle and is increasingly sensitive to changing climate. Earlier spring photosynthesis can increase carbon uptake over the growing season or cause early water reserve depletion that leads to premature cessation and increased carbon loss. Determining the start and the end of the growing season in ENFs is challenging due to a lack of field measurements and difficulty in interpreting satellite data, which are impacted by snow and cloud cover, and the pervasive “greenness” of these systems. We combine continuous needle‐scale chlorophyll fluorescence measurements with tower‐based remote sensing and gross primary productivity (GPP) estimates at three ENF sites across a latitudinal gradient (Colorado, Saskatchewan, Alaska) to link physiological changes with remote sensing signals during transition seasons. We derive a theoretical framework for observations of solar‐induced chlorophyll fluorescence (SIF) and solar intensity‐normalized SIF (SIF_relative) under snow‐covered conditions, and show decreased sensitivity compared with reflectance data (~20% reduction in measured SIF vs. ~60% reduction in near‐infrared vegetation index [NIRv] under 50% snow cover). Needle‐scale fluorescence and photochemistry strongly correlated (r² = 0.74 in Colorado, 0.70 in Alaska) and showed good agreement on the timing and magnitude of seasonal transitions. We demonstrate that this can be scaled to the site level with tower‐based estimates of LUE_Pand SIF_relativewhich were well correlated across all sites (r² = 0.70 in Colorado, 0.53 in Saskatchewan, 0.49 in Alaska). These independent, temporally continuous datasets confirm an increase in physiological activity prior to snowmelt across all three evergreen forests. This suggests that data‐driven and process‐based carbon cycle models which assume negligible physiological activity prior to snowmelt are inherently flawed, and underscores the utility of SIF data for tracking phenological events. Our research probes the spectral biology of evergreen forests and highlights spectral methods that can be applied in other ecosystems. 

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2. Phenology of Photosynthesis in Winter‐Dormant Temperate and Boreal Forests: Long‐Term Observations From Flux Towers and Quantitative Evaluation of Phenology Models

   https://doi.org/10.1029/2023JG007839  

   Bowling, David R.; Schädel, Christina; Smith, Kenneth R.; Richardson, Andrew D.; Bahn, Michael; Arain, M Altaf; Varlagin, Andrej; Ouimette, Andrew P.; Frank, John M.; Barr, Alan G.; et al (May 2024, Journal of Geophysical Research: Biogeosciences)

   Abstract We examined the seasonality of photosynthesis in 46 evergreen needleleaf (evergreen needleleaf forests (ENF)) and deciduous broadleaf (deciduous broadleaf forests (DBF)) forests across North America and Eurasia. We quantified the onset and end (Start_GPPand End_GPP) of photosynthesis in spring and autumn based on the response of net ecosystem exchange of CO₂to sunlight. To test the hypothesis that snowmelt is required for photosynthesis to begin, these were compared with end of snowmelt derived from soil temperature. ENF forests achieved 10% of summer photosynthetic capacity ∼3 weeks before end of snowmelt, while DBF forests achieved that capacity ∼4 weeks afterward. DBF forests increased photosynthetic capacity in spring faster (1.95% d⁻¹) than ENF (1.10% d⁻¹), and their active season length (End_GPP–Start_GPP) was ∼50 days shorter. We hypothesized that warming has influenced timing of the photosynthesis season. We found minimal evidence for long‐term change in Start_GPP, End_GPP, or air temperature, but their interannual anomalies were significantly correlated. Warmer weather was associated with earlier Start_GPP(1.3–2.5 days °C⁻¹) or later End_GPP(1.5–1.8 days °C⁻¹, depending on forest type and month). Finally, we tested whether existing phenological models could predict Start_GPPand End_GPP. For ENF forests, air temperature‐ and daylength‐based models provided best predictions for Start_GPP, while a chilling‐degree‐day model was best for End_GPP. The root mean square errors (RMSE) between predicted and observed Start_GPPand End_GPPwere 11.7 and 11.3 days, respectively. For DBF forests, temperature‐ and daylength‐based models yielded the best results (RMSE 6.3 and 10.5 days). 

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3. What is the fate of xylem‐transported CO ₂ in Kranz‐type C ₄ plants?

   https://doi.org/10.1111/nph.15908  

   Stutz, Samantha S.; Hanson, David T. (June 2019, New Phytologist)

   Summary High concentrations of dissolved inorganic carbon in stems of herbaceous and woody C₃plants exit leaves in the dark. In the light, C₃species use a small portion of xylem‐transported CO₂for leaf photosynthesis. However, it is not known if xylem‐transported CO₂will exit leaves in the dark or be used for photosynthesis in the light in Kranz‐type C₄plants.Cut leaves ofAmaranthus hypochondriacuswere placed in one of three solutions of [NaH¹³CO₃] dissolved in KCl water to measure the efflux of xylem‐transported CO₂exiting the leaf in the dark or rates of assimilation of xylem‐transported CO₂* in the light, in real‐time, using a tunable diode laser absorption spectroscope.In the dark, the efflux of xylem‐transported CO₂increased with increasing rates of transpiration and [¹³CO₂*]; however, rates of¹³C_effluxinA. hypochondriacuswere lower compared to C₃species. In the light,A. hypochondriacusfixed nearly 75% of the xylem‐transported CO₂supplied to the leaf.Kranz anatomy and biochemistry likely influence the efflux of xylem‐transported CO₂out of cut leaves ofA. hypochondriacusin the dark, as well as the use of xylem‐transported CO₂* for photosynthesis in the light. Thus increasing the carbon use efficiency of Kranz‐type C₄species over C₃species. 

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4. Elevated light and CO₂ levels increase photosynthetic rates of diverse snow algal communities from the North Cascades

   https://doi.org/10.1111/nph.70332  

   Weigel, Brooke_L; Castillo, Guadalupe; Pata, Honu_K; Young, Jodi_N; Kodner, Robin_B (July 2025, New Phytologist)

   Summary Microalgae adapted to near‐zero temperatures and high light levels live on snowfields and glaciers worldwide. Snow algae have red‐colored pigments that darken snow surfaces, lowering its albedo and accelerating snowmelt. Despite their importance to the cryosphere, we know little about controls on snow algal productivity and biomass.Here, we characterize photophysiology from diverse natural field‐collected populations of alpine snow algae from the North Cascades of Washington, USA, where the major red‐bloom producing generaChlainomonas,Sanguina, andRosettawere present. We tested short‐term physiological responses of snow algae to light (0–3000 μmol m⁻² s⁻¹) and CO₂levels (0–1600 ppm), allowing us to determine the saturating light and CO₂levels for snow algal community net photosynthesis.All snow algal communities surveyed were adapted to extremely high light levels (3000 μmol m⁻² s⁻¹). In addition, photosynthesis rates of all the snow algal communities responded strongly to increasing CO₂levels. At current atmospheric CO₂levels (420 ppm), snow algal net photosynthesis rates were onlyc.50% saturated.Together, these results suggest the primary productivity of important bloom‐forming snow algal communities in alpine ecosystems will likely rise as atmospheric CO₂concentrations increase, regardless of potential changes in available light levels. 

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5. Synchronous Marine and Terrestrial Carbon Cycle Perturbation in the High Arctic During the PETM

   https://doi.org/10.1029/2020PA003942  

   Cui, Ying; Diefendorf, Aaron_F; Kump, Lee_R; Jiang, Shijun; Freeman, Katherine_H (April 2021, Paleoceanography and Paleoclimatology)

   Abstract The Paleocene‐Eocene Thermal Maximum (PETM; 56 Ma) is considered to be one of the best analogs for future climate change. The carbon isotope composition (δ¹³C) ofn‐alkanes derived from leaf waxes of terrestrial plants and marine algae can provide important insights into the carbon cycle perturbation during the PETM. Here, we present new organic geochemical data and compound‐specific δ¹³C data from sediments recovered from an early Cenozoic basin‐margin succession from Spitsbergen. These samples represent one of the most expanded PETM sites and provide new insights into the high Arctic response to the PETM. Our results reveal a synchronous ∼−6.5‰ carbon isotope excursion (CIE) in short‐chainn‐alkanes (nC₁₉; marine algae/bacteria) with a ∼−5‰ CIE in long‐chainn‐alkanes (nC₂₉andnC₃₁; plant waxes) during the peak of the PETM. Although δ¹³C_n‐alkanesvalues were potentially affected via a modest thermal effect (1‰–2‰), the relative changes in the δ¹³C_n‐alkanesremain robust. A simple carbon cycle modeling suggests peak carbon emission rate could be ∼3 times faster than previously suggested using δ¹³C_TOCrecords. The CIE magnitude of both δ¹³C_n‐C19and δ¹³C_n‐C29can be explained by the elevated influence of¹³C‐depleted respired CO₂in the water column and increased water availability on land, elevatedpCO₂in the atmosphere, and changes in vegetation type during the PETM. The synchronous decline in δ¹³C of both leaf waxes and marine algae/bacteria argues against a significant contribution to the sedimentary organic carbon pool from the weathering delivery of fossiln‐alkanes in the Arctic region. 

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