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  1. 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 (StartGPPand EndGPP) 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 (EndGPP–StartGPP) was ∼50 days shorter. We hypothesized that warming has influenced timing of the photosynthesis season. We found minimal evidence for long‐term change in StartGPP, EndGPP, or air temperature, but their interannual anomalies were significantly correlated. Warmer weather was associated with earlier StartGPP(1.3–2.5 days °C−1) or later EndGPP(1.5–1.8 days °C−1, depending on forest type and month). Finally, we tested whether existing phenological models could predict StartGPPand EndGPP. For ENF forests, air temperature‐ and daylength‐based models provided best predictions for StartGPP, while a chilling‐degree‐day model was best for EndGPP. The root mean square errors (RMSE) between predicted and observed StartGPPand EndGPPwere 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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    Free, publicly-accessible full text available May 1, 2025
  2. Abstract

    Snowfall is an important driver of physical and biological processes in alpine systems. Previous work has shown that surface deposition of snow can vary for reasons not directly related to precipitation processes and that this variance has consequence for water budgets in snow-dominated terrestrial systems. In this work, measurements were made over several winter seasons in a forest–meadow ecotone in the Rocky Mountains of southeastern Wyoming. Two groups of measurements—both with wind-exposed and sheltered precipitation gauges—were analyzed. Reasonable agreement between snow deposition from a Hotplate gauge (exposed) and snow deposition from a SNOTEL pillow gauge (sheltered) is reported. The other result is that snow deposition is enhanced at an exposed gauge that was deployed on the leeward side of a forest–meadow edge. The enhancement is approximately a factor of 2 and varies with wind direction and speed and with upwind forest coverage. The enhancement is greater than was documented in an earlier investigation of Rocky Mountain snow deposition; however, in that study measurements were conducted above tree line.

     
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  3. Abstract

    Sublimation is an important hydrological flux in cold, snow‐dominated ecosystems. In high‐elevation spruce‐fir forests of western North America, spruce beetle outbreaks have killed trees, reduced the canopy, and altered processes that control sublimation. We evaluated two hypotheses related to effects of disturbance on sublimation in this ecosystem: (1) the dominant source for sublimation is canopy intercepted snow and (2) the loss of canopy following a beetle disturbance leads to less total sublimation. To incorporate uncertainty hierarchically across multiple data sources and address phenomenological parsimony, Bayesian statistics were used to analyze 17 years (2000–2016) of winter eddy covariance flux data at the Glacier Lakes Ecosystem Experiments Sites AmeriFlux sites where a spruce beetle outbreak caused 75–85% basal area mortality. Our analysis revealed that resistances to sublimate snow from the canopy were an order of magnitude less than from the snowpack, and the maximum snow loading capacity in disturbed canopies was reduced to 34% of its pre‐outbreak value. Total sublimation has decreased since 2010, 2 years after the main outbreak, declining 24% (with a 95% credible interval, C.I., between 18% and 38%) during 2014–2016 due to a 32% decrease in canopy sublimation. Snowpack sublimation only increased 3% over this period. With less total sublimation, the forest retained 6.1% (4.5–12.3% C.I.) more snowpack mass or equivalently 4.4% (3.2–8.8 C.I.) of the annual precipitation. Considering tree growth and ecological succession are slow in spruce‐fir forests, this decrease in sublimation should persist as an increased snowpack for decades, with substantial impacts on catchment hydrologic processes and potentially streamflow.

     
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