Talvinen et al. (2026) Beyond cloud cover: Low- and high-altitude clouds have distinct impacts on tree sap flow and transpiration

Identification

• Journal: Agricultural and Forest Meteorology
• Year: 2026
• Date: 2026-04-09
• Authors: Sini Talvinen, Yann Salmon, Jose Gutiérrez López, Maj-Lena Finnander Linderson, Štěpánka Řehořková, Ladislav Šigut, Abhay Devasthale, Ilona Ylivinkka, Caroline Greiser, Ekaterina Ezhova, Johannes Quaas, Natalia Kowalska, Marian Pavelka, Stanislav Juráň, Eric Larmanou, Teemu Paljakka, Claudia Mohr, Ilona Riipinen, Radovan Krejci
• DOI: 10.1016/j.agrformet.2026.111182

Research Groups

• Department of Environmental Science, Stockholm University, Stockholm, Sweden
• Bolin Centre for Climate research, Stockholm University, Stockholm, Sweden
• Institute for Atmospheric and Earth System Research, University of Helsinki, Helsinki, Finland
• Department of Forest Sciences, Forest Ecology and Management, University of Helsinki, Helsinki, Finland
• Department of Forest Ecology and Management, Swedish University of Agricultural Sciences, Umeå, Sweden
• Department of Earth and Environmental Sciences, Lund University, Lund, Sweden
• Department of Matters and Energy Fluxes, Global Change Research Institute of the Czech Academy of Sciences, Brno, Czech Republic
• Meteorological Research Unit, Research and Development, Swedish Meteorological and Hydrological Institute, Norrk¨oping, Sweden
• Department of Physical Geography, Stockholm University, Stockholm, Sweden
• Institute for Meteorology, Universit¨at Leipzig, Leipzig, Germany
• Laboratory of Ecological Plant Physiology, Global Change Research Institute of the Czech Academy of Sciences, Brno, Czech Republic
• PSI Center for Energy and Environmental Science, Paul Scherrer Institute, Villigen, Switzerland
• Department of Environmental Systems Science, ETH Zurich, Zurich, Switzerland

Short Summary

This study investigates how different cloud types affect tree transpiration in European boreal and temperate forests using long-term sap flow and surface-based cloud observations. It reveals that low-altitude clouds significantly suppress transpiration, while high-altitude clouds have a negligible effect, with declining low-altitude cloud cover potentially increasing annual transpiration by 0.6–1.2 mm in boreal forests.

Objective

• To resolve the impact of clouds on sap flow relative to the impacts of vapor pressure deficit (VPD) and soil moisture.
• To investigate whether low- and high-altitude clouds have different impacts on sap flow and its drivers.
• To estimate the magnitude of potential changes in transpiration due to current trends in different cloud types.

Study Configuration

• Spatial Scale: Five forest sites across boreal and temperate Europe: Norunda (Sweden), Svartberget (Sweden), Hyyti¨al¨a (Finland), Bílý Kˇríˇz (Czech Republic), and R´ajec (Czech Republic).
• Temporal Scale: Long-term measurements, with sap flow data spanning 2016–2023 (varying by site), surface-based cloud observations from 2017–2023, and satellite cloud data (CLARA-A3) from 1982–2020. The analysis focuses on boreal summertime (June-August) daytime measurements.

Methodology and Data

• Models used: Piecewise Structural Equation Modelling (SEM), linear mixed-effect models, univariate regression analysis (Ordinary Least Squares and Total Least Squares), and CLARA-A3 satellite derivatives for long-term cloudiness trends.
• Data sources:
  • Long-term tree sap flow measurements (thermal heat balance, heat pulse, thermal dissipation, tissue heat balance methods) for Scots pine, Norway spruce, and Silver birch.
  • Surface-based remote sensing cloud observations (Cloudnet data portal, lidar ceilometer Vaisala CL51) for cloud base height and cloud fraction.
  • Satellite data (CLARA-A3) for long-term cloudiness trends.
  • Meteorological data: relative humidity, air temperature, total incoming photosynthetic photon flux density (PPFD), diffuse PPFD, global incoming shortwave radiation (SW), and precipitation.
  • Volumetric soil water content (soil moisture) at 20–30 cm depth.
  • Clearness Index (CI) as a cloud proxy.

Main Results

• Overall cloudiness reduced maximum sap flow by 7% to 40% compared to clear-sky days, but sap flow occasionally exceeded clear-sky levels under specific cloudy conditions.
• Low-altitude clouds significantly suppressed transpiration by limiting incoming radiation, with sap flow clearly decreasing as low-altitude cloud cover increased across all boreal sites and species.
• High-altitude clouds had a negligible impact on sap flow; under overcast high-altitude cloud conditions (cloud fraction > 0.8), sap flow was similar to clear-sky days.
• Structural Equation Modelling (SEM) revealed a direct pathway from cloudiness to sap flow, even when accounting for vapor pressure deficit (VPD) and soil moisture, and models including cloud parameters outperformed simpler models.
• VPD was the most important driver of sap flow, but the clearness index ranked 2nd to 4th in importance, sometimes exceeding soil moisture.
• Satellite data (CLARA-A3, 1982–2020) showed decreasing trends in low-altitude cloud fraction over boreal forests (e.g., 0.66% per decade in FI-Hyy, 0.48% per decade in SE-Nor).
• A simplified model-based estimate suggests that the observed decline in low-altitude cloudiness could lead to a potential increase in transpiration equivalent to approximately 0.6–1.2 mm of precipitation annually in boreal forests.

Contributions

• This is the first study to combine long-term sap flow data with surface-based cloud measurements to differentiate the impacts of cloud cover and altitude on tree sap flow and transpiration.
• It provides empirical evidence that low- and high-altitude clouds have distinct effects on tree transpiration, a differentiation largely overlooked in previous ecohydrological studies.
• The study demonstrates that cloudiness, when differentiated by type, can be a significant driver of sap flow, comparable in some cases to VPD and soil moisture, challenging the common practice of treating cloud cover as a simple binary or undifferentiated variable.
• It quantifies the potential magnitude of transpiration changes in boreal forests due to observed long-term declines in low-altitude cloud cover, highlighting a novel mechanism for climate feedbacks.
• The findings underscore the critical importance of separating cloud types for a more accurate understanding of biosphere–atmosphere interactions and hydrological cycling in a changing climate.

Funding

• Knut and Alice Wallenberg foundation (project CLIVE, grant No 2022.0104)
• Horizon Europe programme (Grant Agreement No 101137680, project CERTAINTY)
• The Ministry of Education, Youth and Sports of the Czech Republic (AdAgriF; CZ.02.01.01/00/22_008/0004635)
• CzeCOS program (grant No LM2023048)
• Swedish Research Council, Vetenskapsrådet (grant No 2021-05143)
• Research Council Finland (grant No 323843 and 357263)
• Research Council of Finland (ACCC Flagship, grant No 357902)
• Business Finland project CARBON+
• Swedish Research Council FORMAS (grant No 2021-01993)

Citation

@article{Talvinen2026Beyond,
  author = {Talvinen, Sini and Salmon, Yann and López, Jose Gutiérrez and Linderson, Maj-Lena Finnander and Řehořková, Štěpánka and Šigut, Ladislav and Devasthale, Abhay and Ylivinkka, Ilona and Greiser, Caroline and Ezhova, Ekaterina and Quaas, Johannes and Kowalska, Natalia and Pavelka, Marian and Juráň, Stanislav and Larmanou, Eric and Paljakka, Teemu and Mohr, Claudia and Riipinen, Ilona and Krejci, Radovan},
  title = {Beyond cloud cover: Low- and high-altitude clouds have distinct impacts on tree sap flow and transpiration},
  journal = {Agricultural and Forest Meteorology},
  year = {2026},
  doi = {10.1016/j.agrformet.2026.111182},
  url = {https://doi.org/10.1016/j.agrformet.2026.111182}
}