Several studies have used field-based data (e.g., eddy covariance flux tower, tree-ring isotopes) to evaluate local changes in forest WUE and GPP 29, 30, 31. WUE has been widely used to assess forest–climate interactions 24, 25, 26, and especially forest responses to changing environmental conditions (e.g., precipitation declines, temperature rise, periodicity and intensity of droughts, CO2 fertilization) 16, 27, 28. WUE per unit area increases as more carbon is gained and less water is lost. The trade-off between forest ecosystem GPP and ET is defined as water-use efficiency (WUE = GPP/ET) 21, 22, 23. In summer, the higher ET rates can be linked to an increase in forest transpiration (which represent ~60% of land ET 5) driven by warmer temperature and heat waves 20. The warmer temperatures during spring and autumn can also increase the loss of soil moisture and plant transpiration, resulting in higher ET rates 17, 18, 19. However, some studies have reported that the spring and autumn carbon gain is offset by carbon loss caused by higher summer respiration rates 15, 16. GPP in temperate forests has increased in the spring and autumn because of the prolonged growing season triggered by global heating 14. For example, global heating and extreme weather can directly alter forest GPP and ET 11, 12, 13. The dynamics of both are strongly driven by environmental conditions 9, 10. The water lost during transpiration and evaporation from ecosystems is defined as evapotranspiration (ET). The amount of carbon assimilated by an ecosystem during a given period is defined as gross primary production (GPP). These processes mitigate anthropogenic CO2 emission and help to buffer rainfall patterns at various spatial scales 6, 7, 8 because of the returned water vapor back to the atmosphere. Atmospheric CO2 assimilation sequesters up to 12% of anthropogenic carbon emissions, and transpiration is responsible for returning up to 40% of local precipitation to the atmosphere 5. During photosynthesis, the trees assimilate atmospheric CO 2 as biomass, and release the soil water drawn from the roots back into the atmosphere as water vapor. Photosynthesis drives the exchange of carbon and water between forest canopies and the ambient atmosphere. Overall, 20% of total gross primary productivity across all European forest core areas was offset by forest areas that exhibited a net decrease in productivity.įorest ecosystems play an important role in global and regional carbon and hydrological cycles by acting as carbon sinks and as sources of water vapor via transpiration 1, 2, 3, 4. However, productivity increases during spring and autumn were not sufficient to compensate for summertime decreases in 25% of core forest areas. Enhanced productivity drove increased water-use-efficiency (the ratio of gross primary productivity to evapotranspiration). Both parameters increased during the early spring and late autumn in nearly half of the total undisturbed core forest area (3601.5 km2). Here, we analyse MODIS satellite data to assess monthly trends in gross primary productivity and evapotranspiration across undisturbed core forest areas in Europe between 20. However, high resolution and large spatial scale observational evidence of such responses in undisturbed core forest areas is lacking. Phenological responses of vegetation to global warming impact ecosystem gross primary production and evapotranspiration.
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