What is the compound effect of re/af-forestation and extreme heat on summer land-atmosphere coupling across Europe?

Through soil moisture and vegetation exchanges, land-atmosphere coupling contributes significantly to the evolution of extreme events. Land use/land cover changes (LUC) modify local land surface properties that control the land-atmosphere mass, energy, and momentum exchanges. The Flagship Pilot Study LUCAS (Land Use & Climate Across Scales) provides a coordinated effort to study LUC using an ensemble of 11 regional climate models (RCMs). In the first phase of the project, three reanalyses-driven experiments were performed for continental Europe: eval (with each RCM using its standard land use / land cover distribution), forest (maximised forest cover), and grass (trees replaced by grassland. An analysis of the impact on the coupling between temperature and evapotranspiration is performed using the usual correlation metric, while a new coupling metric based on the product of normalised variables was developed to analyse the coupling between extreme heat (TX90p) or heat wave (TX90p for at least five consecutive days) and evapotranspiration (LH) or soil moisture (TX90p*LH or TX90p*SMOIS). Whenever its values are lower than -1, then LH (SMOIS) is concurrently in deficit, and soil is uncoupled from the atmosphere. Conversely, when its values are greater than 1, then land-atmosphere coupling occurs.

For all RCMs, a positive correlation between near-surface maximum temperature and latent heat prevails over northern Europe, while the negative correlation dominates over southern and southeastern Europe. Forestation (forest-grass) will lead to higher correlations between latent heat and near-surface maximum temperature due to the different transition zone belt locations and weaker correlations in the grass experiment.

Extreme heat and evapotranspiration are positively coupled in forests across the whole continent except in the Mediterranean.  In the grass experiment, the Mediterranean areas are negatively coupled in most models, whilst northern Europe is positively coupled. This coupling (positive/negative) is amplified under heat wave events. Overall, forestation induces increased coupling in central Europe.  In the forest experiment, extreme temperature and soil moisture are negatively coupled across Europe, indicating that the increase in evapotranspiration is associated with the ability of the trees to source water from deeper soil layers.  In the grass experiment, the ensemble mean shows very weak un/coupling in central/ southern Europe, indicating the inability of grasses to source water in deeper soil layers and a broadening of the transition zone.

 

Acknowledgements

The authors wish to acknowledge the financial support  from the Portuguese Fundação para a Ciência e Tecnologia, (FCT, I.P./MCTES) through national funds (PIDDAC): UID/50019/2025 and LA/P/0068/2020 (https://doi.org/10.54499/LA/P/0068/2020), DHEFEUS (https://doi.org/10.54499/2022.09185.PTDC), and through project references https://doi.org/10.54499/UIDB/00239/2020, https://doi.org/10.54499/UIDP/00239/2020 ,  LS, RMC, AR, and DCAL are supported by FCT, financed by national funds from the MCTES through grant UI/BD/154675/2023, and https://doi.org/10.54499/2021.01280.CEECIND/CP1650/CT0006, https://doi.org/10.54499/2022.01167.CEECIND/CP1722/CT0006, and https://doi.org/10.54499/2022.03183.CEECIND/CP1715/CT0004, respectively