Vegetation-heatwave correlations and contrasting energy exchange responses of different vegetation types to summer heatwaves in the Northern Hemisphere during the 1982–2011 period

Heatwave is a serious threat to society and can lead to grave consequences. Although it is well known that persistent large-scale circulation anomalies are the key to generate heatwaves, land-atmosphere interactions have also been suggested to intensify and propagate heatwaves. Vegetation plays a vital role in land-atmosphere interactions, modulating energy and water exchange through various pathways. However, vegetation impacts on surface energy exchange during extreme events such as heatwaves, and the attribution of effects to different vegetation types, is complex and poorly understood. In this study, we found strong interannual correlations between summer heatwaves and various plant function types (PFTs), based on the Global Heatwave and Warm-spell Record (GHWR) and leaf area index (LAI) products from satellites during 1982–2011. In Central Europe and the southern and southeastern part of North America, where the land cover is dominated by grasslands, temperate deciduous forests or temperate needleleaf forests, heatwaves tend to occur more frequently in years with lower LAI. In contrast, in the northwestern and northeastern part of North America, where boreal evergreen forests dominate, higher-than-normal LAI is associated with an increase in heatwave occurrence. These findings are in general supported by composite analyses of extreme LAI years in these PFT regions and heatwave characteristics therein. We speculate that the different surface heat flux responses over different PFTs during heatwaves may explain the above relationships. Focusing on North America, and using various datasets including those generated by the Global Land Data Assimilation System (GLDAS) with three different land surface models (CLM, MOS, NOAH), three reanalysis datasets (MERRA-2, NOAA-CIRES-DOS, NCEP/NCAR), and also observations from an extensive network of flux towers, we found that when temperate and boreal evergreen forests are greener, positive sensible heat anomalies increase significantly during heatwaves. Meanwhile, over boreal evergreen forests, changes in latent heat anomalies are much smaller than the positive sensible heat anomalies, suggesting that a greener boreal evergreen forest may prolong and amplify heatwaves significantly. This generates a positive feedback mechanism that begins with higher LAI in generally warmer years in these temperature-limited regions, thus sustaining the strong positive heatwave-LAI correlation. In contrast, for temperate needleleaf forests, temperate deciduous forests and grasslands, strong positive latent heat anomalies with cloud, precipitation and evaporative cooling feedback during high-LAI years appear to suppress heatwaves regionally. Our study revealed the interannual relationships between heatwaves and vegetation as well as the underlying energy exchange processes for different vegetation types and background climate conditions, with implications for the management of forest resources in view of worsening heatwave severity under the future climate.