Abstract
Abstract. In the context of the first phase of the Coordinated Regional Climate Downscaling Experiment in the European domain (EURO-CORDEX) flagship plot study on Land Use and Climate Across Scales (LUCAS), we investigate the biophysical impact of afforestation on the seasonal cycle of soil temperature over the European continent with an ensemble of 10 regional climate models. For this purpose, each ensemble member performed two idealized land cover experiments in which Europe is covered either by forests or grasslands. The multi-model mean exhibits a reduction of the annual amplitude of soil temperature (AAST) due to afforestation over all European regions, although this is not a robust feature among the models. In the Mediterranean, the spread of simulated AAST response to afforestation is between −4 and +2 ∘C at 1 m below the ground, while in Scandinavia the inter-model spread ranges from −7 to +1 ∘C. We show that the large range in the simulated AAST response is due to the representation of the summertime climate processes and is largely explained by inter-model differences in leaf area index (LAI), surface albedo, cloud fraction and soil moisture, when all combined into a multiple linear regression. The changes in these drivers essentially determine the ratio between the increased radiative energy at surface (due to lower albedo in forests) and the increased sum of turbulent heat fluxes (due to mixing-facilitating characteristics of forests), and consequently decide the changes in soil heating with afforestation in each model. Finally, we pair FLUXNET sites to compare the simulated results with observation-based evidence of the impact of forest on soil temperature. In line with models, observations indicate a summer ground cooling in forested areas compared to open lands. The vast majority of models agree with the sign of the observed reduction in AAST, although with a large variation in the magnitude of changes. Overall, we aspire to emphasize the biophysical effects of afforestation on soil temperature profile with this study, given that changes in the seasonal cycle of soil temperature potentially perturb crucial biochemical processes. Robust knowledge on biophysical impacts of afforestation on soil conditions and its feedbacks on local and regional climate is needed in support of effective land-based climate mitigation and adaption policies.
Highlights
There is currently a strong policy focus on afforestation as a possible greenhouse gas mitigation strategy to meet ambitious climate targets (Grassi et al, 2017)
In order to explain the inter-model spread in annual amplitude of soil temperature (Sect. 3.4), we examine the changes in surface energy balance components with respect to differences in landuse parameters across Regional climate models (RCMs) (Sect. 3.2) and the response of soil moisture content to afforestation in summer (Sect. 3.3)
In CCLM-TERRA, CCLM-VEG3D, CCLM-CLM4.5, CCLMCLM5.0 and RegCM-CLM4.5, the soil heating is decreased with afforestation in summer over the Mediterranean and Scandinavia, because the increased available radiative energy is compensated by the increased sum of turbulent heat fluxes
Summary
There is currently a strong policy focus on afforestation as a possible greenhouse gas mitigation strategy to meet ambitious climate targets (Grassi et al, 2017). In the context of the Land-Use and Climate, IDentification of robust impacts (LUCID) model intercomparison project, de Noblet-Ducoudré et al (2012) diagnosed the LUC effects over North America and Eurasia between the present and the pre-industrial era They found that deforestation caused a systematic surface albedo increase across all seasons, leading to a reduction in available energy accompanied by a decrease in the sum of turbulent fluxes. Lejeune et al (2015) used a state-of-the-art RCM to explore the biophysical impacts of possible future deforestation on Amazonian climate They demonstrated that the projected land cover changes for 2100 could increase the mean annual surface temperature by 0.5 ◦C and decrease the mean annual rainfall by −62 mm yr−1 compared to present conditions. We compare the simulated impact on annual amplitude of soil temperature (AAST) with observational evidence based on FLUXNET paired sites, classified as forest or open land (Sect. 3.5)
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