Aerodynamic resistance and Bowen ratio explain the biophysical effects of forest cover on understory air and soil temperatures at the global scale

The microclimate dynamics under forest crown fundamentally drive plant community responses to global warming. The understory air and soil temperatures are two of the most important components of forest understory microclimate. However, there is rare method to reasonably evaluate the joint effects of forest cover on the understory air and soil temperatures. In this study, we combined a novel three-layer energy balance model and intrinsic biophysical mechanism model to evaluate the biophysical effects of forest on air (ΔTa) and soil temperatures (ΔTs) under forest crown at the global scale. Observations from in situ paired expariments and eddy covariance sites from FLUXNET 2015 were used for validations over the globe. The warming effect caused by low albedo and cooling effect caused by large aerodynamic roughness of forest lands mainly explain the global patterns of ΔTs and ΔTa, which indicate mostly a net cooling in low latitudes, but show opposite directions in large parts of temperate and high latitudes. The ratios of aerodynamic resistance of sensible heat fluxes between upper and lower layers show a positive relationship with ΔTs and ΔTa. The Bowen ratio is negatively related to ΔTs, but is positively related to ΔTa, respectively. Additionally, we examined a new indicator, which is composed of both ΔTs and ΔTa and regulated by the aerodynamic resistance parameters, to evaluate the joint biophysical effects of forest on understory air and soil temperatures. This study fills the gap in modeling the biophysical effects of forest on air and soil temperatures under forest crown over the global scale and improves our understanding of the mechanisms governing the biophysical effects of global forest cover on understory microclimate