The impact of forest canopy structure on the understory microclimate: a physics-based approach and sensitivity analysis

Forest canopies can buffer or amplify macroclimate temperature extremes in their understory depending on structural and ecological parameters such as canopy height, canopy openness, and species composition. Forest management practices that alter these parameters have a strong impact on understory temperature extremes, with potential repercussions on forest resilience to climate change. Physics-based microclimate models offer a means to explore the effects of forest canopy structure on understory temperatures. Compared to empirical models, they allow to extrapolate results outside the range of canopy structure and climatic conditions that have been observed so far. Although physics-based models may differ in their representation of soil-vegetation-atmosphere interactions, they all rely on a priori knowledge of meteorological data above the forest (also called climate forcing), including air temperature, relative humidity and wind speed timeseries. This raises challenges when conducting sensitivity analyses on structural parameters because canopy structure influences the microclimate not only inside and below the forest canopy but also above it. Employing the same climate forcing for all scenarios of canopy structure is thus deemed inappropriate and adjustments of climate forcing must be accounted for. In this study, we propose a new physics-based modelling approach to perform sensitivity analyses of canopy structure on understorey microclimate that incorporates the feedback of a change in canopy structure on the climatic conditions above it. This approach relies on existing theories on the similarity of turbulent flux-gradient relationships within the atmospheric surface boundary layer between different scalars (e.g. air temperature and humidity) and wind speed, adapted to rough surfaces such as forest canopies. This approach is tested against datasets collected in various forest ecosystems across Europe and applied to explore the impact of canopy structure, in particular canopy density and clumping, on understory microclimate. Our approach will advance our understanding of the intricate relationships between canopy structure, boundary layer dynamics and microclimate, offering insights for more effective forest management strategies.