Are microclimate extremes always buffered in the understory, irrespective of tree density? Consequences for tree seedling survival.

Tree seedlings have their leaves very close to the ground and their roots are very shallow. They therefore experience more severe heat and water stress during hot summers than mature trees. As droughts and heatwaves increase in severity and frequency, the growth and survival of tree seedlings thus become more difficult, impairing forest regeneration in many regions. In response, forest managers are increasingly shifting from thinning regimes that promote light availability and seedling growth to regimes that promote seedling survival and the buffering of climate extremes. However, the identification of such thinning regimes is not trivial because the mechanisms underpinning the impact of canopy cover on understory microclimate, although all well understood, can have opposite effects on climate extremes. In particular, wind attenuation and water consumption by the remaining adult trees can sometimes create conditions for seedlings in the understory hotter and drier than in an open field. This has led researchers to hypothesize the existence of thinning thresholds beyond which forest canopies transition from buffering to amplifying climate extremes. Metrics such as leaf area index (LAI), crown aggregation and canopy height have emerged as critical factors, as well as other local factors such as species composition or water availability. Here, we use a physics-based model of forest hydrology, physiology and microclimate (MuSICA), in combination with microclimate observations from a variety of forest types across Europe, to address the following questions. (1) Is the threshold in LAI and/or crown aggregation below which summertime temperature and evaporative demand become amplified in the understory generic, or is this threshold site-specific? (2) How does understory microclimate evolve during heatwaves depending on the structure of the canopy above and the duration of the heatwave? (3) How does this translate in terms of plant water and heat stress for understory species?