Higher soil moisture increases forest temperature buffering

Forests can buffer the understory against temperature extremes often creating cooler microclimates during warm summer days compared to temperatures outside the forest. The buffering of maximum temperatures in the understory results from a combination of canopy shading and air cooling through soil water evaporation and plant transpiration. Therefore, buffering capacity of forests depends on canopy cover and soil moisture content, which are increasingly threatened by more frequent and severe canopy disturbances and soil droughts. The extent to which this buffering will be maintained in future conditions is unclear due to the lack of understanding about the relationship between soil moisture and air temperature buffering in interaction with canopy cover and topographic settings. We explored how soil moisture variability affects temperature offsets between outside and inside the forest on a daily basis, using temperature and soil moisture data from 57 sites in temperate broadleaved forests in Central Europe over four climatically different summer seasons. Daily maximum temperatures in forest understories were on average 2°C cooler than outside temperatures. The buffering of understory temperatures was more effective when soil moisture was higher, and the offsets were more sensitive to soil moisture on sites with drier soils and on sun-exposed slopes with high topographic heat load. Based on these results, it can be expected that, in a warmer climate with longer dry periods, the soil-water limitation of forest buffering will become more prevalent and will likely lead to changes in understory communities. Our results support the inclusion of soil moisture in models and predictions of forest microclimate, understory biodiversity and tree regeneration, aiming to provide a more precise estimate of the effects of climate change.
This research was funded by FORMAS [project 2021-01993 to CG], the Czech Science Foundation [projects GACR 20-28119S and GACR 23-06614S] and the Czech Academy of Sciences [project RVO 67985939]. We thank everyone helping with the data collection.