Both day and night warming reduce tree growth in extremely dry soils

Trees in global forests are exposed to warming climate, the rate of which is different between day and night, and associated with soil drought. Previous studies commonly show that forest growth responds positively to daytime warming but negatively to night warming. However, it remains unclear whether such asymmetric responses of forest growth to day and night warming still exist in extremely dry soils. Here, based on the long-term records of the normalized difference vegetation index and ring-width index at 2294 forest sites across the Northern Hemisphere, we found that the rising daytime maximum temperature ( T _max ) reduces stem growth but the rising nighttime minimum temperature ( T _min ) lowers canopy greenness when the soil is drier than a threshold. We further discuss three mechanisms that could drive such negative impacts. For example, data from experimental studies showed that the shifted biomass allocation from wood to leaves is one important mechanism driving the reductions of wood growth under day warming. These findings indicate that climate warming could negatively affect tree growth in extremely dry soils, regardless of whether temperature rises during the daytime or at night. Thus, understanding the interactions of water and temperature on the sub-diurnal scale is critical for improving our ability to predict the forest dynamics under future climate change.