Experimentally disentangling drought from heat stress effects in European beech trees

Hot summer droughts are becoming increasingly frequent in temperate biomes world-wide. In summer 2018, several weeks of drought paired with constantly high temperatures led to significant forest decline and mortality in mature trees in central Europe. In many regions, European beech was one of the most drought-sensitive tree species in 2018 with many trees showing partial or complete crown dieback by the end of the year, preceded by early leaf browning in mid-summer. So far it is disputed, if these symptoms were solemnly driven by a direct drought effect from dried-out soils, or if they were additionally amplified by hot mid-day temperatures and high atmospheric vapor pressure deficits (VPD).

In a recent study we therefore aimed to disentangle the effects of soil drought, air temperature and VPD on beech seedlings in a full-factorial experiment. Two-year old beech saplings were exposed to either cooler (daily max. temperature 24°C) or warmer (daily max. temperature 35°C) temperatures in walk-in phytotrons. Within each temperature treatment, half of the saplings were grown at either high or low relative humidity resulting in the same low or high mid-day VPD in both temperature treatments (0.7 vs. 2 kPa).Finally, half of the saplings from each temperature-VPD treatment combination where exposed to drought by stopping irrigation in July, with only compensatory water addition among pots to ensure the same dry-out rate among treatments (i.e. no faster soil drying at high VPD). Across all temperature-VPD treatment combinations, drought led to very similar decreases of stomatal conductance, photosynthesis and pre-dawn leaf water potential. There was only a tendency for an approximately 1 week earlier decline of F_v/F_m (leaf fluorescence) under drought at high VPD conditions, pointing at slightly faster stress occurrence at the leaf-level at very low air humidity. Neither high temperatures, nor high VPD induced stress symptoms at the leaf or whole-tree level in well-irrigated saplings. Overall, air temperature and VPD had no significant effect on the saplings' survival time under drought, with saplings from all temperature-VPD combinations dying on average after about 100 days into drought. Therefore, at the same decreasing rate of soil water availability, we did not find additional negative effects of warm temperatures or high VPD on tree survival, indicating that European beech is per se quite robust against heat and high VPD stress.