Vapour Pressure Deficits in Mixed Beech and Oak Forests of the Western Carpathians, Romania

Central European mixed beech-forests will suffer from higher temperatures and dryer conditions during climate change in subsequent decades. To understand the dominance tipping point from beech to oak, we conducted vpd-measurements in mid-summer 2019 along three hypsometrical transects in the natural beech-oak ecotone forests of Milova, Maciova, and Eşelniţa. Air temperature and humidity were measured in the interior of oak, two ecotone, and beech forests, in 2 m above ground (understorey) to calculate vpd-values. Additionally, the vpds of the herb layer (0.5 m) and the shade canopy (7 m) were measured in the same oak and beech forests. The day-time mean on radiation days were by 2.2 hPa higher in the understorey oak forests than the beech forests (oak 11.3; ecotone1 13.5; ecotone2 10.4; beech 9.1). Especially in the second half of the day, oak forests are more xeric than beech forests, while the vpd-extremes differ even more. Mean vpds of the shade canopy (oak 12.7; beech 9.7) as well as of the herb layer (oak 10.8; beech 7.7 hPa) are also significantly different between oak and beech forests. Furthermore, shade canopy and herb layer are significantly different from the respective understorey (p<0.001), besides that of the beech-herb-layer. Among these microclimatic tipping points, beech is losing its dominance for the benefit of oaks, a fact that will probably also takes place with time in western Central European forests with strictness of laws of nature.