Vertical temperature and air humidity gradients in beech and oak forests, and the forest interior climate created by beech

Abstract

Forests dominated by European beech are characterized by a humid and relatively cool forest interior climate with low light intensities on the forest floor, which harbors an adapted herb layer vegetation. With the aim to quantify the air temperature and humidity (RH) conditions in the beech forest interior in direct comparison to the conditions in thermophilic oak forests at the drought-induced range limit of beech, we conducted air temperature and RH measurements in mid-summer at three height levels (7 m, 2 m, and 0.5 m above ground) in 12 forest stands in the natural beech-oak ecotone in Western Romania. On cloudless mid-summer days, the shade canopy layer of the oak forests (7 m) was at noon usually up to 3 °C warmer than the beech shade canopy; the daily maxima differed even by up to 6 °C (extremes: 32.1 °C and 27.5 °C in the beech and oak shade canopies, respectively). RH reached by about 4 percentage points lower values in the shade canopy of the oak forests than in the beech forests; the RH minima differed by up to 9 percentage points (28.2 vs. 37.4%). Vapor pressure deficit (VPD) was between noon and midnight by 2–6 hPa higher in the oak than in the beech shade canopy; even more differed the VPD maxima (38.7 vs. 22.6 hPa). We conclude that closed beech forest canopies have a considerable microclimatic buffering capacity that can reduce the severity of climate warming impacts on the vitality of the herb layer and the beech shade crown.