Spring and autumn phenology in sub-mesothermal beech stands from the southwestern extremity of the Carpathians

Abstract

The research carried out was aimed at assessing the phenological behavior of beech (Fagus sylvatica L.) in the southwestern area of the Carpathians, in sub-mesothermal forest sites, which are different from the majority mesothermal ones of this species. The data obtained can be used for predicting the phenological changes of the species, especially in the Carpathian area, under the future influence of expected climate change. Assessments for spring and autumn phenology (buds burst-BB and foliation, flowering and leaves senescence) were carried out in 3 phenological research areas located at 200 m, 700 m and 1200 m. At each altitude level, 100 trees of I and II Kraft classes were phenologically characterized: 50 trees on the south-facing slope (sunny exposure) and 50 on the north-facing slope (shade exposure). The phenological data were interpreted in relation to climate data (average temperature) recorded in each area by a HOBO U23 Pro v2 sensor. The site conditions of sub-mesothermal climate in the low altitude area led to DOY (day of the year) for BB (buds burst) values below 100. The phenological differentiation of populations is evident in relation to the altitude, while the influence of the exposure is much lower. The gradient values based on altitude sectors highlighted the nonlinearity of the development of foliation phenophase: 1.6 days/100 m at low altitude, where the beech is under the impact of the sub-mesothermal climate and 4.4 days/100 m between 700 and 1200 m altitude. Thus, it has been proven that the foliation onset depends on the cumulation of temperatures in relation to the DOY moment and on the values recorded throughout the vegetative rest. The altitudinal gradient values which are influencing the flowering also differentiate the stands. The leaves senescence has a delay of 1.8 up to 2.4 days per 100 meters altitude, and the length of the vegetation season is reduced more sharply in the upper half of the analyzed altitudinal transect. Our data may be used for predicting the phenological changes, especially in the Carpathian area, under the expected climate change.