No abstract

European oak species have long been considered relatively resistant to different disturbances, including drought. However, several recent studies have reported their decline initiated by complex changes. Therefore, we compared mature sessile oak trees (Quercus petraea (Matt.), Liebl.) infested versus non-infested by hemiparasitic yellow mistletoe (Loranthus europaeus Jacq.) during the relatively dry vegetation season of 2019. We used broad arrays of ecophysiological (maximal assimilation rate Asat, chlorophyll a fluorescence, stomatal conductance gS, leaf morphological traits, mineral nutrition), growth (tree diameter, height, stem increment), and water status indicators (leaf water potential Ψ, leaf transpiration T, water-use efficiency WUE) to identify processes underlying vast oak decline. The presence of mistletoe significantly reduced the Ψ by 1 MPa, and the WUE by 14%. The T and gS of infested oaks were lower by 34% and 38%, respectively, compared to the non-infested oaks, whereas the Asat dropped to 55%. Less pronounced but significant changes were also observed at the level of photosystem II (PSII) photochemistry. Moreover, we identified the differences in C content, which probably reduced stem increment and leaf size of the infested trees. Generally, we can conclude that mistletoe could be a serious threat that jeopardizes the water status and growth of oak stands.

At present, temperate forest ecosystems are endangered by both abiotic and biotic factors. The effects of abiotic components, e.g., meteorological variables, are constantly studied. However, the detailed mechanisms affecting the phenology of plants are still unknown. Two meteorological variables (air temperature and cumulative precipitation) were analysed for the period from 1995 to 2020 in order to determine which factor which has a more significant effect on onset of the full-flowering (FF) phenophase. A set of nine forest herbs, representing different phenological groups from the viewpoint of flowering, was examined (early spring: Petasites albus and Pulmonaria officinalis; mid-spring: Carex pilosa and Dentaria bulbifera; late spring: Fragaria visa and Galium odoratum; early summer: Veronica officinalis; mid-summer: Mycelis muralis; and late summer: Campanula trachelium). Temperature-sum requirements and temporal trends in the onset of FF were also studied. The research conducted at the Ecological Experimental Station in the Kremnické vrchy Mountains (central Slovakia) at an altitude of 500 m asl. Our results show that the air temperature correlated more significantly with the date of onset of FF (r > 0.6, p < 0.001) than with precipitation. On average, the air-temperature sums, calculated for the threshold temperatures of 0 °C and 5 °C, increased from 142.9 °C (Petasites albus) to 1732.9 °C (Campanula trachelium) and from 223.4 °C (Petasites albus) to 1820.8 °C (Campanula trachelium), respectively. Temporal trends in the onset of FF over the last 26 years confirm shifts to earlier dates for most species (excepting early spring Petasites albus). In spring flowering species, shifts ranged from 2 days (0.07 day/year) for Pulmonaria officinalis to 8 days (0.30 day/year) for Carex pilosa. As for summer species, the onset of flowering shifted more significantly to earlier dates—from 7 days (0.27 day/year) for Campanula trachelium to 12 days (0.46 day/year) for Veronica officinalis. The observed trends were statistically significant (p < 0.05) for five examined species (Carex pilosa, Dentaria bulbifera, Fragaria vesca, Veronica officinalis and Mycelis muralis).