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.
Highlights
European oak species are dominant species in temperate European hardwood forests [1] and among the main components of the European forest economy [2]
We identified 100 adult dominant oak trees with canopies comparably exposed to sun radiation: 50 individuals of non-infested oaks and 50 individuals infested by the hemiparasitic yellow mistletoe (Loranthus europaeus Jacq.), which were comparable for both, the height and diameter
We observed a significant reduction of leaf water potential, transpiration rate and stomatal conductance in infested oak trees, which resulted in the decrease of their CO2 assimilation rate as well
Summary
European oak species are dominant species in temperate European hardwood forests [1] and among the main components of the European forest economy [2]. With average air temperatures significantly rising and significant changes in precipitation regime in Europe over the last several decades [6], conditions at higher altitudes have become more suitable for these species [7]. They have been more often negatively affected and reports about the decline of oak stands suggest that multifactorial processes are responsible for the increased damage [8]. Inappropriate oak forest management [9], cold winters [10] and soil nutrient imbalance [11]
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