Variation in understorey radiation in different types of partial regeneration harvests in mature sessile oak (Quercus petraea [Mattuschka] Liebl.) forests

Understanding the influence of stand structures generated by silvicultural treatments on light conditions is essential for tree regeneration, in particular of light demanding species such as Quercus petraea . Many past studies have quantified average light levels resulting from different types of regeneration harvests. Here, we analysed the impact of regeneration cuts on light heterogeneity, which may be important for competition among regenerating tree species and understorey. To this purpose, we investigated radiation levels and patterns in 57 stands originating from shelterwood harvesting of varying density, and 51 canopy gaps of different sizes in even-aged mature oak forests. Understorey radiation levels were quantified as indirect (ISF), direct (DSF), and total site factor (TSF) at systematically placed sample points. In shelterwoods, strong relationships occurred between ISF, DSF, and TSF and both canopy cover and stand basal area, suggesting that these metrics are suitable indicators of light conditions. In gaps, strong relationships existed between their diameter, area, height of edge trees and average ISF, DSF, and TSF. In gaps > 0.1 ha (in stands of ca. 30 m height), spatial heterogeneity of radiation exceeded that of shelterwoods, suitable to regenerate species with different light requirements. In contrast, shelterwoods provide uniform light conditions corresponding to the current requirements of the prevailing regeneration over large areas. Therefore, shelterwood harvesting is more suitable for regeneration of even-aged oak-dominated stands, whereas gaps are more appropriate for stands where sessile oak is admixed in form of single trees or groups and shall remain an admixed species. • Understorey radiation in shelterwoods can be predicted by stand basal area or canopy cover. • Understorey radiation in canopy gaps can be predicted by gap size and edge tree height. • Light conditions in shelterwoods are more uniform in space but more variable in time than in gaps. • In canopy gaps, spatial heterogeneity of radiation increases with increasing gap size. • Optimal light levels for oak regeneration change abruptly from 20 % to 60 % of light in the open.