Fig. 5

Conceptual illustration of the optimal range of light availability levels to promote regeneration of desired tree species in forest understories. The range of light levels and ontogenetic development covered in different studies on sessile oak regeneration is depicted in the form of boxes that span the observation gradients; the dashed vertical line serves as a reference for tree age representing a time period of 15 years after regeneration initiation. 1 = Brezina and Dobrovolny (2011); 2 = Hauskeller-Bullerjahn (1997); 3 = von Lüpke (1998); 4 = von Lüpke and Hauskeller-Bullerjahn (1999); 5 = von Lüpke and Hauskeller-Bullerjahn (2004); 6 = Ligot et al. (2013); 7 = Annighöfer et al. (2019); 8 = Jaloviar et al. (2014); 9 = Modrow et al. (2020). Light levels ought to be greater than the light compensation point of the desired species to prevent seedling mortality. Light availability should also not exceed levels associated with maximum growth rates (light saturation) of the desired species to prevent conditions that potentially favour competing, more light-demanding species over the desired one. The light levels depicting maximum height growth and light compensation point follow measurements of authors 5, 6 and 9. As indicated by the question marks at the right end of the curves, the precise light levels for these two curves at every age/size of oak plants, which also depends on other stresses (water, nutrients, pathogens, herbivory), are not known and are also not decisive for this conceptual figure