Reversing forest regression: Effects of shrub removal and grazing on thermophilous oak forests herb layer (Central Poland)

Abstract
Thermophilous oak forests rank among the most species-rich forest ecosystems in Central Europe. Preserving their centuries-old, human-shaped biodiversity now requires active management.

This study evaluates the effects of three restoration treatments – shrub removal, controlled grazing, and their combination – on the herb-layer composition of an oak forest. Research was conducted on permanent plots before and after three consecutive years of treatment, assessing changes in species frequency in relation to their functional traits.

All treatments contributed to partial restoration of the characteristic herb-layer composition, with the strongest response in the combined variant and the weakest in the control plots. Initial thinning, applied to enhance light availability, increased species frequency across all variants, though effects were much stronger under management. Species responding most positively included hemicryptophytes and geophytes, mainly CSR strategists, shaded forest and oak forest species, light demanding species and species tolerant to disturbances. All this changes indicate a shift in the composition of the herb layer towards thermophilous oak-forest.

Overall, the combination of shrub removal and grazing proved most effective in restoring the herb-layer composition, while thinning alone produced similar but weaker effects. The historical proximity approach may support short-term forecasting of changes in species frequency.