Modelling Growth and Yield Response to Thinning in Quercus robur L. Stands in NW Spain

Thinning is a key silvicultural practice for managing forests; however, its effects on stand growth and yield remain debated. This study evaluated the growth and yield responses to thinning in even-aged Quercus robur stands in Galicia (NW Spain) using data from three long-term thinning trials established between 1998 and 1999. A randomised complete block design was applied with four thinning intensities from below: control (C, 0% basal area removal), light (L, 15%), moderate (M, 35%), and heavy (H, 55%). Two complementary analytical approaches were implemented using linear mixed-effects models: a state-space approach examining post-thinning stand dynamics and a thinning-effect approach assessing the cumulative stand growth and yield, accounting for both standing and harvested components. The state-space analysis confirmed that thinning produced distinct stand structures in moderate and heavy treatments (M and H), with the largest differences observed in the stand basal area and trees per hectare, while the dominant height remained unaffected. In the thinning-effect approach, the cumulative basal area and volume—excluding and including mortality—followed the pattern L > C > M > H. Overall, the results indicate that light or moderate thinning intensities maintain stand yield and enable intermediate harvests. At the same time, although the mean diameter increased under more intensive thinning, differences in the dominant diameter—approximating potential future crop trees—were not significant, indicating that stronger thinning from below did not necessarily enhance the development of the dominant trees.