Key message <p>Heavy thinning from above promotes coarse root and stem diameter growth in both Scots pine and sessile oak. Coarse root-stem allometry indicates a greater allocation of resources to coarse roots relative to the stem in both species.</p> Abstract <p>The effects of heavy thinning from above on stem and coarse root growth, as well as on coarse root–stem allometry, were investigated in mature Scots pine (<i>Pinus sylvestris</i> L.) and sessile oak (<i>Quercus petraea</i> (Matt.) Liebl.) growing in monospecific and mixed stands in Bavaria, Germany. Thinning was conducted in autumn 2017, and growth was analyzed over a five-year pre-thinning (2013–2017) and post-thinning (2018–2022) period using replicated thinned and unthinned plots. Thinning substantially enhanced diameter growth in both species. On average, stem increments increased by 26–61% and coarse root increments by 2–77% relative to unthinned controls, with the relative growth between stems and coarse roots varying among variants. Coarse root–stem allometry showed species-specific responses: in Scots pine, thinning intensity had the strongest effect, whereas in sessile oak, variation was more closely linked to precipitation and water availability. Across all variants, allometric exponents ranged from 1.59 to 7.14, with sessile oak exhibiting higher values than Scots pine, reflecting greater proportional investment in coarse roots among species and thinning regimes. Overall, heavy thinning from above promoted growth and altered biomass partitioning between stems and coarse roots in both species. Stimulated coarse root development likely improved anchorage, water and nutrient uptake, and resilience to climatic stress, thereby strengthening stand stability. These findings highlight the importance of adapting thinning strategies to species-specific traits and site conditions to enhance forest productivity and adaptability under changing climatic conditions.</p>