Harrowing promotes Scots pine seedling establishment more effectively than mounding under herbivore exclusion in a southern Swedish field trial

In Sweden, browsing by large herbivores hampers seedling establishment and causes substantial growth losses in economically important, regenerating Scots pine (Pinus sylvestris) stands. In addition, suboptimal site conditions often require mechanical site preparation (MSP) to promote seedling establishment by improving substrate quality and reducing vegetation competition and pest damage. Here, we examined the interaction between herbivore exclusion fencing and regeneration method (natural, seed-sown) on sapling height and radial growth, as well as the interplay between exclusion fencing and MSP (harrowing, mounding) on the growth of planted seedlings. There was no significant interaction between regeneration method and fencing for either of the two growth variables (height, radial growth). Both regeneration techniques resulted in similar growth performance regardless of fencing. Fencing caused greater height (+17%) and radial growth (+7%), irrespective of regeneration method. A significant interaction between MSP and fencing was observed for both height and radial growth. Without fence, height and radial growth were similar in both MSP methods. With fencing, harrowing stimulated seedling height growth by 25% and radial growth by 48% compared to mounding, which exhibited similar growth patterns as mounding without a fence. Our findings highlight the interplay between MSP and herbivore exclusion and indicate that harrowing is far superior to mounding when fencing is technically and economically feasible. This study provides actionable guidance for forest managers aiming to promote Scots pine more prominently in the future.