Impact of deer rubbing on pine forests: implications for conservation and management of Pinus pinaster populations

Deer populations have increased considerably in the last decades, modifying the growth and regeneration patterns of many tree species. However, the impact of antler rubbing (fraying) by deer has received very little attention. Here, we analyze the impact of deer rubbing on pines (tree selection, damage intensity, growth and successful reproduction) in a natural (uneven-aged) Pinus pinaster population where red deer (Cervus elaphus) are abundant. We developed a probability model that shows a nonlinear relationship between dendrometric variables (tree height and diameter) and the probability of rubbing damage. Intermediate-sized trees were clearly selected, reaching maximum probability of being rubbed for trees 1.5 m high and 11 cm basal diameter. However, intensity of rubbing showed a more linear pattern, where small-sized trees were more heavily damaged, followed by medium and large trees. Interestingly, both rubbed and non-rubbed trees significantly grew in basal diameter whereas growth in height and dbh (diameter at 1.3 m high) was strongly negatively affected by rubbing. This dissimilar growth affected plant shape and tree quality and prevented young trees from advancing onto the reproductive stage. Reproduction success (probability of cone production) was strongly reduced (fivefold) when rubbing affected > 25 % of the trunk circumference. Sustainability of rubbing was defined based on plant shape and reproduction success through an increasing 0-5 damage rank. In 6 years the average rubbing damage increased from 3.1 to 4.1, reaching unsustainable levels of rubbing. Recommended management practices include tree protection and adding natural or artificial decoy posts of the most deer-preferred diameters and heights.