The effects of local neighbourhood diversity on pest and disease damage of trees in a young experimental forest

Pests and diseases causing defoliation and crown discolouration are important risks threatening the vitality of forests, especially in the early stages of forest development. Mixing different tree species in a forest stand has been described as a possible solution to tackle this problem through the mechanism of associational resistance. However, most research up till now has focused on mature forests. We assessed three different damage symptoms related with tree crown condition, i.e., branch and shoot damage, defoliation, and crown discolouration, for nine common tree species at two sites of a recently established tree species diversity experiment in Belgium. The assessment was done in two subsequent years. A tree's damage degree was influenced by the site characteristics and the timing of the assessment, and the species identity of the target tree was more important than the effect of local neighbourhood diversity per se in explaining a tree's damage degree. Our results only partially support the hypothesis that trees in more diverse young plantations show less crown damage. Nevertheless, some particular mixtures resulted in reduced damage degrees.