Community-based biotic effects as determinants of tree resistance to pests and pathogens

In the light of the global increase in forest pests and diseases, accompanied by withdrawal of chemical pesticides and fungicides, we review recent advances in how forest tree resistance to pests and diseases might be influenced by manipulation of characteristics of the trees' community of associated plants and other organisms. These include associational resistance (AR), effected by manipulation of tree species composition or richness, genotypic diversity, or by tree density/frequency of resistant species or genotypes, use of hybrids, and strategic use of bacterial and fungal symbionts to engender systemic induced resistance, or defense priming. Due to the rotation times in forest production systems, none of the community-based actions that attempt to disrupt a tree-pest or tree-pathogen interaction, can be used in a rapidly mobilised targeted response to currently known pests or pathogens or to unknown ones that are yet to emerge. The only exception is the possible use of non-pathogenic or mutualistic organisms to induce systemic resistance or prime tree defence systems against attack. Due to the range of permutations of participating species and assemblages and the idiosyncratic nature of their ecological interactions and processes, it is not currently possible to formulate predictive rules to protect forests using only these community-based methods. It is however possible to use them as part of a long-term prophylactic strategy by structuring future forests to increase their general resistance, and reduce the probability of impacts of pests and diseases, e.g. via diversification of tree species and genotypes.