Indirect plant-trait mediated effects of tree diversity on herbivores

Levels of herbivory in plant species mixtures are often lower than in plant monocultures and stand diversification is recommended as a strategy to reduce pest damage in agriculture and forestry. Several mechanisms can explain this associational resistance phenomenon in mixtures, e.g. host plant dilution and increase in natural enemies. But for some herbivores plant mixtures represent preferred habitats and their damage is higher in mixed stands than in monocultures (associational susceptibility). In order to use plant diversification effectively as a strategy to reduce insect pests, we need to be able to predict when associational resistance or associational susceptibility is likely to arise. This project will explore a novel mechanism which might potentially explain variation in pest responses - variation in leaf quality in species mixtures due to interactions with intraspecific neighbours (e.g. through shading). This project will explore leaf traits, including chemical and physical defences, as well as nutrient levels, on focal trees in species mixtures and monocultures using several tree diversity experiments (Satakunta experiment in Finland and ClimateMatch experiment in the UK). Associated levels of herbivory as well abiotic variables such as light levels will also be measured. A meta-analysis of published studies which explored the effects of tree diversity on plant quality for herbivores will also be conducted. The project will enable generalisations as to the conditions when plant quality in tree mixtures is likely to increase causing associational susceptibility instead of associational resistance. The project will provide student with meta-analysis and field work skills.