Diversity of forest management promotes parasitoid functional diversity in boreal forests

Abstract

Intensive forest management leads to forest homogeneity and compromises biodiversity conservation for the sake of a single commodity provision: wood biomass. However, forest biodiversity supports multiple ecosystem functions, with the regulation of insect pest populations among the less understood. We studied how parasitoid functional diversity and composition were affected by forest management and the application of harvesting practices directed to the promotion of biodiversity through the emulation of natural disturbances, i.e. prescribed burning and retention forestry. We used a large-scale, replicated ecological experiment initiated in Finland in 2000, where forest structure was manipulated with several harvesting regimes in combination with prescribed fire. We sampled parasitoid flies (Diptera: Tachinidae) attacking herbivorous insects, and compared their functional diversity and trait composition between distinct habitats shaped by management and in association with plant functional composition. At the local scale, parasitoid taxonomic and functional diversity were positively related to plant functional diversity. Variation in parasitoid functional composition was linked to the different habitats created by forest management and was associated with plant functional composition. Diversity of forest management, involving prescribed fire, variable tree retention and the preservation of old-growth forest, increases habitat diversity in boreal forests. This management leads to higher forest heterogeneity at multiple spatial scales, increasing plant structural diversity and cascading up to higher trophic levels, resulting in higher parasitoid functional diversity. This result gives further support for the importance of preserving natural early successional and old-growth boreal forests, with potential benefits of improved natural biological control across the forest landscape.