Abstract

Plant pests, particularly non-native species, can have a profound impact on their host plants. Trees are the foundation of forest ecosystems and serve as important structures in agricultural landscapes and urban areas. In Sweden, forests cover around 70 % of Sweden’s land area and serve as an important national resource for the economy and for biodiversity. Consequently, introductions of non-native tree pests can cause significant economic and environmental damage.
Pest risk analysis (PRA) provides the foundation and scientific evidence to inform decisions to mitigate and manage the introduction of non-native plant pests into new areas. PRA involves structured and standardized procedures to assess the risks posed by non-native pests to a country or territory. There are general guidelines on the assessment of impact, including economic, environmental, and social aspects, as well as both direct and indirect effects.
Regarding environmental impacts, two major constituents of a natural ecosystem are highlighted in environmental impact assessments of plant pest invasions; biodiversity and ecosystem services. However, there is no consensus of how environmental assessments in PRAs should be performed, and different methods are used in different assessments schemes. Overall, within pest risk assessments, there is a need for a more comprehensive understanding of the environmental consequences of plant pest invasions.
The aim of this review is to summarize the main values provided by trees and compile the available information on the biodiversity and the ecosystem services to which trees contribute. All major tree species and genera found across various landscapes in Sweden were included in the report, including forests, agricultural land, and urban environments.
In terms of biodiversity, we define values at risk based on the number of native species associated with individual tree species. We also identify valuable habitats where trees play an important structural role, such as old-growth forests, protected forest areas, and habitat types as defined in the Habitat Directive. Concerning ecosystem services, we summarize the contributions of trees within ecosystems identified as important for providing these services. These ecosystem services primarily cover provisioning, regulating, and cultural services. The contribution of trees for these services are described across various ecosystems, including forests, agro-ecosystems, semi-natural grasslands and urban areas. When possible, we present case studies that investigate the effects and magnitude of impacts that tree pests and pathogens have on biodiversity and the provisioning of ecosystem services. These examples, along with the reviews of the main ecosystem services provided by trees, aims to provide guidance for estimating risks in the context of pest risk assessments.