Background
Non-native tree species (NNT) whose natural range is outside Europe have long been part of the cultural development of the European forest landscape, providing numerous benefits as well as posing risks to biodiversity and other ecosystem services. On the one hand, NNT are valued for their timber properties, high growth rates and resistance to drought for improving forestry or adapting forests to climate change. On the other hand, they can potentially negatively impact the environment and economy, particularly when they spread into protected areas, become established there and can only be controlled at great expense under risk management. Given their (potential) negative impacts, some NNT are classified as invasive in multiple European countries based on the results of risk assessments. The ones classified as invasive may be included in regional, national, or EU legislation, which may result in imposing restrictions on their cultivation. However, the methods applied in risk assessments across Europe were not specifically developed for NNT, and countries differ in their approaches. Existing methods may therefore not sufficiently identify the ecological risks associated with NNT unless they explicitly address the characteristics of tree species and site-specific aspects of forest management. Moreover, country-specific approaches may hamper the harmonisation of information and hinder risk assessments that extend across European borders. No studies to date have investigated the methods of risk assessments for NNT used in European forestry. Since NNT can have both risks and benefits, a careful and scientifically sound risk assessment is thus vital to provide clear evaluations for management, policy decisions and scientific purposes.
Research objectives
The main purpose of this thesis was to improve risk assessment approaches for the use of NNT in European forestry. In this context, the first objective was to review current risk assessment methods in Europe in terms of their suitability in identifying the ecological risks associated with NNT, thereby supporting both forest and risk management decisions (first objective). Based on the analysis of existing methods, it became obvious that it is necessary to improve the data base for risk assessments, e.g. by using forest inventory data (second objective), and to establish new criteria for assessing the risks of NNT (third objective). In summary, the main research objectives were as follows:
(1) analyse the methods of existing risk assessment schemes in Europe for their practical applicability and consistency for potentially invasive non-native tree species.
(2) strengthen the evidence base for risk assessments of widespread non-native tree species in Europe, using systematically collected data from forest inventories.
(3) develop a risk assessment method that permits a more generalisable consideration of the costs and benefits of using non-native tree species in forests.
Methods
(1) Analysis of existing risk assessment tools
Several risk assessment tools currently used in Germany and neighbouring countries were analysed for their practical applicability and consistency using four NNT (Fraxinus pennsylvanica Marsh., Paulownia tomentosa (Thunb. Ex Murray), Pseudotsuga menziesii (Mirb.) Franco, and Quercus rubra L.) as case studies. Literature reviews were carried out to collect the required information on the invasion biology of the four NNT in Europe. Different methods were applied by assessing the tools’ criteria for each NNT using the information derived from the literature review based on the same reference area (Germany).
(2) Strengthening the evidence base for risk assessments
The relevance of using large scale forest inventory data for risk assessments was demonstrated using the two potentially invasive NNT, Quercus rubra and Pseudotsuga menziesii, in Germany as case studies. For this purpose, the establishment success of natural regeneration was quantified in terms of cover and height classes using national forest inventory data for Germany. The current extent of spread into protected forest habitats was investigated using a regional dataset for the State of Baden-Württemberg (south-west Germany).
(4) Developing a risk assessment method for NNT
First, basic principles and steps were identified and formulated for the development of a new methodological framework in order to assess the risks of NNT. Subsequently, four workshops were conducted with interdisciplinary groups of experts, public authorities, and stakeholders from the areas of forest conservation, silviculture, and nature conservation. Workshop participants were encouraged to evaluate each component of the proposed method and suggest improvements.
Results and discussion
Using different tools to classify risks for the same NNT yielded inconsistent results for all NNT. Different criteria are used in the methods and/or similar criteria are weighted differently. In most cases, no differentiation is made between the risks posed by NNT at different sites and ecosystem types. When data quality was poor, the precautionary principle (of considering only the worst observed effect) was typically applied without ranking the available ecological studies by their evidence. As a result, observations of small case studies are often extrapolated to large spatial scales by providing one single risk classification, i.e., typically ‘invasive’ or ‘potentially invasive’. Such a single undifferentiated risk classification is unlikely to provide meaningful guidance for a wide range of different ecosystems and regions.
Large-scale forest inventories can provide valuable data across a range of different forest types to support the risk assessments of widespread NNT in forests. Based on the assessment of Pseudotsuga menziesii and Quercus rubra, there was no evidence of high establishment and spreading potential for the majority of forest types in Germany. Natural regeneration of both NNT has been reported in a small proportion of protected forest habitats. Semi-natural forests with sufficient light in the understory and competitively inferior tree species can be considered most sensitive to invasion. To mitigate any potential negative effect of both NNT, management approaches may involve buffer zones around sensitive ecosystems. When natural regeneration of NNT is systematically recorded, the approach could also be applied in other countries or regions.
A new methodological framework was developed to mitigate risks associated with the use of NNT in European forestry while taking advantage of their ecosystem services. In contrast to the previously developed risk assessment approaches, the proposed method takes different ecosystem sensitivities to NNT into consideration as well as existing silvicultural methods to control or exclude potential risks. The framework comprises eight steps and is based on the existing knowledge as well as collecting new data. In addition to the use of the proposed method, several changes of environmental policy and forest management are recommended in order to achieve positive outcomes in the sustainable management of NNT.
Conclusions
The analysis of existing risk assessment tools (first objective) has shown that the results of the different risk assessment methods applied in Central Europe cannot be used as a reliable decision support tool for both forest and risk management of NNT. To strengthen the evidence base for risk assessments (second objective), forest inventories can provide important data for assessing the establishment and spreading potential of widespread NNT across a range of sites, thus identifying sensitive ecosystem types. The risk assessment criteria developed in this thesis enable NNT with a low current risk to be identified and considered for planting. The criteria thus provide a framework for integrating risk mitigation into forest management and represent an important step towards reliable, Pan-European risk assessments of NNT (third objective).
The knowledge derived from such risk assessments should be made available for various stakeholders. In addition, clear communication is necessary between practitioners, policymakers, and the public about the risks of NNT regarding different forest types, sites and regions, as well as available management options and uncertainties in the data. At the same time, further monitoring of NNT and more research on potential impacts are required to continuously improve the information basis for risk assessments. To strengthen the benefits of NNT while mitigating their risks, new political approaches based on unifying principles are needed in Europe. These issues need to be addressed to arrive at risk assessments that are of high practical value for the responsible use of NNT in European forestry.