Abstract
In Europe, some non-native tree species (NNT) are classified as invasive because they have spread into semi-natural habitats. Yet, available risk assessment protocols are often based on a few limited case studies with unknown representativeness and uncertain data quality. This is particularly problematic when negative impacts of NNT are confined to particular ecosystems or processes, whilst providing valuable ecosystem services elsewhere. Here, we filled this knowledge gap and assessed invasion risks of two controversially discussed NNT in Germany (Quercus rubra L., Pseudotsuga menziesii (Mirb.) Franco) for broad forest types using large scale inventory data. For this purpose, establishment success of natural regeneration was quantified in terms of cover and height classes. The current extent of spread into protected forest habitats was investigated in south-west Germany using regional data. Establishment was most successful at sites where the NNT are abundant in the canopy and where sufficient light is available in the understory. Natural regeneration of both NNT was observed in 0.3% of the total area of protected habitats. In forest habitats with sufficient light in the understory and competitively inferior tree species, there is a risk that Douglas fir and red oak cause changes in species composition in the absence of management interventions. The installation of buffer zones and regular removal of unwanted regeneration could minimize such risks for protected areas. Our study showed that forest inventories can provide valuable data for comparing the establishment risk of NNT amongst ecosystem types, regions or jurisdictions. This information can be improved by recording the abundance and developmental stage of widespread NNT, particularly in semi-natural ecosystems.
Highlights
Humans have historically influenced forests in many different ways
The number of protected habitats with established non-native tree species (NNT) may be higher than originally indicated (e.g. Hys 2018). This is one of the first studies in Europe that analyses systematic forest inventory data to assess natural regeneration of NNT in different forest types to carry out a risk assessment
Based on the large-scale assessment, we found no evidence for high establishment and spread potentials of Douglas fir and red oak seedlings in the majority of forests in Germany
Summary
Humans have historically influenced forests in many different ways. One of these influences is the change in tree species composition through introduction and promotion of non-native tree species, i.e. with a natural range outside Europe (hereinafter NNT). Locally observations of negative impacts are generalised and extrapolated to larger spatial scales by providing one single risk classification (Bartz and Kowarik 2019; Bindewald et al 2020) These uncertainties in methods can hamper responsible NNT management, when a species has negative impacts in one type of ecosystem, whilst providing valuable ecosystem services elsewhere (e.g. Robinia pseudoacacia L.) (Vítková et al 2017). If data are poor, invasiveness is frequently assumed following the precautionary principle, but without clear scientific evidence (Aven 2011; Davis et al 2011; Strubbe et al 2011) Such approaches are not transparent and reliable (Bartz and Kowarik 2019), and they can exacerbate conflicts of interest in the use and management of NNT (e.g. Ammer et al 2014; Dickie et al 2014; van Wilgen and Richardson 2014; Vítková et al 2017)
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