Forest inventory-based assessments of the invasion risk of Pseudotsuga menziesii (Mirb.) Franco and Quercus rubra L. in Germany

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.

Site‐specific risk assessment enables trade‐off analysis of non‐native tree species in European forests

“…In addition, repeated inventories can be used to determine how the state of NNT is developing over time. Such data can be derived from regional and national forest inventories (NFI), regional datasets covering protected areas (e.g., Bindewald et al, 2021 ; Oswalt et al, 2015 ; Steinmetz & Bauhus, 2016 ; Verheyen et al, 2007 ; Wagner et al, 2017 ), or from assessment reports of habitats valuable for nature conservation (e.g., EU Habitats Directive, Campagnaro et al, 2018 ). However, the usefulness of this method is limited to NNT that have already reached a certain distribution (Klemmt & Neubert, 2011 ).…”

“…For example, concern has been expressed that red oak competes with native tree species in semi‐natural oak forests in Germany (Nehring et al, 2013 ). Although a recent study showed that red oak has spread into certain protected oak forest communities in southwest Germany, there is still insufficient data on the stage of establishment and the actual (long‐term) impacts on biodiversity in these sensitive ecosystems to draw firm conclusions, for example, on its competitiveness (Bindewald et al, 2021 ).…”

“… Silvicultural adaptation strategies should focus primarily on NNT that currently pose no risks, or NNT that pose risks only in some environmental contexts, and these risks can be controlled (NNT of group b and c, Figure 2 ). At the same time, these NNT need to be continuously monitored, for example, as part of forest inventories (Bindewald et al, 2021 ). Forest authorities should assess and report the extent and distribution of NNT, as well as their actual or potential effects.…”

“… Site : A location, habitat, or ecosystem type characterized by a specific assemblage of species, a specific abiotic environment (Bland et al, 2018 ), and a specific objective of management (Nyssen et al, 2016 ). If there are multiple different sites (e.g., forest communities), it may be useful to aggregate them into certain groups (e.g., major forest types) that could be relevant for understanding the context‐dependence of impacts (Bindewald et al, 2021 ). Site‐specific risk assessment (SSRA) : A stratified assessment of risks posed by NNT, which distinguishes between different locations, habitats, or ecosystem types (herewith defined for the first time).…”

Morphological variability of Douglas fir (Pseudotsuga menziesii (Mirb.) Franco) cones in the context of seed extraction

“…Most of the introduced tree species in Poland originate from North America (McDowell et al 2000) and Asia, and some from Europe (Szymanowski 1959, Białobok and Chylarecki 1965, Bellon et al 1977, Feliksik and Wilczynski 2003, St Clair et al 2005), e.g. Italy (Marziliano et al 2015, Ravaioli et al 2019, Marchi and Cocozza 2021, Germany (Marziliano et al 2015, Bindewald et al 2021, and Poland (Giedrowicz et al 2020).…”