Non-native Tree Research Articles

Limited integration of biodiversity within climate policy: Evidence from the Alliance of Small Island States

Climate change and biodiversity loss are deeply intertwined anthropogenic global crises, for which forests provide powerful nature-based solutions. Biodiverse forests are more resilient to climate change than monocultures, thereby enhancing long-term carbon storage and ecosystem-based adaptation. Awareness of these interdependencies is slowly growing, but we know little about how countries are considering biodiversity within climate policies. Island and low-lying coastal states are particularly vulnerable to climate change and biodiversity loss. Here we assessed if and how the members of the Alliance of Small Island States (AOSIS) are integrating biodiversity into their national climate action plans through forest-based climate solutions. Our analysis shows that these solutions are a missed opportunity for tackling the twin crises together. Only five of the 39 countries explore co-benefits and synergies between forest-based climate solutions and biodiversity conservation measures. An additional nine mention them separately. Among these 14 countries, only a narrow range of interventions were proposed. While 28 AOSIS members prioritised forests for combating climate change, mostly for mitigation, only three prioritised their unique and globally important biodiversity. This omission is potentially risky, since mitigation measures, such as planting rapidly growing non-native trees, can have negative outcomes for biodiversity. Climate action plans must place a greater emphasis on concrete and measurable targets that create synergies with biodiversity conservation, including through the protection of old-growth forests and forest restoration. Our results highlight that forums such as the United Nations Climate Change Conferences need to continue pushing for a stronger integration of biodiversity into climate policies.

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

Non‐native tree species (NNT) are used in European forestry for many purposes including their growth performance, valuable timber, and resistance to drought and pest or pathogen damage. Yet, cultivating NNT may pose risks to biodiversity, ecosystem functioning, and the provisioning of ecosystem services, and several NNT have been classified as invasive in Europe. Typically, such classifications are based on risk assessments, which do not adequately consider site‐specific variations in impacts of the NNT or the extent of affected areas. Here, we present a new methodological framework that facilitates both mitigating risks associated with NNT and taking advantage of their ecosystem services. The framework is based on a stratified assessment of risks posed by NNT which distinguishes between different sites and considers effectiveness of available management strategies to control negative effects. The method can be applied to NNT that already occur in a given area or those NNT that may establish in future. The framework consists of eight steps and is partly based on existing knowledge. If adequate site‐specific knowledge on NNT does not yet exist, new evidence on the risks should be obtained, for example, by collecting and analyzing monitoring data or modeling the potential distribution of NNT. However, limitations remain in the application of this method, and we propose several policy and management recommendations which are required to improve the responsible use of NNT.

Non-native plant drives the spatial dynamics of its herbivores: the case of black locust (Robinia pseudoacacia) in Europe

Non-native plants typically benefit from enemy release following their naturalization in non-native habitats. However, over time, herbivorous insects specializing on such plants may invade from the native range and thereby diminish the benefits of enemy release that these plants may experience. In this study, we compare rates of invasion spread across Europe of three North American insect folivores: the Lepidoptera leaf minersMacrosaccus robiniellaandParectopa robiniella, and the gall midgeObolodiplosis robiniae, that specialize onRobinia pseudoacacia. This tree species is one of the most widespread non-native trees in Europe. We find that spread rates vary among the three species and that some of this variation can be explained by differences in their life history traits. We also report that geographical variation in spread rates are influenced by distribution ofRobinia pseudoacacia, human population and temperature, thoughRobinia pseudoacaciaoccurrence had the greatest influence. The importance of host tree occurrence on invasion speed can be explained by the general importance of hosts on the population growth and spread of invading species.

Are street trees friendly to biodiversity?

Urban areas are home to more than half the world’s population, but also habitats for a wide range of plant and animal species. While street trees in urban areas have been recognized as important for human well-being, however, how they can contribute to wildlife conservation is less explored. Here we compiled a database of street tree inventories in China, that included species diversity, abundance and origin information of street trees from 59 cities to explore how different cities rank with regards to the use of native and diversity of street trees, which are both considered beneficial to urban biodiversity. We found the most abundant species contributed an average of 35.8% of all street trees in the studied cities, and non-native species contributed an average of 40.6% of the street trees. Most cities are dominated by only a few species of trees, and a large proportion of these trees are non-native tree species, indicating that streetscapes are likely not friendly to biodiversity. Our proposed ranking schedule provides an easy tool for classifying cities according to their street tree wildlife friendliness, while also providing clear management directions on how to improve city tree composition for biodiversity conservation. To build a sustainable society in which nature and humans can coexist, we recommend that city planners should consider biodiversity conservation as a core value of urban planning. Specifically, we encourage the planting of more native trees and use of a more diverse set of species capable of attracting wildlife, thus promoting biodiversity in cities. Furthermore, awareness of biodiversity friendly tree planting systems needs special attention in developing regions and densely populated areas. We emphasize that more regional research needs to go into identifying local species that can be used as street trees while simultaneously functioning as wildlife attractants.

Cryptogamic epiphytes and microhabitat diversity on non-native green ash (Fraxinus pennsylvanica Marsh., Oleaceae) in urban habitats

With the increased planting of non-native trees within urban environments there is a need for investigating the impacts they may have on the indigenous biodiversity. In this study, we explored the diversity of epiphytic lichens and bryophytes as well as the tree-related microhabitats on planted, non-native green ash Fraxinus pennsylvanica and compared it to that of indigenous Fraxinus excelsior and Quercus robur. We conducted sampling on trees of similar growing conditions and size within two cities of eastern Germany (Dresden and Dessau-Roßlau). In our analysis we did not find any significant differences in epiphyte diversity and abundance. By contrast, microhabitat diversity was significantly higher on F. pennsylvanica than on the indigenous tree species, which we attribute to the pioneer character of F. pennsylvanica with faster ageing. Our results underline a low impact of F. pennsylvanica on epiphytic lichen and bryophyte diversity, while indigenous animals might even benefit from the higher diversity and frequency of microhabitats on trees of this species. Therefore, its use as an ornamental tree should not be generally rejected in urban environments.

Chemical Traits that Predict Susceptibility of Pinus radiata to Marsupial Bark Stripping.

Bark stripping by mammals is a major problem in managed conifer forests worldwide. In Australia, bark stripping in the exotic plantations of Pinus radiata is mainly caused by native marsupials and results in reduced survival, growth, and in extreme cases death of trees. Herbivory is influenced by a balance between primary metabolites that are sources of nutrition and secondary metabolites that act as defences. Identifying the compounds that influence herbivory may be a useful tool in the management of forest systems. This study aimed to detect and identify both constitutive and induced compounds that are associated with genetic differences in susceptibility of two-year-old P. radiata trees to bark stripping by marsupials. An untargeted profiling of 83 primary and secondary compounds of the needles and bark samples from 21 susceptible and 21 resistant families was undertaken. These were among the most and least damaged families, respectively, screened in a trial of 74 families that were exposed to natural field bark stripping by marsupials. Experimental plants were in the same field trial but protected from bark stripping and a subset were subjected to artificial bark stripping to examine induced and constitutive chemistry differences between resistant and susceptible families. Machine learning (random forest), partial least squares plus discriminant analysis (PLS-DA), and principal components analysis with discriminant analysis (PCA-DA), as well as univariate methods were used to identify the most important totals by compound group and individual compounds differentiating the resistant and susceptible families. In the bark, the constitutive amount of two sesquiterpenoids - bicyclogermacrene and an unknown sesquiterpenoid alcohol -were shown to be of higher levels in the resistant families, whereas the constitutive sugars, fructose, and glucose, as well individual phenolics, were higher in the more susceptible families. The chemistry of the needles was not useful in differentiating the resistant and susceptible families to marsupial bark stripping. After artificial bark stripping, the terpenes, sugars, and phenolics responded in both the resistant and susceptible families by increasing or reducing amounts, which leveled the differences in the amounts of the compounds between the different resistant and susceptible classes observedat the constitutive level. Overall, based on the families with extreme values for less and more susceptibility, differences in the amounts of secondary compounds were subtle and susceptibility due to sugars may outweigh defence as the cause of the genetic variation in bark stripping observed in this non-native tree herbivory system.

Contrasting responses of saproxylic beetles and plants to non-native tree invasion reveal feedback mechanisms between trophic levels

Invasive, alien trees threaten native biodiversity, but detailed information about the patterns and mechanisms of diversity loss remain unknown. We explored the impact of an invasive tree on vascular plants and saproxylic beetles. We compared their species richness, community composition, and selected biological characteristics between stands of invasive black locust (Robinia pseudoacacia), a nitrogen-fixing clonal tree, and native oak-dominated, broadleaf forests in the SE Czech Republic and W Slovakia.Compared to native forests, R. pseudoacia stands were characterised by low canopy cover, high light and soil nutrient availability, and similar deadwood volumes. R. pseudoacacia had species-poor, dense understorey vegetation composed of tall, nitrophilous herbs. Saproxylic beetles, on the other hand, were similarly diverse and abundant in both R. pseudoacacia stands and native forests. Their communities were mainly shaped by forest structure, as overall beetle diversity and guilds exploiting more decayed wood benefited from the open canopies and high deadwood volumes. Richness and abundance of threatened plants and beetles requiring fresh wood, together with total richness of threatened beetles were, however, substantially lower in R. pseudoacacia stands.The adverse impacts of R. pseudoacacia on plants partly contrasted with its much less pronounced effects on saproxylic beetles. In intensively exploited landscapes, R. pseudoacacia stands can offer refuge to saproxylic beetles. However their lower diversity and lower incidence of threatened species show that R. pseudoacacia contributes to homogenization and impoverishment of plant and insect assemblages. Robinia stands thus should be eliminated from protected areas and minimalised wherever biodiversity conservation is of concern.

Agreement levels of London tree officers towards the benefits and costs of urban forests

Public perceptions of urban forests have been widely investigated, but the professional practitioners’ views have received little attention. Surveying the opinions of tree professionals could inform and fine-tune urban forest policies and practices. The opinions of 79 tree officers in London were evaluated. Our questionnaire assessed the levels of agreement with 66 statements stating the benefits and costs associated with urban trees and green space (referred to as urban forests). It allowed quantitative analysis of responses, accompanied by qualitative assessment of written comments. Respondents expressed high agreement with benefits related to environmental quality and human well-being. The regulating benefits were appreciated at a weaker level. The comments indicated keen awareness of underlying factors of benefit delivery, their conditional nature and practical limitations, and sound scientific evidence. Statements concerning costs received more divergent views with lower agreement scores. This result was attributed to the belief that planting design and management could minimise costs. Cluster analysis identified five officer clusters harbouring disparate latent mindsets, which could be subsumed under two aggregate groups based on the agreement ratio between benefits and costs. The mainstream view did not seek a lot more native than non-native trees. Awareness of the practical limitations of native species could have dampened the desire. Working experience and age influenced the agreement levels far more than other socio-demographic factors. The findings implied potential obstacles to urban forest management for regulating benefits due to officers’ preference for other functions and cost reduction.

Abscisic acid induces somatic embryogenesis and enables the capture of high-value genotypes in Douglas fir (Pseudotsuga menziesii [MIRB.] Franco)

Douglas fir (Pseudotsuga menziesii) is one of Europe’s most important non-native tree species due to its drought tolerance as well as timber quality and yield. To obtain superior seed from selected parental trees, breeding programs had been established in seed orchards. Douglas fir seed is used as source material for somatic embryogenesis with the aim to select elite genotypes invaluable for clonal mass propagation. To improve given protocols for somatic embryo initiation, we used immature Douglas fir zygotic embryos as explants and abscisic acid (ABA) as plant growth regulator in contrast to the application of auxins and cytokinins. With ABA supplementation, induction frequencies were slightly but in mean higher than with auxin/cytokinin, showing also a strong genotype effect. This offered the possibility to capture SE cultures from otherwise recalcitrant crosses. Furthermore, we observed remarkable differences between the two sets of plant growth regulators concerning the morphological development of the explants, including the absence of non-embryogenic callus by using ABA as inducer. This simplifies the detection of events and the handling of the obtained cultures. Nevertheless, a histological approach suggested, that the same competent cells are addressed by the different hormonal stimulation. Besides, we studied the influence of different points in time of cone harvest, two different basal media and different genetic backgrounds of the explants as well as the maturation ability of the induced embryogenic cultures. In sum, we were able to improve the first steps of somatic embryogenesis and to maintain a significantly higher number of high-value genotypes.

Tree species composition and soil properties in pure and mixed beech-conifer stands drive soil fungal communities

Soil fungi, especially the functional guilds of saprotrophs and symbiotrophs, play a central role in ecosystem processes such as decomposition and plant nutrient acquisition. Fungal communities are influenced by soil properties and management strategies such as tree species selection. Yet, the implications of the enrichment of temperate forests consisting of tree species in their native range with non-native tree species on soil fungal diversity and their functional groups are unknown. Here, we studied fungal communities in 40 plots (2500 m2 size per plot) located in two regions differing in site conditions (nutrient content, soil moisture and climate) in forests composed of European beech, spruce and Douglas-fir (non-native) and mixtures of beech with either spruce or Douglas-fir. We hypothesized that fungal community structures are driven by soil properties and tree species composition, generally resulting in higher fungal diversity in mixed than in mono-specific forests. We further hypothesized that Douglas-fir has a negative effect on ectomycorrhizal fungal species richness compared to native species, whereas saprotrophic fungal richness is unaffected. Fungal DNA barcoding and dissimilarity analyses showed significant separation of fungal communities between nutrient-rich and nutrient-poor regions and among forest types. Mycorrhizal species richness did not vary with forest type, but the relative abundance of mycorrhizal species was lower in Douglas-fir and mixed beech-Douglas-fir forests than in spruce or beech-spruce mixture forests. Conifer forests contained higher relative abundances of saprotrophic fungi than mono-specific beech forests. Among 16 abundant fungal orders in the soil, two containing saprotrophic fungi (Tremellales and Hymenochaetales) were enriched in conifer forests, regardless of site conditions and tree species mixture. The other fungal orders, including those dominated by mycorrhizal fungi (Russulales, Boletales, Atheliales and Cantharellales) showed variable patterns depending on site conditions and tree species. Our results show the impact of tree species composition and soil properties on soil fungi. The response of fungal guilds and taxa to forest types and soil properties can potentially influence nutrient fluxes.

The Use of an Unmanned Aerial Vehicle for Tree Phenotyping Studies

A strip of 20th-century landscape woodland planted alongside a 17th to mid-18th century ancient and semi-natural woodland (ASNW) was investigated by applied aerial spectroscopy using an unmanned aerial vehicle (UAV) with a multispectral image camera (MSI). A simple classification approach of normalized difference spectral index (NDSI), derived using principal component analysis (PCA), enabled the identification of the non-native trees within the 20th-century boundary. The tree species within this boundary, classified by NDSI, were further segmented by the machine learning segmentation method of k-means clustering. This combined innovative approach has enabled the identification of multiple tree species in the 20th-century boundary. Phenotyping of trees at canopy level using the UAV with MSI, across 8052 m2, identified black pine (23%), Norway maple (19%), Scots pine (12%), and sycamore (19%) as well as native trees (oak and silver birch, 27%). This derived data was corroborated by field identification at ground-level, over an area of 6785 m2, that confirmed the presence of black pine (26%), Norway maple (30%), Scots pine (10%), and sycamore (14%) as well as other trees (oak and silver birch, 20%). The benefits of using a UAV, with an MSI camera, for monitoring tree boundaries next to a new housing development are demonstrated.

50 Grades of Shade

AbstractCities increasingly recognize the importance of shade to reduce heat stress and adopt urban forestry plans with ambitious canopy goals. Yet, the implementation of tree and shade plans often faces maintenance, water use, and infrastructure challenges. Understanding the performance of natural and nonnatural shade is critical to support active shade management in the built environment. We conducted hourly transects in Tempe, Arizona, with the mobile human-biometeorological station MaRTy on hot summer days to quantify the efficacy of various shade types. We sampled sun-exposed reference locations and shade types grouped by urban form, lightweight/engineered shade, and tree species over multiple ground surfaces. We investigated shade performance during the day, at peak incoming solar, at peak air temperature, and after sunset using three thermal metrics: the difference between a shaded and sun-exposed location in air temperature (∆Ta), surface temperature (∆Ts), and mean radiant temperature (∆TMRT). Air temperature did not vary significantly between shade groups, but ∆TMRT spanned a 50°C range across observations. At daytime, shade from urban form most effectively reduced Ts and TMRT, followed by trees and lightweight structures. Shade from urban form performed differently with changing orientation. Tree shade performance varied widely; native and palm trees were least effective, while nonnative trees were most effective. All shade types exhibited heat retention (positive ∆TMRT) after sunset. Based on the observations, we developed characteristic shade performance curves that will inform the City of Tempe’s design guidelines toward using “the right shade in the right place” and form the basis for the development of microclimate zones (MCSz).

Does Information and Deliberation Affect Ecosystem Service Judgments? Evidence from Tree Plantation Expansion in Argentina

Non-native tree plantations have become a significant presence on the landscape of northeastern Argentina. This study investigated perceptions of eucalyptus plantations in two Argentine communities: Ubajay, a community with significant historical plantation presence and forestry industry employment, and La Criolla, a community transitioning to a greater focus on plantation forestry. The purpose of this study was to determine effects of providing scientific results to community members and facilitating discourse on participant judgments of plantations’ ecosystem service effects and the acceptability of plantations as a land use. Following information provision and discussion, La Criolla participants judged plantation impacts more negatively than did their Ubajay counterparts. Our findings suggest that ecosystem service impacts are likely to be judged less negatively by those who receive socioeconomic benefits from plantations. Analysis of these changing community judgments emphasizes the importance of engaging the public with deliberative practices and exposure to the best available scientific information.

Biotic threats for 23 major non-native tree species in Europe

For non-native tree species with an origin outside of Europe a detailed compilation of enemy species including the severity of their attack is lacking up to now. We collected information on native and non-native species attacking non-native trees, i.e. type, extent and time of first observation of damage for 23 important non-native trees in 27 European countries. Our database includes about 2300 synthesised attack records (synthesised per biotic threat, tree and country) from over 800 species. Insects (49%) and fungi (45%) are the main observed biotic threats, but also arachnids, bacteria including phytoplasmas, mammals, nematodes, plants and viruses have been recorded. This information will be valuable to identify patterns and drivers of attacks, and trees with a lower current health risk to be considered for planting. In addition, our database will provide a baseline to which future impacts on non-native tree species could be compared with and thus will allow to analyse temporal trends of impacts.

Evaluating Strategies for the Management of Douglas-Fir in Central Europe

In view of the increasing demand for forest resources in Europe, it is an option to foster the use of non-native tree species that can keep pace with the rapidly changing environmental conditions, such as Douglas-fir (Pseudotsuga menziesii). Thus, sufficient knowledge of how to manage such introduced species is highly required. In this study, we investigate theoretical silvicultural management options of Douglas-fir for forests in central Europe. We follow a three-step approach: (i) we collect the current central European management practices based on 434 Douglas-fir stands managed by 19 forest companies in Eastern Austria and Southern Germany using a survey. (ii) We calibrate and validate a Douglas-fir parameter set for the tree growth simulator MOSES so that we are able to (iii) simulate the silvicultural management options of Douglas-fir management. Our simulation results suggest: in mixed stands, Douglas-fir should be planted in mono-species patches. This leads to about six times higher productivity compared to a random arrangement. Natural regeneration is possible but requires active management at further development since the productivity might decrease up to 86% when growing in association with the highly competitive native tree species, Common beech (Fagus sylvatica). Intensive tending, as well as thinning, yields a surplus stem volume production of more than 30% in comparison with a moderate intervention. Even if our simulation results were not validated in the field, this analysis suggests that modeling as a heuristic tool is a useful instrument for forest managers in the decision-making process.

The Influence of Tilia tomentosa Moench on Plant Species Diversity and Composition in Mesophilic Forests of Western Romania–A Potential Tree Species for Warming Forests in Central Europe?

Climate change challenges important native timber species in Central Europe. The introduction of non-native tree species originating from warmer climates is one option to make Central European forests compatible to global warming. This, however, requires an assessment of the species’ growth requirements, and of its impact on biodiversity in its native ranges. Silver lime (Tilia tomentosa), a moderately drought-tolerant, thermophilous tree species of South-eastern Europe is considered suitable for the future. Along three elevational transects in western Romania, we assessed the impact of changing climate and local site conditions on the abundance of this tree species and contrasted plant species diversity and composition of lime-dominated forests with mesophytic oak and beech forests. Local site conditions and disturbance histories shaped the distribution pattern of silver lime. When dominant, it reduced plant species diversity within stands due to its dense canopy. For shade-tolerant, mesophytic species, though, lime forests provided an additional habitat and extended their range into warmer environments. Thus, silver lime may have the potential as an admixed tree species forming a transitory meso-thermophilous habitat in the future. At the same time, silver lime may be limited under increasing drought frequency.

Seasonal Spectral Separation of Western Snowberry and Wolfwillow in Grasslands with Field Spectroradiometer and Simulated Multispectral Bands

Woody plant encroachment (WPE), the expansion of native and non-native trees and shrubs into grasslands, has led to degradation worldwide. In the Canadian prairies, western snowberry and wolfwillow shrubs are common encroachers, whose cover is currently unknown. As the use of remote sensing in grassland monitoring increases, opportunities to detect and map these woody species are enhanced. Therefore, the purpose of this study is to identify the optimal season for detection of the two shrubs, to determine the sensitive wavelengths and bands that allow for their separation, and to investigate differences in separability potential between a hyperspectral and broadband multispectral approach. We do this by using spring, summer, and fall field-based spectra of both shrubs for the calculation of spectral separability metrics and for the simulation of broadband spectra. Our results show that the summer offers higher discrimination between the two species, especially when using the red and blue spectral regions and to a lesser extent the green region. The fall season fails to provide significant spectral separation along the wavelength spectrum. Moreover, there is no significant difference in the results from the hyperspectral or broadband approach. Nevertheless, cross-validation with satellite imagery is needed to confirm the current results.

The Ecosystem Service Provision Index as a generic indicator of ecosystem service supply for monitoring conservation targets

At present, most governments, particularly in low and middle-income countries, are not effectively using ecosystem services (ES) indicators for conservation monitoring, which is in part due to the complexity of current indicators. This study seeks to contribute to this implementation gap by illustrating the performance of a remote sensing based indicator, the Ecosystem Services Provision Index (ESPI) (based on the Normalized Difference Vegetation Index), as a feasible indicator for monitoring ES supply. We examined ESPI’s performance by analyzing its spatial congruence (regression and cluster analysis) with four type-specific ES indicators, namely timber provision, forage provision, opportunities for recreation and water regulation, for different types of temperate native forests, non-native tree plantations, shrublands and grasslands, within a southern district in Chile. We additionally compared values of ESPI and ES indicators across land uses and covers by means of ANOVA and Tuckey test (post-hoc). The regression analysis showed medium to low fitting of ES indicators to linear and non-linear variations of ESPI. Unexplained variation can arise from different dispersion sources: a) uncaptured variation in specific ES flow by variation in their corresponding indicators (specific ES indicators and ESPI), b) distances between parametric values of different ES indicators and ESPI, and c) mismatches between ESPI and ES indicator scales and resolutions. Cluster analysis showed ESPI-ES hotspot congruence of 62% for water regulation, followed by the ES bundle (56%), recreation opportunities (38%), timber supply (36%) and forage provision (32%). The values of ESPI, water regulation and recreation opportunities were significantly higher on old growth forests. Our study contributes to establish to what extent remote sensing indicators can be used as a proxy of ES supply, which is relevant for the adoption of these indicators in monitoring conservation outcomes when specific ES indicators are not available or they lack the necessary spatial and temporal coverage.

Between approval and disapproval: Citizens’ views on the invasive tree Ailanthus altissima and its management

While cities are invasion hotspots, the view of urban residents on non-native species is critically understudied – an important knowledge gap since strategies on biological invasions could gain power by integrating human values, attitudes and perceptions. How citizens perceive the non-native tree Ailanthus altissima (tree of heaven) is unknown despite its abundance in many cities globally and its classification as invasive in many countries. In a quantitative survey with closed questions, we analysed (i) whether residents of Berlin, Germany knew the widespread species, (ii) how they perceived it in different urban situations, (iii) how they accepted different management strategies of it, and (iv) how the sociodemographic background of respondents predicted their preference and acceptability ratings. In total, we surveyed 196 respondents. Most respondents recognized the tree in a photograph, but few provided its correct name. Citizens’ preferences differed significantly among four urban contexts in which the species was shown, with prevailing approval for trees as a component of designed green spaces and less pronounced preferences for wild-grown trees in other urban spaces. When respondents were asked to indicate how the tree should be managed (three options), we found the most support for removal in problematic cases (‘adaptive on-site’ strategy); some support was found for the ‘leave alone’ strategy and least support for the ‘complete removal’ management strategy. Practitioners with expertise in urban landscaping were more critical of Ailanthus than laypeople. Ordinal logistic regression analyses showed that respondents with a ‘close to nature’ behaviour and attitude had a more positive view on Ailanthus and expressed more support for ‘leave alone’ management. Results demonstrate the importance of citizens’ context dependent views about a widespread invasive species, spanning from approval to disapproval in different situations. We conclude that urban management strategies concerning Ailanthus would gain support from citizens when combining multiple approaches: (i) to control the species in case of realized negative impacts; (ii) to prevent the invasion of the species in areas of conservation concern; and (iii) to develop novel approaches of integrating wild Ailanthus trees into urban green spaces. These insights could support management measures that need to be established due to the EU-Regulation on Invasive Alien Species.

Usefulness of non-native invasive tree species wood residues for pellet production

We produced pellets from five invasive non-native tree species growing in Slovenia on a laboratory pelleting device, namely: wild chestnut (Aesculus hippocastanum), boxelder maple (Acer negundo), black locust (Robinia pseudoacacia), thorny locust (Gleditsia triacanthos) and tree of heaven (Ailanthus altissima), as well as mixtures of the raw material from the above non-native invasive species and spruce (Picea abies) in the ratios 70:30 and 50:50. Under the same production conditions, we produced a total of 15 different types of pellets. The most important physical and mechanical properties (water content, bulk density, mechanical stability and ash content) were determined for the pellets produced. The results were compared with the limits defined in the international standard SIST EN ISO 17225-2:2014. The water content and bulk density of all produced pellet types met the requirements of the standard for the highest quality class A1. The mechanical durability of the pellets produced did not meet the requirements of the standard and did not exceed 96.5% (which is the limit value for quality class B). The results suggest that black locust, thorny locust and tree of heaven have the highest potential for further optimization of the pelleting process.