Non-native Tree Research Articles

A biosecurity perspective on urban trees in public and private spaces and trees available from nurseries

Urban trees are of high importance for healthy urban ecosystems. However, some trees are invasive plants that may ‘escape’ from urban areas into natural environments. Furthermore, invasive forest pests often arrive and establish in urban environments before they spread to forest ecosystems. Therefore, knowledge of tree species composition in urban areas is important for the biosecurity of cities and surrounding forests. Despite this, urban tree inventories often lack information on trees on private properties. To assess the potential role of urban areas as a source of invasive trees as well as invasive pests of trees, we compared the species composition of urban trees in public and private spaces in several Swiss cities and in tree nurseries (henceforth: ‘nurseries’) in Switzerland. We analysed the various data sets we compiled regarding invasive tree species, and examined how many trees are potential hosts for invasive quarantine pests of trees. We found no statistically significant difference in tree species composition in public and private urban areas, but the proportion of non-native trees was higher in private than in public spaces. Tree species available in nurseries and those present in cities were not significantly different, with larger urban tree inventories having a greater overlap with the species available in nurseries. Five of eight tree species considered invasive whose sale was banned since September 2024 were still on sale ten months before the ban came into force. All eight tree species were present in urban tree inventories. Importantly, between 91 % and 97 % of all urban trees in public and private spaces were suitable hosts for at least one quarantine forest pest. Given the pivotal role that urban trees play for the health of urban and peri-urban ecosystems, knowledge of tree species composition is essential from a biosecurity perspective. Our findings suggest that the combination of public urban tree inventories and trees available from nurseries can provide an estimate of the composition of private urban trees and can be useful for invasive species monitoring.

The disruption of birds' double mutualistic interactions in novel ecosystems.

Non-native trees disrupt ecological processes vital to native plant communities. We studied how forests dominated by Acacia dealbata and Eucalyptus globulus affect the role of birds as dual pollinators and seed dispersers in a region heavily impacted by these two non-native species. We compared bird-plant interactions in the native and in the two non-native forest types. We constructed a multilayer regional network for each forest type and evaluated differences in network dissimilarity between networks. We also calculated the bird's importance in connecting processes and variables associated with module diversity. To determine how the networks react to changes in species richness, we did a simulation of species richness gradient and link percentage for each forest type. The number of birds acting both as pollinators and seed dispersers was higher in native than in non-native forests. However, birds in non-native forests still play a crucial role in maintaining the ecological services provided to native plant communities. However, the eucalyptus network exhibited a concerning simplification, forcing bird species to fully exploit the few remaining resources, leaving little room for structural adjustments and limiting the ecosystem's ability to withstand further species loss. These findings highlight how non-native trees may trigger cascading effects across trophic levels.

A living inventory of planted trees in South Africa derived from iNaturalist

Trees have a multifaceted influence on ecosystems globally. Treescapes have been manipulated by humans over millennia for ecological, economic, and cultural reasons that have changed over time and continue to change. In South Africa tree planting over three centuries has radically affected the composition of treescapes, contributing important ecosystem services, but also disservices. Rapid global change calls for diverse interventions to create more resilient ecosystems. Many nature-based solutions involve manipulating tree cover in rural and urban landscapes. There is a need for a spatially-explicit database of planted trees in South Africa to serve as the foundation for policy and management decisions.We used the community science platform iNaturalist to create a comprehensive database of planted trees in South Africa. Records were carefully checked to verify the accuracy of taxon identifications, locality data, and categorization as planted rather than wild-growing trees.The cleaned database contained 35,303 records of 805 planted tree taxa; over 90 % of records were identified to species level. Almost a third of taxa (32.2 %) belong to three families: Fabaceae (97 taxa), Myrtaceae (80 taxa) and Arecaceae (78 taxa). Rarefaction and extrapolation curves suggest fairly comprehensive sampling, but several regions are under-sampled. Non-native taxa dominate, with Melaleuca viminalis having the most records. The Western Cape has the highest number of records and taxa, particularly in urban areas. Both native and non-native trees provide key ecosystem services, but non-natives dominate.The database provides the first spatially-explicit open-access resource for guiding decisions on tree planting and the management of planted trees in South Africa. It offers a snapshot of tree planting trends, predominantly from recent years, leading to some underrepresentation of historically planted species. The database has numerous potential uses, including guiding management of trees pests and diseases, urban greening initiatives, monitoring for new invasions, and planning nature-based solutions.

Soil drought sets site specific limits to stem radial growth and sap flow of Douglas-fir across Germany.

Soil drought during summer in Central Europe has become more frequent and severe over the last decades. European forests are suffering increasing damage, particularly Norway spruce. Douglas-fir (Pseudotsuga menziesii (Mirbel) Franco), a non-native tree species, is considered as a promising alternative to build drought-resilient forests. The main goal of this study was to investigate the intraannual radial stem growth and sap flow performance of Douglas-fir along a precipitation gradient across Germany under severe drought. Sap flow and stem radial changes of up to ten trees each at four sites with different precipitation regimes were measured in combination with volumetric soil water content during the growing season of 2022. Measurements of stem radial changes were used to calculate the trees' stem water deficit, a proxy for tree water status and drought stress. The severe summer drought of 2022 led to an early growth cessation and a significant reduction in daily sap flow at all four sites monitored. We could identify a site-specific threshold in soil water availability ranging between 21.7 and 29.6% of relative extractable water (REW) under which stem water reserves cannot be replenished and thereby inhibiting radial growth. We could also demonstrate that at this threshold, sap flow is heavily reduced to between 43.5 and 53.3%, and for a REW below 50%, sap flow linearly decreases by 1.1-2.0% per 1% reduction in REW. This reduction tends to follow the humidity gradient, being more pronounced at the most oceanic characterized site and suggesting an adaptation to site conditions. Even though Douglas-fir is considered to be more drought stress resistant than Norway spruce, growth and sap flow are greatly reduced by severe summer drought, which became more frequent in recent years and their frequency and intensity is likely to increase. Our results suggest that timber production of Douglas-fir in Central Europe will decline considerably under projected climate change, and thus pointing to site specific growth constraints for a so far promising non-native tree species in Europe.

Bird Community Responses to Restoration along Las Vegas Wash, Nevada

In a 12-year study of seasonal bird use along Las Vegas Wash (Clark County, Nevada), an area undergoing intensive channel stabilization and riparian and wetland restoration, we found that total bird abundance and species richness significantly increased. In the late phase of the project, total bird abundance was greatest during the nonbreeding season and winter transition, and species richness was greatest during the fall transition. Species associated with riparian shrubs decreased, likely as a response to large-scale removal of non-native shrubs and the lag in recovery of a native shrub understory. Species associated with trees and the mid-story had a mixed response, with detections of those species previously most closely associated with non-native trees showing a negative trend, while those associated with native overstory plants showing an increasing or mixed trend. Detections of wetland and aquatic species increased significantly throughout the project. This study shows differential restoration responses of species based on their natural history, seasons of primary use of the site, and the lag time in recovery of particular vegetation elements that are used by various species assemblages.

Balancing Risks and Benefits: Stakeholder Perspective on Managing Non-Native Tree Species in the European Alpine Space

For centuries, non-native tree (NNT) species have been planted throughout Europe for ecosystem services including timber and urban greenery. Public interest in NNTs has recently increased due to their potential role in climate change adaptation as alternatives to vulnerable native forest tree species. However, opinions regarding the benefits and risks of European NNTs differ. Understanding stakeholder perceptions is crucial for guiding adaptive forest management, especially in sensitive ecosystems like the European Alpine Space. To assess awareness and perception, a structured questionnaire was administered to 456 respondents from six countries in the European Alpine Space. Most respondents were aware of the origin of native and NNT species in their area. NNTs and invasive-NNTs were primarily found in urban regions, with a perceived increase in their occurrence over the past 25 years. With some exceptions, such as Pseudotsuga menziesii (Mirb.) Franco, the most common NNTs were generally perceived as potentially invasive. The perception of the invasiveness of NNTs correlated with their perceived risks and benefits on ecosystem services. The respondents who were unconcerned about invasiveness believed NNTs had a positive impact on provisioning services like timber, while those concerned about invasiveness perceived their negative effects on regulating cultural ecosystem services such as native biodiversity and landscape aesthetics. Overall, most respondents were conservative, opposing the promotion of NNTs, even in biodiversity-poor areas. Most stakeholders also believe that NNT regulations should prioritize their sustainable use and management rather than focusing solely on an invasive-centric narrative.

Exploring the multifunctional landscapes model in areas dominated by non-native tree plantations

Multifunctional landscapes offer an integrated approach to production, conservation, and human well-being. However, the challenges of implementing them in territories where plantations dominate are yet not well understood. This is the case in Chile, where plantations of non-native pines and eucalypts are extensively planted in its South-Central regions for timber and pulp. The resulting landscape homogenization, environmental degradation, and increased wildfire risk have caused and exacerbated conflicts, impacting biodiversity and the well-being of local communities and the Indigenous Mapuche Peoples.After the mega-wildfires in the region in 2023, science-policy discourse promoted the multifunctional landscapes model as a way to increase resilience. But what does this multifunctionality mean in challenging social-ecological contexts? Here, we aim to explore and deconstruct the multifunctional landscapes model in the context of the complex social-ecological systems of South-Central Chile. In this study, a review of the literature and semi-structured interviews with regional experts were used to better understand the challenges and opportunities presented by multifunctional landscapes.The results show a need to deepen the knowledge of how to move the model into practice, such as how to identify and decide compatible activities in the landscape. The thematic analysis of the interviews showed that restoration and water security are shared goals across the diverse actors in South-Central Chile. However, there were significant differences in knowledge, experiences and resources. While a number of landscape initiatives exist in the region, significant work is needed to build a common vision before the potential of multifunctional landscapes can be realized.

Alien Trees and Shrubs with the Complements Saksamaa (Germany) and Saksa (German) in Early Written Language and Folklore

The study of alien species has become more and more relevant today. In particular, the influence of alien species on the local nature is studied, but more and more studies have also begun to appear on how alien species affect the local language and culture. In this article, we took a look at non-native trees and shrubs that have been called “Saksamaa” and “Saksa” [German]. Germany was synonymous with foreign countries in early literature. We investigated the motives behind the names given to these species, how long they were in circulation and how these names were later replaced. For this purpose, we studied lexicons, archival texts and historical literature. „German“ complemented plant names are most common in Estonian and Finnish – with fir, beech, larch, elder, Persian walnut tree, poplar, Swedish whitebeam and false spiraea coinciding. The prevalence of plant names can be divided into three: a) names found only in the early written word, which are absent in the oral tradition; b) vernacular plant names with the suffix “German” which were entered into dictionaries and supported by official terminology; c) individual fop names with the suffix “German” collected from the people. Alien species that were planted as fruit trees, ornamental trees, medicinal plants and forest trees were called by these names, but imported pharmaceutical drugs, fruits and wood were also called “German”. Mainly, non-ntive species were named after a local tree, to which “German” was added in front of the name. Later, those names were either replaced by adaptations of German plant names or new Estonian plant names were created. Several trees could be called by one name. For example, larches, firs and alien spruce species, which differed from the local spruce by the silvery colour of their needles, have been called the “German spruce”. In the 17th and 18th centuries, all foreign trees that were frost-resistant could be called “German trees.” Back in the 20th century, people spontaneously called foreign trees that were different from domestic trees by the names of “German”.

The potential of non-native tree species to provide major ecosystem services in Austrian forests.

Forestry is facing an unprecedented challenging time. Due to climate change, major tree species, which until recently fulfilled major ecosystem services, are being lost and it is often unclear if forest conversion with other native or non-native tree species (NNT) are able to maintain or restore the endangered ecosystem services. Using data from the Austrian Forest Inventory, we analysed the current and future (2081-2100, RCP 4.5 and RCP 8.5) productivity of forests, as well as their protective function (avalanches and rockfall). Five different species change scenarios were considered for the replacement of a tree species failing in the future. We used seven native tree species (Picea abies, Abies alba, Pinus sylvestris, Larix decidua, Fagus sylvatica, Quercus robur and Quercus petraea) and nine NNT (Pseudotsuga menziesii, Abies grandis, Thuja plicata, Pinus radiata, Pinus contorta, Robinia pseudoacacia, Quercus rubra, Fraxinus pennsylvanica and Juglans nigra). The results show that no adaptation would lead to a loss of productivity and a decrease in tree species richness. The combined use of native and NNT is more favorable than purely using native species in terms of productivity and tree species richness. The impact of the different species change scenarios can vary greatly between the different environmental zones of Austria (Alpine south, Continental and Pannonian). The Pannonian zone would benefit from the use of NNT in terms of timber production. For the protection against avalanches or rockfall in alpine regions, NNT would not be an advantage, and it is more important if broadleaved or coniferous trees are used. Depending on whether timber production, protective function or tree species richness are considered, different tree species or species change scenarios can be recommended. Especially in protective forests, other aspects are essential compared to commercial forests. Our results provide a basis for forest owners/managers in three European environmental zones to make decisions on a sustainable selection of tree species to plant in the face of climate change.

The role of a non-native forest in the stopover ecology of migratory passerines

During their journey, migratory birds need stopover sites where they can replenish their energy stores. Mosaic forests of agricultural areas, often planted with non-native trees, can provide opportunities for birds to do this. In the present work, I sought to answer the question of the importance of these habitats for migratory birds. For this reason I studied the stopover ecology of 15 species of long- and short-distance migratory passerines. I calculated the minimum stopover duration and the extent of fat accumulation per species and by dividing species into different groups by habitat use and migration distance. For three species, I also estimated their possible flight ranges based on the accumulated fat, body mass and wing length. My results show that the planted oleaster forest has a different role in the stopover habits of the species studied. The body mass of the birds typically did not changed significantly during the time they spent in the area. Forest and farmland species spent the longest time in the area. Despite the possibly high rate of intra- and inter-specific competition, the area can provide sufficient food for birds throughout the whole period. Short-distance migrants stored less fat than long-distance migrants, probably due to the different migration strategies. Flight distances varied according to the migratory habits of the species. From a conservation biology point of view, the study highlighted the role of these habitats in bird migration.

Distinct metabolites affect the phloem fungal communities in ash trees (Fraxinus spp.) native and nonnative to the highly invasive emerald ash borer (AGRILUS PLANIPENNIS).

Emerald ash borer (EAB, Agrilus planipennis) is an invasive killer of ash trees (Fraxinus spp.) in North America and Europe. Ash species co-evolved with EAB in their native range in Asia are mostly resistant, although the precise mechanism(s) remain unclear. Very little is also known about EAB or ash tree microbiomes. We performed the first joint comparison of phloem mycobiome and metabolites between a native and a nonnative ash species, infested and uninfested with EAB, in conjunction with investigation of larval mycobiome. Phloem mycobiome communities differed between the tree species, but both were unaffected by EAB infestation. Several indicator taxa in the larval gut shared a similarly high relative abundance only with the native host trees. Widely targeted metabolomics revealed 24 distinct metabolites in native trees and 53 metabolites in nonnative trees, respectively, that differed in relative content between infested and uninfested trees only in one species. Interestingly, four metabolites shared a strong relationship with the phloem mycobiomes, majority of which affected only the native trees. Collectively, our results demonstrate a complex interplay between host tree chemistry and mycobiome, and suggest the shared relationships between the mycobiomes of the native host tree and EAB may reflect their shared co-evolution.

Functional traits in soil-living oribatid mites unveil trophic reorganization in belowground communities by introduced tree species

Biodiversity loss and its potential threat on ecosystem functions call for a critical evaluation of human impacts on forest ecosystems. Management practices based on stand diversification offer a possible solution to biodiversity loss due to monoculture plantations, and these practices often involve planting introduced tree species. Although introduced non-native tree species may provide high economic returns, they may also form novel ecosystems and threaten local biodiversity, but this has been little studied. Here, we combined a taxonomic and trait-based approach and investigated communities of oribatid mites (Oribatida, Acari) across forest types of both native and introduced tree species in Northern Germany. Both trophic and life-history traits of oribatid mites were evaluated using native European beech (Fagus sylvatica) as reference, compared to native Norway spruce (Picea abies), introduced Douglas fir (Pseudotsuga menziesii) and beech-conifer mixtures. The abundance and diversity of oribatid mites were generally similar among monocultures of European beech, Norway spruce and Douglas fir. By contrast, species and trait compositions of oribatid mite communities were shifted to include more primary decomposers and more surface-living oribatid mites in Douglas fir, resulting in a trophic reorganization with less predators than in European beech forests. These results suggest that oribatid mites maintain a similar level of trait diversity regardless of forest type, but the changes in trophic guild composition and vertical distribution indicate greater availability of litter-based resources in Douglas fir than in European beech forests. The similar abundance and diversity of oribatid mite communities in Douglas fir mixed stands as in native European beech forests points to mixed forests as a promising management option for future forestry. Overall, our trait-based analyses provided insights into the changes of soil biota composition, revealing the impact of introduced tree species on the structure and functions of soil animal communities.

Use of nonnative, invasive tree logs for commercial mushroom production

Abstract Removal and disposal of nonnative trees is expensive and time-consuming. Using these nonnative trees as a substrate to produce edible mushrooms could diversify farming operations and provide additional income to small-scale farmers. This research compared the production of shiitake mushrooms (Lentinula edodes) on nonnative tree logs to shiitake mushroom production on native oak (Quercus L.) logs, which are the traditional substrate. In a 2-yr study, we evaluated nonnative tree species as alternate substrates for growing shiitake mushrooms at farms in northern Florida and southern Georgia. A mix of native Quercus spp. and nonnative trees was targeted for removal on participating farms. Five nonnative tree species were initially tested for their ability to produce edible mushrooms, either shiitake or oyster (Pleurotus ostreatus var. florida). Of the nonnative trees we tested: Chinaberry (Melia azedarach L.), Chinese tallowtree [Triadica sebifera (L.) Small], silktree (Albizia julibrissin Durazz.), earleaf acacia (Acacia auriculiformis A. Cunn. ex Benth.), and paperbark tree [Melaleuca quinquenervia (Cav.) S.F. Blake], only T. sebifera produced shiitake mushrooms, and none produced native Florida oyster mushrooms. In on-farm trials, Quercus spp. logs produced more total mushrooms and more mushrooms per log and had a higher total mushroom yield per log. However, mushrooms produced on T. sebifera logs had higher mean weight per mushroom. Edible mushroom fungi can be used to recycle invasive, nonnative T. sebifera and transform their biomass from waste into an income-producing resource.

Identifying the generalizable controls on insect associations of native and non-native trees.

Trees growing outside their native geographic ranges often exhibit exceptional growth and survival due in part to the lack of co-evolved natural enemies that may limit their spread and suppress population growth. While most non-native trees tend to accumulate natural enemies over time, it remains uncertain which host and insect characteristics affect these novel associations and whether novel associations follow patterns of assembly similar to those of native hosts. Here, we used a dataset of insect-host tree associations in Europe to model which native insect species are paired with which native tree species, and then tested the model on its ability to predict which native insects are paired with which non-native trees. We show that native and non-native tree species closely related to known hosts are more likely to be hosts themselves, but that native host geographic range size, insect feeding guild, and sampling effort similarly affect insect associations. Our model had a strong ability to predict which insect species utilize non-native trees as hosts, but evolutionarily isolated tree species posed the greatest challenge to the model. These results demonstrate that insect-host associations can be reliably predicted, regardless of whether insect and host trees have co-evolved, and provide a framework for predicting future pest threats using a select number of easily attainable tree and insect characteristics.

Higher establishment of nonnative trees with increased harvest intensity in strip cuttings

Higher establishment of nonnative trees with increased harvest intensity in strip cuttings

Lepidoptera species richness and community composition in urban street trees

The triple threats of climate change, habitat loss, and environmental pollution have stimulated discussion on how urban areas can be modified to both mitigate heat increases and provide habitat for wildlife such as insects. The strategy of using trees to reduce temperatures has been adopted by numerous cities. However, the majority of street trees planted around the world are non-native. Studies conducted in non-urban areas have demonstrated in comparison to native plants, non-native plants are less likely to support native insect diversity. Here we use a database approach to quantify the number of native Lepidoptera species associated with 76 of the most common street tree species planted in Vancouver, Canada. We tested the prediction that compared to non-native trees, native street trees will support a higher diversity and unique community of native Lepidoptera. As predicted, native street trees were associated with five times as many native Lepidoptera species, and the Lepidoptera communities supported by native vs. non-native street trees were distinct. There was no difference in native Lepidoptera associations between broadleaf vs. coniferous street trees. These results are consistent with studies that have used active sampling techniques to investigate insect richness on a smaller subset of native and non-native tree species. Collectively, these data provide good evidence that the planting native instead of non-native trees will help stem the loss of insect diversity in urban areas

Salient features and ecosystem services of tree species in mountainous indigenous agroforestry systems of North-Eastern Tanzania

Indigenous agroforestry systems in tropical mountainous environments provide crucial ecosystem services, but these ecosystems are also facing some challenges. A loss of diversity and native tree species in the overstory layer has been a growing concern in agroforestry worldwide, yet the drivers behind it remain inadequately understood. We hypothesize that the choice of overstory tree species is closely linked to the ecosystem services required by farmers, their livelihood strategy, and the salient features of each system. We, therefore, investigated four different farming systems in the mountains of northeastern Tanzania, i.e., the Kihamba on Mt. Kilimanjaro, Ginger agroforestry in the South Pare mountains, and Miraba and Mixed spices agroforestry in the West and East Usambara. In 82 farms, we collected data on the structure, tree species composition (both native and non-native), diversity, and associated provisioning ecosystem services as identified by smallholder farmers. Our results indicate that although all studied systems are multi-layered with three or four vertical layers, they have notable differences in their salient features concerning structure, composition, and diversity. The unique climate, landscape setting, soil, historical background, and economic opportunities that exist in each region contribute to those differences. Our findings indicate that the Kihamba system had the highest number of native tree species, and the largest diversity in species used for provisioning services, followed by Ginger agroforestry. No native species were used in Miraba or Mixed spices agroforestry, where a limited number of non-native tree species are planted mainly for fuel and timber or as a crop, respectively. Our findings regarding reported provisioning ES corroborate our hypothesis and imply that policies to increase resilience and restore the native tree species cover of the agroforestry systems of Tanzania can only be successful if knowledge of the ES potential of native species is increased, and interventions are tailored to each system’s ES needs for conservation as well as livelihood.

The Range Potential of North American Tree Species in Europe

European forest ecosystems are projected to change severely under climate change especially due to an anticipated decline in the distribution of major tree species in Europe. Therefore, the adaptation of European forests appears necessary and urgent. While spontaneous adaptation mechanisms bear a large self-guided potential, we focus on quantifying the potential of management-guided mechanisms. Besides other possible tree species groups for adaptation, non-native tree species from North America have a long tradition in Europe, yet their full distribution potential is not completely revealed. We applied an ensemble species distribution model approach to six North American species, using combined occurrence data from the native and naturalized ranges to gain more insights into the species suitability in the introduced area in 2070 (2061–2080) under the emission scenarios RCP 4.5 and 8.5. Our findings support the assumption that there is unreported species potential in the introduced area beyond their current distribution. Next to northeastern range shifts projected for all species, we identified Abies grandis, Liriodendron tulipifera, Quercus rubra, and Robinia pseudoacacia with increasing range potentials in the future. P. ponderosa and P. menziesii var. menziesii are projected to show a steady and decreased range potential under RCP 4.5 and 8.5, respectively.

Making smart use of woody alien plants

It is unquestionable that some alien woody plant species are a major issue for biodiversity conservation. However, being alien species generally adapted to warmer conditions than natives, they could be particularly suited to cope with the new environmental and climatic conditions that are also forecasted in the coming decades in Sicily. This poses an issue that is not easy to deal with, whether trying to use these species for the advantages they could bring, for instance in the new reforestation activities, while avoiding them becoming a serious problem for biodiversity conservation in natural and seminatural areas. Here, we present some study cases of non-native tree species which represent emblematic examples in that regard: Acacia saligna and A. cyclops, Eucalyptus spp., Leucaena leucocephala, Opuntia ficus-indica and Ailanthus altissima. These tree species are very invasive in some ecological contexts, while being useful to meet ecological services in others. A sound planning activity could help distinguish where planting these species could be not only possible but also desirable, from areas where their presence should be totally avoided.

The relationship between the abundance of non-native tree species and the community structure of resident woody species in tropical montane forest of West Usambara Mountains, Tanzania

Non-native tree species are globally claimed as liable to the encroachment of natural forests in their proximity resulting to the loss of biodiversity. This study was conducted in Magamba Nature Forest Reserve (MNFR) in West Usambara Mountains of Tanzania. A total of 96 (20 m x 20 m) plots were established to record all woody plants with Diameter at Breast Height (DBH) ≥ 5 cm while two nested plots of 2 m x 5 m were set to measure any plant with DBH ≥ 1 cm but less than 5 cm. Furthermore, two nested plots of 1 m x 1 m were established within the bigger plot in order to assess and record seedlings (DBH < 1 cm). Generalized Linear Models (GLMs) were used to assess the relationship between total abundance, species diversity, richness, evenness, and biomass of resident species and the abundance of Acacia melanoxylon and Eucalyptus grandis. Findings show a significant (p < 0.05) decrease of abundance of all resident woody species, adults, saplings and seedlings with an increase in the abundance of A. melanoxylon. The study also showed that the species richness and diversity of all resident species decreased with increasing the abundance of non-native species (p < 0.05). The abundance of E. grandis did not show significant (p > 0.05) association when assessed against the total abundance, species diversity, richness and evenness of all the resident woody species. This non-significant result may not necessarily mean no effect. This is because the P-value does not explain the size of the effect in biological data. Further analysis revealed that, the biomass of resident species was negatively related to the abundance of non-native species A. melanoxylon (p = 0.003) and E. grandis (p = 0.038). The negative relationship between the abundance, richness, diversity, evenness, and biomass of the resident woody species and the abundance of non-native species portrayed in this study raise an alarm. Therefore, there is need for setting monitoring and control mechanism to stop further spread of the non-native tree species in MNFR and hence, save the remaining biodiversity of the area.