Impact of human disturbances on the diversity of alien trees and shrubs in a university landscape in India

Mapping invasive alien trees in water towers: A combined approach using satellite data fusion, drone technology and expert engagement

The high ecological impacts of many invasive alien trees have been well documented. However, to date, we lacked synthesis of their economic impacts, hampering management actions. Here, we summarize the cost records of invasive trees to (I) identify invasive trees with cost information and their geographic locations, (II) investigate the types of costs recorded and sectors impacted by invasive trees and (III) analyze the relationships between categories of uses of invasive trees and the invasion costs attributed to these uses. We found reliable cost records only for 72 invasive trees, accumulating a reported total cost of $19.2 billion between 1960 and 2020. Agriculture was the sector with the highest cost records due to invasive trees. Most costs were incurred as resource damages and losses ($3.5 billion). Close attention to the ornamental sector is important for reducing the economic impact of invasive trees, since most invasive trees with cost records were introduced for that use. Despite massive reported costs of invasive trees, there remain large knowledge gaps on most invasive trees, sectors, and geographic scales, indicating that the real cost is severely underestimated. This highlights the need for further concerted and widely-distributed research efforts regarding the economic impact of invasive trees.

Can Sentinel-2 be used to detect invasive alien trees and shrubs in Savanna and Grassland Biomes?

Five major invasive alien tree species in European Union forest habitat types of the Alpine and Continental biogeographical regions

The absence of keystone indigenous trees inhibits bird recovery up to a decade after invasive tree removal from riparian habitats

Over 430 alien tree species worldwide are known to be invasive, and the list is growing as more tree species are moved around the world and become established in novel environments. Alien trees can simultaneously bring many benefits and cause substantial environmental harm, very often leading to conflicts over how they should be managed. The impacts grow over time as invasions spread, and societal perceptions of the value of alien trees also change as understanding grows and as values shift. This leads to a dynamic environment in which trade-offs are required to maximise benefits and minimise harm. The management of alien tree populations needs to be strategic and adaptive, combining all possible management interventions to promote the sustainable delivery of optimal outcomes. We use examples, mainly from South Africa (where issues relating to invasive alien trees introduced for forestry have received most attention), to argue for holistic and collaborative approaches to alien tree management. Such approaches need to include bold steps, such as phasing out unsustainable plantation forestry that is based on highly invasive species, and in which the costs are externalised. Furthermore, it would be advisable to impose much stricter controls on the introduction of alien trees to new environments, so that problems that would arise from subsequent invasions can be avoided.

Invasive alien organisms can impact adversely on indigenous biodiversity, while riparian invasive alien trees (IATs), through shading of the habitat, can be a key threat to stream invertebrates. We ask here whether stream fauna can recover when the key threat of riparian IATs is removed. Specifically, we address whether IAT invasion, and subsequent IAT removal, changes benthic macroinvertebrate and adult dragonfly assemblages, for the worse or for the better respectively. Natural riparian zones were controls. There were statistically significant differences between stream reaches with natural, IAT-infested and IAT-cleared riparian vegetation types, based on several metrics: immature macroinvertebrate taxon richness, average score per macroinvertebrate taxon (ASPT), a macroinvertebrate subset (Ephemeroptera, Plecoptera, Trichoptera and Odonata larvae; EPTO), and adult dragonfly species richness. Reaches with natural vegetation, or cleared of IATs, supported greater relative diversity of macroinvertebrates than reaches shaded by dense IATs. Greatest macroinvertebrate ASPT and EPTO were in reaches bordered by natural vegetation and those bordered by vegetation cleared of IATs, and the lowest where the riparian corridor was IATs. Highest number of adult dragonflies species was along streams cleared of dense IATs. Overall, results showed that removal of a highly invasive, dense canopy of alien trees enables recovery of aquatic biodiversity. As benthic macroinvertebrate scores and adult dragonfly species richness are correlated and additive, their combined use is recommended for river condition assessments.

The global database of invasive trees and shrubs has been updated, resulting in a total of 751 species (434 trees and 317 shrubs) from 90 families (Rejmanek and Richardson 2013 Divers Distrib 19:1093–1094). This database is used to assess major trends in human-assisted exchanges of dendrofloras among 15 major geographical regions. Areas most invaded by non-native trees are Pacific Islands (136 species), Southern Africa (118), Australia (116), and North America (114). Areas most invaded by non-native shrubs are North America (98), Australia (87), Pacific Islands (71), and Europe (61). The most important sources of invasive trees are Asia (122–146 species, depending on how many Eurasian species are considered to have been introduced only from Europe), Australia (81), and South America (81). The most important sources of invasive shrubs are Asia (103–118), Europe (68), and South America (54). Mean number of native geographical regions for invasive trees is 1.64, while the mean number of invaded regions by trees is 2.51. The difference is smaller for shrubs: 1.60 versus 2.11. Asia is the major source of invasive Rosaceae shrubs, as well as invasive Arecaceae and Oleaceae species. South America and Australia are major sources of invasive Fabaceae trees. North America and Europe are major sources of invasive Pinaceae. Most of the invasive Salicaceae are of Eurasian origin. The identified trends will very likely continue in this century. Because of increasing interactions with many states in Asia, even more invasive woody species will be introduced from this part of the world.

Both invasive alien trees and agricultural conversion have major impacts on biodiversity. We studied here the comparative impact of these two types of land transformation on a wide range of surface-active arthropod species using pitfall traps, with evergreen sclerophyllous natural vegetation (fynbos) as the control. The study was in the Cape Floristic Region, a global biodiversity hotspot, where alien trees are of major concern and where vineyards replace natural fynbos vegetation. Surface-active arthropods were selected as they are species rich, relatively immobile, and occur in high abundance. We hypothesized that the impact of the two types of land cover transformation would produce similar qualitative and quantitative effects on the arthropods. We also compared the results in the transformed and natural areas with those in areas cleared of alien trees. Arthropod species richness in cleared areas was higher than in vineyards and more similar to that in natural fynbos, while alien trees had the lowest. Overall abundance scores were highest in cleared areas, closely followed by fynbos, then vineyards and lowest in alien trees. Several species were restricted to each vegetation type, including alien trees. In terms of assemblage composition, all vegetation types were significantly different, although fynbos and vineyards grouped, suggesting that vineyards have less impact on the arthropod community than do alien trees. When rare species were excluded, vineyards and cleared sites grouped, indicating some recovery but only involving those species that were common and habitat tolerant. Our results suggest that vineyards retain a greater complement of indigenous species than alien trees, but that clearing of these aliens soon encourages establishment of indigenous species. Although there were significant differences in soil moisture and litter depth within and between vegetation types, we did not record them as significantly affecting species richness or abundance, even in alien vegetation, an encouraging sign for restoration.

Tree invasions into native grasslands and shrublands are global phenomena, with alien tree invasions occurring on nearly every continent. We hypothesized that wood nutrient concentrations are a key trait to enable successful tree invasions in regions limited by soil nutrient availability, such as the fynbos vegetation of the Cape Floristic Region of South Africa. We sampled wood and bark of a height range of indigenous and alien woody species within the Cape Floristic Region. We used linear regression and phylogenetic analyses to assess associations between wood nutrients and tree height and analysis of variance to assess how wood nutrients vary across vegetation types. Alien trees had lower wood nutrient concentrations than many native trees, and nutrient concentrations, especially not only N and P, but also Mg and K, were negatively associated with tree height. These results suggest that recent tree invasions into the nutrient‐poor fynbos shrublands may be a result of low wood nutrient concentrations that allow the alien trees to successfully overtop the native vegetation. This indicates that wood nutrient concentrations may be a useful trait to predict whether an alien tree species has a greater potential of becoming invasive in regions with limited soil nutrients.

Invasion by invasive alien trees is a major threat to the biodiversity of the fynbos biome in South Africa. The environmental factors influencing the intensity of invasion are, however, largely unknown. In particular, the effects of soil nutrients on the invasion of alien trees in the fynbos biome have not been investigated. The Catabolic Theory predicts that trees will be more competitive than shrubs and herbaceous plants where anabolic nutrients (e.g. Mg, Mn and B) are comparatively more abundant than catabolic nutrients (e.g. P, Cu and Zn). We tested this theory at seven study sites by comparing soil nutrient concentrations in non-invaded and adjacent invaded sub-sites. Soil samples from 275 plots across 17 sub-sites were analysed for pH, Na, Mg, K, Ca, P, S, C, B, Mn, Cu and Zn. The results corroborated the Catabolic Theory. An index of anabolic to catabolic nutrients was greater in all ten sub-sites invaded by invasive alien trees than in sub-sites without invasion. Nutrients strongly related to tree invasion included B and P. As per the Catabolic Theory, poverty of B at certain sites is likely to constrain anabolism and consequently tree establishment. Some non-invaded sites had B concentrations of 0.02 mg kg−1, which is an order of magnitude less than critical deficiency thresholds in an agronomic context. By contrast, richness of P (as a result of prior fertilization) in fallow agricultural fields would, as per the Catabolic Theory, promote catabolism and the competitive strength of shrubs and/or herbaceous plants relative to trees. Practical implications of these findings include: 1) the mapping of soil properties such as B and P to reveal the inherent propensity of landscapes in the Western Cape to be invaded by trees; and 2) the application of B-binding substances such as illite and vermiculite to constrain invasion of trees in fynbos.

Invasion by alien woody plants strongly affects plant-animal interactions, often reducing invertebrate and bird abundance, thereby altering pollination and ultimately influencing plant reproduction. However, it remains unclear whether invasive alien trees also affect interactions typically considered antagonistic, such as nectar robbing. This study investigates the effects of invasive alien trees on insect communities and how these, in turn, affect the occurrence and frequency of nectar robbing in bird-pollinated Erica discolor and Erica unicolor. Floral-visitor observations were recorded at 27 uninvaded and 31 invaded fynbos (a Mediterranean-type shrubland) sites along the southern Cape coastal mountains, South Africa, from April to June 2023. We assessed factors affecting (1) nectar-robbing insect abundance, (2) the rate of nectar robbing, and (3) the impact of nectar robbing on pollination rate. Although overall robbing rates did not differ between invaded and uninvaded sites, the species composition of nectar robbers shifted. The Cape honey bee (Apis mellifera capensis) dominated robbing in uninvaded sites, whereas small solitary bees dominated robbing in invaded sites. Robbing appeared to have no effect on bird pollination rates in either Erica species. This study is the first to directly compare the widely used cumulative measure of robbing rate with observed robbing rate, revealing a significant positive relationship between the two. Together, these findings offer insights into how invasive alien trees can alter fynbos ecosystems and emphasise the critical role of insect communities in shaping plant-animal interactions.

Biological invasions are a serious threat to nature conservation and sustainable forest management. Invasive trees can reduce biodiversity, modify nutrient cycles and transform native forests into novel ecosystems determined by invaders. Almost all European trees form ectomycorrhizal (ECM) symbiosis with fungi, which is crucial for tree development. However, the impact of invasive trees on ECM fungi in native forests has scarcely been studied.We aimed to determine how an invasion of the widespread alien trees Robinia pseudoacacia (which hosts arbuscular mycorrhizal (AM) fungi), Prunus serotina (which forms AM associations and potentially can ECM symbiosis), and Quercus rubra (ECM-dominated, can form AM associations) influences symbiotic fungi in soil. We collected soil samples along a gradient of increasing density of invasive species in 81 stands in pine-dominated forests in Western Poland, identifying fungi through high-throughput sequencing of the ITS2 rDNA, a barcode region for fungi. Each invasive tree was examined independently.Robinia pseudoacacia had the strongest negative impact, almost eliminating ECM fungi from the soil. Among soil compounds, N-NO3 and N-NH4 significantly influenced diversity and taxa composition of ECM fungal communities. Uninvaded pine stands near R. pseudoacacia had significantly lower richness and abundance of ECM fungi, and several-fold higher N-NO3 concentrations, than uninvaded stands elsewhere. That indicates, that the negative impact of Robinia trees extends beyond stands containing Robinia to influence surrounding forests. Prunus serotina had significant negative impact on ECM taxa abundance, but not on ECM taxa richness and diversity indices, while Quercus rubra enhanced the abundance and richness of ECM fungi, but had no significant influence on the diversity indices.

Se presenta la primera base de especies invasoras arbóreas y arbustivas de la Provincia de Córdoba y se analiza en el contexto de las invasiones por leñosas a escala nacional y mundial. Se identificaron 34 especies leñosas exóticas invasoras. La mayoría son originarias de Europa, Asia y Medio Oriente, introducidas como ornamentales. Los ecosistemas más invadidos son los pastizales (39%) seguido por bosques y matorrales (32%). La mayoría (59%) se dispersan por animales (aves) y viento 35%). A excepción de dos especies, la mayoría son también invasoras en otros lugares del mundo. Noobstante, solo la mitad de las invasoras de Córdoba se registraban como invasoras de Sudamérica. Esto último refleja la magnitud en la cual se está subestimando la invasibilidad del continente Sudamericano. Dos especies (Cupressus macrocarpa y Zanthoxylum armatum) se incorporarían como nuevas especiesen la base mundial de leñosas invasoras. Se detalla la información faltante en dos bases nacionales, esperando contribuir a la actualización de estos valiosos documentos. Finalmente se listan seis especies nativas de Argentina que estarían invadiendo comunidades vegetales de Córdoba. Esperamos que esta base de datos aporte al desarrollo de estudios básicos y aplicados en la ecología de las invasiones biológicas.

Financial and human resources are insufficient to address environmental and socio-economic threats by invasive alien trees; besides, the financial feasibility of value chains using the biomass (hence contributors to trees removal) is disputed. In South Africa, we study the supporting role of carbon pricing mechanisms for value chains using biomass from invasive alien trees: do such value chains meet carbon credit issuance conditions (eligibility), and are carbon-related revenues significant enough to make a difference? We target three value chains and apply certified methodologies to quantify emissions reductions and incentives levels: biochar, lump charcoal for stoves, chips for steam production in industrial boilers. We find that eligibility depends on standards as their approach and criteria differ, yet the trend is supportive. Besides, the fact that invasive alien trees are an environmental liability has encouraged standards to adopt a more flexible approach and issue credits on the sole condition that clearing operations are sustainable and regardless of other considerations in terms of climate change mitigation impact. Carbon incentives vary greatly among projects and hold potential to make a difference with up to 20% of the final product value. This translates into an 8‐95% increase in the maximum affordable biomass supply cost to break even and depending on value chains and assumptions (e.g. market value of carbon credits). For biochar specifically, the prospects of very high-value carbon credits due to the alleged contribution as Carbon Dioxide Removals (CDR) would secure profitability even with very low biochar market prices, which is possibly a game changer to support biochar production in a context where willingness to pay by farmers/users remains low. Overall, our study shows that carbon markets could contribute to the management/eradication of invasive alien trees despite debatable mitigation impacts, but that value chains must also rely on solid markets for their end products to break even.