Biol Invasions (2014) 16:473–481 DOI 10.1007/s10530-013-0606-9 ORIGINAL PAPER Tree invasions: patterns, processes, challenges and opportunities David M. Richardson • Cang Hui • Martin A. Nun˜ez • Anı´bal Pauchard Received: 1 October 2013 / Accepted: 13 November 2013 / Published online: 4 December 2013 Ó Springer Science+Business Media Dordrecht 2013 Abstract Tree invasions have escalated in impor- determinants of invasiveness and invasibility), but tance in the last few decades (more species, greater also the full spectrum of human perceptions and values area invaded, more types of impacts, increasing that frames biological invasions as an environmental complexity of management challenges), and are problem. This editorial provides background and increasingly studied from many perspectives. This summarizes the main outputs from a workshop held research spans many disciplines, including ecology, in Argentina in September 2012 that set out to population biology, genetics, remote sensing, ecolog- summarize current knowledge on key topics and to ical modelling, risk analysis, resource economics and, determine the most important challenges facing increasingly, the humanities. There has been sub- researchers and managers. The sixteen papers in the stantial progress in understanding patterns and pro- special issue of Biological Invasions span disciplines, cesses, but many unanswered questions remain. Only a geographic regions and taxa and provide novel few invasive trees have been well studied, many of insights on pathways and historical perspectives, them in only a small part of their invasive range. detection and monitoring, determinants of invasive- Invasive trees often have substantial impacts, espe- ness, function and impact, and the many challenges cially when they invade formerly treeless vegetation. that face managers. Trees have several features that make them useful for understanding key aspects of biological invasions (the Keywords Introduction pathways Á Impacts Á Invasibility Á Invasiveness Á Tree invasions D. M. Richardson (&) Á C. Hui Centre for Invasion Biology, Department of Botany and Zoology, Stellenbosch University, Private Bag X1, Matieland 7602, South Africa Introduction e-mail: [email protected] Trees did not feature prominently on national, regional M. A. Nun˜ez and global lists of the most important invasive plants Laboratorio Ecotono, INIBIOMA, CONICET, University Nacional del Comahue, Quintral 1250, 8400 Bariloche, until fairly recently. In the past few decades, however, Argentina hundreds of tree species have become invasive (Richardson and Rejma´nek 2011; Rejma´nek and A. Pauchard Richardson 2013), and many now feature on lists of Facultad de Ciencias Forestales, Institute of Ecology and Biodiversity (IEB), Universidad de Concepcio´n, the most widespread and damaging of all invasive Concepcio´n, Chile species (Weber 2003; Richardson and Rejma´nek 123 474 D. M. Richardson et al. Table 1 Major players contributing to knowledge of tree invasions Journals Subject categories Countries Funding Organizations Biol Invasions (198) Environmental sciences/ecology (1,537) USA (816) NSF (110) Stellenbosch U (45) Plant Ecol (78) Biodiversity conservation (484) Australia (154) USDA (26) US Forest Service (44) Biol Cons (58) Plant sciences (195) South Africa (89) EU (18) Rice U (41) Divers Distrib (54) Forestry (155) Canada (85) CIB (14) US Geol Survey (38) J Ecol (50) Evolutionary biology (89) France (76) ARC (13) U Florida (33) Ecology (47) Biochemistry molecular biology (61) Germany (66) ASCR (9) Oregon State U (31) Oecologia (46) Physical geography (61) New Zealand (62) NSERC (9) Colorado State U (30) Rest Ecol (46) Zoology (40) Britain (60) WfW (9) U Cape Town (30) Ecol Appl (42) Genetics heredity (24) Spain (51) NC (8) U Wisconsin (28) J Appl Ecol (39) Agriculture (20) Japan (50) CAS (7) UC Davis (27) The top 10 journals, ISI Web of Science subject categories, countries, funding agencies and organizations of the 1,537 publications in the Ecology category ([2,000) were listed from a search on Web of Science (5 November 2013) using ‘tree*’ and ‘invasi*’ as search terms. Numbers in brackets are the number of publications. NSF National Science Foundation, CIB DST-NRF Centre of Excellence for Invasion Biology, ARC Australian Research Council, ASCR Academy of Sciences of the Czech Republic, NSERC Natural Sciences and Engineering Research Council of Canada, WfW Working for Water Programme, South Africa, NC Nature Conservancy, CAS Chinese Academy of Sciences. Some studies report on range expansions of native tree species within, or adjacent to the native range of the species 2011). Invasive trees have substantial and growing the massive invasion debt created by a century or more impacts on biodiversity, ecosystem functioning, and of dissemination and planting of non-native trees is human livelihoods in many regions. However, many now leading to the manifestation of many large-scale species are still useful, in some areas and in some invasions. Hundreds of papers now document diverse contexts, for the same reasons for which they were aspects of tree invasions (Table 1), including histor- introduced and disseminated, or for totally new uses. ical accounts of introductions and plantings, the Few of the major invasive trees are exclusively emergence of problems with particular taxa and desirable or undesirable; this creates complex, con- approaches for dealing with them, detailed studies text-specific problems for researchers, society and on the ecology of invasions and the effects of invasive managers. species, and the application of modern molecular The ultimate reason for the escalation of problems techniques to reconstruct invasion histories and with tree invasions worldwide is the rapid increase in unravel fundamental aspects of their dynamics. The the human-mediated transport and dissemination of human dimensions of invasions have begun to be thousands of species for a wide range of purposes, explored by historians, sociologists, philosophers and especially forestry, agroforestry and ornamental hor- economists. ticulture. Vast plantations of alien trees now dominate Despite the increasing attention to trees as invasive many regions of the world. Alien trees are also species worldwide, problems associated with these increasingly conspicuous as ornamental and amenity invasions are increasing in magnitude and complexity. plants and in different forms of agroforestry, and other There are very few success stories—where the extent species are widely used for food production. Large- and impacts of tree invasions have been reduced to the scale projects are championing the use of trees, mainly satisfaction of all stakeholders. Many introduced tree non-native species, for the provision of goods and species are both crops of commercial, cultural or services, especially in developing countries (Low aesthetic importance in some parts of the landscape, 2012). The massive propagule pressure, in many cases but serious pests in others, creating vexing conflicts of coinciding with disturbances in receiving environ- interest which thwart or paralyse management efforts. ments, is driving the emergence of invasions in many In September 2012 a workshop was held on Isla regions. The long time lag for tree invasions (Kowarik Victoria Island, Nahuel Huapi National Park near 1995) has probably given the false impression that Bariloche, Argentina, to assess the state of knowledge trees are less invasive than some other plant forms, but of tree invasions worldwide. The meeting aimed to 123 Tree invasions 475 synthesize current knowledge in selected areas and to applied to other taxa. For example, life-history traits determine the most important challenges facing that confer invasiveness in Pinus (Rejma´nek and researchers and managers. The phenomenon of alien Richardson 1996) and other conifers (Richardson and tree invasions was discussed and debated from many Rejma´nek 2004) have been successfully used to angles and perspectives during the 4-day meeting. The predict invasiveness in woody plants in general overall aim was to move the research agenda for tree (Rejma´nek et al. 2013). However, much work remains invasions beyond the elucidation of case studies to be done to test whether insights from model groups towards a more general understanding of the wide and systems can be reliably applied much more widely range of factors involved in mediating the outcome of (Kueffer et al. 2013). Such insights are urgently introductions and shaping the options for manage- required to improve our ability to screen species for ment. The workshop was attended by 22 participants invasiveness—both new introductions and species from at least nine countries (as reflected by their already widely planted and which may constitute a current affiliations) who brought diverse interests and substantial ‘‘invasion debt’’ (sensu Essl et al. 2011). experience with tree invasions. Tree invasions into formerly treeless or tree-poor This special issue of Biological Invasions com- systems are relatively easy to detect and map, using prises a collection of 16 papers that arose from conventional as well as new high-tech methods. This deliberations at the workshop. The papers provide a paves the way for obtaining data at levels of accuracy reasonable cross-section and flavour of issues from the and at spatial scales that have hitherto been unavail- frontline of research on tree invasions. Early versions able to population biologists and modellers for study- of all papers were presented