Current knowledge, status, and future for plant and fungal diversity in Great Britain and the UK Overseas Territories

Colin Clubbe1 | A. Martyn Ainsworth2 | Sara Bárrios1 | Keith Bensusan3 | Juliet Brodie4 | Paul Cannon2 | Ted Chapman5 | Alison I. Copeland6 | Marcella Corcoran1 | Michele Dani Sanchez1 | John C. David7 | Trevor Dines8 | Lauren M. Gardiner9 | Martin A. Hamilton1 | Thomas Heller1 | Peter M. Hollingsworth10 | Nicola Hutchinson8 | Theo Llewelyn11,12 | Laura Lowe Forrest10 | Kevin J. McGinn13 | Stephanie Miles5 | Katherine O'Donnell14 | Nancy Woodfield-Pascoe15 | Tim C. G. Rich16 | Fred Rumsey4 | Jolene Sim17 | Struan R. Smith18 | Nicola Spence19 | Andrew Stanworth20 | Peter Stroh21 | Ian Taylor22 | Clare Trivedi5,23 | Alex D. Twyford10,24 | Juan Viruel1 | Kevin Walker25 | Jo Wilbraham4 | Julian Woodman26 | Michael F. Fay1,27


| INTRODUC TI ON
In this article, we summarize current knowledge of the status and threats to the plants and fungi of Great Britain and the UK Overseas Territories (UKOTs; former British colonies that have elected to remain under British sovereignty). The UK Government reaffirmed its close relationship with UKOTs in its 2012 White Paper setting out its commitment to work with the UKOTs to address the challenges we face, with a strong focus on the environment and biodiversity (UK Government, 2012). The 25-year Environment Plan (UK Government, 2018) contains renewed commitments to improve the environment of the UK and UKOTs at species and site levels. The UK's ratification of biodiversity-related Multilateral Environmental Agreements has been extended to many UKOTs (CBD, 2020;CITES, 2020;Ramsar, 2020).
UKOTs are remarkably diverse, consisting of islands and peninsulas spread across the world's oceans (Figure 1). Together they have a surface area approximately seven times that of mainland UK and are collectively home to many endemic species (McPherson, 2016), with some estimates putting this at 94% of known endemic British species (Churchyard et al., 2014). In contrast, the UK endemic flora is composed of relatively large numbers of apomictic taxa in a small number of genera; most of the sexual species also occur elsewhere in Europe. In addition, due to many species being at the edge of their range in the UK, they are often more abundant elsewhere in their range (Wigginton, 1999). The bryophyte flora similarly shows low levels of endemism, but includes a higher proportion of rare, widely disjunct species for which the UK has globally significant populations.
The UK and the UKOTs together host a globally unique flora and mycota constituting an important component of the world's plant and fungal diversity. Our review highlights the knowledge gaps relating to the biodiversity of some of the remotest islands and archipelagos in contrast to the often-detailed knowledge, particularly of the flora, of Britain. In the face of global change, for conservation to be effective, we have a responsibility to know and understand the biota of these geographically scattered parts of the world. The gaps identified here are often the same as those for other remote and understudied parts of the world, and lessons learned with rare oceanic island endemics will inform conservation elsewhere. Global biodiversity faces unprecedented threats with 75% of terrestrial and 66% of marine environments already severely altered by human activities (UN, 2020), we need transformational change to stop biodiversity loss. To conserve and sustainably use biodiversity, we need to know what it is and what state it is in.
We hope this review will promote a new renaissance in plant and fungal exploration and as we plug the information gaps for the UK and particularly for UKOTs this will lead to a greater realization of the importance of retaining our remaining biodiversity and foster a greater recognition of the importance of these far-flung parts of the world.  (Wigginton, 1999).

| Geographical coverage
We treat St Helena, Ascension, and Tristan da Cunha separately because of their individually unique biodiversity. We exclude the Sovereign Base Areas of Akrotiri and Dhekelia due to the difficulty of disaggregating data from the island of Cyprus.

| Taxonomic coverage
For plants, we review available information for vascular plants, bryophytes (hornworts, liverworts, and mosses), and freshwater, terrestrial, and marine algae. For fungi, we review Basidiomycota, Ascomycota (including lichenized fungi), and other groups for which data are available.

| HIS TORIC AL OVERVIE W
The flora of mainland UK is one of the most studied worldwide and is well represented in herbaria. With c. 620,000 specimens, the British and Irish Herbarium at the Natural History Museum, UK (BM; herbarium acronyms follow Index Herbariorum, NYBG, 2020), holds material of 100% of native genera and >99% of native species, often collected throughout their British and Irish range (Natural History Museum, 2020). The Sloane Herbarium of c. 80,000 specimens, perhaps the largest pre-Linnean collection, provides a significant repository of first British records. The herbarium at the Royal Botanic Gardens, Kew, UK (K), includes an estimated >185,000 UK plant collections. Among the c. 3,000,000 specimens held in the Herbarium at the Royal Botanic Garden   Sell and Murrell (1996, 2014. These collections are of great importance for studies of historical distributions and changes in the face of climate change. Major publications have focused on the origin of the UK flora and its history since prehistoric times (e.g., Godwin, 1956;Proctor, 2013), and distributions documented in detail (e.g., Preston, Pearman, & Dines, 2002;Stace, 1997Stace, , 2019. Some studies highlighted changes in the vascular plant flora due to anthropogenic and other pressures (e.g., Braithwaite, Ellis, & Preston, 2006;Preston et al., 2002).
The bryophyte flora is well studied and documented, with major herbarium collections at BM, E, CGE, and NMW, the last housing BBSUK (the British Bryological Society herbarium), the voucher repository for all new vice-comital records. Four comprehensive hectad-level distribution atlases have been produced (Blockeel, Bosanquet, Hill, & Preston, 2014;Hill, Preston, & Smith, 1991, 1992, 1994. The seaweed flora of the UK is also one of the most studied; BM holds material of virtually every species recorded from the UK, with collections being used for studies of rare and non-native species (e.g., Brodie, John, Tittley, Holmes, & Williamson, 2007;Brodie, Wilbraham, & Pottas, 2013;Brodie, Wilbraham, Pottas, & Guiry, 2016). There is an atlas of red, green, and brown seaweeds for Britain and Ireland (Hardy & Guiry, 2003) and a provisional red data list (Brodie et al., 2013).

| UKOTs fungi
The mycota of the UKOTs remains largely unknown, and few territories have full checklists or species guides. Some regional lists exist (e.g., Minter, Rodríguez-Hernández, & Mena-Portales, 2001), but data on specific territories are difficult to access for the non-specialist. Waterston (1947) (Detheridge et al., 2020) revealed low numbers of Basidiomycota and high numbers of Capnodiales (Ascomycota), an order usually associated with leaf surfaces. Kohlmeyer and Kohlmeyer (1977) published a key to filamentous higher marine fungi in Bermuda.

| British plants
The

Box 1 Hybridization in vascular plants
Natural hybridization has been intensively studied in the British flora, with Stace (1975) and Stace, Preston, and Pearman (2015) representing landmark publications documenting natural hybrids, their parentage, distribution, cytology, and ecological attributes.
This unrivalled resource has been used to assess the frequency of hybrids and the factors that affect their persistence in nature.
Although most of the 909 natural hybrids are spontaneous, 152 hybrids have become established following introductions . Hybrids are particularly common in a few genera, with over half of the native hybrids being found in Epilobium, Euphrasia, Rosa, Rumex, and Salix.
The extensive knowledge base of hybridization in the British flora has led to it being used as a baseline comparison for the prevalence of hybridization in animals (Mallet, 2005), one of eight floras used in a global meta-analysis of plant hybridization (Mitchell et al., 2019), and a model for studying hybridization under global change (Vallejo-Marín & Hiscock, 2016). In addition, focused research on hybrids has been instrumental in understanding the evolutionary processes that shape the outcome of hybridization.
Gaultheria antarctica × G. plumula is a natural hybrid reported from the Falkland Islands (Heller et al., 2019). Hybridization of native endemics with non-native taxa (often introduced for horticulture) is an increasing problem. In Bermuda, there is concern over the hybridization of the endemic Juniperus bermudiana with J. virginiana (Adams, 2008;Adams & Wingate, 2008). In the Cayman Islands, hybridization with an introduced relative is considered a threat to the genetic distinctiveness of the endemic Cordia sebestina var.
as aliens in Ireland. Conversely, 15 taxa are native in Ireland but considered alien in Britain. For information about non-native plants in Britain, see Box 2.
There are differences between this and other lists, largely due to the inclusion (or not) of apomictic taxa [see, e.g., Stace (2019) vs. Sell and Murrell (1996, 2014]. Thirty-one non-native species (5%) were listed, but the numbers may be higher and will include naturalized species that are thought indigenous.

| UKOTs plants
There has been no systematic analysis of the status of the UKOTs flora, and data are not yet centralized to enable this analysis to be completed. Data presented here are the first attempt at a synthetic analysis based on specimen records available in online databases. We used specimen data from Plants of the World Online (POWO, 2020) supplemented by selected checklists, resolving synonymy issues as far as possible to produce a summary of vascular plants from each territory (  (Baardseth, 1941), who noted that endemism for marine macroalgae was relatively high (40%). Chamberlain (1965) reported 40 marine algal for Gough Island. South Georgia has a unique and diverse seaweed flora with 127 species (Wells, Brewin, & Brickle, 2011). Price and John (1980) recorded 52 species for Ascension, increased to 82 by Tsiamis et al. (2017).

| British fungi
Estimates of the numbers of UK species range from 12,000 to 20,000 species to, more recently, 18,000 to 20,000 species (e.g., Dines & McCarthy, 2014;NBN, 2020). However, there is still no comprehensive annotated checklist of British mycota. Significant milestones include checklists for British agarics and boleti (Dennis,

Box 2 Established aliens in Britain
As a result of intensive management of much of the British landscape for agriculture, forestry, and horticulture, non-native plant species have become a significant part of the vascular plant flora. Stace and Crawley (2015) reported that, at 57%, the proportion of alien plants in the UK is probably the highest in Europe. Increasing numbers of alien species are becoming established, and the BSBI publishes regular updates on "adventives and aliens" (e.g., Berry, 2020) and their latest figures indicate that 66% of the species in Britain are non-native (Table 1).
There are alien taxa in other terrestrial groups. A few bryophyte species (c. 15) are thought to be introductions through horticulture.
Two early imports, Orthodontium lineare (first record 1920) and particularly Campylopus introflexus (1941), have achieved extensive British ranges significantly impacting native flora. Lophocolea semiteres and L. bispinosa are proving invasive, but their impact is difficult to assess. In the marine environment Brodie et al. (2016) listed 31 seaweed species considered non-native to Great Britain. One of these, Japanese wireweed (Sargassum muticum), is now widespread following its arrival in the 1970s; where it is spreading on the Scottish coast it is considered to present a significant threat to marine biodiversity (Gaywood, Boon, Thompson, & Strachan, 2016).
Not all non-native species represent a threat to native flora. Many archaeophytes are treated as important from a cultural or conservation point of view, e.g., snake's-head fritillary (Fritillaria meleagris) and Tenby daffodil (Narcissus obvallaris), or are familiar "wild plants", e.g., shepherd's purse (Capsella bursa-pastoris) and the sycamore (Acer pseudoplatanus). A few non-natives represent a major threat to native habitats such as Rhododendron ponticum (CABI, 2019a) and Impatiens glandulifera (CABI, 2019b). They can also have impacts on the economy (e.g., Reynoutria japonica (CABI, 2019c) and human health, e.g., Ambrosia artemisiifolia (CABI, 2019d). Aquatic weeds (e.g., Crassula helmsii, Ludwigia grandiflora, and Myriophyllum aquaticum) that spread by small fragments through water courses represent a serious problem, and these were the first to be banned from sale in the UK in 2014 (National Archives, 2020). The impact of climate change on alien plants is being investigated. There is some evidence that ornamental species that were previously not a threat in the UK now have the potential to spread into the wider environment. For example, the tree of heaven (Ailanthus altissima) is beginning to spread in the urban heat island of London. Some also may have the potential to provide resilience to climate change, e.g., holm oak (Quercus ilex) and cherry laurel (Prunus lusitanica), as well as providing ecosystem services for native invertebrates (Salisbury et al., 2017(Salisbury et al., , 2020. Thirty-seven species of lichens/lichenicolous fungi were considered as endemic/probable endemic by Woods and Coppins (2012), 14 of which have since been found elsewhere. Assessing endemism for lichens is complicated by the fact that the sterile/sorediate morph and the fertile morph of a species can be visually distinct (e.g., Ertz, Coppins, & Sanderson, 2018).

TA B L E 2
Of the 39 Basidiomycota with British holotypes described between 2000 and 2019, evidence was found for all but eight also occurring overseas. At this stage these should not be regarded as endemics because some are inconspicuous and only known from the type locality and most are still poorly known, even in Britain. Spooner and Roberts (2005) noted the existence of >300 (mostly inconspicuous microfungal) species described from Britain with no known records elsewhere, but concluded that these data highlight our poor understanding of fungal distributions; few endemic fungi are likely to occur in Britain.

| UKOTs fungi
There are huge gaps in our knowledge of UKOTs fungi.

| G LOBALLY S I G NIFI C ANT PL ANTS . FUNG I AND HAB ITATS OF THE UK AND U KOTs
In the face of global (largely anthropogenic) change, actions to mitigate the effects of the environmental challenges we face are widely recognized to be urgently needed if we are to avoid major losses to biodiversity and destruction/degradation of habitats. However, it is also generally perceived that conserving representative populations of all species is not achievable, and decisions need to be made about priorities for action.
One approach to this process of "triage" (e.g., Hayward & Castley, 2018, and references therein) is to assess the significance of the biota of countries, regions, or habitats against those of other parts of the world, and the significance can be assessed in various ways, depending on the aims and desired outcomes. In a broad sense, these can be divided into those that focus on (a) species richness; (b) phylogenetic distinctiveness; (c) presence of unique or rare genetic variation within species/populations; and (d) the regional or global significance of the ecosystems/habitats.
Species richness is a relatively easy measure and it has led, for example, to the recognition of global biodiversity hotspots (sensu Myers, Mittermeier, Mittermeier, da Fonseca, & Kent, 2000).
However, this approach generally fails to include an assessment of the evolutionary distinctiveness of the species (treating all species as equal), the presence of unique or rare genetic variation, or the rarity of the habitat types that are formed by the species. In recent decades, assessment of phylogenetic distinctiveness of taxa has been increasingly used to overcome the first of these issues, and phylogenetic diversity (PD) sensu Faith (1992) links diversity and

Box 3 Established aliens in the UKOTs
Invasive Alien Species are an increasing threat to the plants of the UKOTs and rank as the second most common threat to species listed on the IUCN Red List (IUCN, 2020). Although there has been no systematic analysis of the non-native flora across the UKOTs since Varnham (2006), many territories report an increasing number of alien plants in their floras and rank invasive species as one of the top three threats to biodiversity (e.g., Clubbe, Hamilton, & Corcoran, 2010;Gray, Pelembe, & Stroud, 2005;GSGSSI, 2016;Hughes et al., 2020;Kingston & Waldren, 2003, 2005Lambdon, 2012;Rojas-Sandoval & Acevedo-Rodríguez, 2015). Although there is uncertainty in these data, Table 3 indicates that the territories with particularly high non-native components are Ascension (82%), St Helena (79%), British Indian Ocean Territory (77%), South Georgia (70%), Tristan da Cunha (68%), and Bermuda (63%). Recent work coordinated through the GB Non-Native Species Secretariat has identified increased biosecurity as a key solution and a horizon scanning exercise has prioritized actions and identified likely next species threats (GBNSS, 2020).
Research and control of invasive species is underway in many territories. In South Georgia, a Non-Native Plant Management Strategy is being implemented controlling 41 non-native species (GSGSSI, 2016). In the Falkland Islands, 14 high-risk invasive plants have been identified for control (Lewis, 2014) with Berberis microphylla (calafate) of particularly concern and the focus of current control activities. In Bermuda, a significant proportion of the landscape is covered in invasive vegetation which presents the biggest threat to the survival of the indigenous flora (Government of Bermuda, 2020), and Wolsak, Wingate, and Cronk (2018) (Smyth, Waldren, & Kingston, 2010). Because of its aggressive growth and the decline in its wood being harvested for fuel, it has formed dense canopies across the island and is preventing native species regeneration (Smyth, 2008), including the endemic Abutilon pitcairnense (Smyth et al., 2010). A study in Montserrat used prediction mapping of the spread of Psidium guajava and Cryptostegia madagascariensis which showed a near complete overlap with the habitat requirements of the endemic Rondeletia buxifolia and if left unchecked could result in its extirpation (Jones, Clubbe, & Hamilton, 2012;Stow, 2008 Table 3). PD is also likely to be low, but this remains to be rigorously tested, although the British vascular flora has been used as a test case for a variant of the EDGE approach (Pearse et al., 2015) and St Helena's fragile flora containing unique monotypic genera has recently been reviewed (Lambdon & Cronk, 2020).
Many species in the British flora have been studied genetically and the dominant pattern seen is that the genetics of British populations is a subset of that found elsewhere in the range, but relatively few detailed studies exist for species from the UKOTs (see Box 5 for more detail).  Table 4 summarizes IPAs and IFAs in Britain.
These incorporate new criteria to assess sites for direct protection of fungal diversity and include lists of fungal assemblages. Some British sites are of international significance, e.g., those with outstanding assemblages of hyper-oceanic lichens, nitrogen-sensitive grassland fungi, and fungi inhabiting dead beech (Fagus sylvatica).
Two UKOTs have completed IPA assessments. Upson (2012) identified 17 IPAs across the Falkland Islands. These are now listed as key sites of biodiversity interest (FIG, 2016a), providing a guide for implementing the Biodiversity Framework (FIG, 2016b). Following the revised IPA concept for tropical regions (Darbyshire et al., 2017), 18 Tropical IPAs (TIPAs) have been identified in the British Virgin Islands which support concentrations of globally and nationally threatened plants and nationally threatened habitats (BVI TIPAs National Team, 2019). Preliminary work to identify TIPAs has been undertaken in Turks and Caicos Islands (Hardman et al., 2012) and Montserrat (Jones, 2008).
Other site-based prioritizations have been conducted in most territories to declare protected areas and reserves. For example,

Box 4 Managing the threats to Fraxinus excelsior in the UK
Fraxinus excelsior (European ash) is a large tree, native to the UK and found across much of mainland Europe. In Britain, ash is the second most abundant tree species in small woodland patches and the third most abundant in larger areas of forest. Also found in hedgerows, by roads and railways and in urban environments (Thomas, 2016), it is estimated that there are 125 million ash trees in woodlands and 27-60 million ash trees outside woodlands in the UK (Department for Environment, Food & Rural Affairs, 2015;Forestry Commission, 2013. Ash is highly valued for its wood and supports biodiversity and ecological function. Almost 1,000 species are associated with ash, of which 45 are believed to have only ever been found on ash (Mitchell et al., 2014a(Mitchell et al., , 2014b (Coker et al., 2019). The total cost of ash dieback to the UK has been estimated at £14.6 billion based on the cost of dealing with the impacts of the disease (e.g., felling), replanting, and loss of ecosystem services (Hill et al., 2019).
Ash is also under threat from the emerald ash borer

St Helena's unique cloud forest is protected as the Peaks National
Park (St Helena Government, 2020). Ascension's Green Mountain National Park conserves remaining cloud forest (Ascension Island Government, 2020;Duffey, 1964). The British Virgin Islands' Systems Plan includes 21 terrestrial National Parks (Gardner, Smith-Abbott, & Woodfield, 2008) and Montserrat's key biodiversity is conserved in the Centre Hills Reserve (Young, 2008).
Bermuda successfully re-afforested the Nonsuch Island Nature Preserve, a more than 50-year effort to establish a pre-colonial forest (Wingate, 1985).

| British plants
The

Box 5 Genetic diversity in UK and UKOTs vascular plants and bryophytes UK plants
DNA barcoding data exist for all UK native seed plants. Initial work produced a complete DNA barcode dataset for the flora of Wales (de Vere et al., 2012), which has since been expanded to all of Britain, with a full three-locus DNA barcode of rbcL, matK, and ITS2 available for multiple individuals from 1,016 taxa; data for at least one locus are available for c. 1,500 taxa (de Vere et al., unpublished). These data provide a resource for plant identification and studies of ecological processes and interactions. For bryophytes, the liverwort flora has been DNA barcoded for rbcL, matK, psbA-trnH, and ITS2 (RBGE, unpublished), resulting in the detection of several new species (e.g., Bell et al., 2012); further potential new species await a follow-up study (Forrest et al., unpublished).
There are many studies looking at patterns of genetic diversity in British species, and >1,000 taxa have some genetic study, on British or non-British populations (Ruhsam et al., 2018). The British flora has been profoundly impacted by past glaciations, resulting in large-scale changes in distributions and many native species have undergone dramatic and recent changes in range in response to climate change (Wang et al., 2014). This recent history has limited the opportunity for unique highly divergent genetic lineages to evolve. There are some cases of divergent lineages being restricted to the UK, but the dominant signal is that species present in the UK have a subset of the genetic variation found elsewhere, as seen in, e.g., Calluna vulgaris (Rendell & Ennos, 2002), Cypripedium calceolus (Fay et al., 2009), and Silene nutans (Martin, Touzet, Van Rossum, Delalande, & Arnaud, 2016). Next-generation sequencing approaches are being increasingly used to investigate diversity in British species (e.g., Primula farinosa; Theodoridis et al., 2017).
Despite the limited evidence for historically isolated genetic lineages, there is a well-established body of literature showing genetic adaptation to environmental conditions in Britain, and widespread species often show differential adaptations across their British range. Population differences are often driven by environmental heterogeneity, with Britain having considerable variation in rainfall, temperature, elevation, and soil type. For example, reciprocal transplant experiments in Nardus stricta revealed reproductive variation between Scottish populations, with clear home-site advantage related to elevation (Miller & Cummins, 2014).
A provisional Red Data list for UK seaweeds revealed that 34% are DD, indicating that further work is needed to determine species distribution and status (Brodie & Wilbraham, 2014). Threats in Britain include habitat loss, harvesting, non-native species, and climate change (Brodie et al., 2016;Yesson, Bush, Davies, Maggs, & Brodie, 2015).
Among freshwater habitats, there are no undamaged rivers in lowland England and Wales and most ponds and streams are biologically degraded (Freshwater Habitats Trust, 2013). Distribution and ecological data are lacking for most groups of freshwater algae; e.g., Rhodophyta are largely restricted to streams and rivers and are sensitive to changes in water quality; their populations have almost certainly declined drastically, but little monitoring has been conducted. Stoneworts are among the most severely threatened plants in Britain and are the only algal group to have a published Red Data book (Stewart & Church, 1992). Of the 30 known species in Britain, 17 are nationally rare or extinct (Stewart, 2004); the number of sites with extant populations has declined by >60 percent in the last 30 years (Lambert, 2009). The most serious threat is nutrient enrichment (nitrates and phosphates) of water systems (Stewart, 2004), and detrimental management and encroachment and competition by successional vegetation are also factors (Stewart, 2004). Most recently, Stewart and Hatton-Ellis (2020)  a great diversity of desmids; they are at risk given their sensitivity to water quality, but we lack the data necessary for assessing their status adequately.

| UKOTs plants
A global Red List for UKOTs is far from completion, and many territories are undertaking Red Listing assessments, concentrating initially on rare and endemic taxa. Currently 515 taxa have been globally assessed, representing c. 21% of the flora (Table 6). Of these, 135 taxa  (Burton, 2008) and the Falklands Islands (Broughton & McAdam, 2002;Upson, 2012) and are in progress in several other territories. At a national level, 27% of the Cayman Islands flora and 21% of the Falkland Islands floras are threatened, comparable to the global figure of 21% for vascular plants (Bachman et al., 2016).
Bermuda tested a new rapid assessment tool which has the potential to accelerate assessments across the UKOTs (Bachman, Walker, Bárrios, Copeland, & Moat, 2020). There is one approved Red List for non-lichenized fungi in Britain (Ainsworth et al., 2013) (James, Hawksworth, & Rose, 1977) and the UK has international responsibility for the conservation of 30 of these species (Ellis, 2016;Woods & Coppins, 2012). These communities are now highly fragmented due to habitat loss through land-use inten- Among more widespread threats like climate change and pollution, ash dieback presents a new serious threat for epiphytic lichens.

| UK fungi
Thirty percent of UK lichens occur on ash and the negative effect of ash dieback on lichens has been predicted to be comparable to that of climate change (Ellis et al., 2014). However, an increase in average occupancy for UK lichens since 1970 suggests communities may be starting to recover from major losses prior to this (Outhwaite, Gregory, Chandler, Collen, & Isaac, 2020).
Collection continues to fill gaps, mainly of rare, specialized, and taxonomically complex species and those that do not reliably produce seed in the UK.
Greater emphasis is being placed on increasing intraspecific sampling depth, conserving large collections that capture genetic diversity within and between populations across species ranges (Willis et al., 2018). The UK National Tree Seed Project developed an innovative sampling strategy to capture genetic diversity at national, eco-geographical, and individual mother plant scales (Kallow & Trivedi, 2017). Seventy-five native species have been collected,

| CON CLUS IONS
Detailed knowledge of biodiversity is a fundamental requirement for wise management of natural resources and to harness society support for conservation. This review has highlighted many gaps in our knowledge of the plants and fungi of the UK and UKOTs. Exploration and research on the vascular plant flora of the UK have received much attention over the last 100+ years, and we can almost say that we have a complete baseline knowledge, and work on monitoring trends and changes is ongoing. In contrast, work across the UKOTs is still in the exploration phase with some territories lacking sufficiently up-to-date records of their flora to enable effective conservation measures in a changing world. There is an urgent need to secure significant resources to complete this exploratory phase so that territories can more effectively conserve and sustainable utilize their floras. For non-vascular plants, particularly algae, the knowledge base is patchier with many gaps evident. However, the biggest gaps remain in our knowledge of fungi, particularly for groups other than lichens and Basidiomycota.

ACK N OWLED G M ENTS
The authors and trustees of the Royal Botanic Gardens, Kew, and the Kew Foundation would like to thank the Sfumato Foundation for generously funding the State of the World's Plants and Fungi project.
We are grateful to Amanda Cooper for preparing the map in Figure 1 and to Rafaël Govaerts and Nicholas Black for supplying data from POWO. We thank Quentin Cronk and a second anonymous reviewer for valuable comments on the first draft of this manuscript. We wish to thank several colleagues (Natasha de Vere, Richard Ennos, Paul Kirk, Jonathan Krieger, Iain Macdonald, Alan Paton, and Suzanne Sharrock) in the UK who made useful comments on earlier drafts of this review. We thank the many colleagues across the UKOTs who have been at the forefront of botanical and fungal exploration for many years and who we have collaborated with in many projects, the data which have enabled us to undertake this review, and to Fred