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A Comprehensive Guide to the Lichens
The concept of a lichen species can no longer be restricted to the classical view of a mutualistic association between a single fungal partner and one photosynthetic partner. Traditionally, species delimitation in lichens relied primarily on the mycobiont, as the fungal component governs morphology, development, and secondary chemistry. However, recent research has demonstrated that the lichen thallus is a complex multi-organism symbiotic system, hosting multiple algal lineages, yeasts, bacteria, and even viruses Morillas et al. (2022).
Within this modern framework, a lichen species represents a multi-species symbiotic unit, whose structure and function emerge from dynamic interactions among the mycobiont, one or more photobionts, and the lichen microbiome. This microbiome—comprising diverse microorganisms inhabiting the surface or internal layers of the thallus—contributes directly to ecological performance, stress tolerance, metabolic capabilities, and the production of bioactive compounds.
Thus, a lichen species may be understood as a fungus-defined symbiotic system, whose identity and ecological behavior are fundamentally shaped by its associated microbial consortium. Consequently, species boundaries in lichens should be delimited not solely by fungal morphology or genetics, but also by considering the composition, stability, and functional integration of the full symbiotic community that collectively forms a persistent and coherent thallus.
Ref:
Morillas, L., Roales, J., Cruz, C., & Munzi, S. (2022). Lichen as multipartner symbiotic relationships. Encyclopedia, 2(3), 1421-1431.
Lichen morphology comprises a suite of specialized structural architectures that integrate fungal and photosynthetic partners into a coherent thallus. Fundamental forms include crustose thalli, intimately adpressed to substrates; foliose thalli, exhibiting dorsiventral leaf-like lobes; and fruticose thalli, characterized by three-dimensional, filamentous or pendulous branching. Key internal elements include the cortex, a protective fungal layer; the photobiont zone, where algal or cyanobacterial cells are strategically positioned for optimal light capture; and the medulla, a loose hyphal matrix facilitating gas exchange and water retention. Some lichens also develop rhizines or holdfasts for anchorage, forming highly adaptive, ecophysiologically efficient morphotypes.
Lichen symbiosis constitutes a specialized, evolutionarily refined consortium in which a fungal mycobiont orchestrates a persistent association with a photosynthetic photobiont, typically a green alga or cyanobacterium. The mycobiont constructs a morphologically integrated thallus—a regulated microhabitat that buffers environmental extremes—while the photobiont furnishes photosynthates and, when cyanobacterial, biologically fixed nitrogen. This bidirectional metabolic complementarity enables lichens to occupy ecological niches inaccessible to solitary organisms. Although traditionally viewed as a dual partnership, this foundational symbiosis now serves as the core scaffold for more intricate, multi-organism lichen holobionts displaying exceptional physiological resilience and biochemical innovation.
Lichen growth forms represent evolutionarily optimized architectures enabling symbiotic function across diverse habitats. Crustose lichens form tightly adherent, often inseparable crusts that penetrate substrates, maximizing stability in extreme or exposed environments. Foliose lichens exhibit dorsiventral, leaf-like lobes with distinct upper and lower cortices, allowing enhanced gas exchange, photobiont productivity, and flexible substrate attachment. Fruticose lichens develop three-dimensional, filamentous or pendulous axes with high surface-area-to-volume ratios, optimizing light interception and atmospheric nutrient acquisition. Together, these morphotypes reflect adaptive solutions to microclimatic constraints, substrate specificity, and ecological niche differentiation within lichenized fungal lineages.
Lichen reproduction operates through both sexual and asexual pathways, reflecting the dual biological nature of the symbiosis. Sexual reproduction is governed by the fungal mycobiont, producing spores within apothecia, perithecia, or other ascomatal structures; these spores must subsequently acquire a compatible photobiont to reconstitute a functional lichen. Asexual propagation employs specialized diaspores such as soredia, isidia, and thallus fragments, each containing both partners and enabling rapid, symbiotically intact dispersal. These propagules facilitate colonization across diverse substrates and environmental extremes, ensuring evolutionary persistence, genotypic diversification, and efficient maintenance of the lichen holobiont.
Lichen secondary metabolites encompass an exceptionally diverse repertoire of approximately 1,115 known lichen substances, including polyketides, depsides, depsidones, dibenzofurans, terpenoids, and pulvinic-acid derivatives. Synthesized chiefly by the mycobiont via specialized biosynthetic gene clusters, these compounds accumulate in the cortex or medulla as crystalline or cell-associated deposits. They function in photoprotection, antioxidant defense, antimicrobial activity, and deterrence of herbivory. Iconic metabolites—such as usnic acid, atranorin, and gyrophoric acid—hold major taxonomic value and support species delimitation through chemical tests and chromatographic profiling. This remarkable chemical diversity underpins the ecological resilience and evolutionary success of lichen holobionts.
Lichen pigments constitute a diverse suite of secondary metabolites and photoprotective compounds that generate the remarkable chromatic range observed in lichen thalli. These include parietin, usnic acid, melanins, and various anthraquinones, each contributing distinct absorbance properties and ecological functions. Pigments modulate light penetration, dissipate excess radiation, and provide defense against herbivory, microbial attack, and oxidative stress. Spatial heterogeneity in pigment distribution—across cortices, medullary tissues, or reproductive structures—produces colours spanning vivid yellows and oranges to deep browns and blacks. This biochemical diversity reflects adaptive responses to microclimatic regimes, substrate exposure, and evolutionary pressures shaping lichen photophysiology.
Lichens demonstrate remarkable climate adaptation, thriving in habitats ranging from arid deserts to polar tundra. Their symbiotic structure enables flexible metabolic responses: they can enter prolonged dormancy during drought, rapidly reactivating when moisture becomes available. Many species adjust their photosynthetic partners or pigment production to cope with shifting temperatures and solar radiation. Lichens also serve as sensitive indicators of climate change, with shifts in their distribution, growth rates, and community composition reflecting altered environmental conditions. By adapting physiologically and structurally, lichens persist where few organisms can, offering valuable insights into ecosystem resilience under changing climates.
Lichen ecology reflects the remarkable capacity of lichen holobionts to exploit microhabitats across an extraordinary spectrum of environmental conditions. Occupying substrates ranging from bark and rock to soil, metal, and anthropogenic surfaces, lichens respond sensitively to moisture regimes, irradiance levels, temperature fluxes, and atmospheric chemistry. Their microhabitat preferences—sun-exposed rock faces, shaded bark fissures, fog-influenced coasts, alpine scree, biological soil crusts—shape community composition and biogeographic patterns. Through nitrogen fixation, mineral weathering, and organic-matter accumulation, lichens substantially influence ecosystem processes. Their high responsiveness to pollutants further positions them as premier bioindicators in ecological monitoring and climate-change assessments.
Lichens growing on tree bark are a striking and informative feature of Kew’s landscape. The exceptional diversity of tree species throughout the garden creates a wide range of bark chemistries, textures, and microhabitats, offering ideal conditions for supporting varied lichen communities. These differences allow lichens to establish across many parts of Kew, from the Arboretum and Woodland Garden to formal avenues and specialised collections. Each area reveals distinct assemblages shaped by light, moisture, and substrate. Together, they demonstrate how tree diversity enhances ecological richness, making Kew an outstanding place to observe and appreciate the complexity of lichen–tree interactions.
Lichens are among the most effective pioneer species in terrestrial ecosystems, capable of colonising bare substrates such as rock, bark, soil crusts, and newly exposed surfaces after disturbance. Their unique symbiosis enables them to tolerate extreme environmental stress while initiating key biological processes. By secreting organic acids, lichens contribute to the gradual weathering of rock, facilitating the formation of early soils. Their thalli trap dust, retain moisture, and create microhabitats that support bacteria, fungi, algae, and small invertebrates. Through these functions, lichens pave the way for subsequent plant establishment, playing a foundational role in ecological succession and landscape development.
At the Royal Botanic Gardens, Kew, soil and moss surfaces support a discrete yet ecologically important community of lichens. On open, lightly disturbed ground, genera such as Cladonia and Collema form miniature landscapes of cups, squamules, and softly lobed thalli. These lichens frequently grow among mosses, benefiting from retained moisture while contributing to soil stabilisation and nutrient cycling. Typical habitats include grassy margins, sandy paths, heath-like patches, and mossy banks in less-managed areas of the Gardens. Together, soil- and moss-dwelling lichens at Kew illustrate early stages of ecological succession and highlight the Garden’s role as a refuge for ground-level biodiversity within an urban setting.
In the woodland habitats of the Royal Botanic Gardens, Kew, epiphytic lichens form a rich and sensitive layer of biodiversity on tree bark and woody substrates. Mature oaks, limes, maples, and willows host diverse assemblages of crustose, foliose, and fruticose lichens, including genera such as Parmelia, Hypogymnia, Physcia, Lecanora, and Usnea. These lichens respond strongly to bark chemistry, humidity, light, and air quality, making them valuable ecological indicators. Sheltered woodland interiors, veteran trees, and shaded trunks provide particularly suitable microhabitats. The diversity of epiphytic lichens at Kew reflects both improving urban air quality and the ecological continuity of its historic woodland landscape.
Lichens constitute a significant component of the lithic biota across Kew, although their ecological presence is frequently underestimated. These resilient, symbiotic organisms colonise a wide range of lithological substrates, developing complex assemblages characterised by diverse pigmentation, thallus morphology, and microstructural patterning. Their distribution reflects the interplay of substrate mineralogy, hydric conditions, irradiance levels, and temporal stability—parameters that collectively shape distinct lichen communities throughout the garden. On sandstone, flint, limestone, and anthropogenic stone constructions, lichens mediate biochemical weathering processes while simultaneously generating microhabitats that support invertebrate fauna and microbial consortia. Growth form and community composition provide valuable bioindicators of atmospheric quality and local microclimatic regimes.
Across the Rock Garden, alpine environments, pathways, commemorative structures, and historic masonry, lichens exemplify the capacity of life to colonise and persist on ostensibly inhospitable surfaces. Careful observation of these subtle thalli allows visitors and researchers alike to discern the extended ecological narratives inscribed upon Kew’s stonework—narratives that reveal long-term processes of resilience, adaptation, and the intricate intersections between biological activity and geological substrate.
Lichens at the Royal Botanic Gardens, Kew, are strongly shaped by moisture and light, both influenced by London’s relatively mild, rainy climate. Frequent rainfall and high humidity favour many epiphytic and corticolous lichens, allowing them to remain physiologically active for much of the year. Shaded, moisture-retaining habitats such as woodland interiors support shade-tolerant species, while more open areas host lichens adapted to higher light and periodic drying. Bark texture, canopy cover, and orientation further modify local conditions. The balance between moisture availability and light exposure across Kew creates a mosaic of microhabitats, supporting diverse lichen communities and reflecting the subtle ecological gradients typical of London’s urban landscape.
Urban lichens at the Royal Botanic Gardens, Kew, tell a living story of London’s environmental history. During the nineteenth and early twentieth centuries, severe air pollution from coal smoke and sulphur dioxide caused many sensitive lichen species to disappear from the city. By the 1960s, Kew—like much of London—had become a near lichen desert. Following clean air legislation and the decline of sulphur pollution, lichens gradually returned. Today, however, nitrogen pollution from traffic, agriculture, and urban activities represents a new ecological pressure. The tolerance thresholds of many lichen species to nitrogen enrichment remain poorly understood and require further monitoring and research. More than 200 lichen species are now recorded at Kew, continuing to serve as sensitive indicators of London’s changing air quality.
Lichens at the Royal Botanic Gardens, Kew, play a quiet but vital role in supporting biodiversity. Growing on trees, soil, rocks, and historic structures, they create microhabitats that shelter invertebrates, retain moisture, and contribute organic matter to developing ecosystems. Lichens also support food webs by providing resources for insects and microorganisms, particularly in woodland and less-managed areas. Their sensitivity to environmental conditions helps maintain ecological balance and signals habitat quality. By conserving diverse lichen communities, Kew not only protects these remarkable symbiotic organisms but also strengthens wider urban biodiversity, demonstrating how even small-scale organisms underpin the health and resilience of a UNESCO World Heritage landscape.
Lichens at the Royal Botanic Gardens, Kew, quietly record the changing quality of London’s air. Once severely reduced by coal smoke and sulphur dioxide, many species vanished during the peak of industrial pollution. Their gradual return since the Clean Air Acts reflects improving conditions, yet a less visible story is still unfolding. Nitrogen pollution, fine particulates, and changing microclimates now shape modern lichen communities in ways that remain poorly understood. Subtle shifts in species composition across trees, walls, and pathways hint at new environmental pressures. Studying these patterns at Kew offers an opportunity to reveal an undiscovered chapter of urban air quality, written not in instruments, but in living, sensitive organisms.
Historic buildings, walls, and stone features within the Royal Botanic Gardens, Kew provide important substrates for lichen colonisation. Crustose and foliose lichens gradually establish on brick, stone, and mortar, responding to surface chemistry, texture, moisture retention, and light exposure. At Kew, these communities reflect both the age of structures and the improvement of London’s air quality since the mid-twentieth century. While lichens add ecological and educational value, their presence also raises conservation questions, requiring careful monitoring to balance biodiversity with heritage preservation. As a UNESCO World Heritage Site, Kew offers a unique setting where lichen growth records environmental change while informing sustainable management of historic fabric.
Lichens at the Royal Botanic Gardens, Kew, change subtly with the seasons, reflecting shifts in moisture, light, and temperature. In autumn and winter, frequent rainfall and high humidity revive their colours, making greens, yellows, and greys appear brighter and more vivid. During drier spring and summer periods, many lichens become paler or duller as they partially desiccate, entering a dormant state. Rehydration after rain quickly restores their activity and colour. Seasonal leaf cover in woodland areas also alters light availability, influencing growth and visibility. These seasonal transformations highlight lichens as dynamic organisms, responding rapidly to London’s changing weather while remaining firmly rooted in place.
Lichens at the Royal Botanic Gardens, Kew display a remarkable variety of textures, shapes, and surface features that aid identification. Smooth, cracked, powdery, or warty surfaces reflect different growth strategies and environmental adaptations. Crustose lichens form thin, tightly attached layers on bark and stone, while foliose species develop leaf-like lobes with distinct upper and lower surfaces. Fruticose lichens grow as branched or hair-like structures, often hanging from twigs. Surface features such as soredia, isidia, apothecia, and fissures provide further diagnostic clues. Together, texture and form reveal how Kew’s lichens respond to light, moisture, substrate, and air quality across the Garden.
Colour is a practical first clue for quickly recognising lichens at the Royal Botanic Gardens, Kew. Bright yellow and orange lichens often indicate sun-exposed, nitrogen-enriched habitats, while pale greys and whites are common on clean bark and stone. Greenish tones typically appear when lichens are wet and actively photosynthesising, fading as they dry. Dark browns and blacks may signal species adapted to high light or exposed surfaces. Although colour alone is not sufficient for precise identification, it provides an effective starting point in the field. Combined with growth form, substrate, and habitat, colour helps visitors and researchers rapidly narrow down likely lichen groups at Kew.
Microscopic characters are essential for accurate lichen identification and reveal features invisible to the naked eye. Examination of thin sections under a microscope shows the internal structure of the thallus, including the cortex, photobiont layer, and medulla. Reproductive structures provide key diagnostic traits: the shape, size, and septation of ascospores; the form of asci and paraphyses; and the anatomy of apothecia or perithecia. Pycnidia and conidia, when present, offer further clues. These microscopic features, combined with external morphology and chemistry, allow reliable species identification and help clarify relationships among closely related lichen taxa.
Spot tests are simple chemical reactions used to help identify lichen species by revealing diagnostic compounds. Small drops of reagents—commonly potassium hydroxide (K), sodium hypochlorite (C), and para-phenylenediamine (P)—are applied to the thallus or medulla. Colour changes such as yellow, red, orange, or purple can indicate specific secondary metabolites. Under UV light, some lichens fluoresce brightly, providing additional clues. While spot tests do not replace microscopic or molecular analysis, they are valuable field and laboratory tools. Combined with morphology, substrate, and habitat, chemical indicators greatly improve the accuracy of lichen identification.
Rapid identification of common lichens in the Royal Botanic Gardens, Kew can begin by grouping species according to their substrate.
Stone lichens are often crustose, tightly attached, and pale grey, white, yellow, or black, commonly belonging to genera such as Lecanora, Caloplaca, and Verrucaria.
Twig and branch lichens are frequently fruticose or small foliose forms, including Ramalina, Usnea, and Evernia, adapted to light and air movement.
Tree trunk lichens show greater diversity, with foliose and crustose genera such as Parmelia, Physcia, Graphis, and Lecidella.
On historic surfaces, slow-growing crustose species dominate, reflecting substrate chemistry and long-term environmental conditions.
This substrate-based approach provides a practical rapid key for garden exploration.
Interpreting lichen anatomy in the field relies on careful observation rather than microscopes. Features such as growth form—crustose, foliose, or fruticose—provide the first anatomical clues. The presence of an upper and lower cortex, rhizines, lobes, or branching patterns helps distinguish major groups. Surface structures like soredia, isidia, apothecia, and fissures can often be seen with a hand lens. Colour changes when wet or dry also reflect internal anatomy and water storage. By combining these visible traits with substrate and habitat, field observers can interpret lichen anatomy effectively and make confident preliminary identifications before laboratory study.
At the Royal Botanic Gardens, Kew, fruticose and filamentous lichens represent two visually distinct growth forms. Fruticose lichens are three-dimensional, shrubby or hair-like, and often branched, growing upright or hanging from twigs and branches. Common examples include Usnea, Ramalina, and Evernia, which thrive in well-lit, well-ventilated habitats and respond sensitively to air quality.
In contrast, filamentous lichens consist of fine, thread-like strands closely associated with moist, shaded surfaces, sometimes resembling algae. These forms lack the robust branching of fruticose lichens and are typically found in humid microhabitats. Distinguishing these growth forms helps observers interpret habitat conditions and lichen ecology across Kew.
Although algae and lichens may look similar in the field, they are fundamentally different organisms. Algae are single-celled or simple multicellular photosynthetic organisms that grow directly on damp surfaces, often forming green films or slimy layers. Lichens, in contrast, are stable symbiotic associations between a fungus and a photosynthetic partner (an alga or cyanobacterium). This partnership produces a structured thallus with distinct forms—crustose, foliose, or fruticose—that is usually dry, firmly attached, and slow growing. Lichens lack the slimy texture typical of algae and persist through drought by entering dormancy. Observing texture, structure, attachment, and response to drying helps reliably distinguish lichens from algae in natural and garden settings.
Many lichens appear deceptively similar, especially to beginners. Grey foliose species such as Parmelia and Hypogymnia are often confused; checking the underside for rhizines and whether the lobes are hollow can quickly separate them. Yellow-orange lichens may belong to Xanthoria (foliose) or Caloplaca (crustose), so noting growth form is essential. Dark crustose lichens on stone are frequently mistaken for algae—true lichens feel dry and firmly attached. Using a hand lens to examine texture, reproductive structures, and attachment, and always considering substrate and habitat, greatly reduces confusion and leads to more confident lichen identification.
Crustose lichens on stone are among the most common yet challenging lichens to identify at the Royal Botanic Gardens, Kew. These species form thin, tightly attached crusts that cannot be removed without damaging the substrate. Colour is a useful first clue: pale grey and white crusts often belong to genera such as Lecanora or Lecidella, while yellow and orange patches suggest Caloplaca or Xanthoria. Black or dark crusts on damp stone are frequently Verrucaria. Surface texture, cracking patterns, and the presence of apothecia or perithecia provide further guidance. Combined with substrate type and moisture conditions, these features allow reliable preliminary identification in the field.
Lichens are increasingly recognised as valuable indicators of cultural heritage condition. Their presence, absence, species composition, and growth forms on stone, brick, and historic surfaces reflect environmental quality, material properties, microclimate, and management history. Some lichens signal long-term stability and low disturbance, while others indicate pollution, moisture imbalance, or surface decay.
In heritage sites such as the Royal Botanic Gardens, Kew, lichen studies help conservators distinguish between natural patina and active biodeterioration. By integrating lichen ecology with conservation science, these organisms provide a non-invasive, cost-effective tool for monitoring and safeguarding historic structures within living landscapes.
Lichens are among the most effective biological indicators of pollution, widely used to monitor air quality in urban and semi-natural environments. Because they absorb water and nutrients directly from the atmosphere, lichens are highly sensitive to sulphur dioxide, nitrogen compounds, heavy metals, and particulate pollution.
Changes in lichen species diversity, abundance, and community composition provide clear evidence of pollution levels and long-term environmental trends. At sites such as the Royal Botanic Gardens, Kew, lichen-based monitoring has documented both historical pollution impacts and ecological recovery following improvements in air quality, supporting research, conservation, and environmental management.
Epiphytic species—especially lichens growing on tree bark—are powerful tools for biomonitoring environmental quality. Because they obtain water and nutrients directly from the atmosphere, these lichens respond rapidly to air pollution, nitrogen deposition, heavy metals, and climate change.
Changes in species presence or absence, abundance, cover, and community composition provide clear signals of environmental stress or recovery. In urban and semi-natural settings, long-term monitoring of epiphytic lichens supports assessments of air quality trends, ecosystem health, and evidence-based conservation and management decisions.
At the Royal Botanic Gardens, Kew, lichens are an integral part of conservation science, linking biodiversity research with practical management. Lichen surveys document species richness and habitat continuity across trees, rocks, and historic structures, providing indicators of ecological quality. Because many lichens are sensitive to disturbance, air pollution, and climate change, they help guide conservation priorities and site management. Integrated with Kew’s long-term records and collections, lichen research informs habitat protection, monitoring programmes, and evidence-based decisions that safeguard both living landscapes and built heritage within this UNESCO World Heritage Site.
At the Royal Botanic Gardens, Kew, lichen photography opens a window onto hidden micro-worlds often overlooked by the naked eye. Using macro lenses and simple hand lenses, photographers reveal intricate colours, textures, and growth forms that define lichen diversity. These images support identification, documentation, and public engagement, transforming crusts on bark, stone, and walls into striking visual stories. At Kew, lichen photography bridges science and art, helping visitors appreciate the beauty, complexity, and ecological significance of these remarkable symbiotic organisms within a UNESCO World Heritage Site.
At the Royal Botanic Gardens, Kew, lichens play an important role in climate change research as sensitive indicators of environmental change. Their growth, distribution, and species composition respond quickly to shifts in temperature, rainfall patterns, air quality, and nitrogen deposition. Long-term lichen records at Kew allow scientists to track ecological change over decades, revealing both past pollution impacts and recent recoveries linked to improved air quality. Today, lichen studies at Kew help interpret how climate change interacts with urban environments, historic landscapes, and biodiversity, contributing valuable evidence for conservation planning within this UNESCO World Heritage Site.
At the Royal Botanic Gardens, Kew, DNA barcoding has become central to modern lichen taxonomy. By sequencing standard markers such as ITS, mtSSU, and RPB genes, Kew researchers resolve cryptic species, clarify synonymy, and stabilise names that morphology alone cannot. Integrated with the world-leading Fungarium collections and historical type material, molecular data link past records to present diversity. This approach strengthens biodiversity inventories, supports conservation assessments, and underpins accurate identification for ecology, bioindication, and heritage science within this UNESCO World Heritage Site.
At the Royal Botanic Gardens, Kew, lichens are interpreted not only as biological organisms but also as part of human cultural history. Across Europe and beyond, lichens have featured in traditional dyes, folk medicine, perfumery (such as oakmoss), and rural knowledge systems. Kew’s collections and archives preserve historical references that link lichen species to textile colouring, wound treatments, and symbolic meanings in folklore. By presenting these stories alongside modern science, Kew highlights how lichens connect ecology, culture, and traditional knowledge, enriching our understanding of their value beyond taxonomy and conservation.
The future of lichen research at the Royal Botanic Gardens, Kew lies at the intersection of taxonomy, ecology, conservation, and public engagement. Building on more than 150 years of historical records, Kew is uniquely placed to study long-term changes in lichen diversity linked to air quality, climate change, and urban pressures. Advances in molecular methods, imaging, and environmental monitoring will refine species concepts and reveal hidden diversity. Integrating field surveys with digital tools and citizen science can expand data coverage across the Gardens. Together, these approaches position Kew as a leading centre for understanding lichens as indicators of environmental change and as key contributors to urban biodiversity and heritage conservation.
Observing lichens at the Royal Botanic Gardens, Kew should always be done with care and respect for both nature and heritage. Use a hand lens or camera rather than touching or removing specimens, as lichens grow slowly and are easily damaged. Stay on designated paths and avoid disturbing soil, bark, or historic surfaces. Never scrape lichens from trees, stones, or buildings, especially within this UNESCO World Heritage Site. Record observations through notes or photographs, and share findings via approved citizen-science platforms. By observing without collecting and by respecting garden rules, visitors help protect Kew’s lichen diversity while still enjoying close engagement with these remarkable organisms.