Lichenpedia of Kew 

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 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 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 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.

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 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.

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.

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, 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.

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.

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.

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.

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.

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.

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 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.

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, 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, 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.

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.