Tracking global change using lichen diversity: towards a global‐scale ecological indicator

Lichen Biodiversity and Near-Infrared Metabolomic Fingerprint as Diagnostic and Prognostic Complementary Tools for Biomonitoring: A Case Study in the Eastern Iberian Peninsula

In the 1990s, a sampling network for the biomonitoring of forests using epiphytic lichen diversity was established in the eastern Iberian Peninsula. This area registered air pollution impacts by winds from the Andorra thermal power plant, as well as from photo-oxidants and nitrogen depositions from local and long-distance transport. In 1997, an assessment of the state of lichen communities was carried out by calculating the Index of Atmospheric Purity. In addition, visible symptoms of morphological injury were recorded in nine macrolichens pre-selected by the speed of symptom evolution and their wide distribution in the territory. The thermal power plant has been closed and inactive since 2020. During 2022, almost 25 years later, seven stations of this previously established biomonitoring were revaluated. To compare the results obtained in 1997 and 2022, the same methodology was used, and data from air quality stations were included. We tested if, by integrating innovative methodologies (NIRS) into biomonitoring tools, it is possible to render an integrated response. The results displayed a general decrease in biodiversity in several of the sampling plots and a generalised increase in damage symptoms in the target lichen species studied in 1997, which seem to be the consequence of a multifactorial response.

Lichens as spatially transferable bioindicators for monitoring nitrogen pollution

Excess nitrogen is a pollutant and global problem that harms ecosystems and can severely affect human health. Pollutant nitrogen is becoming more widespread and intensifying in the tropics. There is thus a requirement to develop nitrogen biomonitoring for spatial mapping and trend analysis of tropical biodiversity and ecosystems. In temperate and boreal zones, multiple bioindicators for nitrogen pollution have been developed, with lichen epiphytes among the most sensitive and widely applied. However, the state of our current knowledge on bioindicators is geographically biased, with extensive research effort focused on bioindicators in the temperate and boreal zones. The development of lichen bioindicators in the tropics is further weakened by incomplete taxonomic and ecological knowledge. In this study we performed a literature review and meta-analysis, attempting to identify characteristics of lichens that offer transferability of bioindication into tropical regions. This transferability must overcome the different species pools between source information - drawing on extensive research effort in the temperate and boreal zone - and tropical ecosystems. Focussing on ammonia concentration as the nitrogen pollutant, we identify a set of morphological traits and taxonomic relationships that cause lichen epiphytes to be more sensitive, or more resistant to this excess nitrogen. We perform an independent test of our bioindicator scheme and offer recommendations for its application and future research in the tropics.

NH3 concentrations below the current critical level affect the epiphytic macrolichen communities - Evidence from a Northern European City

Acidophytic, oligotrophic lichens on tree trunks are widely considered to be the most sensitive biota to elevated concentrations of atmospheric ammonia (NH₃). We studied the relationships between measured NH₃ concentrations and the composition of macrolichen communities on the acidic bark of Pinus sylvestris and Quercus robur and the base-rich bark of Acer platanoides and Ulmus glabra at ten roadside and ten non-roadside sites in Helsinki, Finland. NH₃ and nitrogen dioxide (NO₂) concentrations were higher at the roadside than non-roadside sites indicating traffic as the main source of NH₃ and nitrogen oxides (NO_x). The diversity of oligotrophs on Quercus was lower at the roadside than non-roadside sites, while that of eutrophs was higher. The abundance and presence of oligotrophic acidophytes (e.g., Hypogymnia physodes) decreased with increasing NH₃ concentration (2-year means = 0.15-1.03 μg m^-3) especially on Q. robur, while those of eutrophic/nitrophilous species (e.g., Melanohalea exasperatula, Physcia tenella) increased. The abundance of some nitrophytes seemed to depend only on bark pH, i.e., their abundances were highest on Ulmus, which had the highest average bark pH. Overall, the results of lichen bioindicator studies may depend on tree species (bark pH) and lichen species used in calculating indices describing the air quality impact. Nevertheless, Quercus is recommended to be used to study the impact of NH₃ alone and in combination with NO_x on lichen communities, because the responses of both oligotrophic acidophytes and eutrophic species can already be observed at NH₃ concentrations below the current critical level.

More than you can see: Unraveling the ecology and biodiversity of lichenized fungi associated with leaves and needles of 12 temperate tree species using high-throughput sequencing

Currently, lichen surveys are generally based on the examination of fruiting bodies. Lichens in the mycelial stage, in spores, or awaiting conditions for fruiting body formation are usually overlooked, even though they are important for maintaining biodiversity and ecosystem functions. This study aimed to explore the lichenized fungal community composition and richness associated with leaves and needles of 12 temperate tree species using Illumina MiSeq-based amplicon sequencing of the internal transcribed spacer (ITS) 2 region. Picea abies harbored the highest richness and number of lichenized fungal species. We found that the lichenized fungus Physcia adscendens dominated the leaves and needles of the most temperate tree species. Eleven lichenized fungal species detected in this study were recorded for the first time on leaves and needles. In addition, we identified Athallia cerinella, Fellhanera bouteillei, and Melanohalea exasperata that are on the German national red lists. Lichenized fungal richness was higher in conifer compared to broadleaf trees. Overall, tree species (within coniferous trees) and tree types (broadleaved vs. coniferous trees) harbored significantly different lichenized fungal community compositions pointing out the importance of host species. Diversity and community composition patterns of lichenized fungi were correlated mainly with tree species. Our study demonstrates that the diversity of foliicolous lichens associated with leaves and needles of 12 temperate tree species can be appropriately analyzed and functionally assigned using the ITS-based high-throughput sequencing. We highlighted the importance of conifers for maintaining the biodiversity of foliicolous lichens. Based on the discovery of many red list lichens, our methodological approach and results are important contributions to subsequent actions in the bio-conversation approaches.

Functional Traits in Lichen Ecology: A Review of Challenge and Opportunity

Community ecology has experienced a major transition, from a focus on patterns in taxonomic composition, to revealing the processes underlying community assembly through the analysis of species functional traits. The power of the functional trait approach is its generality, predictive capacity such as with respect to environmental change, and, through linkage of response and effect traits, the synthesis of community assembly with ecosystem function and services. Lichens are a potentially rich source of information about how traits govern community structure and function, thereby creating opportunity to better integrate lichens into 'mainstream' ecological studies, while lichen ecology and conservation can also benefit from using the trait approach as an investigative tool. This paper brings together a range of author perspectives to review the use of traits in lichenology, particularly with respect to European ecosystems from the Mediterranean to the Arctic-Alpine. It emphasizes the types of traits that lichenologists have used in their studies, both response and effect, the bundling of traits towards the evolution of life-history strategies, and the critical importance of scale (both spatial and temporal) in functional trait ecology.

The response of plant functional traits to aridity in a tropical dry forest

Drylands are experiencing an overall increase in aridity that is predicted to intensify in the future due to climate change. This may cause changes in the structure and functioning of dryland ecosystems, affecting ecosystem services and human well-being. Therefore, detecting early signs of ecosystem change before irreversible damage takes place is important. Thus, here we used a space-for-time substitution approach to study the response of the plant community to aridity in a Tropical dry forest (Caatinga, Brazil), and infer potential consequences of climate change. We assessed plant functional structure using the community weighted mean (CWM) and functional diversity, measured through functional dispersion (FDis), along a 700 km climatic gradient. We studied 13 functional traits, reflecting strategies associated with establishment, defense, regeneration, and dispersal of the most abundant 48 plant species in 113 sampling sites. Spearman correlations were used to test the relation between aridity and single-trait functional metrics. Aridity was a major environmental filter of the plant community functional structure. We found a higher abundance of species with deciduous leaves, zoochorous dispersal, fleshy fruits, chemical defense exudation and spinescence, and crassulacean acid metabolism towards more arid sites, at the expense of species with evergreen and thicker leaves, autochory dispersal, and shrub growth-form. The FDis of leaf type and thickness decreased with aridity, whereas FDis of fruit type, photosynthetic pathway, and defense strategies increased. Our findings provide functional indicators to early detect climate change impacts on Caatinga structure and functioning, to timely adopt preventive measures (e.g. conservation of forest remnants) and restoration actions (e.g. introduction of species with specific functional traits) in this threatened and unique ecosystem.

Assessing Ecological Risks from Atmospheric Deposition of Nitrogen and Sulfur to US Forests Using Epiphytic Macrolichens

Critical loads of atmospheric deposition help decision-makers identify levels of air pollution harmful to ecosystem components. But when critical loads are exceeded, how can the accompanying ecological risk be quantified? We use a 90% quantile regression to model relationships between nitrogen and sulfur deposition and epiphytic macrolichens, focusing on responses of concern to managers of US forests: Species richness and abundance and diversity of functional groups with integral ecological roles. Analyses utilized national-scale lichen survey data, sensitivity ratings, and modeled deposition and climate data. We propose 20, 50, and 80% declines in these responses as cut-offs for low, moderate, and high ecological risk from deposition. Critical loads (low risk cut-off) for total species richness, sensitive species richness, forage lichen abundance and cyanolichen abundance, respectively, were 3.5, 3.1, 1.9, and 1.3 kg N and 6.0, 2.5, 2.6, and 2.3 kg S ha⁻¹ yr⁻¹. High environmental risk (80% decline), excluding total species richness, occurred at 14.8, 10.4, and 6.6 kg N and 14.1, 13, and 11 kg S ha⁻¹ yr⁻¹. These risks were further characterized in relation to geography, species of conservation concern, number of species affected, recovery timeframes, climate, and effects on interdependent biota, nutrient cycling, and ecosystem services.

Modeling the provision of air-quality regulation ecosystem service provided by urban green spaces using lichens as ecological indicators

The UN Sustainable Development Goals states that urban air pollution must be tackled to create more inclusive, safe, resilient and sustainable cities. Urban green infrastructures can mitigate air pollution, but a crucial step to use this knowledge into urban management is to quantify how much air-quality regulation can green spaces provide and to understand how the provision of this ecosystem service is affected by other environmental factors. Considering the insufficient number of air quality monitoring stations in cities to monitor the wide range of natural and anthropic sources of pollution with high spatial resolution, ecological indicators of air quality are an alternative cost-effective tool. The aim of this work was to model the supply of air-quality regulation based on urban green spaces characteristics and other environmental factors. For that, we sampled lichen diversity in the centroids of 42 urban green spaces in Lisbon, Portugal. Species richness was the best biodiversity metric responding to air pollution, considering its simplicity and its significative response to the air pollutants concentration data measured in the existent air quality monitoring stations. Using that metric, we then created a model to estimate the supply of air quality regulation provided by green spaces in all green spaces of Lisbon based on the response to the following environmental drivers: the urban green spaces size and its vegetation density. We also used the unexplained variance of this model to map the background air pollution. Overall, we suggest that management should target the smallest urban green spaces by increasing green space size or tree density. The use of ecological indicators, very flexible in space, allow the understanding and the modeling of the provision of air-quality regulation by urban green spaces, and how urban green spaces can be managed to improve air quality and thus improve human well-being and cities resilience.

Do Different Teams Produce Different Results in Long-Term Lichen Biomonitoring?

Lichen biomonitoring programs focus on temporal variations in epiphytic lichen communities in relation to the effects of atmospheric pollution. As repeated surveys are planned at medium to long term intervals, the alternation of different operators is often possible. This involves the need to consider the effect of non-sampling errors (e.g., observer errors). Here we relate the trends of lichen communities in repeated surveys with the contribution of different teams of specialists involved in sampling. For this reason, lichen diversity data collected in Italy within several ongoing biomonitoring programs have been considered. The variations of components of gamma diversity between the surveys have been related to the composition of the teams of operators. As a major result, the composition of the teams significantly affected data comparability: Similarity (S), Species Replacement (R), and Richness Difference (D) showed significant differences between “same” and “partially” versus “different” teams, with characteristics trends over time. The results suggest a more careful interpretation of temporal variations in biomonitoring studies.

May the Diversity of Epiphytic Lichens Be Used in Environmental Forensics?

Epiphytic (tree inhabiting) lichens, well-known biomonitors of atmospheric pollution, have a great potential for being used in environmental forensics. Monitoring changes in biodiversity is a useful method for evaluating the quality of an ecosystem. Lichen species occurring within an area show measurable responses to environmental changes, and lichen biodiversity counts can be taken as reliable estimates of environmental quality, with high values corresponding to unpolluted or low polluted conditions and low values to polluted ones. Lichen diversity studies may be very useful in the framework of environmental forensics, since they may highlight the biological effects of pollutants and constitute the base for epidemiological studies. It is thus of paramount importance that great care is taken in the interpretation of the results, especially in the context of a rapidly changing environment and facing global change scenarios. For this reason, it seems advisable to produce several zonal maps, each based on different species groups, and each interpreted in a different way. This exercise could also be a valid support in the framework of a sensitivity analysis, to support or reject the primary results. In addition, a clear and formal expression of the overall uncertainty of the outputs is absolutely necessary.

Selecting lichen functional traits as ecological indicators of the effects of urban environment

Air pollution and the urban heat island effect are known to directly affect ecosystems in urban areas. Lichens, which are widely known as good ecological indicators of air quality and of climatic conditions, can be a valuable tool to monitor environmental changes in urban environments. The objective of this work was to select lichen functional traits and functional groups that can be used as ecological indicators of the effects of urbanization, with emphasis in the Southern subtropics, where this had never been done. For that, we assessed lichen functional composition in urban sites with different population density, which was considered as proxy for grouping sites in two levels of urbanization (low and medium/high). This a priori grouping was based on their significantly differences on air pollutants and land cover. Urbanization and air pollution showed to affect all lichen functional traits, with different responses depending on the functional group. Medium/high density urbanization was associated to an increase on the mean relative abundance of lichens with chlorococcoid green algae, foliose narrow lobes, soredia as the main reproduction strategy, pruinose thallus and containing secondary metabolites for chemical protection. Lower density urbanization showed a higher relative frequency of cyanolichens and lichens with Trentepohlia as the main algae, loosely attached crustose thallus and isidia as the main reproductive structure. The differences found on photobiont and growth form traits in response to the environmental variables used as proxies of microclimatic conditions (forest cover and number of trees around the sampling units), enabled us to detect the urban heat island effect (drier conditions in more urbanized sites).