Anthropogenic activities have increased nitrogen (N) emissions. However, higher N deposition is associated with negative fallouts in ecosystems, highlighting the necessity to define its critical load (CLO). Lichens, with their lack of roots, stomata, and cuticles, are helpful air pollution indicators and can be used to establish N CLO. Several studies report epiphytic lichens’ reaction to N deposition, but few have also focused on their recovery. In this study, I reproduced a deposition gradient to investigate epiphytic lichen biomass response to long-term N deposition. As such, whole trees were fertilized with 0.6, 6, 12.5, 25, or 50 kg N/ha/year from 2006 to 2012. In 2012, after six years of irrigation, Hypogymnia species and A. sarmentosa had altered biomass, and based on those results, a CLO below 6 kg N/ha/year is recommended for epiphytic lichens in the boreal forest. I also investigated the recovery of epiphytic lichens nine years after the simulated N deposition ceased. In 2021, a high increase in lichen biomass was recorded, suggesting that lichen growth rate is faster than previously assumed by other studies. However, biomass recovery was lower in high N treatments in Hypogymnia sp., Bryoria sp., and A. sarmentosa, suggesting those species recall the historical treatments possibly due to retained impairments. Since differences among treatments increased in the nine years after the simulated N deposition ceased, my results indicate that short-term negative effects of N deposition can have long-lasting effects on the epiphytic lichen community. Thus, I stress the importance of considering the recovery capability of different species during CLO determination to avoid underestimating the impact of N deposition.

Globally, more than half of the world's population is living in urban areas and it is well accepted that human activities (e.g. climate warming, pollution, landscape homogenization) pose a multitude of threats to ecosystems. Largely, human-related impacts on biodiversity will hold consequences for larger ecological processes and research looking into human impacts on sensitive epiphytic lichen and moss communities is an emerging area of research. While seemingly small, lichen and moss communities exist on nearly every terrestrial ecosystem on Earth and contribute to whole-system processes (e.g. hydrology, mineral cycling, food web energetics) worldwide. To further examine human impacts on epiphytic communities, I conducted three studies examining urbanization and climate warming effects on epiphytic lichen and moss biodiversity and ecology. In the first study I revisited a historic urban lichen community study to assess how urban lichen communities have responded to regional air quality changes occurring over the last nearly two decades. I further investigated, for the first time, the biodiversity of urban tree canopy-dwelling lichen communities in a native coniferous tree species, Pseudotsuga menziesii. I found that urban parks and forested areas harbor a species rich community of lichens epiphytes. Further, I found evidence for the distinct homogenization of urban epiphytic lichen communities, suggesting that expanding beyond simplistic measures of biodiversity to consider community composition and functional biodiversity may be necessary when assessing the ecology and potential ecosystem services of epiphyte communites within urbanizing landscapes. Next, I present the first tall tree canopy study across a regional gradient of urbanization near Portland, Oregon, USA. I found that tall tree canopy epiphyte communities change dramatically along gradients of increasing urbanization, most notably by the transitioning of species functional groups from sensitive, oligotrophic species to a dominance of urban-tolerant, eutrophic species. The implications these dramatic shifts in species composition have on essential PNW ecosystem processes, like N-fixation and canopy microclimate regulation, is still not well understood and is difficult to formally evaluate. However, I find strong evidence that native conifer trees in urban areas may provide a diversity of essential ecosystem services, including providing stratified habitat for epiphyte communities and their associated micro arthropod communities and the scavenging of atmospherically deposited nutrients. Future work is needed to understand how losses in canopy N fixation and species with large biomass (both lichens and bryophytes) will affect nutrient and hydrologic cycling in the PNW region, which continue to undergo rapid growth and urbanization. The final chapter investigates the impacts of passive warming by Open Top Chambers (OTCs) in moss-dominated ecosystems located on the Western Antarctic Peninsula, an area of increasing climate warming. I compared species-specific temperature effects, moss canopy morphology, sexual reproductive effort and invertebrate communities between OTC and control moss communities for two moss species, Polytrichastrum alpinum and Sanionia uncinata, that make up over 65% of the terrestrial vegetative cover in the area. I found distinct reproductive shifts in P. alpinum under passive warming compared to controls. Moss communities under warming also had substantially larger total invertebrate communities than those in control moss communities, and invertebrate communities were significantly affected by moss species and moss reproductive effort. Further, substantial species-specific thermal differences among contiguous patches of these dominant moss species were revealed. These results suggest that continued warming will differentially impact the reproductive output of Antarctic moss species and is likely to dramatically alter terrestrial ecosystems dynamics from the bottom up. This combined work provides a diverse contribution to the field of epiphyte ecology and biology by providing new insights on how human impacts will affect epiphyte lichen and moss communities across diverse ecosystems, in light of a rapidly changing planet.

Felling of old trees as well as changes in land use and management of wooded pastures has led to the decline of biodiversity associated with old deciduous trees in Europe. This also applies to Sweden’s most species rich tree, the oak tree (Quercus robur), which changes with age and develops, for example, a rougher bark structure, deeper bark fissures and cavities. These give rise to several different microhabitats that favor the presence of lichens, mosses and wood-dependent organisms. To preserve the biodiversity, environmental conditions linked to a high diversity need to be identified and described, of which indicator species are a simplified way of measuring diversity. Epiphytic lichens are well-used indicators of environmental conditions because of their habitat specificity, longevity and sensitivity to air quality and humidity. In this study, the impact of regional (latitude, longitude and dominated landscape) and local (diameter and sun exposure) environmental variables on species richness and occurrence of epiphytic lichens on coarse oak trees (Quercus robur) were analyzed in meadows and pastures in southern Sweden with generalized linear models (GLM). A total number of 2346 oak trees were included in the study, of which about 47 % had presence of at least one of the studied lichen species. All environmental variables demonstrated significant effects on species richness, of which the regional had the greatest effect. Also with the species-specific analyses the same effect of the regional variables was found, while single species showed different requirements for the local factors. The results indicate that global warming may move the Swedish species composition more to the Northeast and that increased homogenization of the landscape may threaten the diversity attached to oaks. The results also demonstrate the importance of preserving a varied landscape with trees of different sizes, ages and different levels of sun exposure to accommodate a greater diversity of epiphytic species, but also other species linked to oak trees.

Epiphytic lichens are under considerable grazing pressure due to invertebrate lichenivores, mostly gastropods. Preferences of tree-climbing gastropods affect vertical distribution of lichens, their habitat preferences and whole area of distribution. Higher grazing pressure is expected on sites with basic subsoil and sufficient sources of calcium, where the abundance of gastropods is higher. Most of researches about the topic of grazing pressure were therefore conducted on such sites, although this hypothesis has never been formally tested. Epiphytic communities are also affected by pH and other properties specific to its photophyte. Nevertheless, it is not known, if the same applies for grazing gastropods and how is the grazing pressure affected by tree species. I have addressed these questions in my thesis. I set an 80-days-long field transplantation experiment in the mixed forest of Moravský kras (South Moravia, Czech Republic). Glass fiber meshes with lichen thalli were placed on sixty trunks of three tree species (Carpinus betulus - hornbeam, Abies alba - fir, Fagus sylvatica - common beech) on both calcareous (limestone) and acidic (granodiorite) sites. Abundance and diversity of gastropods were measured. Determined grazing pressure was higher than pressure detected by other authors and was...

1 Abstract Forests are the native Central European vegetation, which have dominated in the landscape for the last c. 10,000 years. Stands with an oak and hornbeam dominance occupied lower elevations before human colonization, beech and silver fir-beech forests middle elevations and spruce stands at higher elevations. Only a few remnants of forests, which can be regarded as primeval or with a minimal impact of forest management, have survived in densely populated Central Europe. Examples of the most preserved primeval forests are Rothwald (Austria), Białowieża (Poland, Belorussia), Stužica/Stuzhytsia (Slovakia, Ukraine) and Boubín (Czech Republic). Although these sites are small and isolated, they are local diversity centers for many organisms, mainly for fungi, lichens and bryophytes, refugia for numerous endangered species and some of them have their last localities there. Epiphytic and epixylic lichens are an ideal model group for studies about forests because they sensitively indicate management, continuity, heterogeneity and age of a woodland. Therefore they could help us to answer many important questions about the conservation of natural forests. This thesis comprises several different points of view on lichens in Central European forests and its aim is to join these heterogeneous fields into one...