Wrocław, situated in the south-western part of the country, is one of the biggest and oldest towns in Poland, occupying an area circa 293 km2 and inhabited by about 674 000 permanent residents (GUS 2022) and since 2022 by over 200 000 immigrants. The city is located in a flat area formed by the Odra River and its five tributaries which connect with the river in the city. A compactly built-up centrum, comprising old downtown, factories, large housing estates built mostly in the period of 1960s-1990s, and strongly developed network of streets, covers about 30% of the city area. About 17% of the centrum surface is occupied by urban greenery (four large parks and few smaller ones, tree-lined walking routes − Lewicki 2014). Typical for the city center are tree-covered lawns between buildings.

The climate is transitional, between oceanic and continental with short and mild winters. The average annual temperature is about 10°C, and the annual temperature amplitude is 20.5°C. Within the centre of Wrocław, an urban heat island is detected, raising the annual mean temperature by 1°C, and in windless and cloudless night, even exceeding 9°C (Szymanowski & Kryza 2009). The average annual relative air humidity in the city centre is about 6% lower than outside it, and on clear nights in summer it is even 40% lower. In the years 2013-2023 the average annual temperature in Wrocław showed fluctuations, ranging between 8.92°C and 11.5°C but did not appear a clear upward trend. However, such trend was visible in winter and autumn months: January, February, March, September and October. Since 2019 average months temperature between December and March was in Wrocław never below 0°C. In these months, the days with precipitation accounted for 40-51% (Szokalska 2018-2023).

In the period 2019-2023, the level of SO2 and CO in the air within Wrocław did not exceed the lower thresholds of the assessment (respectively: 50 µg/m3: 5 µg/m3), while for NO2 the permissible level for average annual concentrations (32 µg m3) was exceeded and 1-hour concentrations were in the range of 140-100 µg/m3. A large load of air pollution is suspended dust, PM10, PM 2.5, for which the daily permissible level was observed to be exceeded; average annual concentration of PM2.5 dust was above 17 µg/m3 (Ostrycharz et al. 2024).

The concept of the described research was based on the re-examination of 109 research plots previously studied in 2013. Then an initial network of 100 × 100-m plots (squares) was established over the whole area of Wrocław. Next, based on a 1-m surface digital terrain model (LIDAR−originated) determining the canopy of trees, all the squares where trees existed were selected. From that set, 500 research plots were randomly drawn using computer program (Fudali & Szymanowski 2019). For the current research, the location of the plots was limited to the city center, because this area seemed to be the most affected by environmental changes related to the increase in temperature, both as a result of global warming and the urban heat island. To test the working hypothesis, 37 locations in the city center parks (P), 33 – from those situated within housing estates with tree-covered lawns (B), 29 – from those covering housing estates mixed with streets (the latter occupied 10-40% of the plot area – B-U) and 10 – from those covered mostly with streets (U) were chosen from the remaining plots located in the city center which had a mixed nature of the land use. In every research plot all trees with a girth of more than 30 cm were studied at the height range of 0.8-1.2 m above ground level to find presence of bryophyes and, then, to record a phytosociological releves, each covering 30 × 40 cm. The area occupied by the individual species was noted in % coverage and later, during analyses, converted into the size of the area covered with the species in [dm2]. Species than could not readily be identified in the field were sampled for determination in the laboratory. The moss and liverworts nomenclature follows Hodgetts et al. (2020).

Since the bryophytes occurring on tree trunks have varying colonization abilities to other substrates (e.g., soil, rocks, rotten wood) two types of epiphytes are usually distinguished in the contex of bryophyte ecology: obligatory (inhabiting only tree trunks) and facultative (colonizing tree trunks and other substrates). Classifying a species as an obligatory or facultative epiphyte always relates to regional conditions (Barkman 1958). In the presented study the group of the facultative species was additionally divided on epiphytic-epilithical that colononize tree trunks and rocklike habitats (e.g., walls and concrete) and multisubstrate ones. The species affiliations to one of these groups were based on bryological data published for Wrocław and its vicinity to date (Fudali & Żołnierz 2019 and literature quoted therein). During field surveys the presence of 11 species which usually occur in other habitats and appear incidentally on the trees was noted, but they were omitted in further analyses which concerned typical epiphytes.

The aim of the analysis was to assess the changes in the number of trees inhabited by epiphytes and in the area occupied by these epiphytes on tree trunks in the research years 2013 and 2023 in the urban areas of Wrocław. The study aimed to identify whether the changes that occurred are statistically significant in different spatial groups classified according to the urban-use complex variable. In the analysis, the nonparametric Kruskal-Wallis test was used, which allows for the comparison of the median between more than two groups in the case of data that do not meet the assumptions of normality. The analysis was performed using the SciPy© library, in the Python v10 environment.

In the general assessment, the number of research plots in which bryophytes were recorded on tree trunks (in the tree segment studied) as well as the number of inhabited trees and the size of the area occupied by the bryophytes increased visibly (Table 1). This upward trend refers also to all the types of land use but there were quantitative differences among them.

The largest percentage increase in the number of trees with bryophytes was calculated for U (79%) and B-U (73%) while the smallest for P (50%). But when only obligatory epiphytes were considered – the largest percentage increase appeared in B (146%). However, in P the number of trees covered with bryophytes (both in the entire diversity of species and when only the obligatory epiphytes are considered) remained the largest. The Kruskal-Wallis test showed that statistically significant changes in this parameter were observed only for two urban-use complexes: B and P (Table 2). In relations to the area of tree trunk covered by epiphytes, the largest percentage increase was observed for P (227%) and B-U (216%) and only for these types was statistically important (Table 2). But when only obligatory epiphytes were considered – the largest percentage increase appeared in B (437%) and U (346%).

It is worthy to point out, that in both years of research the percentage share of obligatory epiphytes in the total area of the bryophytic cover on tree trunks was similar, reaching: 19.6%: 9.8% respectively (Table 1).

Not all species recorded on tree trunks showed an upward trend in the number of the research plots and inhabited trees (Table 3). This trend is visible among most of the obligatory epiphytes (64%) and most of facultative epiphytes with epiphytic-epilithic preferrences (67%) and in a case of Hypnum ‌cupressiforme ‌Hedw. which represent the facultative multisubstrates epiphytes. But the majority of the latter showed opposite tendency or lack of changes.

Comparison of the total number of research plots in which individual species were recorded and number of colonized trees in the analysed years (Table 3) exhibits three dynamic trends among epiphytes (both obligatory and facultative): 1) spatial expansion (increase in the number of plots and trees) found for 13 species (including eight obligatory epiphytes), but strongly marked only in a case of five taxa (three obligatory and two facultative epiphytes); 2) spatial regression (decrease in the number of plots and trees up to no occurrence in 2023) – six species (three obligatory epiphytes); and 3) stagnation (no changes in the number of plots and trees) – five (three obligatory epiphytes). Group of species classified as appearing expansion tendency comprises five taxa not recorded in the plots studied in 2013, including three species (obligatory epiphytes: Pteryginandrum filiforme Hedw., Ulota ‌bruchii ‌Hornsch. ex Brid. and facultative Leucodon ‌sciuroides ‌(Hedw.) Schwägr.) not reported from Wrocław so far (Fudali 1998, 2005, 2019; Fudali & Żołnierz 2019). The epiphytic bryoflora was therefore subject to dynamic species shift in the period studied. This is also evident when comparing on which plots species showing tendency of stagnation occurred in 2013 and 2023. Only in the case of Dicranum ‌tauricum ‌Sapjegin these were the same plots. Similar mobility among epiphytic bryophytes and tends to temporal shifts in species composition was found in the studies from southern Belgium (Hutsemékers et al. 2023).

A slightly different picture of the dynamic trends of some epiphytic bryophytes emerges from the comparative analysis of the changes in the area of the trunks covered by the individual species and its percentage share in the overall epiphyte’s cover (Table 4). 17 species showed increase in cover area but only nine of them also increase in % of share. It is striking that among remaining ones there are such species as obligatory Dicranoweisia ‌cirrata ‌(Hedw.) Lindb. and facultative Orthotrichum ‌diaphanum ‌Sw. ex anon., because these species exhibited visible increase in a number of plots and inhabited trees.

Based on the analyses presented so far, it can be accepted that quantitative changes in the occurrence of epiphytic bryophytes were caused by strong expansion of several species which were: obligatory epiphytes – Orthotrichum ‌pumilum ‌Sw. ex anon, Dicranoweisia ‌cirrata, Platygyrium ‌repens ‌(Brid.) Schimp. and facultative ones – Hypnum ‌cupressiforme and Orthotrichum ‌diaphanum. The two latter showed the greatest increase in the number of inhabited trees and the total cover area. It should be noted that most of the other obligatory epiphytes have also shown an upward trend but weakly expressed.

Regarding changes in species diversity, an upward trend in the total number of obligatory epiphytes appeared, from 10 to 13. The number of facultative species has not changed.

Previous analyses (Table 1) showed some differences in the intensity of the quantitative changes in the epiphytic bryophytes occurrence in various urban-use complexes concerning the number of plots, inhabited trees and the cover area, what agrees with the preliminary assumption. These differences are also evident when comparing the number of epiphytic species and their diversity (Table 5).

The number of epiphytes was the highest in P and did not changed, amounting 19. A slight increase, one species, was recorded in B and B-U and amounted 12: 13 respectively, but in U a decrease from nine to five was noted. The highest percentage of permanent species was in P (74%) and the lowest in U (45%), while in B and B-U remained on similar levels, 67%: 63% respectively. These differences show that the intensity of epiphytic species exchange depended on the type of urban-use complex.

Among the species classified as the most expansive some differences in the intensity of changes between parks and built-up areas were found (Table 6).

In parks visibly stronger expansion than in other urban-use complexes exhibited Hypnum ‌cupressiforme, Dicranoweisia ‌cirrata and Platygyrium ‌repens, while Orthotrichum ‌diaphanum expanded most intensively in the built-up area. Orthotrichum ‌pumilum seems to spread without such pronounced preferences to one of the urban-use complexes. Among the species showing the regression in 2023 two facultative epiphytes, Orthotrichum ‌pallens ‌Bruch ex Brid. and Amblystegium ‌serpens ‌(Hedw.) Schimp. showed the greatest decrease in records in built-up area and streets, but obligatory epiphytes: Lewinskya ‌affinis ‌(Schrad. ex Brid.) F.Lara, Garilleti & Goffinet, Jochenia ‌pallescens ‌(Hedw.) Hedenäs, Schlesak & D.Quandt and Dicranum ‌tauricum ‌Sapjegin lost most records in parks.

Regarding some authors’ opinion that distribution of epiphytic bryophytes in cities is greatly depending on air humidity, temperature and level of air pollution (i.e., Stapper & Kricke 2004; Larsen et al. 2007; Żołnierz et al. 2022) the ecological characteristics (Dierssen 2001) of the species showing different dynamic tendencies (newly found, not re-found, showing clear regression and strongly expansive) were compared to find the possible reasons of the revealed changes in the epiphytic bryophytes diversity during the last 10 years (Table 7). The use of the species’ general ecological indicators in such analysis undoubtedly limits its accuracy but allows for the indication of trends.

Among newly found epiphytes there was no thermophytic species althought Pteryginandrum filiforme, new for Wrocław, is characterized by Dierssen (2001) as species able to survive in a range of temperatures from moderate to high. Thermal requirements of not re-found species and these with clear regression also do not indicate that increase in temperatures had a direct impact on their dynamics. In the group of strongly expanding species there are similar numbers of mesothermic and thermophytic species. The latter, Orthotrichum ‌diaphanum and O.  ‌pumilum, had already occurred in Wrocław before 2000 (Fudali 1998, 2001). Thus, last warming itself does not seem to be the driving factor causing the changes in the species diversity. However, the fact that two highly expansive species are thermophilous should not be neglected.

Most of the newly found epiphytes are able to live in a wide range of air humidity, from moderately wet to dry, similarly as many of these in regression and not re-found. The groups of expanding species and these showing regression consist on species with different requirements for air humidity which does not allow to indicate this factor as decisive for the observed bryofloristical changes.

In relation to light intensity factor, a dominant share of photophytes is visible in the group of newly found species and these in expansion, while among the epiphytes appearing regression there are both sciophytic and eurytopic epiphytes, similar to the group of the species not recorded in 2023. Thus, a possible increase in the availability of the light, for example as a result of trees removal (observed in 32 research plots – 29% of all) could facilitate the expansion or colonization of some photophytic species.

Regarding ecological requirements in relations to bark pH all the dynamic groups exhibit a clear diversity. Species with opposite requirements and eurytopic ones have similar shares in there. The only exception was the group of expanding species in which there were not acidophytes. Is the latter a reaction on a possible decrease in acidification of the urban environment due to reducing SO2 emissions? Among the newly found species there are three classified as sensitive to the air pollution (obligatory Lewinskya speciosa (Nees) F.Lara, Garilleti & Goffinet, Pteryginandrum filiforme and facultative Leucodon ‌sciuroides) and there is no species described as resistant, what can be interpreted as an indicator of the air purity improvement. Expansive spread within built-up areas of subneutral Orthotrichum ‌diaphanum, which before 2000 was noted in Wrocław mostly on walls not on tree trunks (Fudali 2001) could also be a reaction to the decrease in acidification of trees bark. The presented data have agreed with observation by Hutsemékers et al. (2023), who reported that the first group reacting on air purity improvement were the resistant epiphytes and they expanded their range. The inflow of new species, sensitive to the air pollution, occurred in the later stages.

Summarizing this analysis, it seems highly probable that last changes evidenced in the species composition were affected mostly by an improvement of the air purity. Changes in the species diversity during the studied period seem to be weakly related to warming. Similar to analyses of Hutsemékers et al. (2023) who also did not show a significant impact of warming on changes in the species composition of epiphytic bryophytes in southern Belgium in 1980-2020. The air pollution decrease, with high probability, might have caused a clear expansion of resistant to pollution epiphytes such as: Hypnum ‌cupressiforme, Dicranoweisia ‌cirrata and Orthotrichum ‌diaphanum, which were previously present in that area.

But, in my opinion, the possible impact of increased temperatures and frost−free winter months, observed in Wrocław since 2019, on expansion of Orthotrichum ‌pumilum and O.  ‌diaphanum, classified as thermophilous species, should be also considered. In the past these species were recorded in Wrocław on tree trunks rather rarely and the visible increase in their frequency and abundance definitely occurred only after 2011 (Fudali 2012, 2018). A high percentage of days with precipitation during warmer winters may have additionally favored their growth. Unfortunatelly, the impact of climate change on the physiology of epiphytes is very poorly researched. One of the few available studies is an experiment by Song et al. (2012) that brought different conclusions. To determine how climate change in subtropical mountain forests in China would affect the growth and health of epiphytic bryophytes researchers transplanted species from higher elevations to lower elevations, simulating in that way predicted for that region future thermal and humidity conditions. After two years they noted remarkably reduced rates of growth and detrimental effects on the health.

Looking for other reason of the observed changes in the epiphytic bryophyte’s diversity and occurrence, a possible impact of the removal of old trees and pruning of tree crowns should not be ignored. These treatments lead to increase the amount of light reaching the trunks. Half of the new species and almost all of the expansive ones were photophytic.