Comparison of Bird-Species Richness Between 1987 and 2024 Reveals the Urban Forest as a Stable Biodiversity Refugium in a Dynamic Urbanized Landscape

Abstract

Urban forests provide many ecosystem services in urbanized landscapes, including biodiversity conservation. The sustainable management of urban forests requires a thorough understanding of biodiversity changes in the context of rapid urbanization. As biodiversity in rapidly changing urban areas is very dynamic, we need a better understanding of long-term biodiversity changes in urban forests. Birds are very good bioindicators of urban forest biodiversity because they are strongly habitat-sensitive. However, a major knowledge gap exists in long-term trends in bird diversity in temperate urban forests. This study analyzed a comparison of bird-species richness in a temperate Central European urban forest over a time span of 37 years. Bird-counts using the standard line-transect method conducted in 2023–2024 were compared with older field data from 1987 gained using the same method in a lowland hardwood floodplain forest in the Czech Republic. The results revealed significant faunistic similarities in the bird-species diversity of an urban forest during the 1987–2024 period. The high local alpha diversity of the bird community (42 nesting bird species) as well as the relatively high long-term stability in bird richness indicated the importance of the studied urban forest as a stable biodiversity refugium in a dynamic urbanized landscape. Therefore, urban forests can be considered very stable biodiversity refugia in dynamically changing urban areas.

1. Introduction

European temperate forests have been influenced by forest management for more than 200 years [1]. Since the end of the 19th Century, forests in the close vicinity of many European towns have been transformed into urban forests, providing various cultural ecosystem services for both local residents and visitors of heritage cities [2].

The ecosystem services of urban forests are based on the biodiversity functioning of these managed forest ecosystems in urbanized landscapes [3]. Thus, understanding biodiversity patterns in UFs is vital for sustainable urban forest management strategies aimed at maintaining and developing ecosystem services as well as for urban planning aimed at urban green infrastructure [4].

Birds are valuable bioindicators in environmental assessments of how management practices affect biodiversity in urban forests [5,6]. These animals are charismatic and relatively easy to detect in forest ecosystems [7]. We have good ecological knowledge about the relationships between bird diversity and forest habitats [8]. Birds are very sensitive to ecological changes in all types of urban habitats, and bird studies in urban forests should support better ecological knowledge of urban forest biodiversity [9].

Urbanized landscapes are highly fragmented, and thus, there is consensus that birds living in urban areas may have strong dispersal abilities for successfully colonizing new habitats [10]. The level of fragmentation in urbanized landscapes is closely connected to different levels of urbanization [11]. Generally, bird-species richness decreases with increasing urbanization [12], but birds respond non-linearly to specific fragmentation thresholds along urbanization gradients [13]. For example, ground-nesting bird species are locally negatively impacted by intensive grass management in urban parks, and insectivorous bird species decline in city parks due to the scarcity of food sources [14]. The highest bird-species diversity is usually connected with intermediate levels of urbanization [15], an observation supporting the traditional intermediate disturbance theory with biodiversity peaks [16]. Urban forests should be situated in the middle of such patterns of urbanization gradients, especially if we consider urbanization as the disturbance of a natural area [17]. The authors of a study [18] conducted on 32 Swedish cities and towns confirmed that urbanization is a strong driver of bird-community homogenization in natural forest habitats and highlighted those natural habitats along the entire urbanization gradient are needed to maintain local bird diversity [19].

A recent study on the habitat-specific diversity of Central European birds [20] highlighted that some heterogenous urban habitats hosted very similar numbers of bird species as natural forest habitats. This finding was consistent with the result of an earlier study [21] which confirmed some ecological similarities in the bird diversity of natural European floodplain forests and large urban parks in a heritage city.

Large urban green spaces have hosted bird communities with high alpha diversity [22]. This greater biodiversity can be explained by the high habitat heterogeneity, as urban green spaces comprise diverse mosaics including old-growth tree stands, shrubs, open green spaces, and meadows [23]. Environmental heterogeneity is a major predictor of high bird-community diversity in European cultural landscapes [24]. In urbanized areas, habitat heterogeneity is significantly supported by the preservation of biological heritage [25]. The main biological heritage features in urban areas are very large old trees [26]. Other types of biological heritage (e.g., remnants of large dead trees standing or lying on the ground, uprooted trees, etc.) are very rare in managed forests [27]. We can observe an exception to this pattern in managed urban forests, which include many very large trees, where such specimens are protected as they are the main features contributing to urban forests’ aesthetic values [28]. Aesthetic enjoyment is an important cultural ecosystem service provided by urban forests [29].

Urban forests are located in highly dynamic urbanized landscapes. Various ecological and socioeconomic drivers have an impact on cities and their surroundings. Land cover and land use in urban areas change rapidly on both temporal and spatial scales. Additionally, changes in the biodiversity of urban forests can occur relatively quickly, reflecting landscape changes and the synantropization of many forest bird species [30]. Despite our current detailed knowledge of bird diversity in urban parks, we still know little about the patterns of bird diversity change in urban forests.

Understanding such long-term changes in urban forests ultimately requires long-term ecological studies [31]. The importance of such studies was first highlighted by the results of long-term bird monitoring in a temperate deciduous forest at the Hubbard Brook Experimental Forest in North America [32]. In Central Europe, several long-term bird studies have been conducted in natural forests, such as the ornithological research that has been ongoing in the primeval Bialowieza Forest since 1975 [33,34]. But long-term bird studies on bird diversity changes in urban green areas are still very rare in Central Europe, and all that that have been conducted are limited to urban parks, which are managed differently than urban forests [35]. The current lack of long-term bird studies in urban forests is a barrier to applying ecological knowledge to sustainable urban forest management.

The fundamental design of this case study of birds in a temperate Central European urban forest was to provide long-term data over a 37-year period by comparing the historic findings of a 1987 bird-species richness study and a 2023–2024 bird-species richness study. The objective of the study was an assessment of the changes in bird-species richness based on faunistic indices investigated using the line-transect census method. If long-term bird studies in European urban forests are very rare, the novelty in this paper can be found in the extraordinarily long period (37-year) applied for the analysis of bird data for the study area.

This study highlighted the extraordinary importance of managed urban forests as long-term stable bird-diversity hot spots within the framework of dynamically changing urban landscapes. The results can be applied to support decision-making in the sustainable management of urban forests in the European temperate zone.

2. Materials and Methods

2.1. Study Area

The study area was the Michalov urban forest (13.73 ha) located in the town of Prerov, Czech Republic (49.4624400 N, 17.4574767 E). The forest habitat type is a Central European temperate lowland (210 m a. s. l.) hardwood forest classified as Ulmi-fraxineta carpini superior [36]. The tree species composition is dominated by Pedunculate Oak (Quercus robur) and European Hornbeam (Carpinus betulus), with admixture of European Ash (Fraxinus excelsior), Small-leaved Lime (Tilia cordata), European White Elm (Ulmus leavis), Norway Maple (Acer platanoides), Sycamore Maple (Acer pseudoplatanus), and Bird Cherry (Prunus padus). The understorey includes Elderberry (Sambucus nigra), Common Maple (Acer campestre), Prickwood (Euonymus europaea), Ivy (Hedera helix), and Midland Hawthorn (Crataegus laevigata). The herbaceous layer is dominated by vernal geophytes such as Snowdrop (Galanthus nivalis), Wood Anemone (Anemone nemorosa), and Concave Corydalis (Corydalis cava) in the spring season. During summer, the herbaceous vegetation is dominated by Wild Garlic (Allium ursinum) and Stinging Nettle (Urtica dioica).

Until the end of 19th Century, the forest in the study area was managed as a standard-with-coppice. The urban forest was created in the beginning of 20th Century as a suburban park. After World War II, the urban forest was not regularly managed (as a legacy of the communistic era in the Czech Republic) until 1992. In the late 1990s (after the political changes in Central Europe), the new era of urban forest management began. The forest was open to people and restored significantly following the principles stated in the Florence Charter. The whole study area was revitalized to restore the former historic landscape composition of the urban park. The urban forest is currently managed as an urban park (with paths for visitors and small recreational areas inside the forest, and some exotic tree species have been planted to increase visitors’ aesthetic experience of the park). Additionally, old trees have been carefully treated or replaced with new plantings, the pathways in the parks were restored, basic amenities for visitors were installed, lawns and flower beds were renewed, and urban forest management, including gardening practices such as regular mowing of large grassy areas, commenced. The main ecosystem service of the urban forest in the study area is recreation for residents as well as visitors of the Prerov town.

2.2. Bird Census

In the 2023 and 2024 breeding seasons, a bird census was conducted in the study area using the line-transect counting method [37]. Birds were detected in the field visually (using binoculars at 10 × 50) or acoustically (by detection of female birdsong without any technology, only using the ornithological skills of the author) or both visually and acoustically in one moment. All birds detected with the line-transect method were counted (excluding randomly occurring birds such as Apus apus and Hirundo rustica) five times per breeding season (from the beginning of March to the first week of July) in the early morning hours (5:00–6:00 a.m.). Each count therefore lasted approximately one hour. The length of the line-transect was 1300 m (Figure 1). The field data from the line-transect census were used to calculate the average number of birds and relative dominance (%) of bird species in the study area (Table 1). Relative dominance means the number of bird individuals of a given species compared (by percent value) to the total number of individuals across all species in the community.

The nest-guild classification (GN: ground nesters, BN: bush nesters, CN: canopy nesters, and HN: hole nesters) was carried out beforehand [38] based on relevant Czech ornithological literature [39].

Table 1. Results of the bird censuses (in 1987 and 2023–2024) for the study area.

Bird Species	Nest Guild 1	Bird Census from 1987 2		Bird Census from 2023–2024 3
		Number of Birds	Dominance [%]	Number of Birds	Dominance [%]
Turdus merula	CN	119	15.7	57	7.0
Turdus philomelos	CN	85	11.2	12	1.5
Streptopelia decaocto	CN	80	10.6	31	3.8
Fringilla coelebs	CN	72	9.5	64	7.9
Serinus serinus	CN	63	8.3	11	1.4
Phylloscopus collybita	GN	37	4.9	52	6.4
Sylvia atricapilla	CN	37	4.9	31	3.8
Hippolais icterina	CN	36	4.8	3	0.4
Coccothraustes coccothraustes	CN	30	4	10	1.2
Parus major	HN	28	3.7	68	8.4
Carduelis chloris	BN	28	3.7	-	-
Cyanistes caeruleus	HN	20	2.6	39	4.8
Sturnus vulgaris	HN	19	2.5	79	9.8
Phasianus colchicus	GN	13	1.7	2	0.3
Passer montanus	HN	12	1.6	29	3.6
Dendrocopos major	HN	12	1.6	28	3.5
Sitta europaea	HN	12	1.6	20	2.5
Columba palumbus	CN	9	1.2	47	5.8
Streptopelia turtur	CN	8	1.1	-	-
Phoenicurus ochruros	HN	5	0.7	-	-
Lanius collurio	BN	5	0.7	3	0.4
Prunella modularis	BN	4	0.5	7	0.9
Erithacus rubecula	BN	3	0.4	12	1.5
Muscicapa striata	HN	3	0.4	6	0.7
Oriolus oriolus	CN	3	0.4	4	0.5
Picus viridis	HN	2	0.3	11	1.4
Anas platyrhynchos	GN	2	0.3	-	-
Passer domesticus	HN	1	0.1	-	-
Cuculus canorus	-	1	0.1	5	0.6
Jynx torquilla	HN	1	0.1	4	0.5
Sylvia curruca	BN	1	0.1	9	1.1
Sylvia communis	BN	1	0.1	5	0.6
Falco tinnunculus	HN	1	0.1	-	-
Columba oenas	HN	1	0.1	-	-
Aegithalos caudatus	BN	1	0.1	16	2.0
Acrocephalus palustris	BN	1	0.1	6	0.7
Turdus pilaris	CN	-	-	27	3.3
Pyrrhula pyrrhula	CN	-	-	2	0.3
Ficedula albicollis	HN	-	-	10	1.2
Parus palustris	HN	-	-	3	0.4
Regulus regulus	CN	-	-	1	0.1
Garrulus glandarius	CN	-	-	9	1.1
Pica pica	BN	-	-	28	3.5
Phylloscopus trochilus	GN	-	-	3	0.4
Luscinia megarhynchos	BN	-	-	15	1.9
Certhia brachydactyla	HN	-	-	6	0.7
Carduelis carduelis	BN	-	-	9	1.1
Dendrocopos medius	HN	-	-	8	1.0
Troglodytes troglodytes	BN	-	-	18	2.2
Total number of species		36		42
Total number of individuals		756		810

¹ Nest guilds: BN—bush nester, CN—canopy nester, GN—ground nester, and HN—hole nester. ² Results from the study [39]. ³ Original results of author.

Table 1. Results of the bird censuses (in 1987 and 2023–2024) for the study area.

Bird Species	Nest Guild 1	Bird Census from 1987 2		Bird Census from 2023–2024 3
		Number of Birds	Dominance [%]	Number of Birds	Dominance [%]
Turdus merula	CN	119	15.7	57	7.0
Turdus philomelos	CN	85	11.2	12	1.5
Streptopelia decaocto	CN	80	10.6	31	3.8
Fringilla coelebs	CN	72	9.5	64	7.9
Serinus serinus	CN	63	8.3	11	1.4
Phylloscopus collybita	GN	37	4.9	52	6.4
Sylvia atricapilla	CN	37	4.9	31	3.8
Hippolais icterina	CN	36	4.8	3	0.4
Coccothraustes coccothraustes	CN	30	4	10	1.2
Parus major	HN	28	3.7	68	8.4
Carduelis chloris	BN	28	3.7	-	-
Cyanistes caeruleus	HN	20	2.6	39	4.8
Sturnus vulgaris	HN	19	2.5	79	9.8
Phasianus colchicus	GN	13	1.7	2	0.3
Passer montanus	HN	12	1.6	29	3.6
Dendrocopos major	HN	12	1.6	28	3.5
Sitta europaea	HN	12	1.6	20	2.5
Columba palumbus	CN	9	1.2	47	5.8
Streptopelia turtur	CN	8	1.1	-	-
Phoenicurus ochruros	HN	5	0.7	-	-
Lanius collurio	BN	5	0.7	3	0.4
Prunella modularis	BN	4	0.5	7	0.9
Erithacus rubecula	BN	3	0.4	12	1.5
Muscicapa striata	HN	3	0.4	6	0.7
Oriolus oriolus	CN	3	0.4	4	0.5
Picus viridis	HN	2	0.3	11	1.4
Anas platyrhynchos	GN	2	0.3	-	-
Passer domesticus	HN	1	0.1	-	-
Cuculus canorus	-	1	0.1	5	0.6
Jynx torquilla	HN	1	0.1	4	0.5
Sylvia curruca	BN	1	0.1	9	1.1
Sylvia communis	BN	1	0.1	5	0.6
Falco tinnunculus	HN	1	0.1	-	-
Columba oenas	HN	1	0.1	-	-
Aegithalos caudatus	BN	1	0.1	16	2.0
Acrocephalus palustris	BN	1	0.1	6	0.7
Turdus pilaris	CN	-	-	27	3.3
Pyrrhula pyrrhula	CN	-	-	2	0.3
Ficedula albicollis	HN	-	-	10	1.2
Parus palustris	HN	-	-	3	0.4
Regulus regulus	CN	-	-	1	0.1
Garrulus glandarius	CN	-	-	9	1.1
Pica pica	BN	-	-	28	3.5
Phylloscopus trochilus	GN	-	-	3	0.4
Luscinia megarhynchos	BN	-	-	15	1.9
Certhia brachydactyla	HN	-	-	6	0.7
Carduelis carduelis	BN	-	-	9	1.1
Dendrocopos medius	HN	-	-	8	1.0
Troglodytes troglodytes	BN	-	-	18	2.2
Total number of species		36		42
Total number of individuals		756		810

¹ Nest guilds: BN—bush nester, CN—canopy nester, GN—ground nester, and HN—hole nester. ² Results from the study [39]. ³ Original results of author.

2.3. Assessment of the Long-Term Change in Bird-Species Richness

Data from the 2023–2024 census were compared to the findings of the 1987 census, which was conducted in the same area using an identical line-transect method [40]. This comparison revealed changes in bird-species richness in the urban forest study area over a 37-year timespan (1987–2024).

Sörensen’s index (QS) was calculated [41] based on the critical value of this index for Central Europe to analyze changes in the species diversity, and Jaccard’s index (Ja) was used to compare the faunistic similarity of the bird communities [42] in the study area between the compared years (1987 and 2023–2024). The data were statistically analyzed using a standard t-test for the two samples with Minitab version 15.1.1. The normality of the data distribution was assessed visually.

3. Results

3.1. Bird-Species Richness of the Urban Forest in 2024

A total of 42 bird species were recorded during the 2023–2024 breeding season in the study area (see Table 1). There were six dominant species (with a dominance value of >5%) in the urban forest’s bird community, as follows: Sturnus vulgaris, Parus major, Fringilla coelebs, Turdus merula, Phylloscopus collybita, and Columba palumbus. This result was predictable in the study area because all the dominant species are common in Central European green spaces in urbanized areas. The high availability of suitable nesting cavities in the old trees in the study area for the first two dominant species and the generally high numbers of the other four dominant species in the cultural landscape clearly explain these results. The dominance of most species is typically low (≤5%), as visible in the dominance distribution curve (Figure 2). The lack of significant breaks in the curve suggests a well-balanced distribution of dominance and species composition in the current bird community in the study area.

3.2. Comparison of Long-Term Changes in Bird-Species Richness over a 37-Year Timespan

A statistical comparison of current bird-species richness (2023–2024) with the older bird-species richness recorded 37 years ago [39], using a t-test, did not indicate any statistically significant differences (p > 0.05).

The bird-species richness in the study area increased—36 bird species were recorded in 1987 and 42 species were recorded in 2023–2024. In 2023–2024, six of the bird species present in 1987 were not detected, whereas fourteen bird species were identified that were not recorded in the earlier study. The results of the analysis of the bird-community changes in the study area over a 37-year period, based on the calculation of the faunal similarity indices, are shown in

Table 2. Changes in bird-species richness in the study area (from 1987 to 2023–2024).

Total Number of Bird Species in 1987	Total Number of Bird Species in 2024	Number of Identical Bird Species in Both Years	QS Index Calculated as a Comparison of Both Years	Jaccard Index (%) Calculated as a Comparison of Both Years
36	42	29	74.4	59.2

. The Sörensen similarity index for species richness reached a value of QS = 74.44, which meant there were significant similarities in species diversity in the compared bird communities for 1987 and 2023–2024. The calculated Jaccard’s faunal similarity index (as a modification of the QS index) revealed a value of Ja = 59.2%, confirming similarities in bird- species richness for 1987 and 2023–2024.

4. Discussion

4.1. Limits of the Methodology Used

The results of this study are clearly limited by the methodology we chose for conducting the bird census. The line-transect method is not considered the best ornithological method for precise bird-counting compared to territory mapping [43]. Traditionally, the method for determining species diversity in birds nesting in a specific location is the mapping of nesting territories combined with direct nest-searching [44]. However, as we aimed to compare the recent results (from 2023–2024) with the previously published data from 1987, we had to use the same method, that is, the line-transect method. On the other hand, a line-transect is an adequate method for determining bird abundance and dominance, which was the focus of our study.

The results of this study do not provide any information about long-term continuous changes in bird diversity, and it is only a comparison of two different periods of time. Nonetheless, such a long-term comparison is valuable because it offers basic insights into the long-term trends in bird-species richness.

4.2. Long-Term Changes in the Urban Forest Bird Communities

Birds are often studied as biological indicators of environmental change in the mosaic of urban habitats, which are highly human-modified [45,46,47]. Urban birds are studied as bioindicators at various geographical scales—from the global scale to the local scale [48]. With increasing time after urban development, bird diversity becomes more distinct from native conditions [49]. Currently, there is scientific consensus that future studies that estimate differences over a greater period of time after both urban development and changes in vegetation cover are needed. This study partly fills this current knowledge gap related to long-term changes in bird-species richness in urbanized areas at the local scale.

In the studied urban forest, bird-species richness was higher in 2023–2024 (42 species) than in 1987 (36 species). The predominant species (with dominance of ≥5%) in 2023–2024 were common bird species, which are the most abundant bird species in urban areas in the Czech Republic [50]. These predominant species in the study area’s bird community were the following six species: Sturnus vulgaris (9.8%), Parus major (8.4%), Fringilla coelebs (7.9%), Turdus merula (7.0%), Phylloscopus collybita (6.4%), and Columba palumbus (5.8%). This is explainable in the study area due to the exceptionally high availability of suitable nesting cavities for the first two dominant species and the generally high numbers of Fringilla coelebs and Turdus merula, as well as Phylloscopus collybita, in the cultural landscape habitats and Columba palumbus in the urban parks and garden habitats in Central Europe. A clear prevalence of subdominant species (D ≤ 4.9%) can be seen, which actually underpins the overall high species diversity in the ornithocenosis in this locality. This situation corresponds with the general trend where in a species-rich community, the dominance of the most abundant species is typically relatively low. The dominance distribution curve (Figure 1), with no significant breaks, suggests a favourable distribution of dominance and a fairly balanced species composition of the bird community in the study area. The species composition of urban bird communities usually refers to an urbanization level within the framework of urbanization gradients—for example, the most abundant species in more urbanized sites in neotropical cities is Passer domesticus, and urban parks in Eastern Europe are dominated by Columba palumbus and Streptopelia decaocto [51]—our findings confirm the latter.

The results of this study revealed that the bird-species richness in the urban forest has been relatively stable (without significant changes) for over 37 years. These indices are related to the conclusion of the recent study [52], which showed that bird assemblages in small urban parks in Slovakia had low stability during a five-year research period and bird-species richness in large parks was more stable.

The bird faunal similarity analyses in this study demonstrated that the nesting bird community in the studied urban forest has not significantly changed for nearly four decades. This important finding is likely related to the relative stability of the habitat: urban forests have been deliberately maintained in almost unchanged form for centuries [53].

The quantitative characteristics (abundance and dominance) of the urban bird species in the study area are highly similar to those of the nesting bird communities in natural broadleaf lowland forests in the Morava River basin [54]. This observation indicates that the human-made mosaic of habitats in urban forests may be as valuable in terms of bird biodiversity as semi-natural lowland forests [55].

Ornithologists have long known that large urban green habitats, such as historic manor parks, are exceptionally valuable locations in terms of bird biodiversity within the cultural landscape [56]. This fact has gained importance, especially in the context of the creation of the European Natura 2000 network of protected areas, where it has been shown that biodiversity in Central European lowland broadleaf forests is strongly linked to the open structure of habitats similar to open “savanna-type” forests [57]. Modern conservation efforts are therefore focused on finding methods for restoring and managing deciduous forests [58]. Urban forests, with large areas resembling open forests with a high proportion of very large and old solitary trees, can contribute to maintaining the diversity of species that were originally associated with historic forest management [59].

4.3. Implications for Urban Forest Management

There is consensus that local factors in urban green habitats are more important drivers of bird taxonomic and functional diversity than regional factors [60]. Many bird studies [61] have highlighted certain types of urban green infrastructure as a main driver affecting avian taxonomic diversity at the local scale. The habitat island approach was used during discussions about connections between the size of urban greening area and habitat diversity, and this is reflected in higher bird-species accumulation [62]. However, past research on urban bird diversity in three Central European countries [63] did not show any significant relationship between species diversity and land area in urban parks and cemeteries; however, bird-species richness was found to be closely related to the area and age of the trees in both urban habitat types. These findings support an emerging idea that the local heterogeneity of urban habitats plays a crucial role in urban bird diversity. The vertical and spatial heterogeneity of vegetation is correlated with park age, which positively influences local bird taxonomic diversity and the abundance of rare bird specialists in urban forests [64].

The habitat heterogeneity of urban environments, resulting from the mosaics of green habitats (i.e., parks, gardens, and cemeteries), built-up areas, and water bodies, is the main factor in greater bird-species richness (and a higher Shannon diversity index) in urban green areas compared to natural green areas [65]. The results of this study confirmed the key role of urban habitat heterogeneity: a greater cover of grass, bushes, and trees is related to increased bird taxonomic diversity, while phylogenetic diversity is positively affected by the presence of grass, trees, and water bodies. The presence of bushes in urban gardens and parks significantly mitigates the negative effects of the high phylogenetic congeniality of birds in urbanized areas [66].

Urban forests are the most heterogeneous habitats within the urban green infrastructure [67]. The high habitat heterogeneity of large urban forests explains the fact that such habitats, as well as urban parks, are biodiversity hotspots in urbanized landscapes [68].

In this study, the most abundant bird guild in the urban forest was hole nesters (Table 1). This finding was consistent with the results of [26], where it was found that urban green areas with larger trees have greater bird diversity. Very large trees are generally considered to be native keystone structures supporting biodiversity in urban green spaces [69]. For example, large historic parks usually host a remarkable number of very large and old trees [70]. Therefore, historic manor parks are biodiversity hotspots for epiphytic bryophytes, fungi, forest plants, and endangered saproxylic beetles [71,72].

Very large trees in urban forests can be considered as a part of their biological legacy [73]. Therefore, the sustainable management of urban forests should include the conservation of biological heritage to maintain biodiversity as a fundamental part of ecosystem service provisioning because the presence of very large trees supports hole-nesting birds, as indicated in this study. If very large trees are important features for the aesthetic values of urban forests, then the cultural ecosystem services of urban forests support biodiversity maintenance. Urban forests, as places where nature and culture meet, are at the forefront of scientific interest in biocultural diversity [74,75].

As this study indicated, urban forests are not only recreational places but also important biodiversity refugia in urbanized landscapes. Urban forests act as important natural safe havens for residents of habitats that have declined in rural landscapes [76]. A better understanding of the trends in biodiversity changes in urban forests can enhance modern approaches to sustainable urban planning in urbanized landscapes, which are expanding worldwide.

The future direction of urban forestry in Europe will undoubtedly increasingly accentuate forest ecosystem services, for which biodiversity is a key prerequisite [77]. Like all ecological issues, the impact of forest management on biodiversity is largely a matter of scale. The widespread implementation of a single type of forest management at the landscape scale would reduce horizontal forest heterogeneity. Therefore, the key to maintaining the biodiversity of temperate urban forests is likely a simple yet effective approach: urban forest biodiversity requires diversity in forestry practices from the local scale to the landscape scale.

5. Conclusions

Studying birds as biodiversity indicators is appropriate in urban forests habitats, as this study confirmed. The results of the study revealed that bird-species richness in an urban forest has been relatively stable (without significant changes) for over 37 years. It supports the emerging awareness of the high importance of urban forests as key structures in urban green spaces for biodiversity maintenance in urbanized areas. If urbanization continues to progress at its current rate, the importance of urban forests as biodiversity refugia will increase.