The pressure on forests around cities is increasing in many parts of the world, with urban growth and intensified land use as key drivers [1
]. As a consequence, forest patches within cities are of increasing importance for biodiversity conservation [4
] and are highly valued by urban dwellers due to their contribution to liveable cities and the wealth of associated ecosystem services they bring [6
]. Complementing the urban green infrastructure with new forests is thus a timely challenge for cities around the globe [8
Yet forest patches in cities differ considerably in terms of their origin and the way in which humans have shaped their development [10
] and could thus contribute differently to urban biodiversity conservation. A traditional conservation focus is on natural forest remnants that many cities encompass within their borders, including in tropical [15
], temperate [17
], and boreal regions [19
]. Natural remnants in cities provide habitats for species of conservation concern [21
], but have often been lost, transformed or fragmented due to urban development [5
Creating new forests by planting trees in cities is well established in restoration ecology, e.g., [26
], ranging from near-natural sites at the urban fringe, e.g., [30
], to anthropogenically altered sites [32
]. Yet such tree plantings and related maintenance require the allocation of considerable resources and do not always yield satisfying results, e.g., when continued maintenance is necessary [31
]. It may thus be useful to consider complementary pathways towards establishing new forests in cities. We focus here on “emerging urban forests” as forests that evolve on urban sites through natural processes (e.g., colonization, and succession). Generally, emerging forests have been defined as secondary, or successional forests, developing in response to severe disturbance or abandonment of previous land use as illustrated in Figure 1
In cities, emerging forests can result from different development paths. The first pathway is the colonization of previously cleared or abandoned urban-industrial land, indicated as ruderal succession in Figure 1
. Outcomes of ruderal succession series have been reported for a range of urban-industrial land-use types [40
], including vacant lots [33
], transportation corridors [48
], post-industrial sites [38
], and landfills [53
]. A second, less acknowledged pathway towards emerging urban forests is spontaneous rewilding of designed greenspaces. This is a parallel to rural landscapes where forests evolve in response to decreased (or ceased) agrarian land use [36
], sylvicultural management [56
] (Figure 1
), or the abandonment of villages and farmsteads [58
]. Although rewilding is likely a common process in many urban greenspaces, resulting wild woods have been mostly reported for old cemeteries in Europe, e.g., [60
]. As an exception, Pregitzer et al. [64
] recently reported successional forests in urban parks in New York City.
We expected that different types of emerging forests in cities harbor different species assemblages due to filtering in terms of divergent seed sources and environmental conditions. While the species composition in successional forests mainly results from colonization from adjacent seed sources, previously planted ornamentals and co-occurring wild species provide additional seed sources in rewilded green spaces. Moreover, succession in rewilded greenspaces starts from developed soils with an increased nutrient availability, whereas ruderal soils have different features, which have been shown to affect biodiversity patterns [42
Making use of successional processes is established practice in restoration ecology [66
] and is often referred to as “passive restoration” [55
]. Since the beginning of this century, rewilding has been a hot topic in ecosystem management and biodiversity conservation [55
] and is also attracting increasing attention in cities [51
]. However, since cities are hotspots for alien plant species, there are concerns that introduced species could have a negative impact on native species in urban forest patches [13
Indeed, historical analyses of urban floras have revealed a steep increase in numbers of alien woody species [76
]. Wild-growing alien species contributed, for example, 16% to Berlin’s woody flora in 1800 but 67% in 1990 [79
]. Remnant forest patches in cities harbor more alien species than in non-urban environments [80
], with the higher numbers of alien species due to edge effects [81
]. Moreover, alien species can dominate successional forests on urban land as was shown first for Berlin [40
] and beyond, e.g., [45
]. Whether emerging urban forests provide habitats for native plant and animal species—despite the generally high occurrence of alien species in cities—is understudied though.
The degree to which alien species are drivers or passengers of change [84
] in urban environments likely depends on the characteristics of the dominant species and the particular ecosystem types [85
]. Yet there is limited evidence about the role of emerging urban forests in large cities and their domination by native or alien tree species. Moreover, comparative analyses of diversity patterns of different types of successional forests in cities are rare; but see [49
Here we analyze biodiversity patterns of emerging urban forests within the city of Berlin, Germany, at two spatial scales. At the city scale, we quantified the area of successional forests on open land (i.e., outside designed greenspaces) and the dominant tree species. Given the high propagule pressure of introduced woody species as a typical urban feature [79
], we expected that most of these forest patches would be dominated by alien tree species.
At the community scale, we analyzed how the alien vs. native status of dominant tree species related to diversity patterns of plants and invertebrates in the ground layer. Our data set included (i) successional forests on previously open land and (ii) rewilded tree stands on old cemeteries. The latter are a prominent example of rewilded urban greenspace and are increasingly important in European cities due to changing burial practices [91
]. As a consequence, parts of many cemeteries in Berlin are no longer used for burials, and natural processes have been allowed to proceed with limited management interference.
We hypothesized a negative relationship between native species richness in the ground layer and alien dominance in the canopy and thus expected native species richness to decrease from (i) plots dominated by a common native tree species (Betula pendula
]) to (ii) plots covered by a mixture of native and alien tree species (native Acer platanoides
, alien A. pseudoplatanus
]) to (iii) plots dominated by Robinia pseudoacacia
, a North American tree species that modifies associated plant assemblages due to nitrogen fixation [94
We adopted a multi-taxon approach to compare responses of vascular plants and two groups of invertebrates (carabid beetles and spiders) to the dominant tree species because (i) different groups of taxa perform differently in early stages of forest succession [96
] and (ii) alien dominance in plant species might affect higher trophic levels if introduced species provide less suitable resources for animal species [97
]. Since endangered species are important indicators of the conservation value of habitats, we tested whether their number differed across forest types.
In detail, we addressed two main research questions at the landscape scale: (1) how important are successional forests that have emerged on previously open land in Berlin (in terms of total area, patch number, patch size)? and (2) to what extent are these forests dominated by native or alien tree species or a mixture of the two?
Addressing the community scale, we further asked: (3) do emerging urban forests that are dominated by native vs. alien tree species differ (a) in terms of plant and invertebrate richness (total, alien, native species, and endangered species), and (b) in the composition of their species assemblages? and (4) is the number of alien plant species in the ground and shrub layers of emerging urban forests negatively related to native plant species richness?
5. Conclusions on the Role of Emerging Forests for Urban Green Infrastructure
Endangered species are important indicators of the conservation value of their habitats but do not represent the total range of targets of biodiversity conservation [178
]. Particularly in an urban context, goals of biodiversity conservation should also include the role of urban nature for liveable environments for urban dwellers [179
]. It is thus necessary to include a range of socially and ecologically based goals when discussing the role of emerging urban forests for developing the urban green infrastructure. Based on previous work [10
], we here highlighted six ways in which emerging forests can contribute to developing biodiverse and liveable cities:
Preserve native biodiversity and populations of endangered species. All types of emerging urban forests harbored a considerable number of native plant and invertebrate species—despite a considerable share of alien species. Their role as habitat for endangered species was limited but may increase with time. Yet most likely, the emerging urban forests will not be able to approach natural forest remains in the near future. This strongly supports the well-established aim of placing the highest priority on protecting natural forest remnants in cities, e.g., [21
], and indicates as well some opportunities for native species in novel urban settings.
Create ecological networks with stepping stones or corridors for plants and animals. While ecological network functions were not studied here, emerging urban forests likely support ecological networks by providing forest patches dispersed over the urban fabric that may be used as stepping stones for birds and other animals [42
]. Since the alien Robinia
forests harbored similar numbers of (endangered) invertebrates as the other forest types, they also contribute to ecological networks, e.g., for pollinators [183
] or at higher trophic levels [95
Facilitate and elucidate the adaptation of ecological systems to urbanization and other environmental pressures. Urbanization as a major driver of change in the Anthropocene period affects all components of urban ecosystems [184
]. In consequence, novel urban ecosystems arise and support the understanding of how species assembly responds to a combination of novel environmental drivers in urban settings [141
]. Allowing emerging forests to develop without intervening in the diversity patterns of alien and native species will provide insights into the adaptation of forest systems to changing urban environments, including interactions with climate change effects; and will allow conclusions to be drawn on the resilience of species and communities to urban pressures, and selection of suitable native or alien species for urban greenspaces.
Re-connect people with nature and support experience of natural elements. The diversity of both species assemblages and structural features of emerging urban forests and their adjacency to urban residents create manifold opportunities to experience natural elements and their dynamics in the neighborhood. This is an important service in times of decreasing experience in nature [186
], with anticipated positive feedbacks to people’s willingness to protect biodiversity [187
], and a strong argument for conserving emerging forests close to places where people live [188
Enhance wilderness in cities. Since wilderness areas significantly decline at a global scale [2
], the aim of promoting wilderness areas in urban environments—complementing the highly managed ecosystems in public and private greenspaces—is on the urban agenda [70
]. Emerging urban forests represent a kind of “novel urban wilderness,” with species assemblages contrasting with the “ancient wilderness” of natural forest remnants but similarly shaped by natural processes [104
]. While ancient wilderness areas are usually located at the urban fringe, emerging urban forests are often integrated into the urban fabric and thus can support access to wilderness in the daily life of urban residents.
Provide ecosystem services for urban people. There is increasing evidence of positive feedback between biodiversity and the provision of ecosystem services in cities [189
]. Emerging forests in particular, including abundant alien tree species, have been shown to provide a range of regulating ecosystem services on vacant land [44
]. Moreover, they constitute informal greenspaces [190
] supporting manifold social uses and cultural services [191
]. Importantly, these ecosystem services are being delivered without the use of resources to produce plants and carry out landscaping and maintenance; thus they have a low CO2
footprint. Integrating emerging forests into the urban green infrastructure therefore also contributes to both climate change mitigation and adaptation.
Although there are many reasons for integrating emerging urban forests into the green infrastructure of cities, some important challenges remain. Studies on the extent to which people value wild vegetation on urban vacant lots have yielded ambiguous results. While some studies revealed prevailing positive valuations of wild vegetation in urban settings [9
], other studies showed that people preferred open succession stages over woody stages [194
]. This is likely related to concerns about safety and to general cross-cultural preferences for semi-open scenery [196
]. Moreover, rubbish and other signs of neglect may reduce the acceptance of emerging urban forests, necessitating “orderly frames” to improve public perception [191
]. As with spontaneous vegetation in general [198
], integrating emerging urban forests into urban green systems thus often requires planning, design interventions, and management to mitigate risks and enhance the opportunities that this novel type of urban woodland offers for the development of biodiverse and liveable green cities. The successful integration of emerging forests in newly established formal parks and greenways in Berlin [103
], the post-industrial landscape of the Ruhr region [50
] and beyond, e.g., [201
], illustrates promising perspectives for enhancing the wild side of urban green infrastructure.