Emerging Urban Forests: Opportunities for Promoting the Wild Side of the Urban Green Infrastructure
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sustainability Article Emerging Urban Forests: Opportunities for Promoting the Wild Side of the Urban Green Infrastructure Ingo Kowarik 1,2,* , Anne Hiller 1,2, Greg Planchuelo 1 , Birgit Seitz 1,2, Moritz von der Lippe 1,2 and Sascha Buchholz 1,2 1 Technische Universität Berlin, Department of Ecology, D-12165 Berlin, Germany; [email protected] (A.H.); [email protected] (G.P.); [email protected] (B.S.); [email protected] (M.v.d.L.); [email protected] (S.B.) 2 Berlin-Brandenburg Institute of Advanced Biodiversity Research (BBIB), D-14195 Berlin, Germany * Correspondence: [email protected]; Tel.: +49-3031471350 Received: 5 October 2019; Accepted: 7 November 2019; Published: 11 November 2019 Abstract: Many cities aim to increase urban forest cover to benefit residents through the provision of ecosystem services and to promote biodiversity. As a complement to traditional forest plantings, we address opportunities associated with “emerging urban forests” (i.e., spontaneously developing forests in cities) for urban biodiversity conservation. We quantified the area of successional forests and analyzed the species richness of native and alien plants and of invertebrates (carabid beetles, spiders) in emerging forests dominated by alien or native trees, including Robinia pseudoacacia, Acer platanoides, and Betula pendula. Emerging urban forests were revealed as shared habitats of native and alien species. Native species richness was not profoundly affected by the alien (co-)dominance of the canopy. Instead, native and alien plant species richnesses were positively related. Numbers of endangered plants and invertebrates did not differ between native- and alien-dominated forest patches. Patterns of tree regeneration indicate different successional trajectories for novel forest types. We conclude that these forests (i) provide habitats for native and alien species, including some endangered species, (ii) allow city dwellers to experience wild urban nature, and (iii) support arguments for adapting forests to dynamic urban environments. Integrating emerging urban forests into the urban green infrastructure is a promising pathway to sustainable cities and can complement traditional restoration or greening approaches. Keywords: biodiversity conservation; cemeteries; endangered species; invasive tree species; plant invasions; passive restoration; rewilding; secondary succession; urban woodland; urban wilderness 1. Introduction The pressure on forests around cities is increasing in many parts of the world, with urban growth and intensified land use as key drivers [1–3]. As a consequence, forest patches within cities are of increasing importance for biodiversity conservation [4,5] and are highly valued by urban dwellers due to their contribution to liveable cities and the wealth of associated ecosystem services they bring [6–9]. Complementing the urban green infrastructure with new forests is thus a timely challenge for cities around the globe [8,10–12]. Yet forest patches in cities differ considerably in terms of their origin and the way in which humans have shaped their development [10,13,14] 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 [15,16], temperate [17,18], and boreal regions [19,20]. Natural Sustainability 2019, 11, 6318; doi:10.3390/su11226318 www.mdpi.com/journal/sustainability Sustainability 2019, 11, 6318 2 of 27 Sustainability 2019, 11, x FOR PEER REVIEW 2 of 27 remnants in cities provide habitats for species of conservation concern [21–24], but have often been regionslost, transformed [19,20]. Natural or fragmented remnants due in tocities urban provid developmente habitats [for5,10 species,25]. of conservation concern [21– 24], butCreating have often new forests been lost, by planting transformed trees inor cities fragmented is well established due to urban in restoration development ecology, [5,10,25]. e.g., [ 26–29], rangingCreating from near-natural new forests sites by pl atanting the urban trees fringe, in cities e.g., [is30 well,31], esta to anthropogenicallyblished in restoration altered ecology, sites [32 –e.g.,34]. Yet[26–29], such ranging tree plantings from near-natural and related maintenance sites at the urban require fringe, the allocation e.g., [30,31], of considerable to anthropogenically resources altered and do sitesnot always [32–34]. yield Yet satisfyingsuch tree results,plantings e.g., and when related continued maintenance maintenance require is the necessary allocation [31, 35of ].considerable It may thus resourcesbe useful toand consider do not complementaryalways yield satisfying pathways result towardss, e.g., establishingwhen continued new maintenance forests in cities. is necessary We focus [31,35].here on It “emerging may thus be urban useful forests” to consider as forests complementary that evolve pathways on urban towards sites through establishing natural new processes forests (e.g.,in cities. colonization, We focus here and succession).on “emerging Generally, urban forests” emerging as forests forests that have evolve been on defined urban assites secondary, through naturalor successional processes forests, (e.g., developingcolonization, in and response succession) to severe. Generally, disturbance emerging or abandonment forests have of previousbeen defined land asuse secondary, as illustrated or successional in Figure1[36 forests,–39]. developing in response to severe disturbance or abandonment of previous land use as illustrated in Figure 1 [36–39]. Figure 1. Pathways to emerging forests in non-urban (above) and urban environments (below). FigureRed triangles 1. Pathways indicate to emerging major drivers forests of in change. non-urban Grey (above) arrows and illustrate urban successionalenvironments processes (below). Red that trianglesmay start indicate from either major open drivers land subsequentof change. toGrey major arrows disturbance illustrate events successional or abandonment processes of that land-use. may startRewilding from coverseither successionalopen land subsequent processes within to majo a matrixr disturbance of existing events vegetation, or abandonment in response toof aland-use. decrease Rewildingor cessation covers in management. successional processes within a matrix of existing vegetation, in response to a decrease or cessation in management. In cities, emerging forests can result from different development paths. The first pathway is the colonizationIn cities, of emerging previously forests cleared can or re abandonedsult from different urban-industrial development land, paths. indicated The as first ruderal pathway succession is the colonizationin Figure1. Outcomes of previously of ruderal cleared succession or abandoned series have urban-industrial been reported for land, a range indicated of urban-industrial as ruderal successionland-use types in Figure [40–44 ],1. including Outcomes vacant of ruderal lots [33 ,succession45–47], transportation series have corridors been reported [48,49], for post-industrial a range of urban-industrialsites [38,43,50–52 ],land-use and landfills types [[40–44],53,54]. Aincluding second, lessvacant acknowledged lots [33,45–47], pathway transportation towards emergingcorridors [48,49],urban forests post-industrial is spontaneous sitesrewilding [38,43,50–52], of designed and landfills greenspaces. [53,54]. This A is second, a parallel less torural acknowledged landscapes wherepathway forests towards evolve emerging in response urban toforests decreased is spontaneous (or ceased) rewilding agrarian of land designed use [ 36greenspaces.,55], sylvicultural This is amanagement parallel to rural [56, 57landscapes] (Figure1 where), or the forests abandonment evolve in response of villages to anddecreased farmsteads (or ceased) [ 58,59 agrarian]. Although land userewilding [36,55], is likelysylvicultural a common management process in many[56,57] urban (Figure greenspaces, 1), or the resultingabandonment wild woods of villages have beenand farmsteadsmostly reported [58,59]. for oldAlthough cemeteries rewilding in Europe, is likely e.g., [ 60a –common63]. As an process exception, in many Pregitzer urban et al. greenspaces, [64] recently resultingreported successionalwild woods forestshave been in urban mostly parks reported in New fo Yorkr old City.cemeteries in Europe, e.g., [60–63]. As an exception, Pregitzer et al. [64] recently reported successional forests in urban parks in New York City. Sustainability 2019, 11, 6318 3 of 27 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 [42,48,65]. Making use of successional processes is established practice in restoration ecology [66–68] 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 [55,69] and is