Secondary wind dispersal enhances long-distance dispersal of an invasive species in urban road corridors

Roads contribute to habitat fragmentation and function as dispersal barriers for many organisms. At the same time many nonnative plant species are associated with road systems, a relationship that has been explained by the availability of disturbed habitats along roadsides and traffic-mediated dispersal of species. By studying secondary wind dispersal (SWD) over paved ground in an urban road corridor, we add the perspective of corridor-specific, but traffic-independent dispersal processes to the complex dispersal systems along roads. We analyzed (1) the seed shadow of an invasive tree Ailanthus altissima along a sidewalk subsequent to a strong wind and (2) the movements of painted samaras of this species released at ground level at the same site to identify the functioning of SWD. For the first experiment, we searched for samaras in the vicinity of an isolated tree three days after a strong wind. For the second experiment, we tracked the movement of the released samaras repeatedly over a period of 9–11 days, approximated probability-distance functions to the frequency distribution of samaras along the transect for different times after release, and related nearby measured wind data to changes in dispersal kernels. Single samaras from an isolated tree formed a seed shadow that extended for a distance of up to 456 m, and fragments of fruit clusters traveled up to 240 m. Forty-two percent of the sampled samaras were moved >100 m. The second experiment revealed that painted samaras released on the ground were moved up to 150 m over the pavement. Dispersal distances increased with time after seed release. A wider distribution of diaspores over the transect was significantly related to higher wind sums. Habitat shifts to safe sites for germination occurred during SWD, and different types of pavement influenced these processes. Smooth-surfaced pavement enhanced SWD, while cobbles with irregular surfaces slowed down or terminated SWD. During the observation period, 17% of released samaras accumulated in patches with a planted tree. Some were recaptured within the median strip and thus must have been lifted and moved over four lanes of heavy traffic. Our results suggest that impervious surfaces within road corridors can function as powerful avenues of wind-mediated long-distance dispersal and may counteract fragmentation of urban habitats. This also offers a functional explanation for the invasion success of Ailanthus at isolated urban sites

Endangered Plants in Novel Urban Ecosystems Are Filtered by Strategy Type and Dispersal Syndrome, Not by Spatial Dependence on Natural Remnants

Understanding the contribution of cities to nature conservation is gaining increasing importance with a globally accelerating urbanization and requires insights into the mechanisms that underlie urban distribution patterns. While a considerable number of endangered plant species have been reported for cities, the spatial dependence of populations of these species on natural remnants versus anthropogenic ecosystems is critically understudied due to deficiencies in population distribution data. To which extent endangered species in anthropogenic ecosystems spatially rely on natural remnants is thus an open question. We used a unique dataset of 1,742 precisely mapped populations of 213 endangered plant species in the city of Berlin and related these point data to habitat patches that had been assigned to natural remnants, hybrid ecosystems and novel ecosystems according to the novel ecosystem approach. By applying point pattern analyses (Ripley’s K function, cross K function, cross pair correlation function) we unraveled the spatial dependence of the populations toward the different ecosystem types. Moreover, we tested how plant traits related to plant strategy and dispersal filter for species occurrence across ecosystems. Differentiating populations on anthropogenic sites revealed that populations in hybrid ecosystems spatially depended on natural remnants, but populations in novel ecosystems (i.e. more than a third of all populations) surprisingly didn’t. A conditional inference tree showed that endangered plant species in novel ecosystems are filtered for ruderal strategy type and wind dispersal syndrome, while competitive and stress-tolerant species were mainly confined to natural remnants. Our results highlight the importance of conserving natural remnants as habitats and seed sources of endangered plants. Yet novel urban ecosystems can support many populations of endangered plant species beyond the adjacency to natural remnants, with hybrid ecosystems likely acting as stepping stones. This indicates a specific contribution of urban ecosystems to biodiversity conservation. Since different filters modulate the species pools of different ecosystem types, novel urban ecosystems are not supposed to substitute fully the habitat functions of natural remnants. Our study thus highlights promising opportunities for involving the total range of urban ecosystem types into urban conservation approaches.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität BerlinBMBF, 01LC1501, BIBS-Verbund: Bridging in Biodiversity Science (BIBS

CityScapeLab Berlin: A Research Platform for Untangling Urbanization Effects on Biodiversity

Urban biodiversity conservation requires an understanding of how urbanization modulates biodiversity patterns and the associated ecosystem services. While important advances have been made in the conceptual development of urban biodiversity research over the last decades, challenges remain in understanding the interactions between different groups of taxa and the spatiotemporal complexity of urbanization processes. The CityScapeLab Berlin is a novel experimental research platform that allows the testing of theories on how urbanization affects biodiversity patterns and biotic interactions in general and the responses of species of conservation interest in particular. We chose dry grassland patches as the backbone of the research platform because dry grasslands are common in many urban regions, extend over a wide urbanization gradient, and usually harbor diverse and self-assembled communities. Focusing on a standardized type of model ecosystem allowed the urbanization effects on biodiversity to be unraveled from effects that would otherwise be masked by habitat- and land-use effects. The CityScapeLab combines different types of spatiotemporal data on (i) various groups of taxa from different trophic levels, (ii) environmental parameters on different spatial scales, and (iii) on land-use history. This allows for the unraveling of the effects of current and historical urban conditions on urban biodiversity patterns and the related ecological functions.BMBF, 01LC1501, BIBS-Verbund: Bridging in Biodiversity Science (BIBS

Untangling the role of urban ecosystems as habitats for endangered plant species

As urbanization accelerates globally, a better understanding of how cities contribute to biodiversity conservation is increasingly pressing. Previous studies reveal that cities can harbor a considerable biological richness, including endangered plant species. Yet, a key question on the urban contribution to plant conservation remains critically open, as little information is available on how populations of endangered plant species occur across different biotope types within cities and to what extent anthropogenically shaped vs. natural ecosystems provide habitats for endangered plants. We analyzed a unique dataset on the exact geographical position of 1742 populations of 213 endangered plant species in the city of Berlin. We first assessed the relative importance of Berlin’s nine major biotope classes as habitats of these species. Second, we applied the novel ecosystem concept to quantify endangered plant populations for natural remnants vs. hybrid vs. novel ecosystems within Berlin. Populations of endangered plant species were generally, although unevenly, associated with specific biotope classes, with forest, grassland, and ruderal biotopes as the most important habitats. Surprisingly, novel ecosystems harbored the highest numbers of total populations, of total species, and of species that were exclusively confined to one type of ecosystem novelty. Quantifying the relative importance of biotope classes and novel vs. (near-)natural ecosystems as habitats of endangered species demonstrates that the urban contribution to biodiversity conservation is best ensured by providing a range of ecosystems. Rather than prioritizing only natural remnants, we thus argue for broad approaches to urban biodiversity conservation that include novel ecosystems.BMBF, 01LC1501, BIBS-Verbund: Bridging in Biodiversity Science (BIBS

Crop seed spillage along roads: a factor of uncertainty in the containment of GMO

Feral populations of crop species along roadsides contribute to the uncertainty regarding the containment of genetically modified (GM) crops, as the feral populations could promote the persistence of transgenes outside of cultivated fields. Roadside populations of several common crop species are known to occur far from arable fields, and the dispersal pathways that promote their recruitment in road verges are unclear. Human-aided dispersal, in particular adhesive dispersal by vehicles, has been suggested as a possible vector, but this has not yet been proven experimentally. We sampled the seed rain from vehicles inside two motorway tunnels in an urban environment to reveal the contribution of crop species to seeds unintentionally dispersed by traffic beyond agricultural production areas. Three species of arable crops, wheat Triticum aestivum , rye Secale cereale and oilseed rape Brassica napus , were among the most frequent species deposited by vehicles inside the motorway tunnels. Each of the three species was clearly more predominant in one direction of traffic. While seeds of Triticum aestivum and Secale cereale were primarily transported into the city, Brassica napus was significantly more abundant in samples from lanes leading out of the city. Seed sources in the local surroundings of the tunnels were virtually nonexistent, and the high magnitude of seed deposition combined with high seed weights suggests a dispersal mechanism different from other species in the sample, at least for Triticum aestivum and Secale cereale . This provides evidence that spillage during transport is a major driver for long-distance dispersal of crops. Our results suggest that seed dispersal by vehicles is the major driver in the recruitment of roadside populations of arable crops, providing a possible escape route for GM crops. Risk management should thus aim at curbing transport losses of GM crops. Roadsides and urban areas are important habitats for feral populations of some common arable crop species, such as Brassica napus , Triticum aestivum and Secale cereale The occurrence of feral crop populations far from arable fields as well as their population dynamics suggest that most roadside populations rely on repeated introductions of seeds due to human-mediated longdistance dispersal. However, the actual dispersal pathways and escape routes remain unknown. Seed dispersal by vehicles from feral roadside populations has been suggested as a relevant mechanism for the colonisation of new sites by Brassica napus in urban area

Zu Mechanismen der Linienmigration von Pflanzen

Linear landscape elements function as important leading structures for plant dispersal. Their ecological relevance results mostly from fostering long-distance dispersal events. While linear distribution patterns of plant species along streams and rivers have been observed since the 19th century, corresponding patterns along roads became much later object of research. Even though such distribution patterns strongly indicate the functioning of roads as dispersal corridors they do not explain the underlying mechanisms. Taking roads as an example, we here illustrate linear plant migration as an outcome of spatial and, in part, functional overlay of varying habitat and dispersal related mechanisms

Emerging Urban Forests: Opportunities for Promoting the Wild Side of the Urban Green Infrastructure

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.BMBF, 01LC1501, Bridging in Biodiversity Science (BIBS

Plant traits, biotopes and urbanization dynamics explain the survival of endangered urban plant populations

With accelerating urbanization, the urban contribution to biodiversity conservation becomes increasingly important. Previous research shows that cities can host many endangered plant species. However, fundamental questions for urban nature conservation remain open: to what extent and where can endangered plant species persist in the long term and which mechanisms underlie population survival? We evaluate the survival of 858 precisely monitored populations of 179 endangered plant species in Berlin, Germany, by assessing population survival throughout different urban ecosystems over a period of 7.6 years on average. By linking population survival to various landscape variables and plant traits, we unravel the underlying drivers. More than one–third of populations went extinct during the observation period. Population survival was inversely correlated to the increase in impervious surfaces in the vicinity following the first 11 years after the fall of the Berlin wall. Additionally, populations in semi‐natural habitats like forests and bogs were surprisingly more prone to local extinction than populations in anthropogenic habitats. Survival was highest for competitive species with a preference for drier soils (Ellenberg indicator for soil humidity). Synthesis and applications. Considerable levels of local population extinction demonstrate that the presence of endangered plants cannot be directly linked with their long‐term survival in cities. However, the survival of remaining populations indicates opportunities for urban biodiversity conservation both within and outside conservation areas. The elucidated links between population survival, urbanization dynamics, biotope class and species traits support urban conservation strategies that reduce the proportion of impervious surface, prioritize conservation management in forests and grasslands and explore the opportunities of green spaces and built‐up areas.TU Berlin, Open-Access-Mittel – 202

Phenology of grassland plants responds to urbanization

Understanding phenological responses of plants to changing temperatures is important because of multiple associated ecological consequences. Cities with their urban heat island can be used as laboratories to study phenological adaptation to climate change. However, previous phenology studies focused on trees and did not disentangle the role of micro-climate and urban structures. We studied reproductive phenology of dry grassland species in response to micro-climate and urbanization in Berlin, Germany. Phenological stages were recorded weekly at the individual plant level for five native grassland species across 30 dry grassland sites along an urbanization and temperature gradient. We estimated 50% onset probabilities for flowering and seed maturation of populations, and analysed variation in onset dates using regression models. Early flowering species significantly advanced flowering phenology with increasing mean air temperature but were little influenced by urbanization. By contrast, late-flowering species showed significant phenological responses to both air temperature and urbanization, possibly because micro-climate was most affected by urbanization in late summer. Surprisingly, not all grassland species showed an advanced phenology with increasing intensity of urbanization. This contradicts observed patterns for urban trees, indicating that phenological shifts in urban areas cannot be generalized from the observation of one growth form or taxonomic group. Growth form appears as a possible determinant of phenological responses. Results suggest that the phenology of dry grassland species may directly respond to the urban heat island, albeit with variable direction and magnitude. This has implications for ecosystem services, shifted allergy seasons, changes of biogeochemical cycles and potential ecological mismatches

Human-mediated dispersal of seeds by the airflow of vehicles

Human-mediated dispersal is known as an important driver of long-distance dispersal for plants but underlying mechanisms have rarely been assessed. Road corridors function as routes of secondary dispersal for many plant species but the extent to which vehicles support this process remains unclear. In this paper we quantify dispersal distances and seed deposition of plant species moved over the ground by the slipstream of passing cars. We exposed marked seeds of four species on a section of road and drove a car along the road at a speed of 48 km/h. By tracking seeds we quantified movement parallel as well as lateral to the road, resulting dispersal kernels, and the effect of repeated vehicle passes. Median distances travelled by seeds along the road were about eight meters for species with wind dispersal morphologies and one meter for species without such adaptations. Airflow created by the car lifted seeds and resulted in longitudinal dispersal. Single seeds reached our maximum measuring distance of 45 m and for some species exceeded distances under primary dispersal. Mathematical models were fit to dispersal kernels. The incremental effect of passing vehicles on longitudinal dispersal decreased with increasing number of passes as seeds accumulated at road verges. We conclude that dispersal by vehicle airflow facilitates seed movement along roads and accumulation of seeds in roadside habitats. Dispersal by vehicle airflow can aid the spread of plant species and thus has wide implications for roadside ecology, invasion biology and nature conservation