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

Urbanisation modulates plant-pollinator interactions in invasive vs. native plant species

Pollination is a key ecological process, and invasive alien plant species have been shown to significantly affect plant-pollinator interactions. Yet, the role of the environmental context in modulating such processes is understudied. As urbanisation is a major component of global change, being associated with a range of stressors (e.g. heat, pollution, habitat isolation), we tested whether the attractiveness of a common invasive alien plant (Robinia pseudoacacia, black locust) vs. a common native plant (Cytisus scoparius, common broom) for pollinators changes with increasing urbanisation. We exposed blossoms of both species along an urbanisation gradient and quantified different types of pollinator interaction with the flowers. Both species attracted a broad range of pollinators, with significantly more visits for R. pseudoacacia, but without significant differences in numbers of insects that immediately accessed the flowers. However, compared to native Cytisus, more pollinators only hovered in front of flowers of invasive Robinia without visiting those subsequently. The decision rate to enter flowers of the invasive species decreased with increasing urbanisation. This suggests that while invasive Robinia still attracts many pollinators in urban settings attractiveness may decrease with increasing urban stressors. Results indicated future directions to deconstruct the role of different stressors in modulating plant-pollinator interactions, and they have implications for urban development since Robinia can be still considered as a “pollinator-friendly” tree for certain urban settings.DFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität BerlinBMBF, 01LC1501A-H, BIBS-Verbund: Bridging in Biodiversity Science (BIBS) - Teilprojekt 1: Koordination, Überbrückung aquatisch-terrestrisch, Pflanze-Boden Interaktionen, Neuartige Ökosystem

Dog Walkers’ Views of Urban Biodiversity across Five European Cities

Contact with nature makes people feel better, live healthier and act more environmentally-friendly. We hypothesized that dog walking, an omnipresent people–nature interaction in cities, translates to a more positive view of urban nature and, subsequently, to more support for conservation initiatives. Insights into such positive side-effects of dog walking are relevant for dog-related urban policies that often focus on negative impacts of dogs (e.g., health risks, disturbance of wildlife). Based on a field survey in five European cities (N = 3717), we analyzed if people who walked dogs regularly valued four urban ecosystem types (park meadows, wastelands, streetscapes, forests), and the plant species diversity within, differently from other people. Opposite to our hypothesis, participants from both groups valued urban ecosystems and their biodiversity very similarly across the cities. Thus, our study does not confirm that regular dog walkers value natural elements more than other people. It thus remains an important challenge for urban planners to balance services and disservices of dog walking in urban greenspaces.EC/H2020/603567/EU/Green Infrastructure and Urban Biodiversity for Sustainable Urban Development and the Green Economy/GREEN SURGEDFG, 414044773, Open Access Publizieren 2019 - 2020 / Technische Universität Berli

Linkages between Phosphorus and Plant Diversity in Central European Forest Ecosystems—Complementarity or Competition?

The phosphorus nutrition status of European forests has decreased significantly in recent decades. For a deeper understanding of complementarity and competition in terms of P acquisition in temperate forests, we have analyzed α-diversity, organic layer and mineral soil P, P nutrition status, and different concepts of P use efficiency (PUE) in Fagus sylvatica L. (European beech) and Picea abies (L.) H. Karst. (Norway spruce). Using a subset of the Second National Soil Survey in Germany, we correlated available data on P in the organic layer and soil with α-diversity indices for beech and spruce forests overall and for individual vegetation layers (tree, shrub, herb, and moss layers). Moreover, we investigated α-diversity feedbacks on P nutrition status and PUE of both tree species. The overall diversity of both forest ecosystems was largely positively related to P content in the organic layer and soil, but there were differences among the vegetation layers. Diversity in the tree layer of both forest ecosystems was negatively related to the organic layer and soil P. By contrast, shrub diversity showed no correlation to P, while herb layer diversity was negatively related to P in the organic layer but positively to P in soil. A higher tree layer diversity was slightly related to increased P recycling efficiency (PPlant/Porganic layer) in European beech and P uptake efficiency (PPlant/Psoil) in Norway spruce. The diversity in the herb layer was negatively related to P recycling and uptake efficiency in European beech and slightly related to P uptake efficiency in Norway spruce. In spruce forests, overall and herb species richness led to significantly improved tree nutrition status. Our results confirm significant, non-universal relationships between P and diversity in temperate forests with variations among forest ecosystems, vegetation layers, and P in the organic layer or soil. In particular, tree species diversity may enhance complementarity and hence also P nutrition of dominant forest trees through higher PUE, whereas moss and herb layers seemed to show competitive relationships among each other in nutrient cycling.DFG, 241127382, PhosForDiv - Phosphatverfügbarkeit als Einflussgröße der Pflanzen-Biodiversität in Waldökosysteme

Historical evidence for context-dependent assessment of Erigeron canadensis invasions in an 18th-century European landscape

Understanding the historical roots of invasion science provides insights into early perceptions of invasive species, allows us to trace the evolution of the discipline over time, and helps contextualize modern research. This paper analyzes work by Christian Ludwig Krause, published 250 years ago, on the invasion of an 18th-century European landscape by Erigeron [Conyza] canadensis (Canadian horseweed), one of the most common invasive species today and a widespread agricultural weed. Here an analysis is conducted of the ecological consequences and underlying mechanisms Krause described, how he evaluated E. canadensis invasions in different land-use systems and how his insights align with existing knowledge. Krause identified copious seed production and long-distance dispersal by wind as key mechanisms for the formation of dominant stands on degraded sandy soils. He recognized various ecosystem services associated with population establishment, such as erosion control, increased soil fertility, and the facilitation of other species. While Krause highlighted the benefits of E. canadensis invasions for the recovery of degraded grasslands and fields, he also acknowledged this introduced species as a troublesome weed in gardens. Thus, Krause’s work is not only an early report on the invasion of a cultural landscape subject to wind erosion but also an early example of a context-dependent invasion assessment, illustrating both positive and negative impacts of the same species in different environments. Krause’s perspective may encourage current assessments of E. canadensis not solely based on its presence or frequency, but on documented ecological and socioeconomic effects and their associated benefits or harms. As Krause impressively demonstrated 250 years ago, these effects can differ starkly in different environments, necessitating multiple responses to the same species

Plant invasions in China : an emerging hot topic in invasion science

China has shown a rapid economic development in recent decades, and several drivers of this change are known to enhance biological invasions, a major cause of biodiversity loss. Here we review the current state of research on plant invasions in China by analyzing papers referenced in the ISI Web of Knowledge. Since 2001, the number of papers has increased exponentially, indicating that plant invasions in China are an emerging hot topic in invasion science. The analyzed papers cover a broad range of methodological approaches and research topics. While more that 250 invasive plant species with negative impacts have been reported from China, only a few species have been considered in more than a handful of papers (in order of decreasing number of references: Spartina alterniflora, Ageratina adenophora, Mikania micrantha, Alternanthera philoxeroides, Solidago canadensis, Eichhornia crassipes). Yet this selection might rather reflect the location of research teams than the most invasive plant species in China. Considering the previous achievements in China found in our analysis research in plant invasions could be expanded by (1) compiling comprehensive lists of non-native plant species at the provincial and national scales and to include species that are native to one part of China but non-native to others in these lists; (2) strengthening pathways studies (primary introduction to the country, secondary releases within the country) to enhance prevention and management; and (3) assessing impacts of invasive species at different spatial scales (habitats, regions) and in relation to conservation resources

Drivers of carbon sequestration by biomass compartment of riparian forests

Riparian forests are expected to play a crucial role in the global carbon (C) cycle but the complex mechanisms of C sequestration in forests remain poorly understood. This study used a comprehensive approach to analyze C sequestration that included the main C compartments in forests, i.e., litterfall, fine roots, and aboveground woody biomass. We aimed at modeling each of them in response to an array of environmental drivers to untangle the functioning of C sequestration by compartment. The study was conducted in a Central European riparian forest that is part of the Donau-Auen National Park in Austria. Carbon sequestration by compartment was correlated with environmental parameters (climate, stream flow, hydrological, spatial, and forest stand parameters) using generalized linear mixed models (GLMM), and the correlations were prioritized by hierarchical partitioning. Our results suggest divergent responses of C sequestration in different ecosystem compartments under dry and wet soil conditions. In particular, dry conditions led to significantly higher C sequestration in aboveground woody biomass (larger distance to the low groundwater table), whereas wetter conditions fostered C sequestration in fine-root (smaller magnitude of fluctuation in the groundwater table) and leaf biomass (smaller distance to the low groundwater table). Fine roots and litterfall responded to short-term variations in climate (mean annual temperature) and flooding parameters (duration of the low to mean Danube River water level in the previous dormant season), highlighting the pivotal role of the dynamic fine-root and leaf biomass compartments for C uptake in forest ecosystems. Consequently, litterfall and fine roots should be considered to improve the sensitivity of C sequestration model responses to climate scenarios.DFG, CI 175/1, Steuergrößen der Kohlenstoffdynamik im Boden und in der Vegetation von Auwälder

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

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