Urban forest restoration ecology: a review from Hamilton, New Zealand

Ecological restoration and biodiversity-friendly management of urban grasslands - A global review on the current state of knowledge

In the face of the global biodiversity decline, ecological restoration measures to actively enhance urban biodiversity and options for biodiversity-friendly greenspace management are high on the agenda of many governments and city administrations. This review aims to summarize and advance the current knowledge on urban grassland restoration by synthesizing research findings on restoration approaches and biodiversity-friendly management measures globally. Indeed, we found restoration approaches to be generally effective in increasing biodiversity; yet, there were variations in the outcomes due to the difference in soil disturbance methods, management regimes, the set of species introduced to a site, and the specific local setting. Based on the reviewed studies, we formulated recommendations for maximizing restoration success of urban grasslands through: i) creating a network of heterogeneous urban greenspaces and enhancing connectivity between them; ii) maintaining the spontaneous vegetation in vacant lots and wasteland sites that can provide habitats for various invertebrate species; iii) evaluating actual soil conditions, soil seed bank, and seed rain before restoration efforts take place since these seed sources could considerably affect the restoration outcomes, iv) preserving nutrient-poor conditions in urban greenspaces instead of introducing nutrient-rich topsoil; v) shifting to less intensive, biodiversity-friendly management in urban greenspaces by reducing mowing frequency and avoiding the use of chemicals; and vi) utilizing native dry grassland species for climate adaptation without irrigation. We further identified knowledge gaps regarding i) city-scale and regional-scale effects of restoration, ii) effects of interventions on multiple taxa and multiple ecosystem services, iii) restoration in small versus mega-cities, and iv) in the global south. These gaps should be addressed in future studies for making general guidelines for urban grassland restoration broadly applicable.

Convergence and divergence in science and practice of urban and rural forest restoration

Forest restoration has never been higher on policymakers' agendas. Complex and multi-dimensional arrangements across the urban-rural continuum challenge restorationists and require integrative approaches to strengthen environmental protection and increase restoration outcomes. It remains unclear if urban and rural forest restoration are moving towards or away from each other in practice and research, and whether comparing research outcomes can help stakeholders to gain a clearer understanding of the interconnectedness between the two fields. This study aims to identify the challenges and opportunities for enhancing forest restoration in both urban and rural systems by reviewing the scientific evidence, engaging with key stakeholders and using an urban-rural forest restoration framework. Using the Society for Ecological Restoration's International Principles as discussion topics, we highlight aspects of convergence and divergence between the two fields to broaden our understanding of forest restoration and promote integrative management approaches to address future forest conditions. Our findings reveal that urban and rural forest restoration have convergent and divergent aspects. We emphasise the importance of tailoring goals and objectives to specific contexts and the need to design different institutions and incentives based on the social and ecological needs and goals of stakeholders in different regions. Additionally, we discuss the challenges of achieving high levels of ecological restoration and the need to go beyond traditional ecology to plan, implement, monitor, and adaptively manage restored forests. We suggest that rivers and watersheds could serve as a common ground linking rural and urban landscapes and that forest restoration could interact with other environmental protection measures. We note the potential for expanding the creative vision associated with increasing tree-containing environments in cities to generate more diverse and resilient forest restoration outcomes in rural settings. This study underscores the value of integrative management approaches in addressing future forest conditions across the urban-rural continuum. Our framework provides valuable insights for policymakers, researchers, and decision-makers to advance the field of forest restoration and address the challenges of restoration across the urban-rural continuum. The rural-urban interface serves as a convergence point for forest restoration, and both urban and rural fields can benefit from each other's expertise.

Urban afforestation: using phytotoxicity endpoints to compare air pollution tolerance of two native Brazilian plants Aroeira (Schinus terebinthifolius) and Cuvatã (Cupania vernalis)

Urban afforestation can mitigate the effects of air pollution, but the suitability of plant species for this purpose needs to be determined according to pollution intensity and climate change. The goal of this study was to evaluate the sensitivity of different phytotoxicity endpoints using two native Brazilian plant species as models, Aroeira (Schinus terebinthifolius) and Cuvatã (Cupania vernalis). The sensitivity parameters evaluated could help in selecting the most air-pollution-tolerant plant species for use in urban afforestation programs. The two plant species were exposed, in a greenhouse, to the combustion gases of a diesel engine for 120 days, with daily intermittent gas exposure. Every 30 days, leaf injury (chlorosis and necrosis), biomass, and physiological/biochemical parameters (proteins, chlorophyll, and peroxidase enzyme activity) were evaluated for both plant species. For the two selected species, the endpoints studied can be ranked according to their sensitivity (or inversely the tolerance) to diesel oil combustion gases in the following order: peroxidase > biomass ≈ chlorophyll > protein > leaf injury. The endpoint responses of higher plants can be used to assess the suitability of particular plant species for use in urban afforestation areas with relatively intense vehicle traffic.

Greening the city: Thriving for biodiversity and sustainability

Urban population and urbanisation are increasing rapidly, mainly in developing countries, usually at the expense of green and blue areas. This trend will decrease the ecosystems' capacity to supply ecosystem services (ES) and threaten human wellbeing. Therefore, it is key to establish greening policies in urbanising areas, which are essential to improve the liveability of cities. Restoring and developing green and blue infrastructures using nature-based solutions is vital to improving urban biodiversity and urban ecosystems. Healthy urban ecosystems have a high capacity to supply regulating (e.g., air, noise, climate and water regulation), provisioning (e.g., food, medicinal plants, biomass) and cultural (e.g., recreation, landscape aesthetics, social cohesion) ES. This multifunctionality can provide diverse environmental, social and economic benefits to urban residents, hence contributing to the sustainability of urban areas. However, urban green and blue areas are also associated with ecosystem disservices (e.g., plant allergies or poisoning, emission of biogenic volatile organic compounds, unpleasant smells), tradeoffs (e.g., increased water consumption, wildfire risk, associated management costs) and implementation barriers (e.g., political motivation, lack of knowledge, time and workload). Overall, the SI published 8 articles from different parts of the world, such as China, the USA, Italy or Spain, focused on important aspects of greening the city (e.g., green roofs, green walls, green infrastructures, sustainable mobility).

Restoration Trajectories and Ecological Thresholds during Planted Urban Forest Successional Development

Successfully reconstructing functioning forest ecosystems from early-successional tree plantings is a long-term process that often lacks monitoring. Many projects lack observations of critical successional information, such as the restoration trajectory of key ecosystem attributes and ecological thresholds, which signal that management actions are needed. Here, we present results from a 65 ha urban temperate rainforest restoration project in Aotearoa New Zealand, where trees have been planted annually on public retired pasture land, forming a 14 years chronosequence. In 25 plots (100 m2 each), we measured key ecosystem attributes that typically change during forest succession: native tree basal area, canopy openness, non-native herbaceous ground cover, leaf litter cover, ground fern cover, dead trees, and native tree seedling abundance and richness. We also monitored for the appearance of physiologically-sensitive plant guilds (moss, ferns, and epiphytes) that may be considered ecological indicators of succession. Linear regression models identified relationships between all but one of the key ecosystem attributes and forest age (years since planting). Further, using breakpoint analysis, we found that ecological thresholds occurred in many ecosystem attributes during their restoration trajectories: reduced canopy openness (99.8% to 3.4%; 9.6 years threshold), non-native herbaceous ground cover (100% to 0; 10.9 years threshold), leaf litter cover (0 to 95%; 10.8 years threshold), and increased tree deaths (0 to 4; 11 years threshold). Further, juvenile native plant recruitment increased (tree seedling abundance 0 to ~150 per 4 m2), tree seedling species richness (0 to 13 per 100 m2) and epiphytes colonized (0 to 3 individuals per 100 m2). These and other physiologically-sensitive plant guilds appeared around the 11 years mark, confirming their utility as ecological indicators during monitoring. Our results indicate that measurable, ecological thresholds occur during the restoration trajectories of ecosystem attributes, and they are predictable. If detected, these thresholds can inform project timelines and, along with use of ecological indicators, inform management interventions.

Native Trees as a Provider of Vital Urban Ecosystem Services in Urbanizing New Zealand: Status Quo, Challenges and Prospects

In New Zealand, over 87% of the population currently resides in cities. Urban trees can face a myriad of complex challenges including loss of green space, public health issues, and harm to the existence of urban dwellers and trees, along with domestic greenhouse gas (GHG) and air pollutant emissions. Despite New Zealand being a biodiversity hotspot in terms of natural environments, there is a lack of knowledge about native tree species’ regulating service (i.e., tree development and eco-physiological responses to low air quality, GHG, rising air temperatures, and drought) and how they grow in built-up environments such as cities. Therefore, we argue for the value of these native species in terms of ecosystem services and insist that they need to be viewed in relation to how they will respond to urban abiotic extremes and climate change. We propose to diversify planted forests for several reasons: (1) to improve awareness of the benefits of diverse planted urban forests; (2) to foster native tree research in urban environments, finding new keystone species; and (3) to improve the evidence of urban ecosystem resilience based on New Zealand native trees’ regulating services. This article aims to re-evaluate our understanding of whether New Zealand’s native trees can deal with environmental stress conditions similarly to more commonly planted alien species.

Tree Species Mapping on Sentinel-2 Satellite Imagery with Weakly Supervised Classification and Object-Wise Sampling

Information on forest composition, specifically tree types and their distribution, aids in timber stock calculation and can help to better understand the biodiversity in a particular region. Automatic satellite imagery analysis can significantly accelerate the process of tree type classification, which is traditionally carried out by ground-based observation. Although computer vision methods have proven their efficiency in remote sensing tasks, specific challenges arise in forestry applications. The forest inventory data often contain the tree type composition but do not describe their spatial distribution within each individual stand. Therefore, some pixels can be assigned a wrong label in the semantic segmentation task if we consider each stand to be homogeneously populated by its dominant species. Another challenge is the spatial distribution of individual stands within the study area. Classes are usually imbalanced and distributed nonuniformly that makes sampling choice more critical. This study aims to enhance tree species classification based on a neural network approach providing automatic markup adjustment and improving sampling technique. For forest species markup adjustment, we propose using a weakly supervised learning approach based on the knowledge of dominant species content within each stand. We also propose substituting the commonly used CNN sampling approach with the object-wise one to reduce the effect of the spatial distribution of forest stands. We consider four species commonly found in Russian boreal forests: birch, aspen, pine, and spruce. We use imagery from the Sentinel-2 satellite, which has multiple bands (in the visible and infrared spectra) and a spatial resolution of up to 10 meters. A data set of images for Leningrad Oblast of Russia is used to assess the methods. We demonstrate how to modify the training strategy to outperform a basic CNN approach from F1-score 0.68 to 0.76. This approach is promising for future studies to obtain more specific information about stands composition even using incomplete data.

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.