Valorization of Urban Street Tree Pruning Residues in Biorefineries by Steam Refining: Conversion Into Fibers, Emulsifiers, and Biogas

Street tree pruning residues are a widely available and currently undervalorized bioresource. Their utilization could help alleviate an increasing biomass shortage and offset costs of the pruning process for the municipalities. In this work, a holistic valorization pathway of pruning residues leading to fibers, oligosaccharides, biogas, and compost is presented. For this, representative mixtures of tree pruning materials from the most prevalent street tree genera (oak, linden, maple) found in Hamburg (Germany) were prepared by shredding and cleaning procedures. Collection of sample material was performed in summer and winter to account for seasonality. A steam-based fractionation was conducted using treatment severities ranging from log R0 = 2.5 to 4.0. At the highest severity, a fiber yield of around 66%, and liquor yield of 26–30% was determined. The fibers were evaluated with respect to their properties for paper product applications, with higher treatment severities leading to higher paper strengths. From the oligosaccharide-rich liquor, emulsions were created, which showed promising stability properties over 8 weeks of storage. The liquors and the rejects from the material preparation also displayed good potential for biomethane production. Overall, the differences between material collected in summer and winter were found to be small, indicating the possibility for a year-round utilization of pruning residues. For the presented utilization pathway, high severity treatments were the most promising, featuring a high liquor yield, good biomethane potential, and the highest paper strengths.

Prioritizing Street Tree Planting Locations to Increase Benefits for All Citizens: Experience From Joliette, Canada

As the climate continues to warm and the world becomes more urbanized, our reliance on trees and the benefits they provide is rapidly increasing. Many cities worldwide are planting trees to offset rising temperatures, trap pollutants, and enhance environmental and human health and well-being. To maximize the benefits of planting trees and avoid further increasing social inequities, a city needs to prioritize where to establish trees by first identifying those areas of greatest need. This work aims to demonstrate a spatially explicit approach for cities to determine these priority locations to achieve the greatest returns on specific benefits. Criteria for prioritization were developed in tandem with the City of Joliette, Canada, and based on nine indicators: surface temperature, tree density, vegetation cover, resilience, tree size and age, presence of species at risk, land use type, socioeconomic deprivation, and potential for active transportation. The City’s preferences were taken into account when assigning different weights to each indicator. The resulting tree planting priority maps can be used to target street tree plantings to locations where trees are needed most. This approach can be readily applied to other cities as these criteria can be adjusted to accommodate specific tree canopy goals and planning constraints. As cities are looking to expand tree canopy, we hope this work will assist in sustaining and growing their urban forest, enabling it to be more resilient and to keep providing multiple and sustained benefits where they are needed the most.

Carbon sequestration potential of street tree plantings in Helsinki

Cities have become increasingly interested in reducing their greenhouse gas emissions, and increasing carbon sequestration and storage in urban vegetation and soil as part of their climate mitigation actions. However, most of our knowledge on biogenic carbon cycle is based on data and models from forested ecosystems even though urban nature and microclimate are very different to those in natural or forested ecosystems. There is a need for modelling tools that can correctly consider temporal variations of urban carbon cycle and take the urban specific conditions into account. The main aims of this study are to examine the carbon sequestration potential of two commonly used street tree species (Tilia x vulgaris and Alnus glutinosa) and their soils by taking into account the complexity of urban conditions, and evaluate urban land surface model SUEWS and soil carbon model Yasso15 in simulating carbon sequestration of these street tree plantings at different temporal scales (diurnal, monthly and annual). SUEWS provides the urban microclimate, and photosynthesis and respiration of street trees whereas the soil carbon storage is estimated with Yasso. Both models were run for 2002–2016 and within this period the model performances were evaluated against transpiration estimated from sap flow, soil carbon content and soil moisture measurements from two street tree sites located in Helsinki, Finland. The models were able to capture the variability in urban carbon cycle due to changes in environmental conditions and tree species. SUEWS simulated the stomatal control and transpiration well (RMSE < 0.31 mm h−1) and was able to produce correct soil moisture in the street soil (nRMSE < 0.23). Yasso was able to simulate the strong decline in initial carbon content but later overestimated respiration and thus underestimated carbon stock slightly (MBE > −5.42 kg C m−2). Over the study period, soil respiration dominated the carbon exchange over carbon sequestration, due to the high initial carbon loss from the soil after the street construction. However, the street tree plantings turned into a modest sink of carbon from the atmosphere on annual scale as the tree and soil respiration approximately balanced photosynthesis. The compensation point when street trees plantings turned from annual source to sink was reached faster by Alnus trees after 12 years, while by Tilia trees after 14 years. Overall, the results indicate the importance of soil in urban carbon sequestration estimations.

The Effects of Residential Street Tree Spacing and Crown Interactions on Crown Dimensions and Canopy Cover

Urban trees provide people with a range of ecosystem services. Trees planted along streets have been a large focus of urban forest research and practice, and municipalities invest significant resources in their survival. However, the optimal spacing of street trees is not addressed in the scientific literature, and existing municipal street tree spacing standards are highly variable and poorly enforced. In this study, we examine variability in crown shape and size for street trees to test for possible interaction effects at closer spacings. We measured variability in crown diameters both parallel and perpendicular to street tree rows to test whether changes in crown dimensions can be explained by interaction effects with neighbouring trees, and whether crown interactions lead to a reduction in total crown projection area (i.e., canopy cover). We measured the crown dimensions and diameter at breast height of 1,338 street trees in Halifax, Canada. We used two-way analysis of variance to test whether crown shape and crown projection area were affected by crown interactions and spacing. We found that the effect of narrower spacing and interactions (i.e., crowns touching/overlapping) among trees translated to crowns extending away from the direction of interaction. We also found that these changing crown dimensions were associated with increases in canopy cover. Urban forest ecosystems are a vital resource for the increasingly urban population. There is a need for empirical research on spacing standards and practices that investigate their influence on the supply of ecosystem services, such as stormwater retention, air pollution removal, and cooling.

Is Street Tree Diversity Increasing in New York State, USA?

Diversity in tree populations is viewed as essential for protecting the public investment in urban trees and for preserving the environmental, social, and economic benefits that these trees provide. It is therefore crucial for officials responsible for the management of municipal trees to know the diversity of their municipal tree populations and whether their efforts to increase diversity have been effective or should be modified. We assessed street tree diversity in New York State, USA by analyzing municipal street tree inventory data from two data sets, the first comprised of 75 inventories collated from municipalities, and the second comprised of 32 sets of inventories conducted at multiple points in time. This analysis builds on two previous papers containing similar assessments by analyzing more current data and by calculating diversity index statistics and relative abundance percentages for prevalent street tree species and genera. Findings indicate that there has been substantial progress to increase street tree diversity in New York State. This progress is correlated with reductions in the dominance of Norway maple (Acer platanoides), the state’s most prevalent street tree species (17% of street trees statewide), and in the dominance of maple (Acer), the state’s most prevalent street tree genus (35% of street trees statewide). Work remains to be done to further increase species and genus diversity so as to meet the challenges posed to municipal street tree populations by invasive pests and climate change. Strategies are proposed for accomplishing this.

A forested urban park : what is the value of Allan Gardens to the City of Toronto?

The purpose of this study was to conduct an assessment of Allan Gardens' urban forest and to investigate the value of environmental and aesthetic benefits it provides the City of Toronto. This project used the Street Tree Resource Analysis Tool for Urban Forest Managers (STRATUM) model to assess forest structure, function, and monetary value of benefits. Soil in Allan Gardens was also investigated to determine the growing conditions for park trees. Results indicate that Allan Gardens maintained 309 trees that provide $60,407 annually in net annual environmental and property value benefits to the City of Toronto. Soil conditions in the park were found to be highly variable, where some locations were highly compacted and may be restricting tree root growth. To sustain and enhance these benefits in the future, Toronto's urban forest requires dedicated management and maintenance that includes new plantings, but prioritized protection and maintenance of existing trees and soil.