Tree Selection for Green Infrastructure

Trees form a critical component of green infrastructure (GI) and provide a wide range of ecosystem services to urban dwellers. However, these valuable services are at risk of being compromised through limited species diversity, inappropriate species selection, impoverished growth environments, and high tree mortality rates in urban areas. Provision of many of these ecosystem services relies on healthy trees and their value often scales with tree size, necessitating tree development to maturity and continued health: trees must thrive, not simply survive.

Despite the centrality of appropriate species selection to the successful delivery of GI schemes, there remains scant guidance for the many actors involved in their establishment. The outcome of this guidance deficit is that urban planners, landscape architects and local authority officers rely heavily on a narrow range of 'traditional' species. This constrains urban forest biodiversity, encourages a plant nursery sector that is only incentivised to supply a narrow range of species and diminishes the resilience of our urban forests to future threats.

Providing tree species selection guidance, underpinned by science and available to all communities tasked with delivering GI projects, has the capacity to transform the long term security of associated ecosystem services. Identifying a range of species that are suitable for different GI scenarios will give those specifying plantings the confidence to try new species, broaden the expectation of diverse plant material from nurseries and act to increase the resilience of vital green infrastructure schemes.

Therefore the primary objective of this project is to develop a decision support tool and guidance for built environment professionals to aid species selection of trees used in urban environments. To ensure that the final outcomes of the project meet the stakeholder requirements, an initial focus group will invite Trees in Design Action Group (TDAG) members to establish the needs of the sector and identify current practice. This will include current species recommendations for contrasting urban planting scenarios (e.g. paved sites, parks, green bridges, rain gardens and streets). TDAG members represent the widest possible range of stakeholders in this sector, including but not limited to urban planners, landscape architects, local authorities, educators and private companies. Further, as TDAG membership is free, there are no boundaries to engagement with this process.

Once stakeholder needs have been confirmed (by April 2016), the academic project partners will strategically evaluate nominated plant material. This will be made available by two of the largest nurseries supplying material to the UK's urban forest (eg. Barcham Trees) as well as several botanical gardens (eg. Royal Botanical Gardens, Kew). Since physiological drought tolerance is a requirement of many urban sites, the leaf turgor loss point will be quantified in approximately 100 species, to supplement existing published ecological data such as shade and flood tolerance.

Synthesis of the data collected and developing a draft tree selection guidance will be the task of a KE Fellow who will be seconded to TDAG (October 2016 to June 2017). Knowledge exchange will be further supported by a series of seminars across the UK to seek feedback on the findings (August to November 2017) and assess likely project recommendations.

The final project outcome will be a written guidance document (and online audio-visual lectures) that will be freely available via the TDAG website to all TDAG members and any other interested parties. This document / website will be updated yearly to reflect the availability of new information, as part of the core aims of TDAG.

A number of communities stand to benefit from developing guidance to select urban trees. Professionals engaged with the delivery of green infrastructure (GI) projects must be able to recommend the most appropriate plant material for any given scheme. One of the prevailing challenges in this task is to be able to match individual species or cultivars to particular sites. Planting sites in the urban environment may be surrounded by impermeable surfaces that limit soil moisture recharge events and generate water deficits in urban trees. Some sites, particularly those integrated with sustainable urban drainage systems, may be regularly inundated with floodwater but at other times become very dry. Other sites such as those on green bridges and in street environments provide limited soil volumes. Often multiple stressors act on the plant at the same time, for example, water deficits and shade frequent occur together in urban canyons. Mitigating the impacts of these various environmental stressors enhances the amenity value of urban trees for the urban community.

Such complex growth environments require the integration of fundamental tree physiology and ecology with knowledge of tree characteristics capable of delivering a range ecosystem services. This project will provide a platform for professionals currently engaged with GI projects to identify key themes relating to their planting sites. After evaluating a wide range of plant material and reviewing published literature relating the tree species' ecology, written guidance will then be produced to specifically address the species selection for GI projects. This will be supported by a series of knowledge exchange events held across the UK, leading to the production of audio-visual material.

It is expected that urban planners, landscape architects, local authority officers will benefit most directly from the project. These professionals will be able to directly apply the findings from this project into their planting recommendations. However, all TDAG members, and the wider public will benefit from the guidance documents produced. Of particular benefit to all engaged with species selection will be the mapping of species to particular planting scenarios based on their physiological drought tolerance and other ecological traits.

Over time, the use of a broader range of species within GI will improve the biodiversity of the urban forest and, therefore, enhance its resilience. This has much more widespread outcomes for society and the long-term security of GI mediated ecosystem services. It is possible a modest commercial advantage will be gained by tree nurseries that are able to bring under-utilised tree species into mainstream cultivation but this will only be a positive artefact of the wider agenda to improve species selection choices by all members of the GI community.

The success of GI initiatives is closely coupled to the efficacy of plant establishment, which is in turn, dependent on appropriate species selection. The impact any GI project, particularly over the longer term hinges on the species planting decision. Given the current paucity of independent advice to landscape professionals regarding species choices for different scenarios, this proposal will benefit large sectors within the GI community. Although this project is focussing on meeting the needs of the UK GI sector, the findings will be applicable to other temperate regions. It is, therefore, hoped that this project's impact would be felt in comparable regions (outside the UK) experiencing similar challenges.

There is good potential to extend this selection model to include other GI components, such as Hardy Ornamental Nursery Stock (HONS). It could also be used to evaluate herbaceous material used in turf-less lawns, green walls and roofs.