The loss of the middle strata has a positive impact on human safety, but are there unintended consequences for ecosystem services and more-than-human entities?  

Urban design increasingly promotes the removal or omission of the middle vegetation strata in urban landscapes to prioritise pedestrian safety.

 

Where have all the shrubs gone?

Principles of Crime Prevention Through Environmental Design (CPTED), and vehicle sightline requirements have been developed to improve safety for pedestrians and users of the public realm. The intent is to provide unimpeded sightlines on footpaths, roads, and public areas. This requires designing-out visual obstructions, maintaining low level planting and a clear understorey.

The unintended consequences of these strategies is the disappearance of the middle or understorey of vegetation – the missing middle.

 

What does the middle strata offer?

The middle strata contributes significantly to the provision of many ecosystem services. These include pollution mitigation, biodiversity, carbon sequestration, noise attenuation, stormwater management, soil fertility, phytoremediation of soil pollutants, urban heat mitigation, and soil stability (1–4).

The middle strata is particularly important for biodiversity. Habitats with complex vegetation structure (layered canopies, shrubs and grasses) harbour higher species abundance and richness (5–7). Research has demonstrated that the presence of shrubs increases abundance and taxonomic richness of herbivore, pollinator, and detritivore bird species (6).

Shrubs also have cultural and aesthetic value in creating landscapes that reflect local character and improve human health and wellbeing. It has also been suggested that their compact forms offer potential value in densifying urban areas where it is increasingly difficult to find space for large trees (1).

(Figure 1: Artist impression including trees and scrubs of proposed development plans for new homes at Bluff Road in Hampton East, Melbourne being undertaken by Homes Victoria. Image Source: Engage Victoria)

 

How does the missing middle impact landscape quality and biodiversity?

In many urban areas, green infrastructure is often composed of isolated tree canopies, turf and low planting. The lack of structural diversity (missing middle) impacts the health and function of urban landscapes, and available habitat for non-human entities. Shrubs provide critical habitat for biodiversity. Limiting understorey vegetation reduces biodiversity of most species in urban green spaces (8). The availability of urban habitat structures, including shrubs, has greatly diminished, creating negative consequences for biodiversity, nutrient cycling, and carbon sequestration (9).

 

More-than-human entities are missing the middle

More-than-human design considerations require landscape architects to think beyond the needs of human users of designed spaces. More-than-human is term coined by David Abram (10) that speaks of nature as a realm that includes and exceeds humankind. It is a worldview that de-centres humans and establishes relationships that are reciprocal and interconnected with non-human entities. Non-human entities become stakeholders in a design process that considers more-than-human amenity needs.

Security and safety needs for more-than-human actors can be the inverse of human needs. Sightline requirements for human safety have discouraged mid-storey planting to minimise places for concealment and maximise passive surveillance. However, mid-storey planting provides refuge, food, and connectivity to more-than-human entities. Dense, structurally complex planting creates ecological corridors that connect more-than-human habitats in a way that is the inverse of human movement networks that are designed to be open and unobstructed. Creating a series of structurally complex, connected landscapes enables safe movement which allows more-than-human entities to find food, breed and establish new territories.

The mid-storey or shrub layer is essential to the structural diversity of habitat patches. Increasing the volume of mid-storey vegetation in urban landscapes improves outcomes for birds, bats, and insects (8). Mid-storey planting should be standard in landscape design for biodiversity enhancement (11). Landscape Architects must start to find ways to fill in the middle.

(Figure 2: Wedge-leaf hob brush. Image source: City of Melbourne Urban Planting Guide)

 

 

Filling in the middle

Contemporary urban ecology research suggests that habitat patch size is less important for biodiversity than spatial configuration, diversity, distribution, and connectedness of green spaces (11,12). This offers enormous design potential for landscape architects, because every designed landscape, no matter how small, could become a meaningful opportunity for habitat creation and connection.

Opportunities for increasing structural diversity would still need to be balanced with safety considerations, but in a more-than-human approach, human safety becomes one parameter in a complex, multi-stakeholder design process that aims to improve the health and functionality of the entire system. Even small greening actions that increase the diversity and complexity of the plant community can result in large increases in diversity within a short time (13).

Increasing mid-storey planting comes with some concerns over management and maintenance. These concerns are being addressed by trial projects including Woody Meadows (14). Woody Meadows are diverse shrub and small tree plantings which are managed by coppicing (hard pruning) to increase density and supress weed growth. Miyawaki forests are another example that provide opportunities for community-led management partnerships. Miyawaki forests that follow a Japanese model of structurally diverse native planting that form four forest layers (canopy, tree, sub-tree, and shrub). They offer opportunities for community-led rehabilitation of degraded areas (15).

 

 

References:

  1. Blanusa T, Garratt M, Cathcart-James M, Hunt L, Cameron RWF. Urban hedges: A review of plant species and cultivars for ecosystem service delivery in north-west Europe. Urban For Urban Green. 2019 Aug 1;44:126391.
  2. Khosravi Mashizi A, and Sharafatmandrad M. Assessing the effects of shrubs on ecosystem functions in arid sand dune ecosystems. Arid Land Res Manag. 2020 Apr 2;34(2):171–87.
  3. Pugh TAM, MacKenzie AR, Whyatt JD, Hewitt CN. Effectiveness of Green Infrastructure for Improvement of Air Quality in Urban Street Canyons. Environ Sci Technol. 2012 Jul 17;46(14):7692–9.
  4. Sjöman H, Ignell S, Hirons A. Selection of Shrubs for Urban Environments—An Evaluation of Drought Tolerance of 120 Species and Cultivars. HortScience. 2023 May;58(5):573–9.
  5. Evans KL, Newson SE, Gaston KJ. Habitat influences on urban avian assemblages. Ibis. 2009;151(1):19–39.
  6. Sharmin M, Tjoelker MG, Esperon-Rodriguez M, Katlav A, Gilpin AM, Rymer PD, et al. Urban greening with shrubs can supercharge invertebrate abundance and diversity. Sci Rep. 2024 Apr 16;14(1):8735.
  7. Shwartz A, Shirley S, Kark S. How do habitat variability and management regime shape the spatial heterogeneity of birds within a large Mediterranean urban park? Landsc Urban Plan. 2008 Mar 3;84(3):219–29.
  8. Threlfall CG, Mata L, Mackie JA, Hahs AK, Stork NE, Williams NSG, et al. Increasing biodiversity in urban green spaces through simple vegetation interventions. J Appl Ecol. 2017;54(6):1874–83.
  9. Le Roux DS, Ikin K, Lindenmayer DB, Blanchard W, Manning AD, Gibbons P. Reduced availability of habitat structures in urban landscapes: Implications for policy and practice. Landsc Urban Plan. 2014 May 1;125:57–64.
  10. Abram D. The Spell of the Sensuous: Perceptions and Language in a More-Than-Human World. New York: Random House; 1996.
  11. Oke C, Lee K, Harrison L, Croeser T, Taylor M. Playbook for Urban Biodiversity [Internet]. Melbourne: Melbourne Centre for Cities, University of Melbourne; 2023 [cited 2024 Apr 16]. Available from: chrome-extension://efaidnbmnnnibpcajpcglclefindmkaj/https://www.unimelb.edu.au/__data/assets/pdf_file/0012/4859634/Playbook-for-Urban-Biodiversity.pdf
  12. Pellissier V, Cohen M, Boulay A, Clergeau P. Birds are also sensitive to landscape composition and configuration within the city centre. Landsc Urban Plan. 2012 Feb 1;104(2):181–8.
  13. Mata L, Hahs AK, Palma E, Backstrom A, Johnston N, King T, et al. Large positive ecological changes of small urban greening actions. Ecol Solut Evid. 2023;4(3):e12259.
  14. Woody Meadow [Internet]. [cited 2025 Mar 21]. Available from: https://woodymeadow.unimelb.edu.au/
  15. Western Australia’s first tiny forest for community-led urban greening [Internet]. [cited 2024 Apr 16]. Available from: https://www.murdoch.edu.au/news/articles/western-australia-s-first-tiny-forest-for-community-led-urban-greening

By Jen Lorrimar-Shanks AILA, Principal Landscape Architect at Josh Byrne & Associates,

Article contributed on behalf of the Australian Institute of Landscape Architects

 

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