Landscape structure influences urban vegetation vertical structure

In this post Matthew Mitchell discusses his recent paper ‘Landscape structure influences urban vegetation vertical structure‘.

The importance of urban vegetation

Odds are that you’re reading this in a city. More than half of humanity today lives in cities, and this is expected to increase to two-thirds by 2050. Living in a city, you almost certainly rely on urban vegetation for key ecosystem services like climate regulation, carbon storage, and recreation. And most likely you also appreciate the biodiversity that urban vegetation harbours.

While we know that vegetation vertical structure – plant density at different vertical heights – affects biodiversity and can impact ecosystem services, we currently know very little about the vertical structure of urban vegetation. For example, how does it vary spatially across cities and what might drive these patterns? This limits our ability to effectively manage urban vegetation.

Fig 1
The urban green spaces of Roma Street Parkland in Brisbane, Australia, exemplify the complex and fine-scale vertical and spatial structure of urban vegetation. Photo credit: Matthew Mitchell

What might drive urban vegetation vertical structure?

Using remotely sensed data from Brisbane, Australia, we found that vegetation vertical structure was best explained by the surrounding landscape structure rather than geophysical, soil, socioeconomic, or urban form variables. This contrasts with studies of urban vegetation spatial extent, where socioeconomic and urban form variables are usually the most important.

Specifically, we found that canopy height, foliage projective cover (a measure of vegetation density), and vegetation vertical diversity all increased as the surrounding amount of vegetation cover increased. These vertical structure variables also increased when patches of urban vegetation cover were more clumped together across the city. These results represent some of the first evidence of the potential drivers of urban vegetation vertical structure.

Interestingly, these broader relationships weren’t constant across the city. For example, the city core, which had low vegetation cover, also had high foliage projective cover and canopy heights. This suggests that there are ways to increase vegetation vertical structure in cities despite low vegetation cover.

Consequences for urban planning

Our results suggest that clearing of urban vegetation generally leads to changes in its vertical structure, although this needs to be tested in other cities. This result is consistent with forest ecosystems, where fragmentation also leads to changes in structural complexity. However, in cities vegetation loss and fragmentation will almost certainly be accompanied by increased human access and activities that could affect vegetation vertical structure. This includes the introduction of novel plant species, removal of hazard trees, management to decrease fire risk, and recreation. The specific processes that link vegetation clearing in cities to changes in vertical structure still need to be identified.

Up until now, urban planning for vegetation has mostly focused on vegetation extent. For example, methods to increase the amount of public green space across cities. Our results suggest that in order to manage the vertical structure of urban vegetation, the spatial arrangement of urban green spaces will also have to be considered. For example, clumping green spaces together might help maximize vegetation vertical complexity which could benefit biodiversity and ecosystem services. We also need to better understand how altering the vertical structure and spatial configuration of urban vegetation impacts how people interact with urban vegetation to obtain ecosystem services. This should allow us to more effectively create sustainable, vibrant, and biodiverse urban landscapes.

Fig 2
Urban green spaces vary widely in Brisbane, from highly managed and manicured yards (left), to more natural, open, Eucalyptus woodlands (right). Photo credit: Matthew Mitchell

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