Shrubbiness: When a tree isn't a tree

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This post is a preview of a sidebar in my upcoming book about the neglected city, Design by Deficit. Also posted on Medium. 

​Syracuse, New York, the city where I used to live, is a shrubby place. By that I mean that unmaintained land within the city quickly becomes colonized by woody vegetation above waist height, but below the height of shade or canopy trees. This is the ecological process of succession, but it’s succession with a twist: it’s being done by what Peter del Tredici calls, “cosmopolitan urban vegetation.” This mix of natives and exotic invasives includes several of the Most Wanted on the list of invasives in the US: shrubby honeysuckles, autumn olive, Japanese knotweed, and of course, public enemy #1, buckthorn. This paper called out Syracuse as one of the only cities in the US where overall tree canopy is increasing, probably because of the buckthorn. That’s a lot of buckthorn. At least it’s too cold here for kudzu ...so far.

Syracuse isn’t alone. Cities in the eastern US, particularly those that have lost industry and population, have shrubby places in part because of this collection of large invasive shrubs, and the rainfall to support them. Numerous other trends converge in shrubbiness: vacant land, abandoned properties, tight municipal budgets that restrict mowing, but also an increased value placed on wildlife habitat and natural-looking landscapes. 

Urban shrubbiness sits at the junction between two established findings about human behavior and vegetation. In this research, tree canopy is the panacea, or nearly so. Tree canopy does it all - we think better, behave better, feel better, and like places and communities better when there are shade trees, to paint this research very broadly. Yes, these are largely studies finding association, not causation, and experiments are hard to design on this topic, but the association is consistent across many studies by many people, and in the end, there’s not much downside to planting trees, as we argue here. But on the other hand, there’s cues to care and loose space, research and theory that argues that we generally perceive apparently unmaintained land (such as that covered with successional vegetation) as out of control and outside the rules, fit for transgressive activities like vandalism. Tall dense vegetation - shrubbiness, again - has been shown in numerous studies to be a less preferred environment that makes us feel insecure, perhaps because we doubt our ability to navigate through it or discern threats within it. 

So shrubbiness is both good and bad, from a human behavior standpoint. Which wins out- the positive or negative impacts, or is it a draw? For that matter, invasives, or the cosmopolitan cocktail of urban vegetation, are a mixed bag ecologically as well. Invasives (like buckthorn) are demonized because they damage biodiversity and don’t do all the good things that their less-competitive native counterparts do, but sometimes, well, a tree’s a tree. They all photosynthesize, and mitigate heat island, and help slow stormwater runoff - or do they? There’s an implied balance sheet here, weighing the good and bad impacts of unmaintained urban understory vegetation in terms of social, economic, and ecological good. 

A tree isn’t a tree when it’s buckthorn vs. white oak, but many of our best and most sophisticated ways to measure urban vegetation in quantity (meaning on more sites that you can visit in person) sees those as the same.

In the lack of distinction lies the mystery; this devil’s in that detail.

What’s the impact of the shrubby city, and how do we find out? What about drier cities or ones without a Syracuse-caliber winter? Is the impact different there? If the environment matters, shouldn’t these differences matter, too? As climate change makes some places wetter as it dessicates others, and we all get hotter and hotter, we may find we want to know whether shrubbiness is working for or against us. Invasive large shrubs aren’t going away. We might as well understand what they’re doing to us.