Lawn grasses, tree cultivars, and other ornamental plant "cultigens" are what most urban greenspace is made of. But what do horticultural plants do to and for the environment? Or for people? Is what they do different from what “natural” plant species – the products of natural selection – do?
Cultigens are plants that are intentionally bred or selected by humans. While the term “cultivar” is more widely known, a cultivar is a certain type of cultigen that has been given a formal name to recognize a human-created sub-species. But whether artificially selected plants have formal names or not, they’re ubiquitous in cities and urban greenspaces, yet they’re poorly studied from an ecological perspective.
As I explained in my last post, cultigens are a gap in the ecological literature, mainly because there are some fundamental differences in the way we need to study cultigens and the way we study other species. In this post, I want to touch on some approaches that come from outside of ecology - environmental history, human-plant geography, and philosophy - that can help us understand the ecological role of cultigens in greenspace.
Environmental history
One of main reasons I spent much of this academic year in Australia is because the urban plants there bear a striking resemblance to my study sites in California cities. Eucalypts and other Australian cultigens are so prevalent in California that some local residents think they’re native. Conversely, iconic California trees such as redwoods are very popular in Australian cities.
It turns out that environmental historians understand this ecological convergence very well. In his 1999 book, Australian historian Ian Tyrrell describes the close relationships between the environmental movement, forestry practices, and the horticultural trade in turn-of-the-century California and Australia. These exchanges were partly influenced by a similar climate that allows many species to thrive in both locations. But they were also driven by the 19th century settlement patterns in the U.S and Australia, the broader context of the American environmental movement and how it intersected with 19th century concepts of nature, and the physical trade routes that allowed plant material to be transported across the Pacific.
It’s not really possible to fully explain the presence of Australian cultigens in America and vice versa without considering all of these factors.
In traditional ecological studies, the presence of species in a particular location is explained by various factors that “filter out” other species. Essentially, ecologists ask the question: why don’t all the world’s species occur everywhere? Why do different species occur in different places? In natural ecosystems this might be because of geologic history (continents that are far apart prevent species migration), or because some species are poorly adapted to the local environment, or because one species out competes another.
For ecosystems dominated by cultigens, we’d also like to understand why some places have more biodiversity than others. Some of the filters that explain biodiversity in natural ecosystems apply to cultivated landscapes, but some don’t. Humans can easily circumvent the distance between continents, for example. In addition, the presence of cultigens introduces others factors we need to consider.
Many of these factors are historical, and require tracing the particular people and circumstances that led to the creation, adoption, and transport of horticultural cultivars and other cultigens. One key step in the study of cultigen ecology is to explore this history.
Human-plant geography
Why are certain plant species selected for human cultivation while others aren’t? If it’s possible to understand this, as geographers suggest it is, we might fill in some major gaps in our ecological understanding of urban greenspaces.
For example, what makes turfgrass cultigens so popular in American culture? This is a critical piece of information for efforts to reduce the area of lawns in urban greenspace, if we think this should be done. To pose this question more ecologically, we're asking what determines the distribution of turfgrass cultigens across urban ecosystems? The usual explanations from natural ecosystems – local climate, natural selection, and dispersal by wind or pollinators - won’t give us a complete picture.
The field of human-plant geography is aimed at answering questions like this in a different way. For cultigens, the ways people perceive and experience plants, and the cultural contexts for concepts about plants, determine how they breed, domesticate, and cultivate them for horticultural and other uses. Humans select for features of ornamental plants that attract them - flowers for example - because they find them aesthetically pleasing, evoke an emotional response, or remind them of a person, place or experience that has meaning to them.
For reasons that I’d say we don’t fully understand, flowers evoke a range of emotions in most people, large trees draw out our fascination with forests, and lawns are both inviting but also exposed and orderly – negating some of our fears of the forces of disorderly nature.
Is this just another way of saying that cultigens provide ecosystems services, i.e. benefits for people? Not entirely. Lesley Head of the University of Melbourne and Jennifer Atchison of the University of Wollongong write “that a certain sort of plant charisma draws in human attention and care” of particular species, and not others. If this is the case, it’s not just humans that are active participants in the relationship between cultivator and cultivated. There’s something about the plants themselves, or their ecological roles, that elicit a human response.
So what is it about these species that leads us to cultivate them, while other species are ignored or actively removed and weeded out of landscapes? And don’t the cultigens themselves, and not just humans, benefit from this interaction?
Cultivation as a symbiosis
There’s an ecological term for a relationship between two types of organisms in which they both benefit. This is a mutualism - a type of symbiosis - and it’s most commonly used to describe the relationships among non-human species.
Yet many have argued that the process of domestication of plants and animals is symbiotic and mutualistic. Cultigens benefit enormously from their relationship with humans. Lawn grasses are heavily fertilized, irrigated, transported, distributed, and cared for by their human partners. Formerly ‘natural’ species that have been hybridized or selected to become successful cultigens have dramatically increased their range beyond their natural habitat. Take the humble Pinus radiata, the Monterey pine, which is an ecological remnant of a past climate now naturally found in just a few small patches along the Pacific coast of North America. Selected as a plantation tree from a few seeds collected by botanist David Douglas in the 1830s, P. radiata is currently the most widely distributed tree cultigen in the world.
The possibility that humans may be equal partners with other highly successful organisms is contrary to the notion that humankind is active and nature is passive. Writing with Dave Kendal, now at the University of Tasmania, and Joslin Moore of Monash University, Anna Wilson of Australian National University pointed out that “if it were ants rather than humans gardening ornamental plants, their relationship would undoubtedly be recognized as a mutualism and one of the wonders of the natural world.”
As a matter of fact, from some vantage points humans may even be on the losing end of these symbioses, making them potentially more parasitic than mutualistic. Given the money, time, energy, and resources poured into lawn maintenance, and the resulting impacts on the environment, are humans really the winners in their relationship with turfgrass cultigens?
Exploring this type of relationship might mean flipping the usual script – instead of focusing only on how people benefit from or influence nature, it might be useful to study how some species influence us to their own benefit, in ways we may not always consciously realize.
In other words, humans are not the only actors in nature. Actor-Network Theory (ANT), developed by philosopher Bruno Latour and others, provides a framework for viewing human-nature relations through a different lens - one in which humans are not always in charge. For an analysis of the role of non-human agency in urban yards, see this new paper led by Jesse Engebretson at the University of Minnesota as part of the American Residential Macrosystem project.
I think that’s all I have space for this week. These last two posts turned out to be much longer than expected. If anyone is still reading – thanks for hanging in there! There’s a lot more to say, and we'll keep exploring the role of cultigens vs. natural/native species in urban greening.