City Flora #1 - The Lawn

Among ecologists and environmentalists, lawns have a well-deserved reputation as "green deserts." This is because, despite being defined by an extensive cover of green plants, they are not very biologically diverse landscapes. Across the world, lawns tend to be sown not only with the same grass species, but also with the same varieties, limiting both genetic and species diversity within and between lawns (Ignatieva and Ahrne 2013). What's more, only a handful of wild plant species are able to elbow their way into growing among the grasses and these are often the same across wide geographic areas (Ignatieva and Steward 2009; Wheeler et al. 2017). This lack of plant diversity in turn limits the insect communities that can find suitable habitat and forage in a lawn. Meanwhile, all the management that is needed to maintain homogenous acres of grass contribute significantly to other environmental problems, such water pollution and scarcity (Robbins et al. 2001; Milesi et al. 2005; Runfola et al. 2013). 

Lawns, in short, are very difficult places for non-human organisms to live in - and yet, some do. Lawns are often biological deserts compared to the native ecosystems that they replace, but they are not lifeless, nor are they entirely controlled by human beings. Different styles of management have different impacts on the physical environment of a lawn and on the biological community that exists there. Some consist of homogenous expanses of thickly, matted grass; others are a mosaic of grass and weeds, shaped by patterns of mowing, trampling, chemical use, sunlight, and human aesthetic preference. Understanding the physical, biological, and cultural mechanisms behind why lawns are the way they are is a crucial first step to addressing the environmental problems they contribute to.

For this first installment of "City Flora," I did a quick survey of the plants living in my parent's lawn in their neighborhood just outside New Haven, Connecticut. What I found was a cast of common, but widely ignored wild plants acting out some really interesting ecological processes that you can observe in your own backyard or in a local park. So come on internet people - let's get down on our hands and knees and touch some grass! 

Beneath the Blade: 

Perhaps the most important factor shaping the ecological character of lawns is disturbance. In ecology, a disturbance is a force or event which, over a relatively short time scale, significantly changes the environmental conditions of an ecosystem and results in changes to the composition and structure of its biological community. The frequency, regularity, intensity, and geographic extent of disturbances can shape the ecological character of a place even in periods between disturbance events. In a previous post, for example, I talked about how fire shapes the plant communities found in Soldier's Delight Natural Environmental Area in Owing's Mill, Maryland. Before European colonization, frequent natural and man-made fires combined with the area's unique, magnesium-rich soils to produce an open barren populated by wildflowers, grasses, and the occasional fire resistance oak. Today, without the benefit of these fires, both the species composition and physical structure of the vegetation community at Soldier's Delight have changed - Virginia pines, which were previously kept out by the fires, are taking over, shading out many of the rare wildflowers and transforming the barrens into brambly pine forests. 

In lawns, natural disturbance events like fire, flooding, storms, and passing herds of large grazing animals are replaced by human management, but they serve the same general function as arbiters of community composition and structure. Regular applications of chemical herbicides act as disturbance events, killing large numbers of plants and opening up opportunities for the survivors to grow and expand. Certain kinds of herbicides are more affective on common "weeds" then they are on lawn grasses, meaning that grasses are able to survive and quickly spread into the area formerly occupied by those weeds. Other herbicides kill grass too, but provide something like an empty slate on which seeds can be aggressively sown by the owner of the lawn, again with the goal of giving the grass a chance to take over and outcompete the weeds. Some weeds have been able to evolve resistance to certain kinds of herbicides, but for the most part, our poisons are pretty affective, and the most biologically homogenous lawns - the most deserted of the green deserts - are those that use the most herbicides (Ignatieva and Steward 2009; Bertonocini et al. 2012; Qi et al. 2016). Conversely, if you want to make your lawn more biodiverse, cutting down on or eliminating herbicide and insecticide use is a great start.

Blackberry (Rubus sp.)

Mowing, the other most common kind of disturbance in a lawn community, is a bit more complicated in its ecological impact. First, and most obviously, mowing a lawn has a significant effect on the physical structure of the plant community by limiting the height to which any plant in that community can grow. In my parent's backyard, there is patch of wild blackberry (Rubus sp.) that was planted years ago by some anonymous bird and from which we might get the occasional ripe, delicious berry if that bird's great grandchildren don't get to them first. Blackberries are very good at colonizing new habitats, both through transport of their seeds by hungry birds and, over shorter distances, by sending out clonal shoots beneath the ground. If my parents didn't keep up a regular mowing schedule, these clonal shoots and the many taller weeds that also grow in that part of the yard would probably take over in just a couple years, transforming their relatively simple, two-dimensional lawn into a complex, three-dimensional bramble patch stuffed with tall, mostly invasive herbaceous plants and grasses. This doesn't happen when they are regularly mowing because taller plants like the blackberry only have a short amount of time to photosynthesize after each resprout before they are cut down again. Many of these sprouts, as a result, are not able to store enough resources to try again and so they eventually die without getting the chance to reproduce themselves. Others pop up again later and the well-established blackberry patch still sends out new clones, but their spread and overall presence in the lawn is much more limited than it would be without the regular disturbance of the mower.

Dwarf Cinquefoil (Potentilla canadensis)

Because it interferes with photosynthesis and reproduction in taller herbaceous and woody plants, mowing selects for a lawn plant community consisting of plants that are small enough to escape the blade. Some of these plants, like the white clover (Trifolium repens), grow vertically, but are small enough that the mower passes them over. Others, like the dwarf cinquefoil (Potentilla canadensis), grow in what botanists call a prostrate growth form - that is, individual plants spread out horizontally, across the ground. In the case of the cinquefoil, growth starts off vertically before stooping back towards the ground as the plant grows. If allowed to spread, the plant's thin, fuzzy stems can reach up to four feet in length! Meanwhile, the height of the cinquefoil remains low (only about 6 inches at the most) and so its distinctive, five-part leaves and showy yellow flowers are left undamaged by spinning mower blades, free to reproduce and photosynthesize to their heart's (or, uh, vascular tissue's?) content.

Common Dandelion (Taraxacum officinale)

Cinquefoils are so small that most people don't pay them very much attention, but if you look closely, you'll start to notice that many larger, more charismatic (or infamous, depending on your outlook) weeds also have prostate growth forms that allow them to escape the mower. This common dandelion (Taraxacum officinale), for example, is growing in a rosette, a circular arrangement of leaves all emerging from a single, deeply set and hard to remove tap root. In environments without regular disturbance, the leaves of the rosette will lift off the ground slightly and point up at an angle; in a high disturbance environment like a lawn, however, the leaves lie flat. This not only protects them from the mower, but also makes the plant less susceptible to damage from trampling.

But of course, the thing that makes dandelions charismatic and/or infamous is not their prostrate growth form - it's their flowers, which do grow on tall, thin stalks that are highly vulnerable to mowing and trampling. So why don't they go the way of the blackberries? Well, the prostrate growth form does help. When a dandelion flower is mown down, the leaves and their photosynthetic cells remain, meaning that the plant survives and can try again at reproducing later on - in fact, an individual dandelion can live over a decade! Mown down dandelions can also reflower later in the growing season, which further increases their chances of successfully reproducing during a stretch of time when you don't feel like mowing. The chance of success for the North American breed of common dandelion is probably further increased by the fact that they can reproduce both sexually and asexually and so don't have to wait around to be pollinated before forming seeds (blackberries can actually do this too). Add all this to their ability to colonize lawns that are in the process of being sown via thousands of windblown seeds and a better question might be "why aren't dandelions covering every unpaved surface in the country?"

White Clover (Trifolium repens)

There are other plants like cinquefoils, clovers, and dandelions that are often able slip beneath the mower blade and colonize our lawns, but in the grand scheme of things, they are a minority. What's more, the extent to which these common weeds are actually present in a given lawn depends a lot on how low the blade on your mower is to the ground and how often you mow. The lower your blade and the more frequently you mow, the less biodiverse your lawn is going to be (Watson et al. 2020). At least here in New England, many of the native plants that would colonize sunny, open habitats like lawns need to grow quite tall before they flower and many of the shorter plants that would have a chance of escaping the mower are excluded due to their existing adaptations to the region's common, precolonial habitat type - dark, shady forests.

Light and Shadow:

Most plants - with a few interesting exceptions - need access to sunlight in order to produce food through photosynthesis. Yet, this does not mean that all plants, if given the choice, will prefer an open, sunny spot over a closed, shady one. Both sunlight and shade have their costs and benefits and different plants balance the tradeoffs that inevitably arise between the two in different ways. As a result, you are likely to find slightly different plant communities in different parts of a lawn depending on how open or shaded they are.

Crabgrass (Digitaria spp.)

One challenge associated with growing on an open landscape is temperature. With the sun beating down from above all day long and not much to block it, plants growing in open areas at temperate and tropical latitudes receive plenty of solar energy for making food, but they also have to deal with extreme heat. This can actually lessen the benefits that a plant may receive from unfettered access to sunlight by affecting the efficiency of the chemical reactions involved in photosynthesis. Of particular significance is the impact of heat on an enzyme called RuBisCO, which plays a crucial role in taking carbon atoms from atmospheric carbon dioxide (CO2) and "fixing" them to what will eventually become energy-storing sugar molecules, the main output of photosynthesis. That's what it's supposed to do anyways. Sometimes, RuBisCO will grab oxygen molecules (O2) instead of CO2 and input that into the relevant chemical pathway, wasting resources and energy without producing sugar molecules at the end of the chain. At 30 degrees C (86 degrees F), there is about a 25% chance of this happening, but that chance goes up as temperatures increase and RuBisCO starts to have a harder time telling the difference between oxygen and carbon dioxide (Sharkey 1988; Busch 2013). To make matters worse, at higher temperatures, plants tend to close their stomate - tiny pores through which CO2 enters and oxygen is released - because they can lose substantial amounts of water through them. This increases the proportion of oxygen to carbon dioxide in the cells of the plant and makes it even more likely that RuBisCO will grab the former over the latter. 

So, what's a sun loving plant to do? Well, some plants have developed a solution to this problem through an alternate photosynthetic pathway called the C4 pathway. It's a bit complicated, but basically, the C4 pathway gets around the problems associated with RuBisCO by creating simple organic molecules that shuttle extra CO2 into the area where RuBisCO is most active. This substantially increases the ratio of CO2 to oxygen in that region and reduces the likelihood of RuBisCO pairing with the latter. Perhaps the most well-known C4 plants are the "warm-season" grasses that are often planted in lawns located in the southern United States. Because they use the C4 pathway, these grasses are able to do most of their growing during the height of the summer and have a particular, competitive edge in the high temperatures of lower latitudes. 

Common Yellow Wood-Sorrel (Oxalis stricta)

Given the effectiveness of the C4 pathway at allowing plants to have their sunlight and eat it too (so to speak), you might expect it to be a fairly widespread adaptation. And yet, this is not the case - only about 3% of terrestrial plants are C4 plants, with the rest using the typical "C3" pathway. The reason for this probably has something to do with the extra energy cost of using the C4 pathway. Remember, in order to keep up the CO2 concentrations in the region of their cells where RuBisCO is most active, C4 plants have to build transport molecules and use them to shuttle extra CO2 in from elsewhere. This requires the use of extra energy in a process that is supposed to store energy for the plant. Under warm conditions, this extra energy expenditure is worth it to combat the elevated waste of energy by RuBisCO, but as temperatures cool at higher latitudes or in the shade of a forest, the returns on a C4 plant's energy investment start to rapidly decline and so does their competitive advantage. 

All this isn't to say that there is a sharp division between open, hot, and sunny habitats dominated by exclusively C4 plants and cooler, shaded habitats where C3 plants take over. 3% is not a very high number and while C4 plants are likely to be a significant part of the plant community in open, sunny habitats (especially in lower latitudes), there are also C3 plants that have found other ways to adapt to these conditions. Many accomplish this by doing most of their growing and reproduction early in the season, when temperatures are still cool and there is less competition from C4 plants that do most of their growing at the height of the summer. Others have evolved physical responses to heat stress. The common yellow wood-sorrel (Oxalis stricta), for example, has adapted to growing in the direct sunlight of open woodlands and meadows by folding its clover-like leaves up during especially hot afternoons, reducing the risk of water loss and tissue damage (Del Tredici 2020). It does the same thing at night and under low light conditions in order to save energy. At least in my parents' lawn, wood-sorrel is common alongside warm-season crabgrass (Digitaria sp.) in sunny, heavily trampled areas near the driveway. (Also, its leaves taste like lemons - that's not really related to the ecology stuff I'm talking about, but I think it's important to mention, because they are delicious).

Common Blue Violet (Viola sororia)

Finally, many C3 plants simply grow in shadier locals where the eccentricates of RuBisCO are less of a problem. Right behind my parent's house, there is a friendly red maple tree that my dad planted when my siblings and I were still little kids, a tree that has grown tall alongside us over the years. The section of the lawn under the tree is shaded from the direct sunlight it would overwise receive each morning from the east and is surrounded by common blue violet (Viola sororia). These lovely, purple wildflowers are native to the eastern United States, where they are well-adapted to growing in the speckled light of open forests, edges, and gaps. They are also, to a point, pretty resistant to mowing and, as a result, are a common sight in the shadier parts of less-intensely managed lawns. In my parent's yard, a clear gradient can be observed in which the violets are the dominate plant right beneath the maple tree, then slowly get smaller and less abundant as you move away from the trunk, eventually disappearing almost entirely when you reach the middle of the yard. 

Ground-Ivy (Glechoma hederacea)

A similar gradient can be observed farther back, where the fence between my parent's yard and that of their rear neighbors is lined with a few Norway maples and a black walnut. Here, the violets are joined by another shade loving species, the ground-ivy (Glechoma hederacea). Like the violets, ground-ivy is a low growing plant with beautiful purple flowers, well adapted to the high disturbance environment of shaded lawns. Unlike the violet, it is nonnative and has a tendency to outcompete native species under the right conditions. Unfortunately, these conditions were often brought about when the species was recommended as an alternative to grass in shaded yards near woodlands. In these cases, the ground-ivy would often spread out of the yard and into the forest, pushing out native groundcover and forming homogenous mats of its own. Today, the ground-ivy is widely considered to be an invasive species.

If there is a wider ecological point to take away from this section, it is that a plant's "environment" is a complicated, multifaceted thing. Adaptations that provide an advantage with respect to one physical parameter will often carry with them disadvantages with respect to another. Any given plant will grow best in a location where these advantages and disadvantages are more or less balanced with respect to its physical surroundings and the competitive abilities of its neighbors. These two entities - the physical environment and the biological community - are the traditionally cited components of an ecosystem. We will finish off this little essay by talking about a third component that is less often discussed in the context of ecosystems, but which cannot be escaped if are talking about lawns - human culture. 

Pearls Before Swine:

Lawns are cultural landscapes, ecosystems shaped by human culture and the land use and management practices associated with that culture. This point may seem so obvious as to hardly be worth mentioning - we all know that lawns probably wouldn't exist if it weren't for human beings that cared enough to plant, mow, weed, and water them - but if we are to fully understand the ecological processes described above and the plant communities associated with them, we have to understand a little about how human preferences contribute to their perpetuation. 

The general appearance and vegetative community associated with the modern lawn was probably inspired in large part by the meadows and pastures of Britain and Continental Europe. Like the lawns we have been discussing here, these habitats would have been defined by high light availability and frequent disturbance in the form of animal grazing and/or flooding if they were near a river. During the Middle Ages, certain wealthy individuals began cutting turf from these meadows and using them to plant the first decorative lawns around castles. Later, lawns became a crucial element in English landscape parks, eventually cementing their place as symbols of social status and stages for recreation among the wealthy. When European settlers colonized other parts of the world, they brought the lawn and its various cultural associations with them, including to the United States. By the twentieth century, the well-kept lawn had become an important symbol middle-class, suburban home ownership, sprouting a multimillion-dollar lawn care industry that persists today (Jenkins 1994; Ignatieva and Ahrne 2013).

Today, the plant community found in any given lawn is impacted as much by the cultural debates around lawns as it is by the sunlight and disturbance which shaped its distant ancestors in the English countryside. My parents, for example, wanted a lawn because they grew up with lawns themselves and they wanted me and my siblings to be able to play as they did. They did not place very much value, however, on maintaining their lawn as a homogenous mat of grass, in part because they had better things to do and in part because they were worried about dumping a bunch of chemicals where their kids were playing. More recently, they have become concerned about the environment impacts of lawns in general and have both participated in no-mow November and ripped up part of their lawn to replace with a pollinator garden. Other people in their neighborhood have navigated these varying cultural pushes and pulls differently, resulting in a mosaic of lawns with different plant communities. 

Squill (Scilla sp.)

In addition to general management practices, the esoteric cultural, social, and aesthetic preferences of lawn owners can have more targeted impacts on the distribution of particular plant species. The most obvious way in which this occurs is via the selection of which grass to plant, but they can also have more subtle impacts on wild and semi-wild plants as well. In my parent's yard, for example, there are a few small patches of beautiful, white and blue flowers in the Eurasian genus Scilla that pop up every year around Easter. We aren't exactly sure where these flowers came from, but whether they were planted by the previous owners of the house or colonized their yard from somebody else's garden, it is clear from their longevity that they are quite good at surviving the conditions of a lawn. And yet, if my parents had wanted to, they probably could have gotten rid of them pretty easily simply by pulling them up or running the mower a couple extra times in early spring. The fact that they haven't done this is a testament to their less intensive management style, but it also probably had something to do with the fact that they had a son whose favorite color is blue and who, as a result, took an instant and intense liking to the flowers.

Or consider the common dandelion. As we've already seen, dandelions have several traits which make them well suited to living in lawn habitats, including the ability to disperse widely and take advantage of temporary disturbances to become established via through their windblown seeds. These seeds are also central to a particular bit of human culture which says that, if you pick a dandelion flower and make a wish before blowing the seeds off of it, the wish will come true. Dandelions are naturally great dispersers, with or without wishful humans present, such that those of us who have engaged in this practice probably play a relatively small part in spreading them around our neighborhoods. On the other hand, I suspect that certain individual dandelions have ended up in different locations than they otherwise would have because I picked one during a family walk and took a few blocks to think about my wish before blowing. It may be a very small part, but childhood wishes, along with the natural abilities and preferences of the dandelion and efforts of people with a less favorable view of the species, all contribute to its distribution in a given neighborhood. 

* * * * *

If it’s starting to seem as if we are leaving ecology behind in favor of some other field — perhaps history or anthropology — well, that’s kind of the point. One thing that I love about urban and suburban ecosystems is how they reveal the fuzziness of the boundaries we set up between nature and culture, wilderness and civilization. When you pick up a dandelion, walk a few blocks, and then blow the seeds, you are acting a seed disperser, just like a squirrel or blue jay does for an oak tree. When you mow your lawn, you are producing a disturbance that regulates what kind of plant community can grow there, much like a fire or herd of large grazing animals does for a grassland. You are engaging in ecological interactions. What exactly the implications of that fact are is a tangent for another day (hopefully soon), but even recognizing the simple truth of it and sitting with that for a moment can be valuable in a time of ecological crisis, when so many of our troubles can be traced back to a failure to see ourselves as part of the natural world. 

I hope that this article and the rest of this series will encourage whoever reads it explore the ecosystem that they are a part of and how they affect and are affected by it. You really are part of one, even if, as we will explore next time, there is hardly a patch of grass to be seen and you are surrounded by pavement…

This article was edited on May 6, 2024 to provide more accurate figures for the rate of oxygen fixation by RuBisCO and to add a few more citations.

Sources:

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