Is There a Place for Weeds in Urban Ecosystems?

Highly urbanized areas have a long history of disturbance. They are a far cry from the natural areas they once displaced, bearing little resemblance to what was there before. In this sense, they are a brand new thing. During the urbanization process, virtually everything is altered – temperatures, soils, wind patterns, hydrology, carbon dioxide levels, humidity, light availability, nutrients. Add to that a changing climate and increased levels of pollution, and the hope of ever seeing such a site return to its original state – whatever that might mean – is crushed.

What then should we consider the natural flora of an ecosystem like this? Certainly it is not the native flora that once stood on the site before it was developed; virtually none of the conditions are the same anymore. If we are defining “natural” as existing with minimal human intervention, then the natural urban flora would be whatever grows wild outside of our manicured landscapes and managed, remnant natural areas. It would be a cosmopolitan mixture of plants that have joined us in our migrations with and without our permission, along with a collection of species that are either extant to the site or have been brought in by wildlife. In many ways it would mirror the human populations of our modern cities – an assortment of residents from around the globe with diverse backgrounds and cultural histories.

In Wild Urban Plants of the Northeast, Peter Del Tredici classifies urban land into three general categories based on their ecological functions: native, remnant landscapes; managed, constructed landscapes; and ruderal, adaptive landscapes. Native, remnant landscapes are generally small areas within city limits that have never been developed. They contain a portion of the native plants that once populated the area, and they require vigilant and regular maintenance to keep non-native plants from invading and to control those that already have. Managed, constructed landscapes include all of the parks and gardens that have been designed and intentionally planted. They require regular maintenance of varying intensity in order to keep them looking the way they are intended to look. Ruderal, adaptive landscapes are abandoned or neglected sites that are populated by plants that have arrived on their own and that maintain themselves with virtually no human intervention. This is where the true, wild urban flora resides.

Prickly lettuce (Lactuca serriola) growing in an abandoned lot.

Many of the plants that make up our wild urban flora are what we commonly refer to as weeds. These weedy plants appear in landscapes throughout our cities, but are generally removed or controlled in all landscapes except the abandoned ones. It is in these neglected sites that weeds have the greatest potential to provide vital ecosystem services, performing ecological functions that are beneficial to urban life.

Not all plants are suited for this role. Spontaneous urban vegetation is particularly suited due to its ability to thrive in highly modified, urban environments without external management. Regardless of provenance, this suite of plants, as Del Tredici points out, seem to be “preadapted” to urban conditions and “are among the toughest on the planet.” A long list of traits has been identified for plants in this category, ranging from seed dispersal and viability to speed of growth and reproduction to tolerance of harsh conditions. Del Tredici summarizes by stating, “a successful urban plant needs to be flexible in all aspects of its life history from seed germination through flowering and fruiting, opportunistic in its ability to take advantage of locally abundant resources that may be available for only a short time, and tolerant of the stressful growing conditions caused by an abundance of pavement and a paucity of soil.”

Abandoned lots flush with weeds, overgrown roadsides and railways, and neglected alleyways colonized by enterprising plants are generally seen as ugly, unsightly eyesores – products of neglect and decline. Some of the plants found in such locations are valued in a garden setting or prized as part of the native landscape in a natural area, but growing wildly among trash and decaying urban infrastructure they, too, are refuse. As Richard Mabey has written: “If plants sprout through garbage they become a kind of litter themselves. Vegetable trash.”

Abandoned chicken coop overtaken by tree of heaven saplings (Ailanthus altissima).

Despite how we feel about these plants or the aesthetics of the locations they find themselves in, they are performing valuable services. Apart from adding to the biodiversity on the site as well as producing oxygen and sequestering carbon – services that virtually all plants offer – they may be preventing soil erosion, stabilizing waterways, absorbing excess nutrients, reducing the urban heat island effect, mitigating pollution, building soil, and/or providing food and habitat for urban wildlife. While cultivated and managed landscapes can achieve similar things, these neglected sites are doing so without resource or labor inputs. They are sustainable in the sense that their ability to provide these services is ongoing without reliance on outside maintenance.

Sites like these should be further investigated to determine the full extent of the services that they may or may not be offering, and in the event that they are doing more good than harm, they should be conserved and encouraged. One service that is receiving more attention, as Del Tredici writes, is phytoremediation – “the ability of some plants to clean up contaminated sites by selectively absorbing and storing high concentrations of heavy metals such as cadmium, lead, copper, zinc, chromium, and nickel in their tissues.” Weed species with this ability include prickly lettuce (Lactuca serriola), lambsquarters (Chenopodium album), and mugwort (Artemisia vulgaris). In an article in Places Journal, Del Tredici gives the example of the often despised, introduced plant, common reed (Phragmites australis) cleaning up the New Jersey Meadowlands by “absorbing abundant excess nitrogen and phosphorous throughout this highly contaminated site.”

In the book, Weeds: In Defense of Nature’s Most Unloved Plants, Richard Mabey writes: “As we survey our long love-hate relationship with [weeds], it may be revealing to ponder where weeds belong in the ecological scheme of things. They seem, even from the most cursory of looks, to have evolved to grow in unsettled earth and damaged landscapes, and that may be a less malign role than we give them credit for.” Perhaps, seeing them in this worthy role, will temper our knee-jerk inclination to demonize them at every turn.

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See Also: Our Urban Planet and Wild Urban Plants of Boise.

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Our Urban Planet

As the human population balloons and cities sprawl, ecological studies in urban areas are following suit. Nature has always been a component of cities – we can’t escape it after all, as hard as we may try – but urban nature (and the enhancement of it) has become increasingly important as the human species continues to urbanize. More and more we are seeing the importance of melding the built environment with the natural one. Our motivations are diverse – albeit largely anthropocentric. But that’s fine. As we make improvements to the live-ability of cities for human’s sake, other living beings benefit. We are finding ways to get along with our neighbors, and we are learning to appreciate and value them as well.

Since 2008, the world’s urban population has outnumbered its rural population, and it is predicted that by 2050, more than two-thirds of humans will be urbanites. Immense resources are required to support such large, concentrated populations, and most of these resources are produced outside of urban areas. This results in an ecological footprint that is significantly larger than the city itself. Additionally, waste and pollution produced within cities negatively effects surrounding areas and beyond in abundant ways.

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In May of this year, Science put out a special issue entitled, “Urban Planet,” which features a series of articles that address some of the latest research in urban ecology and discuss current developments and future research needs – a sort of state of the union address for urban ecology in 2016. A series of 13 articles covered diverse topics including city-integrated renewable energy, innovative solutions to water challenges, transportation and air pollution, and food security in an urban world. Rodent-borne diseases in urban slums, creating sustainable cities in China, and Vancouver’s push to become the “greenest city” were also features of this special issue.

The issue serves to highlight the importance of this field of study and the urgency there is in finding solutions to major environmental challenges. But it also offers hope. Bright minds are working towards solutions to this century’s biggest problems as we look towards a more sustainable future. The introduction emphasizes that “the rise of cities is not…all doom and gloom.” Urbanization has upsides: “consolidating human populations helps shrink our individual environmental footprints, and cities are serving as living laboratories for further improvements.”

Urban ecology is a relatively recent subfield of ecology. In The Ecological Future of Cities, Mark McDonnell and Ian MacGregor-Fors describe how it “arose in the 1990’s out of a need to increase our…understanding of the ecological and human dimensions of urban ecosystems.” Initially the field was mainly concerned with biodiversity and the ecosystem processes and services found within cities. Findings from these studies are now influencing urban planning, design, and management. Such decisions are also informed by more recent studies in the field of urban ecology, which has grown to include “issues of sustainability, environmental quality, and human well-being in urban ecosystems.”

The authors note that our ecological understanding of cities was waylaid because “nature within cites was long considered unworthy of study, except when it involved solving environmental problems that threatened human well-being.” Cities were perceived as unnatural because humans had “disrupt[ed] the natural ecological conditions and processes that scientists [were] attempting to understand.” Today, ecologists recognize that studies in the field of urban ecology help us better understand basic ecological principles, while also providing “valuable information for creating liveable, healthy, and resilient urban environments.”

Studies in urban ecology have also increased our understanding of the mechanisms involved in evolution and adaptation. To illustrate this, the authors offer examples of birds that modified their songs “to communicate at noisy locations” and plants that shifted their seed dispersal strategies to survive in “highly fragmented urban habitats.” The authors also highlight the importance of maintaining or restoring natural vegetation in urban areas in order to help preserve struggling species of plants and animals, citing a study that found that “fewer local plant extinctions occurred in cities that maintained at least 30% native vegetation cover.” Additionally, the authors note that “the scope of urban ecology research extends well beyond city limits,” since urbanization is partly to blame for numerous environmental issues including habitat loss and fragmentation, biodiversity loss, climate change, and invasive species.

In Living in Cities, Naturally, Terry Hartig and Peter Kahn, Jr. address the topic of mental health and urban living. While there is still much to learn about the relationship between the two, it is generally believed that viewing or spending time in nature can help improve one’s mental well-being. As the authors put it, “parks and green spaces” can be viewed as “health resources for urban populations,” and including natural areas and natural processes in the design and creation of cities is necessary “for psychological as well as ecological purposes.”

Green roofs

Green roofs are one way to add green space to urban areas. They help replace vegetation that was removed when buildings were constructed, and they offer numerous environmental benefits.

Interacting with nature in an urban setting can help people develop positive feelings about the natural world and may encourage support for environmental protection. The authors worry that if future generations grow up without an intimate connection to the natural world, elevated amounts of environmental degradation will be seen as normal and a feeling of urgency to protect the environment from continued degradation will fade. This is why including plentiful amounts of green space within cities is essential: “Providing opportunities for people to experience more robust, healthy, and even wilder forms of nature in cities offers an important solution to this collective loss of memory and can counter the shifting baseline.”

This special issue of Science highlights some of the current ecological and environmental research regarding urbanization. For a great introductory look at urban ecology and basic ecological principles, check out the book, Nature All Around Us. Also, expect to see many more urban ecology themed posts on Awkward Botany. Tell your friends.

Trees Are Good For Your Lungs

Trees help reduce air pollution. They do this primarily by pulling gases (like ozone, sulfur dioxide, and nitrogen dioxide) into their leaves and then diffusing them and/or chemically altering them so that they are no longer a direct threat to humans. They also intercept particulate matter, trapping it on the surfaces of their leaves until the wind comes along and blows it away or the rain comes around and washes it into the soil. Trees are filters in this sense, reducing the health threats of our polluted air.

But didn’t I just report on the contribution of urban trees to air pollution via their production of volatile organic compounds? Yes I did. And that remains a possibility; however, according to a study recently published in the journal, Environmental Pollution, the presence of trees is a great benefit to human health despite potential risks. More research is necessary of course, but the consensus so far is that having trees around is a net positive.

Alnus glutinosa, European Alder (photo credit: wikimedia commons)

Alnus glutinosa, European Alder (photo credit: wikimedia commons)

There have been many studies on the relationship between trees and air quality, but little is known about the extent to which human health impacts are avoided and the related money that is saved as a result of air pollution mitigation by trees and forests. With the aid of computer simulations, researchers at US Forest Service and The Davey Institute used 2010 Census data, tree cover maps from the 2001 National Land Cover Database, US EPA’s BenMAP program, and other data to seek answers to these questions. Their analyses – focused at the county level – involved the 48 contiguous United States.

According to their study, trees and forests removed around 17.4 million tons of air pollution in 2010, which resulted in a health care savings of $6.8 billion. 850 human deaths were avoided, and incidences of acute respiratory symptoms were reduced by 670,000. Ozone and nitrogen dioxide experienced the greatest decrease, while the removal of ozone and particulate matter resulted in the greatest health value. Air pollution removal was greater in rural areas compared to urban areas simply because there is more rural area in the US than urban area; however, the removal of air pollution was found to be more valuable in urban areas due to differences in population density. Resulting health benefits and savings are quite dramatic considering that air pollution removal by trees was only found to improve air quality by about 1%.

There were many things left out in this study though, and the researchers acknowledge this. First of all – as stated earlier – trees have the potential to contribute to air pollution. They emit volatile organic compounds which can result in ozone formation, they can reduce wind speeds which concentrates pollutants, and they produce pollen which is a direct contribution to air quality and a major health issue for those with serious allergies.  But trees reduce air pollution in indirect ways as well. For example, by shading buildings, trees can reduce energy demands which results in decreased power plant emissions and a reduction in air pollution.

Quercus sp., Oak Tree (photo credit: wikimedia commons)

Quercus sp., Oak Tree (photo credit: wikimedia commons)

Trees can also be negatively affected by air pollution. When particulate matter collects on leaf surfaces, photosynthesis is compromised, limiting a tree’s ability to take gaseous air pollution into its leaves. Urban trees are stressed in additional ways. For example, trees growing near sidewalks, driveways, and roadways deal with serious soil compaction and are often not receiving optimal amounts of water, which can limit their ability to mitigate air pollution. Thus, environmental factors should be considered when determining the relationship between trees and air quality.

This study was conducted at the county level. The researchers acknowledge that more precise predictions could be obtained if analyses were conducted at a finer scale. “Local-scale design of trees and forests can affect local-scale pollutant concentrations.” So, the number of trees, their concentration and configuration, the length of the growing season, the percentage of evergreen trees vs. deciduous trees, etc. all play a role in the extent of air pollution reduction.

While limitations to the study abound, the researchers assert that this initial analysis gives “a first-order approximation of the magnitude of pollution removal by trees and their effect on human health.” Future studies will provide more accurate approximations, but for now I think it is safe to say that trees are good for our health and worthwhile things to have around.

Boise National Forest

Boise National Forest

This study focused mainly on health issues of the respiratory variety. The positive psychological benefits of plants have been observed in separate studies, and our also worthy of our consideration when determining the health benefits of trees and forests.

Urban Trees: Unlikely Polluters

Trees are central features in urban environments, and their benefits are numerous and well documented. They give off oxygen and sequester carbon, provide food for urban wildlife, help slow storm water runoff, and provide shade which not only keeps us cool in the hot sun but can help increase the energy efficiency of surrounding buildings. And even if they weren’t doing all these things and more, the aesthetic value they add to our concrete jungles alone is worth having them around. So it is a little disconcerting to learn that the trees we benefit so much from may actually be doing us harm by way of increasing levels of air pollution.

It sounds unlikely, but according to researchers at the Institute for Advanced Sustainability Studies in Postdam, Germany, urban trees can contribute to increased levels of tropospheric ozone, a key component of smog. This occurs when trees emit volatile organic compounds (VOCs), special gasses that are meant to attract pollinators, repel insects, and warn nearby trees of ensuing insect herbivory. These biogenic VOCs react with sunlight and nitrogen oxides (another key component of smog and a result of burning fossil fuels) and form ozone. Ozone in high concentrations is particularly harmful to the lungs, aggravating asthma, increasing susceptibility to lung infections, and damaging the lining of the lungs.

Fortunately, according to the study, certain trees contribute significantly less to ozone production than other trees.  Poplars, oaks, and willows, for example, tend to be high emitters of VOCs, whereas birches and lindens emit much less. Planting low VOC emitters in dense urban areas and keeping high VOC emitters scattered throughout the city instead of planted in large groups will help reduce this phenomenon. A recent article at Scientific American points out that cities that are sunnier and warmer have more to worry about than cloudy and cool cities since sunlight and high temperatures speed up the ozone producing reaction.

Despite this unfortunate discovery, trees still have an important role in cities. Apart from placing and planting the proper trees, our focus should be on finding ways to reduce our fossil fuel emissions which remain the major culprit of our polluted air.

River birch (Betula nigra) - Birches were found to low emitters of volatile organic compounds compared to other common urban trees

River birch (Betula nigra) – Birches were found to be low emitters of volatile organic compounds compared to other common urban trees (photo credit: wikimedia commons)

 

 

Square Foot Rooftop Gardening

Square foot gardening is a method of gardening that was described and popularized by Mel Bartholomew. The basic concept is simple: measure out your garden beds into equal squares (4 feet by 4 feet) and then plant individual crops into each square following specific spacing recommendations for each crop. The square foot method is intended to eliminate the inefficiencies of standard row planting, making vegetable crops easier to plant, maintain, and harvest. Bartholomew’s book about square foot gardening was first published in 1981. From that book came a television series on PBS, various other books and updated versions of the original book, a square foot gardening product line, and the Square Foot Gardening Foundation.

As a long time gardener, I had been familiar with Bartholomew’s book and its basic premise for a while but had never read it until recently. I found the book to be basically what I expected: a description of how to garden in squares instead of rows. I can see how this system could be very simple, attractive, and efficient while simultaneously producing decent sized yields; however I felt like Bartholomew’s description of the process made gardening into a very methodical, calculated, and meticulous task bordering on joyless. I’m sure that’s not how he sees it (nor how it really is), but then again, he’s a retired engineer [insert smiley face here].

For a long time I’ve had an interest in green roofs. I even went to graduate school to study them. So when I got to the part in Bartholomew’s book where he talks about square foot gardening on rooftops, I was intrigued. Green roofs (along with rooftop vegetable gardening) have become fairly common in urban areas in the past decade or two. And for good reason. Green roofs offer myriad benefits including mitigating storm water runoff (and the numerous sub-benefits involved with that), reducing the urban heat island effect, increasing a building’s energy efficiency, and re-introducing green space and wildlife habitat that was lost when a building was built.

Vegetable gardening on rooftops is a practical solution for residents of urban areas where space for gardens on the ground is limited. Restaurants – like Noble Rot in Portland, Oregon and Café Osage in St. Louis, Missouri – have found that they can grow some of the produce and herbs they need on their rooftops while simultaneously setting themselves apart from other restaurants. There are also a few urban farming operations on rooftops (Brooklyn Grange and Eagle Street Rooftop Farm for example). Michigan State University (an institution with one of the most prominent green roof research labs in the U.S.) has a research program dedicated to improving rooftop vegetable crop production. So with this recent trend of growing food on rooftops, I was curious to read what Bartholomew was saying about the subject more than thirty years ago, back when green roof vegetable gardening was less than mainstream.

The reality is that square foot rooftop gardening gets only a brief mention in Bartholomew’s book (at least in the first edition – perhaps he has more to say about it in more recent editions), but what he does have to say is relevant.

Rooftops are windy:

“Stay away from plants that grow tall, have delicate stems, or that might be blown over when they are mature and filled with ripening fruit…The wind can be unmerciful to a plant; it whips the leaves about and can dry out the plant in short order.”

Rooftops are hot:

“The other big consideration for rooftop growing is heat buildup…These conditions will naturally affect both the frequency and amount of watering the garden will need.”

Rooftops have weight limits:

“The soil in your rooftop garden should be as light and porous (yet still be water retentive) as possible. Mix in lots of vermiculite and peat moss.”

Each of these three considerations (wind, heat, and weight) continue to be considerations for any vegetated roof whether it includes vegetable crops or not. Yet people are figuring out how to overcome these obstacles, constructing and maintaining incredible rooftop gardens that are both productive and beneficial.

Rooftop Garden - Manhattan, New York ( photo credit: wikimedia commons)

Rooftop Garden – Manhattan, New York ( photo credit: wikimedia commons)

In future posts, I intend to elaborate more on this topic, profiling individuals, groups, and organizations that are making this sort of thing happen. Comment below and share something about your favorite rooftop garden and/or recommend a rooftop garden that should be profiled in an upcoming post.

In the News: Declining Insect Populations

Last week the New York Times published an article about declining populations of insects in the United States, specifically monarch butterflies and wild bees. Monarch butterflies migrate south to Mexico each fall, typically arriving by the millions on the first of November. This year was tragically different, because the monarchs did not arrive on the first, and when they finally began trickling in a week late, there were significantly less of them. In his article, The Year the Monarch Didn’t Appear, Jim Robbins discusses why this and similar scenarios are becoming commonplace.

Increased pesticide use and global climate change are certainly contributing factors in the decline of insect populations; however, Robbins suggests that the loss of native habit is the major culprit. For example: monarch butterflies rely on milkweed (Asclepias spp.); in fact, their larvae feed exclusively on it. No milkweed = no new monarch butterflies. Urban sprawl, farmland expansion, Roundup Ready crops, and herbicide use along roadways all result in declining milkweed populations, as well as declines in the populations of other beneficial native plants.

And that’s not all. “Around the world people have replaced diverse natural habitat with the biological deserts that are roads, parking lots and bluegrass lawns, ” says Robbins, meanwhile landscape plants are selected for their ornamental appeal, “for their showy colors or shapes, not their ecological role.” In support of his argument, Robbins cites studies which found that native oak and willow species in the mid-Atlantic states are hosts to 537 and 456 species of caterpillars, respectively. On the other hand, non-native, ornamental ginkgoes host three.

Insects provide numerous ecosystem services. They help break down waste products, they are pollinators of countless species of plants (including many of our crops), and they are food sources for larger animals (including birds, reptiles, and amphibians)…and this is just the short list. As John Muir said, “When we try to pick out anything by itself, we find it hitched to everything else in the Universe.” Thus, the decline of native insect populations is a concern that should not be taken lightly.

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Asclepias tuberosa – butterfly milkweed

If you haven’t already, please consider including some native plants in your yard. If you don’t have a yard, suggest the idea of landscaping with native plants to your friends. To learn more about monarch butterflies and their plight (including information on how to grow milkweed), visit www.monarchwatch.org.

Plants on Rooftops in South Carolina

Here is a video featuring a couple of folks in South Carolina introducing green roof technology. I have a particular interest in green roofs that stems from my fascination with plants and my interests in urban ecology and being environmentally conscious.  I will eventually post more about green roofs and urban ecology as I have already promised. This should tide you over for now.