Big
City Botany
Plant
resilience and adjusting to urban life
As of 2008, over half of
the world’s population lives in cities. Never before in the history of our
species has this occurred, because the bulk of the world’s population has lived
in rural areas, farming, raising cattle, and providing for their immediate
needs. Now, though, this trend of urbanization is only going to increase in the
coming decades.
As more people move into
cities, and as cities spread farther out and build higher up, they encroach on
the territories of plants, fungi, and other (non-human) animals. Along with
this encroachment comes a new, ‘hot’ area of study: urban ecology. When one
envisions ecological study, one often pictures scientists—pens and notebooks in
hand—trudging through a densely-packed, dimly-lit jungle looking for samples to
collect in order to gather data relevant to their respective academic
interests. One does not imagine a scientist scouring through concrete jungles,
such as New York City or Shanghai. However, this is (more or less) what urban
ecologists do. Because cities continue to spill into the areas in their
periphery, many organisms are eradicated as construction destroys their homes
and food supplies; however, many either become trapped in little pockets of
nature (such as in parks) or adjust to living in and around areas heavily
modified by humans—and some species even thrive in these environments (think of
the New York City subway rat or pigeon).
Often somewhat overlooked
are plants in these areas, since the biological sciences are largely
animal-biased; admittedly, I am in the same boat, as my interest is in urban Herpetology,
the study of reptiles and amphibians in urban environments. Plants can be some
of the most resilient organisms on the planet, however, and their importance
cannot be understated: they, along with other photosynthetic organisms, are
responsible for the continuity of animal life on earth, and we (along with all
other animals) would surely die without them. A testament to their resilience
in this day and age is their ability to adapt to heavily modified environments
as humans continue to wreak havoc on the natural world. Plants must conquer a
variety of issues that are associated with living in urban areas, from dealing
with life in the shadows of skyscrapers to finding enough nutrients in soils
that are often fairly infertile and heavily polluted—among other issues. And
yet, cities are still fairly inundated with plants, whether they be in a large
park, or lining the sides of streets. So, what strategies have plants come up
with in order to deal with these sorts of situations? What have they done to adapt
to urbanization? These are big questions, and there are many answers; let’s talk
about a few.
For one, the species of
flora in a city are not exclusively the ones native to that area. Humans
directly or indirectly introducing non-native species to areas has caused some serious
problems, because non-native species can oftentimes out-compete native species
for resources. This appears to be a trend in urban areas, too. Many studies
have found that, while biodiversity has decreased in urban areas relative to
rural areas, species richness tends to increase in cities (McKinney 2008; Pautasso
2007). Urban and suburban areas have also been central to the spread of
non-native species (Duguay et al. 2007). See the figure below this section for
a bit of data on the trend showing non-natives becoming more predominant in
cities (Godefroid 2001). Without going into much detail—as the focus of this blog
entry will be on plants and how they have adapted to urban environments—it is
important to note that, while species richness appears to increase, we must not
take that as a sign that native plants are doing particularly well in the urban
environment; rather, the increase in non-natives is offsetting the losses of
the natives (Dolan et al. 2011).
 |
| Figure 1: shows the change in numbers of native and alien species in Brussels, Belgium from two different time periods (Godefroid 2001). |
An interesting phenomenon
that occurs in cities is something called the ‘urban heat island effect’: when
a city is much warmer than its surrounding areas due to human influence. It is
caused by the replacement of soil and plants with concrete, asphalt, and other
materials used for roads, buildings, sidewalks, and so forth. These human-built
structures absorb light energy, rather than reflecting it, which causes large
cites to heat up to a higher temperature than the surrounding rural areas. Plants
must deal with these higher temperatures if they are to adjust to living in
urban areas. One aspect of a plant’s life can be greatly affected by this
increase in heat, and that is its phenology, or the timing of an organism’s
life cycle events—such as the timing of when a tree flowers or when a tadpole
goes through metamorphosis. In the case of some plants, the heat island effect
seems to have worked in their favor in the form of extended growing seasons due
in large part to increased air temperature (Menzel & Fabian 1999). This
increase in temperature allows some plants to start growing earlier and to
continue growing when they would normally halt growth due to decreasing temperatures
as the seasons progressed (Lu et al. 2006; Neil et al. 2010). This is one
unexpected example of how urbanization, at least by increasing ambient
temperature, has actually helped some plant species. Along with this comes some
not-so-obvious consequences, such as the relatively innocuous consequence of
extended allergy seasons (Neil & Wu 2006), but there are likely others that
have not been brought into the light just yet. One possible solution to the
issue of the urban heat island effect is implementation of so-called ‘green
roofs’. These are roofs that have been covered completely or partially by
plants and soil in order to create green gaps in an otherwise grey,
concrete-dominated jungle of rooftops. Another way is to simply paint surfaces
in white or other colors that are lighter; this will aid in reflecting more
sunlight.
Another problem that plants
must face is soil pollution. Soils in heavily-urbanized areas are poor and
heavily polluted by heavy metals (Möller et al. 2005; Wilcke et al. 1998); they
are also more hydrophobic—they tend to collect water on their surface, rather
than absorbing it—and acidic than rural soils (Pouyat et al. 1995). Plants
normally need nutrient-rich soils to grow properly, and they are often severely
limited by these nutrients. Many plants also rely on nitrogen-fixing bacteria
in order to attain nitrogen, which is not only incredibly limited to plants,
but also essential in the production of proteins and chlorophyll. Soils in
urban areas are very poor in these limiting nutrients, so it is difficult to
imagine plants that are doing exceedingly well in these areas, despite the environmental
pollutants that they are exposed to. However, many plants can and do exist in
these soils. Surprisingly, we see the emergence of a recurring theme: non-native
species, more so than their native counterparts, tend to be more tolerant of
these more harsh conditions (Godefroid 2001). It is not yet clear how exactly
plants have adapted to living in such unfriendly conditions, but this is definitely
an interesting area of research. The need for conservation in the coming
decades will make this sort of research incredibly valuable in helping to preserve
the native species that inhabit cities—those of which may be unable to adapt
fast enough to changes brought about by anthropogenic climate change and
continuing urbanization.
In order to become a more
informed citizen in the coming years, as science denial permeates the political
and social realm, it is a good idea to understand the impact we have on the
species with whom we share this planet. They, like us, require homes and
resources, and our increasing need for land, as we continue to increase our
numbers and as we continue to become more urbanized, will ultimately compromise
their livelihood. The most pragmatic solutions will need to be implemented in
order to alleviate the harm we cause this planet. Since the bulk of our
discussion started with our leafy cousins, it is prudent to end the discussion
with them, too. That being said, plants face many challenges as we continue to
push them to their limits in the urban jungle; challenges like invasive
species, the urban heat island effect, and poor soil conditions will likely
continue to grow in severity if nothing is done. And these are, by no means, the
only challenges plants face—more are included in the figure below, which shows
changes in conditions on a rural-urban gradient (Johnson et al. 2015). But
innovative people are continuing to fight these issues by implementing their
ideas and becoming more aware of the problems faced by organisms other than
ourselves.
 |
| Figure 2: shows biotic and abiotic changes on a rural to urban gradient (Johnson et al. 2015). |
Works Cited
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Stephens. "Documenting effects of urbanization
on flora using herbarium records." Journal of Ecology 99.4 (2011): 1055-1062.
Duguay, Stéphanie, Felix Eigenbrod, and Lenore Fahrig.
"Effects of surrounding urbanization on non-native
flora in small forest patches." Landscape Ecology 22.4 (2007): 589-599.
Godefroid, Sandrine. "Temporal analysis of the
Brussels flora as indicator for changing environmental
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Menzel, Annette, and Peter Fabian. "Growing
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(2007): 16-24.
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