The Concrete Landscape

                Though they may appear only to consist of monotonous hues of gray, the concrete jungles of urban areas in fact provide a multitude of minute habitats open to those plants who are able to survive in the some of the harshest of conditions. These small habitats – unmaintained gardens, vacant lots, and even small cracks in sidewalks – are all open to colonization by the most opportunistic of pioneers – weeds. Populations of plants that do get established within these urban habitats then have to cope with being trodden on by commuters, heat radiating off of nearby surfaces, and higher levels of pollution, in addition to dealing with simply existing in fragmented habitats.

                Environments that consist of many smaller habitat fragments inherently provide limits to plant population growth. Space itself may be a limiting factor, as large trees and woody shrubs take up a large amount of space that is simply not available in individual small fragments. Separation between fragments may lead to less effective pollination, particularly if pollinators have trouble moving from one habitat fragment to another. Even seed dispersal – the process by which these weeds arrived in this environment in the first place – is limited by many seeds falling outside of suitable patches and not becoming established.  

Figure 1. Crepis sancta. From http://flore.la.rochelle.free.fr

                Crepis sancta, commonly known as hawksbeard, is one such plant that has successfully colonized many fragmented habitats within urban environments, and it has been the target of multiple studies investigating plants in these fragmented, human-dominated environments. C. sancta produces two types of seeds – those designed to be dispersed by the wind, and those designed to simply fall from the flower to adjacent patches of soil. Pierre-Olivier Cheptou and colleagues have studied populations in southern France to look for evidence of evolutionary response of C. sancta to fragmented, urban environments. In one study (Cheptou et al. 2008), they hypothesized that in largely concrete environments, dispersal-type seeds are unlikely to land in new, suitable areas, so producing more non-dispersal-type seeds would be an evolutionary adaptive trait in these fragmented habitats.

                Cheptou et al. first counted the overall proportion of C. sancta seeds that are likely to land in a patch of suitable habitat, and found that in fragmented habitats, fewer seeds ended up in viable areas, meaning that plants in fragmented habitats have higher costs of dispersal. In these fragmented, urban habitats, the authors found that populations of C. sancta produce a higher proportion of non-dispersing seeds compared to contiguous, rural populations. By creating mathematical models of the evolutionary processes that would allow for this shift in reproductive strategy, the authors estimated that the observed difference in non-dispersing seed proportion represents around 12 years of evolutionary divergence, a number that is consistent with construction records for concrete areas near their study populations.

Figure 2. Proportion of non-dispersing seeds of the study populations from Cheptou et al. (2008), showing how fragmented populations generally produce higher proportions of non-dispersing seeds than contiguous populations.

This study (Cheptou et al. 2008) was the first to quantify evolutionary changes in dispersal strategies – toward an increased proportion of non-dispersal-type seeds – showing that C. sancta in fragmented habitats have adapted to better persist in these largely urban areas. Interestingly, as the authors point out, this decrease in production of dispersal-type seeds implies that it is now harder for these plants to colonize new areas, which may exacerbate the impact of continued habitat fragmentation and urban development. If plants in urban environments become less able to disperse to new areas, these plants are likely to have increasingly patchy distributions and be less able to react to ongoing changes in climate. 

                In an ongoing research project here on the Grinnell College campus, my lab group is working to characterize the distribution of multiple plant species within a largely concrete patio environment. Our study area consists of a large courtyard enclosed on all sides by the Noyce Science Center, which serves to isolate this courtyard from much of the surrounding college campus. This courtyard is further divided into many small (less than half of a centimeter wide) cracks between bricks, and seeds that colonized this area were likely carried into the courtyard from elsewhere on the Grinnell College campus by the wind. If the plant populations in our study area have been subject to fragmented environments for multiple generations, dispersal should be very limiting to the distribution of these plants within the courtyard, resulting in patchy populations of these weed species. Alternatively, if the influx of new seeds from outside the courtyard is greater than the production of seeds within it, the distribution of plants will reflect small-scale habitat features that promote seedling germination and establishment, such as the size of the gap between paving stones. By measuring the distribution of plant species within this courtyard, my group aims to determine how isolated these plant populations are from those elsewhere on campus, and how the micro-habitats in the courtyard differ in their ability to support plant growth.

References:

Cheptou, P.-O., O. Carrue, S. Rouifed, A. Cantarel. 2008. Rapid evolution of seed dispersal in an urban environment in the weed Crepis sancta. PNAS 105(10): 3796-3799. 

Between the Cracks: Plant Diversity, Abundance, and Root Health in Paved Areas

"Why doesn't Grinnell have more sidewalks?" students at Grinnell College frequently question while navigating the town. Although paved areas create smooth paths, pavement often creates problems for the surrounding ecological community. While ecological concerns likely did not cause lack of sidewalks in Grinnell, the effect on the surrounding ecological community might be a positive benefit. For instance, pavement decreases the filtration of storm water that would otherwise percolate through the soil. Additionally, paved surfaces reduce gas exchange between the soil and the air. This air exchange may be important for plants to maintain healthy root systems. Some companies marketed paved surfaces with permeable properties because of the negative environmental impacts of paved surfaces. These permeable surfaces allow for greater gas exchange and water filtration. The authors of the paper thought that these properties of permeable pavement might lead to healthier plant root systems for mature Sweetgum trees (Figure 1).

Figure 1. A mature American Sweetgum tree, Liquidambar styraciflua (Arbor Day Foundation).

This paper studies Sweetgum trees to determine the effects of pervious and impervious pavement on their root structure. The scientists created a mini underground laboratory with windows into the soil profile in order to study Sweetgum root structure. They found that pervious pavement did not positively affect any of their measures of Sweetgum performance in comparison to impervious pavement or no pavement at all (Figure 2). 

Figure 2. Mean cumulative (a) new root production, (b) dead root length production and (c) net root length production per cm² tube window until 80 cm depth through time. Open symbols are root production in plots with no pavement (control), black symbols are root production in plots with impervious pavement, and grey symbols are root production in plots with pervious pavement

However, the researchers found that the diameter of the tree trunks were unaffected by pavement installation. The researchers concluded that the negative effects of pavement on Sweetgum root performance likely result from both types of pavements’ prevention of gas exchange between the soil and surrounding air.

The Sweetgum article considers potential factors that may affect my group’s results for our project on the cracks between bricks. Besides crack width and the differences between edge and inner cracks, the differences in soil-air gas exchange could play a role in the performance of plants living in brick crack communities. Additionally, these plants may be nurtured by storm water lacking filtration. In the paper, the authors studied the root systems of ornamental trees. In contrast, my group will only identify the presence and abundance of shoots that arrived on their own (Figure 3). 

 

Figure 3. Different species of plants observed to grow in the cracks of the Noyce Courtyard, my group's experimental site.

My group will not study the root system, which might limit us from observing the negative effects of pavement. The work on pavement and Sweetgum trees shows that the choice to pave and the type of material used tangibly affects surrounding plants. Rather than asking about the effect of different types of concrete on root systems, my group is studying pavement structure by asking how different crack widths affect the diversity and abundance of plant communities.

References

Arbor Day Foundation. 2016. American Sweetgum. Liquidambar styraciflua.

Volder, A., B. Viswanathan, and W.T. Watson. 2014. Pervious and impervious pavement reduce production and decrease lifespan of fine roots of mature Sweetgum trees. Urban Ecology 17: 445-453.