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.