Conservation in Unexpected Places
This research began as an investigation of which plants are best at improving water quality in stormwater basins. When I took a closer look, I wondered whether stormwater basins could not only use plants to remove pollutants from water, but also provide valuable territory for plant conservation. While there are planting lists and seed mixes for detention basins, these urban plant communities have not been studied, and their actual composition and dynamics are unknown. I set out to document the plants that live in stormwater basins.
New Jersey state map, showing the level of impervious surface in each municipality. Source: NJ Geographic Information Network
Stormwater basins play an important role in New Jersey as well as other urban areas. The map of New Jersey shows relative levels of impervious surface. Large areas of the state are heavily paved. As of 2012, impervious surface accounted for over 2000 square kilometers statewide (Lathrop, Bognar and Hasse, 2016). Simultaneously, between 1986 and 2012, 230 square kilometers of wetland were lost in the state. Wetlands are important for many reasons, including storing storm water and as home to high levels of biodiversity.
This New Jersey map shows the distribution of the state’s 20,000 stormwater basins. Source: New Jersey Hydrologic Modeling Database
To contend with runoff from all this impervious surface and reduced wetland capacity for stormwater, we have over 20,000 stormwater basins statewide. Combined, they cover over 65 square kilometers of area. With significant areal coverage and broad distribution throughout the state, I think they can have a role in providing wetland habitat and conserving biodiversity in cities and suburbs.
In my 2018 study, I wanted to answer the primary question: what plants are actually out there in detention basins? Along with that main question, I wanted to know what are the dominant species, are there any rare species, and how many total plant species live there? I also wanted to know whether there were predictable sets of species that live in basins. These are important questions for management, for if there is low diversity, and basins are mostly inhabited by cosmopolitan, non-native species, then basins are not important in conservation. In that case, they should be managed primarily from a hydrology perspective. On the other hand, if basins do have diverse, native communities, then basins may have a role in supporting native plant populations in urban areas. If basins are valuable for conservation, then basins that are currently managed as turfgrass could be converted to a marsh mix. Stormwater basins are already present in the landscape and will accompany all new development, so they represent 65 square kilometers of low-hanging fruit for conservation.
Based on a 2017 pilot study, I expected cattails (Typha species) and giant reed grass (Phragmites australis) to be the dominant species in New Jersey detention basins. I had also seen many other plants, so I thought that there would be three distinct types of basins: cattail-dominated, giant reed grass-dominated, and sedge mix-dominated.
The plants in this detention basin follow a distinctive zonation pattern. In the driest area, in the foreground, grows mugwort (Artemisia vulgaris) . Purple loosestrife (Lythrum salicaria) grows in the area that is intermittently flooded, and cattail (Typha species, top right), dominates the wet area.
Sites. My 2018 study was located in central New Jersey. Route 1, a major suburban highway, runs through urban and suburban areas and became the organizing transect. There are 15 study sites that capture a range of conditions, located both along the highway and in nearby parks. I sampled plants in July and August 2018. I was primarily interested in the plants on the floor of basins that is exposed to periodic flooding, so I followed a line from the inlet pipe to outlet pipe and along perpendicular lines to get a representative sample.
The orange dots are the location of study basins. They all fall along the Rt. 1 corridor. The darker shaded areas of the map have more impervious surface. The basins are distributed throughout areas with different levels of urbanization in order to represent a wide range of conditions.
What did I find?
I found a lot of species. 162 species in 93 genera and 41 families.
73% of species were native species. 45% are specialized wetland plants according to the National Wetland Plant List.
The native species are mostly wetland plants. 61% of native plants are wetland, vs. 25% of non-native plants. I interpret this to mean that the non-native species in these sites are more generalists and could live in lots of different places, but the native plants are more wetland specialists. This shows that detention basins are providing a habitat for wetland specialist plants that wouldn’t be found elsewhere in the urban landscape.
Results from 2018 study. Native species made up about three quarters of the species I found. The figure on the right shows the relative cover of plant species in central New Jersey stormwater detention basins.. Cattails and giant reed grass have the highest cover, followed by purple loosetrife. Outside of the top ten species, all other species accounted for about one quarter of of the cover.
This table represents the top species by abundance in New Jersey detention basins. The table shows whether the plant is native (Yes/No), a conservation value score, and an indication of its wetland status. OBL= obligate, always found in wetland. FAC = facultative, can be found in a range of wetland to dry areas. UPL = upland, always found in dry areas.
As expected, cattails and Phragmites were the top species. They both covered about 25% of all sites. After those two species, there is a drop-off, with Purple loosestrife accounting for 7% cover and Soft rush at 5.39%. Mugwort is a dominant species of the dry slopes, but it appears on this list because it is so dominant at the margins and select areas of the basin floor that remain dry. After these species, there are 17 species with 1-3% cover.
The conservation score comes the Mid-Atlantic Wetland Workgroup. The conservation score ranges from 1-10 and tells us, in broad strokes, how excited to get about the presence of particular species. As we might expect, most of these species have low conservation scores. However, river bulrush has a high score, and 23 other species had a score of 5 or higher.
Now, understanding a little more about the breadth of the diversity, my next steps are to understand which conditions are related to certain species. For example, cattails are known to live in wetter habitats than many of the other plants I found, and can survive in standing water. Giant reed grass may be able to tolerate more salt than other species. Along with plant species, in 2018 I measured soil moisture, standing water, basin topography, and nutrient levels. Understanding the interrelationships between these factors and plant species can guide management decisions in the future.
While invasive species do comprise a large portion of detention basins, they are far from the whole story. It was extremely heartening to find so much biodiversity in places where I least expected it. I saw a lot of insect activity, so I believe that these areas could be promising habitat for pollinators who are also under a lot of pressure from human development. Another encouraging part of this research is that changing from turfgrass to mixed vegetation can be as easy as stopping the mowing. Once you stop mowing, non-grass species that are already there can grow up, and other seeds from the environment come in as well. Nature is just waiting for us to leave it alone in order to regenerate.
