By Dominic Christensen
What is planted along roadside boulevards and how they are maintained is often a heavily debated subject. There are those who are more concerned about accomplishing the perceived primary goal to seed simpler nonnative mixtures to prevent erosion, and some who think the goals and functions of these boulevards should behave similar to a short-statured prairie, and others who are not so opinionated.
What needs to be reflected on before planting is the intended purpose or functions of the area. If our only purpose is to reduce erosion, applying lower diversity mixtures may be adequate. If we want our landscape to fulfill more ecological functions, then planting mixtures with greater diversity will allow for many organisms to flourish. Furthermore, a landscape containing diversity in species and functional groups provides a temporal safeguard against future abiotic and biotic stresses. A higher diversity mixture will also provide spatial insurance when planting across a large area due to micro environmental variability in soil, water, and light characteristics.
Defining a relative scale for the diversity of mixtures for roadsides is important for this discussion. A lower diversity mixture can be defined as <=4 total species, 1 variety per species, and 1 functional group. Medium diversity mixtures can be defined as <8 total species, 1-2 varieties per species, and >2 functional groups. High diversity mixtures can be defined as >8 species, >=2 varieties per species, and >=3 functional groups (see Barot et al., 2017 for variety per species discussion). Vascular plant functional groups compatible with roadside boulevards in Minnesota are C3 grasses, C4 grasses, sedges, rushes, legumes, and non-legume forbs (Figure 1). Sometimes, roadside seed banks contain a diverse array of species and so planting a comprehensive seed mixture may not always be necessary; yet more often than not, abundance of ruderal species (plants avoiding stress and competition and often annuals (Grime, 1974)), and invasive species dominate roadside seed banks. Therefore planting a diverse mixture provides the opportunity to return desirable adapted species to a landscape more quickly than natural seed rain.
Many can agree that our roadsides should be low-maintenance areas, having vegetation that has a short-statured height and grows slowly, thereby reducing visibility impairments and reducing the need for mowing. Low-maintenance also means that these areas should not be fertilized and that herbicide use should be kept to a minimum. Additionally, there seems to be general agreement that these areas should not be so welcoming to animals that they cause more vehicle-animal accidents. We want our roadways to be safe and that includes our road surfaces but also the ecological impacts of our vegetation.
What I often see missing from those who recommend high proportions of native species in seed mixtures is they forget how anthropogenically influenced roadsides are and that some of our more common native species are not adapted to these conditions. Conversely, those opposed to the use of native species can fail to recognize that those species exist that can perform well on roadsides (Figure 2). It is important to remember that many native species take time and effort to establish properly and to manage, which can test our patience. A combination of seeding and transplanting species along a boulevard may be one strategy to incorporate species that establish poorly from seed.
In general, I think it is useful to keep an open mind because there exists both native and nonnative species that perform well in these areas; furthermore our roadsides are currently a mix of native and nonnative species growing together. Yet, it is known that planting native plant species can provide a waterfall effect of benefits for other native local organisms and fulfill more ecosystem processes (Hector & Bagchi, 2007). If we develop mixtures appropriately, I do not see why we should focus on exclusively native or nonnative mixtures. To further convolute things, whether a species is native or nonnative to an area is often debated and so many species and populations do not fall into this dichotomy and are labeled cryptic.
Another issue is that some species sound excellent on paper, but often perform poorly adjacent to the road (Engelhardt, 2016; Engelhardt & Ratliff, 2019). This is not surprising as the cultivar used on the roadside was likely selected and/or bred for biomass in a grazed condition or turfgrass quality in a lawn and not for performance on a roadside. There also are a number of species that exhibit a lot of potential for roadsides, but no commercially available seeds can be purchased. Reestablishing roadside vegetation often covers large areas and seeds of native species and mixtures containing more species are often more expensive, and so this will, in some capacity, constrain higher diversity mixtures. Planting a uniform diverse mixture does not have to be the answer; perhaps designing a patchwork of high-diversity and low-diversity planting zones could be just one option.
It is also important to recognize that our roadsides are often heavily and regularly disturbed, so if we focus or think of our roadside vegetation in a context that is too long-term, then we are never meeting the primary goal in the present. Likewise, if there is too much of an emphasis on meeting the goals for erosion control and aesthetics immediately, then our efforts will likely be short-lived. Balancing the short and long-term goals of revegetation should be carefully thought through (Figure 3). This blog post may leave you with more questions than answers, but I think it illustrates a few of the many complexities of how we consider plant diversity when designing roadside mixtures.
References
Barot, S., Allard, V., Cantarel, A., Enjalbert, J., Gauffreteau, A., Goldringer, I., ... & Porcher, E. (2017). Designing mixtures of varieties for multifunctional agriculture with the help of ecology. A review. Agronomy for sustainable development, 37(2), 13.
Engelhardt, K., & Hawkins, K. (2016). Identification of low growing, salt tolerant turfgrass species suitable for use along highway right of way.
Engelhardt, K. A., & Ratliff, K. (2019). Identification of Low Growing, Salt Tolerant Turfgrass Species Suitable for Use along Highway Right of Way–Field Trials (No. MD-19-SHA/UMCES/7-01).
Grime, J. P. (1974). Vegetation classification by reference to strategies. Nature, 250 (5461), 26-31.
Hector, A., & Bagchi, R. (2007). Biodiversity and ecosystem multifunctionality. Nature, 448(7150), 188-190.