How Mycorrhizae Can Benefit the Performance of Green Roof Plants
Advertisement
Introduction
Arbuscular mycorrhiza under microscope. Photo: Dr. Amy Heim and Dr. Pierre-Luc Chagnon
The word mycorrhizae has Greek origins, with myco meaning fungus and rhiza meaning root (Latef et al., 2016), a fitting name for fungi that live in association with plant roots. The relationship between mycorrhizae and their host is mutualistic, with the mycorrhizae providing nutrients and other benefits to their host, which in turn provides the mycorrhizae carbohydrates and lipids produced through photosynthesis (Jung et al., 2012; Luginbuehl et al., 2017). Mycorrhizae can be divided into four main categories with the majority of land plants being associated with arbuscular mycorrhiza (72%), ericoid mycorrhiza (10%), orchid mycorrhiza (7%), or ectomycorrhiza (1.5%). Additionally, 7% of Earth’s vegetation has inconsistent arbuscular mycorrhiza associations and 8% of plants have no association with mycorrhizae. The presence of arbuscular and ectomycorrhiza may not always be strictly necessary for the survival of their plant host, although the association does tend to improve plant growth and survival. In contrast, the plant species associated orchid and ericoid mycorrhizae require these fungi for survival, with orchid germination being only possible in the presence of orchid mycorrhizae (Brundrett & Tedersoo, 2018).
Cranberries grown on an unirrigated extensive green roof. Photo: Dr. Amy Heim and Dr. Pierre-Luc Chagnon
The Benefits of Mycorrhizae Fungi
Mycorrhizae fungi are known to benefit their plant host in a number of different ways, including increased plant size, drought tolerance, nutrient acquisition, heavy metal tolerance, heat and cold resistance, protection from pathogens and insects, and greater soil aggregate stability (Jung et al, 2012; Latef et al., 2016; John et al., 2017). Some mycorrhizae, such as arbuscular, can act as an extension of a plant’s roots. Their hyphae extend beyond the reach of their host, favouring the acquisition of water, and of low-mobility nutrients (Cavagnaro et al., 2015). Since green roof plant species are often exposed environmental stressors, such as high wind, extreme temperature and drought, the inoculation, or spontaneous colonization, of mycorrhizae on green roof vegetation may improve overall plant performance.
Advertisement
Mixed vegetation extensive green roof in Halifax Nova Scotia. Photo: Dr. Amy Heim and Dr. Pierre-Luc Chagnon
Green roofs are capable of supporting plant species associated with mycorrhizae from all four categories described above, with roof type and maintenance regime determining species composition. Extensive green roofs, those with a substrate depth less than 15 cm, are commonly planted with drought-tolerant herbaceous species, many of which are associated with arbuscular mycorrhizae. Common green roof species with arbuscular associations include those from the family Asteraceae (daisy family), Poaceae (grass family), and Lamiaceae (mint family). Dwarf shrubs from the Ericaceae family (heath family), which are associated with ericoid mycorrhizae, can also flourish on extensive green roofs. For example, green roof researchers in Nova Scotia successfully grew blueberries, cranberries, and crowberries in an unirrigated extensive green roof (Heim & Lundholm, 2022). Orchid mycorrhizae are solely associated with species of Orchidaceae (orchid family), which form dense structures inside roots cells (Brundrett & Tedersoo, 2018). Green roofs do not commonly contain orchids, but they can occur. In 2016, a rare orchid (Anacamptis morio) made its way onto a London green roof (Islington Council, 2016), and a green roof in Zurich is home to 13 different orchid species (Living Roofs, 2023). Plant species linked with ectomycorrhiza are more commonly found on intensive green roofs (substrate depth more then 20 cm) and/or those subjected to frequent maintenance. Specifically, ectomycorrhiza are associated with woody species (John et al., 2017), including many conifers, Myrtaceae (myrtle family) and Rosaceae (rose family).
The most common green roof species are succulents from the family Crassulaceae (stonecrop family), which includes Sedums, Phedimus, and Hylotelephiums. Some species in this family are associated with arbuscular mycorrhizae, while others have inconsistent associations (Brundrett & Tedersoo, 2018). Research by John et al., (2014), observed arbuscular mycorrhizae on the roots of naturally occurring Phedimus spurius and Hylotelephium telephium but no mycorrhizae on the roots of Sedum acre. Nevertheless, research by Chaudhary et al. (2019) on Chicago green roofs fond that Sedum dominant roofs contained substantial amounts of mycorrhizal propagules.
Advertisement
How Mycorrhizae Find There Way To Green Roofs
Urban greening, such as these green tram tracks in Melbourne Australia, can act as a source population of mycorrhizae. Photo: Dr. Amy Heim and Dr. Pierre-Luc Chagnon
Newly Constructed Sedum/Moss extensive green roof at the Swedish University of Agricultural Sciences in Alnarp Sweden. Photo: Dr. Amy Heim and Dr. Pierre-Luc Chagnon
There are several different ways in which mycorrhizae can reach green roofs, with introduction differing by fungal type. Due to their larger spore size, arbuscular mycorrhizae are mainly transported via soil and living organisms such as invertebrates, although wind may be an underappreciated vector of dispersion (John et al., 2017; Chaudhary et al., 2019). They can also reach green roofs when transplanting takes place, with mycorrhizae spreading from the roots of one species to another. The spores of both ericoid and ectomycorrhizae are smaller and easily dispersed by wind (John et al., 2017). For these species, parent populations from urban parks and other green roofs can act as a source population leading to the colonization of newly built roofs. This phenomenon has been demonstrated in the literature with numerous studies observing an increase in fungal populations over time, even on green roofs with no initial colonization (Chaudhary et al., 2019; Henault et al., 2022). In addition to spontaneous colonization, green roofs can also be inoculated with mycorrhizae during or after construction. In such cases, these fungal species have been observed months and years after initial establishment (Young et al., 2015; Molineux 2017).
Although more research is needed, several studies have examined the influence of arbuscular mycorrhizae on the growth of green roof plant species. Young et al., 2015 found that inoculation with mycorrhizae increased the duration of flowering in Prunella vulgaris (heal all) and shoot phosphorus concentration was improved in all inoculated treatments. A green roof study by Mollenyx et al. (2017), observed greater species richness in treatments inoculated with mycorrhizae. Additionally, a drought study comparing treatments with and without inoculation found that over an 88-day period biomass and inflorescence was greater in the inoculated treatments compared to controls (Scjorder et al., 2019). However, other studies reported no effect of mycorrhizal inoculation on plant growth or survival (Ksiazek-Mikenas et al., 2021), indicating a need to further explore this topic, and better understand the context-dependency in mycorrhizal impact on plant growth on green roofs. Overall, the presence of mycorrhizae can be a benefit, and is at times necessary for the survival of green roof plant species.
Conclusion
Research has found that mycorrhizae will naturally colonize green roof systems. This process can be hastened by adding mycorrhizae directly to the green roof substrate. However, research specific to green roof mycorrhizae is needed. Rather than relying on traditional inoculation with commercial inocula, research into specific species of mycorrhizae, with specific traits, that may favour drought tolerance and green roof plant performance could be of benefit to the green roof industry. Ideally, we will get to the point where green roof architects can select the best inocula for their roof, based on roof characteristics and known plant stressors.
Advertisement
Dr. Amy Heim is a Post Doctoral Fellow, Université de Montréal, contact her at Heim.Amy.E@gmail.com, Google Scholar Page.
Dr. Pierre-Luc Chagnon is an Associate Professor, Université de Montréal, contact him at pierre-luc.chagnon@umontreal.ca
References
Brundrett, M. C., & Tedersoo, L. (2018). Evolutionary history of mycorrhizal symbioses and global host plant diversity. New Phytologist, 220(4), 1108-1115.
Cavagnaro, T. R., Bender, S. F., Asghari, H. R., & van der Heijden, M. G. (2015). The role of arbuscular mycorrhizas in reducing soil nutrient loss. Trends in Plant Science, 20(5), 283-290.
Chaudhary, V. B., Sandall, E. L., & Lazarski, M. V. (2019). Urban mycorrhizas: predicting arbuscular mycorrhizal abundance in green roofs. Fungal Ecology, 40, 12-19
Heim, A., & Lundholm, J. (2022). Changes in plant community composition and functional plant traits over a four-year period on an extensive green roof. Journal of Environmental Management, 304, 114154.
Hénault, A., Heim, A., Brisson, J., Dagenais, D., De Bellis, T., & Chagnon, P. L. (2022). Stressful, isolated, yet diverse: Green roofs have rich microbiomes that are not dominated by oligotrophic taxa. Environmental Microbiology Reports, 14(5), 766-774.
Islington Council. 2016. UK first as a rare orchid suddenly appears growing on an Islington Council rooftop. Ecologist: Informed by Nature. Accessed February 15, 2023 from: https://theecologist.org/2016/jun/13/uk-first-rare-orchid-suddenly-appears-growing-islington-council-rooftop
John, J., Lundholm, J., & Kernaghan, G. (2014). Colonization of green roof plants by mycorrhizal and root endophytic fungi. Ecological Engineering, 71, 651-659.
John, J., Kernaghan, G., & Lundholm, J. (2017). The potential for mycorrhizae to improve green roof function. Urban Ecosystems, 20, 113-127.
Jung, S. C., Martinez-Medina, A., Lopez-Raez, J. A., & Pozo, M. J. (2012). Mycorrhiza-induced resistance and priming of plant defenses. Journal of chemical ecology, 38, 651-664.
Ksiazek‐Mikenas, K., Chaudhary, V. B., Larkin, D. J., & Skogen, K. A. (2021). A habitat analog approach establishes native plant communities on green roofs. Ecosphere, 12(9), e03754.
Latef, A. A. H. A., Hashem, A., Rasool, S., Abd_Allah, E. F., Alqarawi, A. A., Egamberdieva, D., Jan, S., Anjum, N.A., & Ahmad, P. (2016). Arbuscular mycorrhizal symbiosis and abiotic stress in plants: a review. Journal of plant biology, 59, 407-426.
Living Roofs. 2023. Orchids on Green Roofs – Switzerland. Accessed February 15, 2023 from: https://livingroofs.org/orchids-on-green-roofs-part-delivering-biodiversity-1/
Luginbuehl, L. H., Menard, G. N., Kurup, S., Van Erp, H., Radhakrishnan, G. V., Breakspear, A., Oldroyd, G., & Eastmond, P. J. (2017). Fatty acids in arbuscular mycorrhizal fungi are synthesized by the host plant. Science, 356(6343), 1175-1178.
Molineux, C. J., Gange, A. C., & Newport, D. J. (2017). Using soil microbial inoculations to enhance substrate performance on extensive green roofs. Science of The Total Environment, 580, 846-856.
Schroeder, R., Mohri, M., & Kiehl, K. (2019). AMF inoculation of green roof substrate improves plant performance but reduces drought resistance of native dry grassland species. Ecological Engineering, 139, 105583.
Young, T., Cameron, D. D., & Phoenix, G. K. (2015). Using AMF inoculum to improve the nutritional status of Prunella vulgaris plants in green roof substrate during establishment. Urban Forestry & Urban Greening, 14(4), 959-967.