Tree species composition of Australia’s urban forests (from Ossola et al., 2020)
Authors: Alessandro Ossola, Samiya Tabassum and Michelle Leishman, 24th September 2021
Compared to other countries, Australia is blessed with a rich diversity of vascular plant species (20,000 and more). Many of our plants are endemic to the red continent or at least parts of it. Western Australia hosts an incredible plant diversity, equalled by only a few biodiversity hotspots in the world.
The rich plant diversity in the Aussie wild is reflected in a relatively high urban plant diversity compared to cities and towns overseas; for instance Australia hosts an impressive 1,200 urban tree species, 25% of the global urban tree diversity (Ossola et al., 2020). Despite this, several issues with Australia’s urban forests and greenspaces, such as low species diversity and the selection of very common species, remain unsolved. These issues can threaten their very survival under the increased impacts from climate change.
The many facets of urban plant diversity
While most people are familiar with the concept of species richness, i.e., the number of plant species planted at a location, many other aspects of diversity need to be considered when planting to ensure urban forests and greenspaces are healthy and resilient into the future. Urban plant diversity can in fact be described by the following factors, ranked from the local to the landscape scale:
- Genetic diversity. In the wild, plants occur in different populations that often have a slightly different genetic make-up compared to each other. Like us, plants have different characteristics (aka traits) that can be linked to genes or sections of their DNA. Like us, there is variation within a species, for example some plants coming from particular provenances or populations (e.g., subspecies, clines, etc.) might have higher resistance to heat or cold, or they might be able to better resist attack from pathogens and diseases. While plants grown from seeds often retain a relatively high genetic diversity – particularly if seeds are sourced from different populations and locations – many other plants propagated vegetatively (i.e., from cuttings, grafting, in-vitro, etc.) have lower – if any – genetic diversity. This can pose a great threat to a horticultural species; if its genetic make-up does not protect a species from a disease or an extreme weather event, this could wipe out entire urban plantings, increasing the risk and maintenance costs of urban greening efforts.
- Cultivar diversity. Many horticultural species undergo a lengthy and skillful process of selection, breeding, crossing, etc., of plant materials to create “new” plants with aesthetic and functional characteristics that may be quite different to that found in nature. As many plants are very “plastic” organisms (i.e., versatile), these characteristics can be changed through careful horticultural practice by blending traits within (cultivars, varieties) and across species (i.e., hybrids). Increasingly, plant species are bred to create varieties that are not only more aesthetically appealing, but also more drought/heat tolerant and more resistant to external stresses. In this way, when considering what to plant and where, it is important to check whether some cultivars might be more appropriate for your location and climate.
- Species richness. The number of species and genera we can plant at a location is probably the easiest action that can be taken to quickly increase urban plant diversity. As seen for the intraspecific (genetic) diversity, having a larger number of planted species can ensure that at least some of them will survive the emergence of a pest outbreak or extreme weather. The 10:20:30 Rule is a useful measure to remember when thinking about how we can create diverse urban plantings (read more about this in our technical guideline ‘How we calculated planting diversity’ in the Which Plant Where plant selection tool based on your personalized species palettes).
- Age structure. Like us, plants are born, they mature, and age until their final days. If a street or avenue is planted with the same species and similarly-aged individuals, these will most likely mature and naturally die of age around the same time, creating a sudden large gap in an urban forest. Because of this, differentiating among plant ages and maximum life spans can offer the greatest insurance that plantings are resilient and that high diversity is maintained over time.
- Gender diversity. Like us, many plants have separate sexes on different individuals (i.e., dioecious species have separate female and male individuals), though most species are monoecious (i.e., male and female flowers on the same plant, or flowers on every plant that contain both male and female parts). A few species reproduce entirely vegetatively. When considering new plantings, it is therefore important to evaluate how the new arrivals will affect the overall demographic structure of an urban plant population. For instance, urban tree plantings in the US in the last century prioritised the use of male trees to reduce unintended smells from fruits rotting on the ground (produced only by female trees). This had the unfortunate consequence of significantly increasing the amount of pollen generated (as only male trees produce pollen) and consequently allergies in people, while also reducing fruit availability for many animals living in cities.
- Structural diversity. A final aspect related to urban plant diversity is the combination of species and cultivars, based on their architecture and life forms. Landscape architects do a great job in designing aesthetically appealing plantings by mixing plants with different shapes, colours, textures, phenologies and more. In many instances, however, planting could benefit from the addition of many more species and cultivars to create more complex and appealing designs (Tabassum et al., 2020). By carefully selecting plants with complementary characteristics, we can dramatically boost the ecological performance of plantings and their resilience to stress and climate extremes (Read the blog ‘What can we learn from natural ecosystems?’). Structural diversity is also important for supporting biodiversity of animals that use urban plants as food or habitat (You can read more in the ‘Why biodiversity is important for cities’ blog).
REFERENCES
Ossola, A., Hoeppner, J.M., Burley, H., Gallagher, R.V., Beaumont, L.J., Leishman, M.R., 2020. The Global Urban Tree Inventory: A database of the diverse tree flora that inhabits the world’s cities. Global Ecology and Biogeography, 29:1907-1914. https://doi.org/10.1111/geb.13169.
Tabassum, S., Ossola, A., Manea, A., Cinantya, A., Fernandez-Winzer, L., Leishman, M.R., 2020. Using ecological knowledge for landscaping with plants in cities. Ecological Engineering, 158, 106049. https://doi.org/10.1016/j.ecoleng.2020.106049.