63 years of global climate change. Image courtesy: NASA via Wikimedia Commons

Authors: Alessandro Ossola and Linda Beaumont, 24 Sept 2021

Climate change leaves virtually no ecosystem unscathed, including our critically important urban forests. While the climate impacts on Australia’s urban forests will be uneven – depending on climate change variability in time and space – some key principles can guide us on how to secure urban forests for climate resilience and sustainability.

Shifting plant species ranges under climate change

Plant species are always ‘on the move’ as, over decades and millennia, they expand and contract their distributional ranges as species acquire new suitable habitat or in turn faces local extinction. Among the many environmental, ecological and human factors that can affect species’ distributions (e.g., soil type, ability to disperse, human introductions, etc.), climate – its variability and trajectory of change – is probably one of the most important factors. Being unable to regulate their internal temperature as humans do, plants are left to the elements. When the climate becomes too hot or too cold, too arid or too wet, some species are unable to perform their vital functions, often struggling or dying.

By using complex computer models, scientists are able to understand what climates a species might survive and thrive in, or under what climate conditions a species might struggle and perish (read more about climate models and species suitability in the blog ‘How can we determine if a species is ‘climate-ready’ for a location?’). These climate suitability models are useful as they can help us to predict where a species might survive under climate change, enabling us to select suitable species for future conditions at particular locations. Climate suitability models suggest that most plant species will be affected by climate change, and they will need to cope with often dramatic changes to the area where they naturally occur. For example, some tropical and sub-tropical species native to northern Australia, such as Pongamia pinnata, are predicted to be able to ‘move southward’ and become more suitable for planting in urban locations in the south as conditions there become hotter (Fig. 1). Other species however, such as Syzygium smithii, are predicted to progressively lose their climatically-suitable habitat, thus restricting their horticultural use to a limited number of urban locations (Fig. 1).

In this way, it is important to understand where the climatically-suitable habitat for a species will be in coming decades, to ensure that only suitable plant species are planted and grown at these locations.

Figure 1: Examples of horticulturally-significant tree species for which climatically suitable area is predicted to expand southward (Pongamia pinnata, Indian beech) and shrink (Syzygium smithii, Common lilly pilly) for 2030 (black areas) and 2070 (orange areas), relative to the baseline (1960–1990) period (grey areas). The centroids the Significant Urban Areas (SUAs) along the eastern seaboard are plotted in pink on the baseline map. Blue arrows indicate predicted species movements under climate change (from Burley et al., 2019).

Urban horticulture and forestry come to the rescue

Species climate suitability models are an important tool to understand how species might fare under current and projected climate change (read more about climate models and species suitability in the blog ‘How can we determine if a species is ‘climate-ready’ for a location?’. However, as these models are largely based on the historical occurrence records from non-urban environments (i.e., the presence of a species at a location), we need to be careful how results should be interpreted. For instance, while some species might be considered unsuitable at certain locations (like the common lilli pilly in western Sydney or western Melbourne), this does not necessarily mean that these species cannot successfully be grown in an unsuitable location. In fact, several of these species could be still grown at these sub-optimal climate locations, provided we give extra care and resources, such as watering during dry periods or shelter from extreme radiation and urban heat (read more about urban microclimates in the blog ‘Urban Heat Islands… or Archipelagos?’).

The question is really whether we want to invest significant resources to maintain species that climate change might ‘push’ beyond their maximum climate tolerances or simply select a better suite of species in the first place, that we know from nature can already survive the climate predicted to occur at a particular location. In this way, urban horticulture, forestry and allied practices can make changes that will influence how urban forests, greenspaces and plantings will survive and thrive under climate change.

Selecting, breeding and growing species for the future. Plant production in both the short and long-term should be adjusted to ensure that climatically suitable species are available for planting in each climate region and urban area. This should go along with the progressive modification of species palettes, planting lists, etc., to ensure that new plants and species used are climatically suitable for the intended period or life expectancy of a green intervention.

While some short-term planting might ‘get away with’ a broader selection of species sub-optimal for a certain climate, the establishment of long-term interventions, such as development of an urban forest, requires more careful consideration of the new climatic norms (i.e., averages) likely to develop under climate changes, as well as the new climatic extremes triggered by new climate regimes (read more about urban droughts in the blog ‘When a drought hits your city…’).

Planting species with future climate in mind. Climate suitability of many species can be significantly expanded by using a careful design of plantings that acknowledges local factors, microclimate and species interactions (Tabassum et al., 2020). For instance, planting heat-sensitive species in the shade of heat-tolerant species can help them survive extreme temperatures. Shade from buildings and trees, as well as appropriate horticultural practices like mulching, can also ensure that drought-sensitive species can survive dry conditions and in drought-prone urban areas.

In this way, it is important to recognise that every urban planting sits within a larger landscape that will also be affected by the same climatic changes. By identifying and controlling the interactions between plants, the environment and the future climate condition, we can ensure that plantings will have a low risk of failure due to climate factors (read more about this in the blog ‘What can we learn from natural ecosystems?’).

Managing urban forests for climate change. Urban forests are dynamic and ever-changing ecosystems. Creating a ‘virtuous climate cycle’ and using an adaptive management approach is therefore needed to ensure long-term success and sustainable outcomes. Approaches that worked in the past might not necessarily work under future climate conditions. In this way, we must take proactive actions to ensure that climate resilience of urban forests is not only maintained, but progressively increased over time. For instance, when replacing trees and plants because of mature age, disease and failure, always and only consider species and cultivars known to survive projected climate conditions at a location. In doing so, you can choose species with ample ‘climate safety margins’ that can ensure better likelihood of survival of extreme weather events such as heatwaves and droughts.

When possible, also think ahead about programs, actions, and policies that can increase urban forest climate resilience (Ossola & Lin, 2021). These include the ability to perform extraordinary and pre-emptive maintenance before extreme weather events (e.g., emergency pre-extreme heat irrigation), and integrated solutions leveraging the ability of communities, NGOs and private businesses to provide extra care and resources to the urban forest in times of need (Lin et al., 2021).

REFERENCES

Burley, H., Beaumont, L., Ossola, A., Baumgartner, J., Gallagher, R., Laffan, S., Esperon-Rodriguez, M., Manea, A., Leishman, M., 2019. Substantial declines in urban tree habitat predicted under climate change. Science of the Total Environment, 685: 451-462. https://doi.org/10.1016/j.scitotenv.2019.05.287.

Ossola, A., Lin, B., 2021. Making Nature-Based Solutions “climate-ready” for the 50°C world. Environmental Science and Policy, 123: 151-159. https://doi.org/10.1016/j.envsci.2021.05.026.

Lin, B., Ossola, A., Ripple, W., Alberti, M., Andersson, E., Bai, X., et al., 2021. Integrating solutions to adapt cities for climate change. The Lancet Planetary Health, 5: e479–86. https://doi.org/10.1016/S2542-5196(21)00135-2.

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.

Resources

When a drought hits your city…

How can we determine if a species is ‘climate-ready’ for a location?

Climate change impacts on urban forests

Not all urban green is equally ‘cool’!

Urban Heat Islands… or Archipelagos?

Our cities need more trees, but some commonly planted ones won’t survive climate change