Dead plants after a dry spell (Image courtesy: Leigh Staas)
Authors: Alessandro Ossola and Michelle Leishman, 24 Sept 2021
Droughts in urban landscapes can decrease the success and performance of urban plantings. Short dry spells and long-lasting droughts can threaten the performance of urban vegetation and the resilience of greenspaces, potentially causing great economic damage (Tabassum et al., 2021a).
A set of integrated solutions, both short- and long-term, can help urban managers, landscape architects and designers to successfully overcome the increasing challenges posed by limited precipitation and water for irrigation, while improving urban sustainability and greening interventions.
Droughts and the curse of climate change
A drought is a period of unusually low precipitation at a location. Because climate varies among cities, droughts depend on local current and historical relative conditions, i.e., a drought event in Darwin is not the same as a drought event in Adelaide, as climate extremes can be different based on their severity (i.e., the extent of water deficit for plants), duration (i.e., the number of days and weeks during which drought conditions are recorded), as well as their frequency (i.e., the number of times drought events reoccur over years and decades).
In many parts of Australia climate change is altering rainfall distribution far exceeding historical changes (Fig. 1). Some areas are becoming wetter overall (i.e., have higher mean annual precipitation) but they can still be affected by seasonal droughts that can cause sporadic yet acute water stress for plants. Other areas are predicted to become drier with longer and more severe drought events, causing new permanent (chronic) climate conditions where only the best adapted plant species can survive and thrive.
Figure 1: Ulmus montana (now Ulmus glabra subsp. montana) was successfully grown in Campbelltown, NSW in the early 1900s. This tree species was thought to grow well at this location despite the fact that “droughty years had been punishing it severely” (Agricultural Gazette of N.S.W., 1908, page 786). Fast forward 100 and so more years, this species is deemed as climatically unsuitable for the entire greater Sydney region based on the Which Plant Where bioclimatic models (read more about models in the ‘How can we determine if a species is ‘climate-ready’ for a location?’ blog).
Urbanisation exacerbates climate extremes
In cities it is also important to consider the local interplay between the built environment and water availability from both rainfall and irrigation, all factors that can affect the water balance for plants. The presence of impervious surfaces can cause localised dry soil conditions even when rainfall is bountiful. On the other hand, irrigation can increase water availability for plants to levels comparable to those recorded in non-drought periods (Fig. 2).
Climate extremes and urban plants
Plant species have different tolerances to soil water availability; some species can cope with both high and low water availability while others can only survive under more limited water conditions (e.g., wetland species able to tolerate waterlogging compared with dryland species unable to tolerate waterlogging). During a drought, plant species that require a lot of water to survive are generally those most impacted by the dry conditions, and particularly if the drought is severe and prolonged over time. On the other hand, plants that can tolerate a wider range of soil water conditions are most likely to be at lower drought risk.
Some plant species have a range of ‘water strategies’ that enable them to conserve precious water resources during times of scarcity (Tabassum et al., 2021b and read our blog ‘Drought tolerators and avoiders’). Some of these strategies comprise:
– the ability to completely shut their leaf pores (stomata) and reduce photosynthesis to a minimum to avoid water loss from their leaves,
– the ability to minimise water losses due to thick and small leaves covered with hairs and waxes which can further reduce water loss,
– the ability to store water in specialised storage organs, and
– the capacity to extend roots deep underground to tap into the water table.
While some of these ‘water strategies’ might allow plants to survive a drought event, these characteristics might not be enough if droughts are extreme, prolonged, or reoccur at high frequency over time.
What you can do!
Managing the negative effects of urban drought in the short term requires quick-to-implement strategies aimed at mitigating its acute impacts on plants. The use of organic and inorganic soil amendments like mulch, bark chips, leaf litter, pebbles, water crystals, gels, etc., can greatly reduce soil/media evaporation and increase soil water retention and porosity (read more about ‘What is the deal with soil additives?’). This can increase water availability for plants experiencing drought conditions. The use of targeted irrigation prior to and during a drought event, if permitted under local water restrictions, can significantly increase the chance of survival for the most vulnerable species until the drought breaks. Proactive and emergency irrigation, however, has to be calibrated by adjusting watering schedules based on the local weather conditions (temperature, vapour pressure deficit) that drive plant evapotranspiration. This can ensure plants have sufficient water while reducing over-irrigation and wasting of a precious resource.
Figure 2: The water balance in urban landscapes is affected by the local precipitation, irrigation and the presence of impervious surfaces (Photo credit: Anton, W Carter, and Dhanashree R Sharma from Wikimedia Commons). The urban water balance affects the environmental drought conditions plants can experience and their short- and long-term survival in our cities.
Long-term strategies to mitigate urban droughts, on the other hand, require greater vision and planning. The progressive substitution of drought-sensitive species with drought-tolerant species should be prioritised in urban forestry strategies as well as planting interventions on private land (read more about ‘The Great Australian (GREEN) Dream’). Greater plant species richness of drought-adapted species and cultivars can ensure most plants will survive urban droughts while reducing risks and future costs. Technical solutions in the built environment – such as de-paving, the use of water tanks, passive irrigation techniques, structural soils and water-sensitive urban designs – can ensure water is maintained in the urban landscape for time of plenty, so that soil water availability is greater during a drought. Similarly, the implementation of nature-based solutions – such as the creation of more structurally complex plantings – can help reduce water loss from the soils to the atmosphere, giving our plants the best chance to ‘feel the rain again’.
REFERENCES
Tabassum, S., Manea, A., Ossola, A., Buyani, T., Blackham, D., Leishman, M.R., 2021a. The angriest summer on record: assessing canopy damage and economic costs of an extreme climatic event. Urban Forestry and Urban Greening, 63: 127221. https://doi.org/10.1016/j.ufug.2021.127221
Tabassum, S., Ossola, A., Marchin, R., Ellsworth, D., Leishman, M., 2021b. Assessing the relationship between trait-based and horticultural classifications of plant responses to drought. Urban Forestry and Urban Greening, 61:127109. https://doi.org/10.1016/j.ufug.2021.127109