Climate-ready urban plantings need to cope with low water availability (Credit: Leigh Staas)
Authors: Samiya Tabassum, Renee Marchin, David Ellsworth & Michelle Leishman, 26 Sept 2021
Plants utilise a range of strategies to cope with drought. Many plants can tolerate drought as they have adaptations to be able to withstand low soil water availability, while other plants try to avoid drought by decreasing the amount of water they use or by acquiring more water from the landscape.
Tolerators vs Avoiders
Drought tolerators are able to withstand drought conditions because they have several adaptations to allow them to survive high water deficits. These adaptations include small, thick leaves, a high turgor loss point (meaning they do not wilt easily under water stress) and high wood density. Drought avoiders, on the other hand, try to avoid drought conditions by either decreasing the amount of water they spend (e.g., by not photosynthesising and in extreme cases by dropping their leaves) or by accessing water from other parts of their body (e.g., many drought avoiders have juicy leaves or specialised water storage organs) and/or the landscape (e.g., by possessing a deep root system that can access ground water).
However, as with many things in life, it’s not always easy to put things into nice neat boxes and plants are no exception! Plants don’t always fit nicely into ‘tolerator’ and ‘avoider’ definitions. In fact, many plants can fall into a grey area where they possess a mixture of ‘tolerator’ and ‘avoider’ traits (e.g., they may have small, thick leaves to tolerate drought as well as a deep root system to avoid drought). This is because these plant traits may have multiple functions (e.g., coping with low soil fertility, or extreme heat, or avoiding being eaten), and so different trait combinations may work in various environments.
What does this mean for plant selection for urban green space?
Drought tolerators and drought avoiders essentially represent two sides of the same coin – they are both ways in which plants can cope with drought. However, whether a plant is a tolerator or avoider can actually affect their suitability for particular urban settings. For example, urban landscapes often have low soil volumes and impermeable surfaces such as pavement and asphalt, which may cause significant water stress for plants. These environments are more suitable for drought tolerators because they can cope better with low water availability and restricted rooting space. Drought avoiders, on the other hand, may find these sites stressful to grow in because they may not be able to get their roots down into the water table. Many drought avoiders also shed leaves in response to drought conditions in order to reduce water loss. Not only does this make plants look unsightly, but the loss of canopy cover will also reduce the evaporative cooling and shading benefits of plants (read more about this in the ‘Not urban Green is equally Cool’ blog).
While some of these effects could be mitigated through irrigation, water restrictions and tight budgets might not make this possible. Instead, drought avoiders may be more appropriate for parks or gardens where restricted rooting space is less likely to be an issue. So the next time you plan an urban greening project, think about the water (i.e., hydraulic) traits of the plants you are using and how they might cope with water stress. Understanding what strategies plants use in response to drought may help improve urban planting outcomes under climate change.
REFERENCES
Marchin, R.M., Ossola, A., Leishman, M., Ellsworth, D., 2020. A simple method to simulate drought on plants. Frontiers in Plant Science, 10:1715. https://doi.org/10.3389/fpls.2019.01715
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