Syzygium and Agapanthus spp. damaged under intense heat and drought. Credit: Alessandro Ossola
Authors: Samiya Tabassum, Renee Marchin, David Ellsworth and Michelle Leishman, 26th Sept 2021
As the effects of climate change continue to intensify, it is likely that we will experience more severe and frequent heatwaves and drought. The outcome of these events is expected to be even more devastating in urban areas due to the additional impact of the urban heat island effect (read more in our ‘Urban heat islands or archipelagos?’ blog). Exposing ill-adapted plants to hot and dry conditions can cause leaf scorching, tissue death, reductions in growth, and ultimately, plant death. Fortunately, many plant species have developed adaptations to help them cope with extremes of heat and drought. Here is a list of common drought and heat tolerance traits you can look for when planning your next urban greening project.
Drought tolerance traits
When faced with drought, many plants opt for one of two strategies.
- They avoid impacts of drought by decreasing the amount of water they lose via transpiration or by increasing the amount of water they gain via their roots. This allows them to maintain their internal water reserves and is commonly known as the ‘drought avoider’ strategy.
Common characteristics of drought avoiders include:
- Deep root systems to help them access groundwater
- Succulent leaves or special water storage organs so they always have water available if soil moisture declines
- Very sensitive stomata (tiny openings on the surface of leaves, much like pores on skin) that can be shut quickly to reduce water loss
- The ability to shed their leaves in response to drought to reduce water loss
- They don’t bother resisting the drought because they have traits which allow them to survive drought conditions. This is commonly known as the ‘drought tolerator’ strategy.
Common characteristics of drought tolerators include:
- Thick, tough or waxy leaves which are very resistant to drying out
- Leaves with a low turgor loss point (also known as wilting point), meaning that leaves can stay turgid even when there is very little water available
- Small sized leaves because they tend to have higher vein density which helps with more efficient water transport to help cool leaves
- Dense wood in order to protect and reinforce their xylem vessels, which transport water from the plant’s roots to its leaves. When the soil is very dry, the pressure in xylem vessels can become so high that air bubbles form (imagine sucking through a straw that is in an empty glass) so that the plant’s transport vessels no longer work.
To make things more complicated, some plants fall into a grey area between drought avoider and drought tolerator because they have traits from both strategies. For more information on the difference between drought tolerators and drought avoiders, please read the blog post ‘drought tolerators vs drought avoiders’.
Heat tolerance traits
Plants that are generally considered heat tolerant may have some or all of the following characteristics:
- Hairy, pale or waxy leaves which help to reflect harsh sunlight, thus reducing the energy load on the leaf (this works the same as putting a reflective sunshade on your car windscreen on sunny days)
- Small, thick and/or lobed leaves which prevent leaf temperatures from rising significantly above air temperatures, thereby reducing heat damage
- Leaves with a high critical leaf temperature, meaning that they can withstand higher temperatures before the leaves start dying (plants from hotter climates generally have higher critical leaf temperatures)
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