Have you ever considered why leaves are so large in the tropics, yet so long and thin in arid areas? In that case let’s take a look at the main drivers of leaf shape.
Leaves have to operate under an incredible range of conditions. Large tropical understory leaves compete with a huge diversity of plants for light, while the leaves of an arid-adapted plant are bombarded with excess light throughout the day. While the tropical tree has ample water supply, arid plants must employ methods to use every scrap of water, and leaf shape is critical to this optimisation.
There is a direct relationship between leaf length and average temperature. Higher temperatures allow more rapid growth of the leaves from the apex (tip), producing longer leaves. Shorter leaves assist in increased sap flow and lower the chance of frost damage in cooler climates.
Tropical plants typically have big leaves, which capture light energy and allow ample surface area for transpiration (plant sweating) that cools the leaf temperature. The tropical-adapted plant is able to sacrifice water to keep cool enough to operate efficiently at high temperatures (see here for a more in-depth discussion). This is in contrast to the long thinner leaves of arid-adapted plants, which need to decrease water loss as much as possible without overheating.
This is because of the boundary layer, which is the layer of relatively still air just above the leaf surface. As leaf size increases, this boundary layer becomes thicker, and slows the rate of transpiration. Transpiration is important for temperature regulation as it cools down the plant. However, water availability will determine whether a plant can sacrifice water for cooling. Hence, tropical plants can sacrifice a large amount of water by having large surface area, while arid plants will thicken the boundary layer with length but decrease the surface area by being narrow.
In alpine areas, plants tend to have small leaves. Rhododendron species in Russia have the smallest leaves in the northernmost regions of their range. In a leaf shape, rather than size, response, Campanula in northern Italy have narrower leaf bases at higher altitudes, thereby reducing leaf size. The small leaves in all these examples reduce the impact of frost damage by increasing sap flow to a smaller area.
Interestingly toothed margins of leaves were found to be more frequent – and more toothed and more highly dissected or deeply toothed – in cooler climates, especially in areas of high rainfall. The toothed margins of leaves allows sites for vigorous photosynthesis in young leaves early in the growing season, effectively kick-starting the trees energy supply after winter.
Temperature is a very important variable for predicting leaf traits, but leaf size is affected by more than just temperature.
Leaf width tends to increase with rainfall. Plants in drier climates have narrower leaves to reduce surface area and limit water loss. Plants in more humid or wetter environments have larger leaves because water loss is less critical and they use transpiration to cool down. Additionally, less seasonal growth environments such as Northern South America on the equator allow plants to invest more into leaf size, resulting in overall larger leaves.
Now, what does this look like on a global scale? What we see is, high temperature regions like central Australia and Africa have very similar, long, thin leaves, like Helichrysum and Dodonaea.
Mean leaf size at a site typically scales with water availability as well as temperature, but other factors such as soil nutrient availability, temperature, light, and water availability are also important factors in plant physiological responses. Here is a map of global predicted leaf size based on several leaf shape limiting factors.
Other interesting leaf shape responses
The size of leaves can also vary within a tree. Those at the top of the canopy tend to be smaller than at the bottom. In this way they don’t shade the leaves below them as much. Leaves at the top of the canopy are rarely lacking in light and so don’t need to be as large.
Pioneer trees, those that grow quickly within gaps in the forest, tend to have very large leaves so they are able to grow quickly and outcompete latter successional species that are slower growing and have smaller trees. In the end however, the later successional species win out as they have a longer life span and other competitive abilities.
Drip tips are present in very humid forests to allow water to more easily flow off the surface of the leaves.
In extreme environments with strong seasonality, some trees lose their leaves altogether (read about Deciduousness here).
Using leaf shape as an environmental indicator
The size and shape of leaves in fossil deposits have been used by scientists to infer climate during geological periods. We can use a similar principle today to to examine the impact of a changing climate on plants through leaf shape changes.
The narrow-leaved hop bush (Dodonaea viscosa) has been used as a monitoring tool in the Mt Lofty and Flinders Ranges of South Australia. The hop bush has narrower leaves in warmer, dryer climates, but interestingly over the last 100 years leaves have narrowed by up to 40% due to warming drying, climate (read more about this example here).
What isn’t known yet is whether these changes are genetic, as an example of microevolution, or plastic as a response to environment. Is it migration of the narrower leaf Dodonaea specimens south? And, will leaf shape be capable of changing further in a worst-case climate scenario?
There are other similar global examples. In China, several species have undergone leaf-shape-shifts. As temperature has increased in cool environments, leaf size has increased as conditions improved with lower frost risk.
From these studies, we expect that as the climate becomes warmer, leaf shape will change. In warmer drier areas, we will start to notice leaves getting narrower, while in alpine areas, leaves will become larger, as growth limiting factors change with climate.
However, more species clearly need to be studied. These studies will allow us to understand which species will be most resilient to changing conditions, and can be used to inform biodiversity management strategies.
Article by Ida Moore and Andrew Lowe
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