Leaf Color and Shape Enhance Cooling Effect

green leaves from a tree with yellow flowers are seen against a light sky

Biological Strategy

Sand drago

AskNature Team

Image: Forest and Kim Starr / Wikimedia Commons / CC BY - Creative Commons Attribution alone

Protect From Light

Access to sunlight is crucial to living systems because it’s the primary energy source for life. However, too much sunlight in the form of ultraviolet radiation (UV) can cause damage to living tissues. Therefore, living systems have strategies to filter out some or all UV radiation. For example, some plants that live in areas exposed to long periods of direct sunlight have reflective surfaces (such as white hairs or powder) that reflect UV light.

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Protect From Temperature

Many living systems function best within specific temperature ranges. Temperatures higher or lower than that range can negatively impact a living system’s physiological or chemical processes, and damage its exterior or interior. Living systems must manage high or low temperatures using minimal energy, which often requires controlling responses along incremental temperature changes. To do so, living systems use a variety of strategies, such as avoiding high or low temperatures, removing excess heat, and holding heat in. Insulation is a well-known example of managing low temperatures by retaining heat using thick layers of hair, fur, or feathers to hold warm air next to the skin.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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The cooling effects of shade trees in subtropical regions are most influenced by foliage density and leaf thickness, leaf texture, and leaf color lightness.

Shade trees provide a cooling effect by blocking sunlight and increasing air moisture, but not all trees offer equal relief from the heat. Tree species with rough, dense foliage and light colored leaves provide the greatest cooling benefits to their surroundings.

During the day, the sun warms the earth as waves of sunlight are absorbed by all that it touches. When sunlight falls on trees, the energy excites atoms in its leaves, triggering and creating heating. Too little heat results in very slow growth, but excess heat can be destructive to temperature sensitive molecules in the plant. Tree species are adapted to their local sunlight regime by producing leaves with varying degrees of reflectiveness; i.e., dark colored leaves absorb the most energy from sunlight, while light colored leaves reflect excess sunlight. For example, conifer forests cover high latitudes where sunlight is limited. By having dark needles, conifers are able to absorb the most energy from sunlight when it is available. In contrast, cacti receive plenty of sunlight in low latitude deserts. To prevent scorching, cacti are light colored to reflect some of the sunlight they receive.

Sunlight also heats the ground where it is exposed, causing it to collect the sun’s energy during the day and release the energy as heat at night. This results in an overall heat increase in exposed areas because nighttime cooling is minimized. Shade trees with dense foliage are able to block sunlight from reaching the ground during the day, preventing the ground from collecting energy. Because shaded areas do not release heat at night, ambient temperatures remain cooler on average than exposed areas receiving the same amount of sunlight.

In addition to general heating, the energy from sunlight also excites water molecules in leaves and transforms them from liquid to vapor, a process called evapotranspiration. Water molecules are unique because they are able to hold a great deal of energy without releasing it as heat. In the vapor state, water molecules have a pronounced cooling effect because they are able to absorb heat from objects they come in contact with, including trees, the ground, and the human body. To maximize this cooling effect, some trees are adapted to allow the greatest amount of evapotranspiration to occur by producing leaves with high surface area. Rough leaves have more surface area than smooth leaves, providing more space for evapotranspiration across the surface of the leaf to occur. This results in greater cooling benefits due to the high humidity surrounding trees with rough leaves.

This strategy was contributed by Melissa Moore Friedman.