How Does Leaf Size Affect Plants?

Large leaves act as solar panels to absorb as much light as possible.

A common trait unifying all plants is their need to photosynthesize for energy production and sustenance -- leaves of all sizes perform this basic and critical function. However, varying locations with wildly different climates dictate leaf size, from huge, elephant-shaped leaves searching for sunlight to tiny leaves exposed to full sunlight most of the year. Does this Spark an idea?

Full Sunlight Exposure

Leaves exposed to full sunlight, such as at the top of a tree canopy, tend to grow smaller and in greater numbers. By generating small leaves, plants reduce their overall surface area exposed to heat and evaporation. As a result, leaves lose less moisture to the surrounding atmosphere and resist high wind damage, compared to large leaves that act as sails on a boat. If your garden trees or shrubs produced uniformly large leaves in a full sunlight position, dehydration would quickly set in as the plant succumbs to widespread dieback. In contrast, plants in south-facing positions, with constant sun exposure, thrive well with small leaves since they retain as much internal water as possible and still grow tall and strong without threat of dehydration.

Shady Conditions

Shade-loving plants still need enough sunlight to photosynthesize properly so these species tend to produce large leaves, much like a solar panel installation. By increasing the leaf's surface area, more ambient light strikes the plant for maximum energy production -- these leaves commonly occur on plants that reside in deep shade from surrounding foliage, such as among multiple dense tree canopies in a large yard. If these shaded plants produced small leaves exclusively, their photosynthesizing abilities would be greatly reduced, making the plant vulnerable to disease, pests and dieback. Growth stunting becomes widespread and you may lose the plant altogether unless you move it to a sunnier location. Because they are in shady conditions, however, moisture loss is not a major threat so leaf size can seem almost exaggerated, such as plants generating leaves several feet in diameter.

Transpiration

Plants exchange gasses with the surrounding environment through small openings called stomata. Referred to as transpiration, plants release air and moisture for basic energy production processes. Found on leaf undersides, stomata gasses must pass through a layer of air called the boundary layer. However, larger leaves have thicker boundary layers, reducing gas exchange processes. To increase transpiration, plants grow smaller leaves that have a thinner boundary layer. As a result, gasses moves faster through the air layer as this exchange cools the plant on hot days, especially for plants in full sunlight positions in your garden.

Overheating

According to the Huntington Library, narrow leaves remain cooler than wide leaves, even when they have the same length. Wind passes around narrow leaves better than wide leaves, allowing transpiration to occur more easily and avoid overheating. These narrow leaves also tend to grow in a vertical fashion so that less surface area is exposed to the hot sun. In contrast, your shaded plants under dense yard tree configurations often grow large and in a horizontal position -- these leaves want any ambient light striking them to avoid frost damage and for optimum photosynthesis.

References

• Plant & Soil Sciences eLibrary: Transpiration - Factors Affecting Rates of Transpiration
• The Huntington Library: Learning From Leaves: A Look at Leaf Size
• U.S. Department of Agriculture Forest Service: Comparing Sun Leaves To Shade Leaves