Transpiration

Figure 1: Beads of water on a leaf, however not transpiration as that is an invisible process.

Transpiration is the process in which plant roots absorb water and then release the water in the form of vapour through the leaves. Transpiration is an important factor in the water cycle as it is one of the major sources of water into the atmosphere. Providing 10% of the total water in the atmosphere, this process is nearly identical to perspiration or sweating in animals. When the roots take in water from the topsoil layer, the water is then converted into vapour and is essentially evaporating off the surface of the leaves.

Plant cells have pores called ‘stomata’ which play part in how much water gets released from the leaves. The stomata open for two reasons; to absorb carbon dioxide (CO₂) from the atmosphere and to take in sunlight, this encourages photosynthesis. Stomata close in events such as drought to prevent the loss of water and eventual death of the plant.

Variables Affecting Transpiration Rates

The rate at which transpiration occurs is varied based on temperature, air movement such as wind, how much moisture is in the soil and surrounding air, the type of plant and land use.

• Temperature plays a major role in the rate of transpiration. As the temperature increases, transpiration will increase due to a higher concentration in sunlight and warm air. However, if temperatures remain high for long periods of time eventually leading to drought, transpiration may go down to conserve water in the plant. Colder temperatures usually lead to very little or no transpiration occur whatsoever.
• Air movement such as wind can aid with transpiration. When the air is still or there is no wind, humidity may buildup around the plant from transpiration eventually decreasing the amount of water being released. When there is wind present, this causes the air to be replaced constantly allowing the plant to transpire.
• Increased humidity has a reduces transpiration. The lower the humidity the easier it is for the plant to release water.
• The type of plant is also a factor in how much a plant will transpire. For example plants grow in arid hot areas such as the desert will transpire less as they aim to conserve water.

Figure 2: In areas where the water table is closer to the surface such as near large bodies of water or on angled terrain, plant roots can access the area beneath the water table, this creates an easier environment for plants to acquire water to aid with transpiration.

• Groundwater availability is the biggest factor for how much a plant transpires. During growing season, when higher volumes of water are entering the top layer of soil, it is much easier for a plant to absorb more water. If a plant is situated near a large body of water or for example on a slope where it may be closer to the water table, it will also be easier for the plant to acquire water. During the dormant or dry season, less water is readily available to plants thus drying out the plants. Figure 2 shows how water table fluctuations effect transpiration.
• The conversion of land by humans can have a detrimental effect on transpiration. When natural land is converted to agricultural land, the vegetation cover is diminished. This decreases transpiration resulting in erosion and an increase in rain runoff thus creating silt buildup in mass bodies of water.