Understanding the Process of Transpiration in Trees

Transpiration is a process similar to evaporation, where water turns into water vapor and is absorbed into the atmosphere. The difference is that transpiration takes place through the leaves of plants. It is part of the water cycle. Precipitation falls onto the land, and then it is absorbed by a plant’s root system. As it travels, the nutrients needed by the plant are removed. Then about 60 percent of the beginning amount of rainfall is transpired back into the atmosphere through small pores on the undersides of leaves. Here the water becomes a vapor to make transpiration possible.

According to the United States Geological Studies (U.S.G.S.) the rate of transpiration varies depending on various factors, such as weather, temperature, soil type and saturation, wind, land slope and how people use and divert the water.

The pores, or openings, on the leaves are called stomata. The stomata are made up of stomas which have guard cells around them. These guard cells regulate when and how the pores open and close. As the stomata release water vapor into the air, they also diffuse carbon dioxide from the atmosphere to be used in creating food through photosynthesis.

As a leaf transpires, during growing seasons, it may give off much more than its weight in water vapor. The United States Geological Studies gives an example of an acre of corn which can send 3000 to 4000 gallons of water per day into the air.

As the temperature surrounding a plant rises, transpiration rate also rises. This is because another act of transpiration is cooling the plant. So as the temperature rises, the stomata opens to allow water to flow through the plant more quickly to help with cooling.


Relative humidity also effects transpiration. If the relative humidity goes up, the air becomes more saturated, so it is less likely to be able to absorb any water vapor from the plant. However, if the relative humidity level goes down, transpiration rate will rise.


Wind is related to relative humidity as far as transpiration rate. The water vapor is able to be absorbed into the air as wind speed increases. The wind helps to bring the relative humidity down, so vapor is accepted into the atmosphere more easily.

Soil moisture

If the area is going through a dry spell, plants may lose leaves due to pre-mature aging. Without these leaves, transpiration rates lower.

Plant type

Each plant is unique in transpiring. Many plants have leaves which allow transpiration to occur easily. Cacti, however, need to conserve the water that they obtain, so they do not have standard leaves that allow transpiration to occur easily,

Other factors that may increase or decrease transpiration are the number of leaves a plant has, as well as the number of stomata, a waxy surface, or cuticle could decrease transpiration rates. Plus the more light that a plant receives can also raise or lower the rate a plant transpires.

Transpiration is one of several processes within a plant that benefits not only the plant itself but he world around it.