How Does Respiration in Trees Work?

Respiration is a process for all living things, including trees, to release energy from the food they produce. For trees, this means converting sugars created through photosynthesis into usable energy to power their metabolic activities. This continuous process sustains a tree’s life, enabling growth and the absorption of nutrients from the soil.

Understanding Tree Respiration

The main purpose of respiration is to convert the chemical energy stored in sugars into a form the tree can use, known as ATP (adenosine triphosphate). This usable energy fuels growth, facilitates the transport of nutrients, and maintains and repairs living tissues. The process occurs within the mitochondria, which are present in all living tree cells from the highest leaf to the deepest root tip.

This conversion is a chemical reaction where glucose (a sugar) combines with oxygen, which breaks down the sugar molecule and releases its stored energy. The byproducts of this process are carbon dioxide and water. Unlike some other tree functions, respiration happens continuously, day and night, as a tree always requires energy to stay alive.

How Different Parts of a Tree Respire

Different parts of a tree are adapted to respire using unique structures. In leaves, tiny pores on their undersides called stomata are the primary sites for gas exchange. While these pores take in carbon dioxide for photosynthesis, they also allow oxygen to enter for respiration and carbon dioxide to exit, particularly at night when photosynthesis ceases.

Stems and branches also respire through small pores in the bark known as lenticels. These structures permit oxygen to reach the living cells within the woody parts of the tree, such as the cambium. This gas exchange is necessary for the metabolic health of the trunk and limbs.

Root respiration powers the uptake of water and mineral nutrients from the ground. Roots absorb oxygen from small air pockets within the soil, so the availability of this oxygen depends on soil conditions. Compacted or waterlogged soils have very little air space, which can suffocate roots, inhibit respiration, and jeopardize the health of the entire tree.

Distinguishing Respiration from Photosynthesis

While respiration and photosynthesis are interconnected, they are opposite processes. Respiration’s primary function is to release energy by breaking down sugars, whereas photosynthesis uses light energy to create those sugars. The two processes are complementary, with the outputs of one serving as the inputs for the other.

The inputs for respiration are sugars and oxygen. Conversely, photosynthesis requires carbon dioxide, water, and light energy to produce sugars and oxygen. This distinction also extends to their timing and location. Respiration occurs in all living cells throughout the tree at all times, while photosynthesis is restricted to cells containing chlorophyll and can only happen when there is light.

Key Factors Affecting Respiration Rates in Trees

The rate at which a tree respires is not constant and is influenced by several environmental and internal factors:

• Temperature: Respiration rates increase as temperatures rise up to an optimal point. If it becomes too hot, the process can become inefficient and place stress on the tree.
• Oxygen Availability: Since oxygen is a required input, any limitation will slow the process. This is a common issue in flooded or compacted soils around the roots.
• Sugar Availability: The amount of sugars produced during photosynthesis directly affects the rate of respiration. When sugar production is high, the tree has more fuel available.
• Tree Condition: Younger, actively growing tissues like branch tips have higher respiration rates to support cell division. During drought, a tree’s metabolic activity, including respiration, may decrease to conserve resources.