Tree Physiology
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Tree Physiology Definition, Processes & Chart
Learn about tree physiology, or the systems and processes that occur within a tree to help it function and grow, such as nutrient uptake and photosynthesis.
What is Tree Physiology?
Tree physiology is the study of various systems and processes that occur within trees, such as nutrient uptake, water transport and photosynthesis. These processes help trees to grow and survive in their environments. Tree anatomy is the study of structures that make up a tree, including leaves, roots, and branches, in which physiological processes take place.
Key Anatomical Structures in Trees
Generally, trees are composed of structures including:
Leaves
Roots
Trunk and branches
These key anatomical features and the physiological processes that occur within them can generally be identified across many tree species, although the characteristics of each anatomical feature may vary substantially depending on the species and their needs. For example, leaf shapes can be wide or thin depending on the amount of light the tree needs and receives. Root systems can also vary in depth or size depending on the tree's needs for accessing nutrients and water in the nearby soil. Tree trunks can also be a variety of widths and heights, and some species of trees can grow up to hundreds of feet tall.
Physiological Processes in Tree Roots
At the bottom of trees are the roots, which are embedded in the soil. Roots help a tree to stay upright and stabilized. They also allow the tree to gather the resources it needs and even interact with organisms in the soil.
Water and Nutrient Uptake
Trees need water and nutrients to support their physiological processes and help them survive. These include macronutrients that are needed in larger amounts, such as potassium and nitrogen, and micronutrients that are needed in smaller amounts, such as zinc. Most of these necessary nutrients are available in the soil, and trees can absorb them using their roots. Tree roots grow and extend outward over time in search of nutrients in the surrounding soil, creating large, complex root systems. Many tree roots will also have root hairs that aid in absorption.
In order to gather certain nutrients more efficiently, trees may develop relationships with other organisms in the soil that can help provide them with the nutrients they need.
Rhizosphere Interactions
The area surrounding a plant's roots is known as the rhizosphere. Within the rhizosphere, organisms such as bacteria and fungi can live near, and even interact with, tree roots.
Mycorrhizae are a beneficial relationship that can form between tree roots and fungi. In this relationship, the fungi can help the roots access nutrients in the soil, and in exchange, the roots provide the fungi with sugars produced by the plant, which contain carbon that the fungi need. Many species of trees have mycorrhizae.
Some interactions between roots and organisms in the rhizosphere can also benefit the tree by improving its defenses against pathogens or its ability to tolerate stresses such as drought.
Physiological Processes in Tree Trunks and Branches
Over time, trees grow upward and develop a main trunk from which branches extend. The trunk and branches contain transport systems that can send resources from the parts of the tree where they are made or acquired, such as the leaves or the roots, to where they are needed.
Water Transport
To deliver water absorbed from the soil by the roots into the rest of the tree, trees use a system of small tubes known as the xylem. To move the water up the tree trunk and branches through the xylem, trees use a passive system that relies on pressure. When water exits the tree through the leaves, it creates negative pressure at the top of the tree. This negative pressure causes water in the xylem to flow upward from the roots toward the top of the tree. Water molecules stick to one another within the xylem, allowing water lower in the tree to follow and take the place of water above it that has moved upward.
Sugar Transport
In addition to bringing water up from the roots, trees also need to deliver the sugars made in the leaves to various parts of the plant where they can be used to make energy. They do so using another system of small tubes known as phloem.
The water, nutrients, and sugars transported in the xylem and phloem are carried in tree sap, the fluids that flow through the tube systems.
Tree Growth
Rings in a tree trunk are a record of the tree's life and its environmental conditions.
Close up color photograph showing concentric rings in a cut tree trunk
Tree trunks form rings as they grow wider over time. A new ring develops over the course of each growing season (spring through fall) as a result of cell division in the cambium of the tree trunk. Rings can provide information about the tree, such as its age, its growth rate, and even its exposure to historical climate events such as droughts.
Shoots, or new growth coming from the trunk or existing branches of the tree, grow from their tips at an area called the apical meristem, where cells divide very quickly to form new tissue. Notably, roots also have apical meristems from which they grow longer over time.
Over time, shoots can develop into branches and eventually form the crown, or top of the tree.
Physiological Processes in Leaves
The leaves of the tree are where the tree collects sunlight and other resources, and uses them to make sugars that the tree can store and eventually use for energy. Many physiological processes happen in the leaves.
Photosynthesis
Photosynthesis is the process by which plants use light, water, and carbon dioxide to produce glucose (a type of sugar). Chloroplasts are the specific part of a plant cell in which photosynthesis takes place. In addition to producing glucose, chloroplasts also produce oxygen as a product of photosynthesis.
Photosynthesis requires light and thus generally only occurs during the daytime.
The glucose made during photosynthesis is stored as starch, and can later be converted into energy for the plant to use through another physiological process known as respiration.
Gas Exchange and Transpiration
The carbon dioxide used to make glucose in photosynthesis is initially found in the atmosphere, and enters the leaves of a tree through stomata, or little pores in the surface of the leaf. Similarly, the oxygen released as a product of photosynthesis exits the leaf through stomata. The physiological process by which these gases move between the atmosphere and the leaf through the stomata is known as gas exchange.
In addition to gas exchange, water vapor is also able to exit the leaves through the stomata in a process called transpiration. In order to regulate the rate at which these processes occur, trees are able to open and close their stomata. Doing so helps trees maintain their water content; for example, in times of high heat and lots of sun, plants may close their stomata to slow transpiration and prevent excessive water loss from their leaves. In addition to the stomata, trees also have a cuticle, or outer layer, on their leaves that helps control water loss.
Other Important Physiological Processes in Trees
Trees use the glucose they create through photosynthesis to produce energy, which they use to fuel other physiological processes in the tree. This process, called respiration, occurs in cells in the roots, trunk, and leaves of the tree, inside a part of the plant cells called the mitochondria. In respiration, trees use glucose and oxygen to produce energy. This process also produces carbon dioxide and water that can be released from the stomata.
Lesson Summary
Trees employ a variety of physiological processes in order to grow and function within their environments. In the roots, trees gather nutrients and water from the soil and can establish relationships with other organisms in the rhizosphere, or area in the soil surrounding the roots, to improve nutrient acquisition. Trees use tubes called xylem and phloem to transport water and sugar. Trees grow wider in their trunks, which creates rings, and also grow longer at the tips of their branches and roots. In a process called photosynthesis, trees are able to create glucose. This process takes place in the leaves, in a tiny part of a plant cell called the chloroplast.
The gases needed for, and created as a result of, photosynthesis are able to enter and exit the leaves in a process called gas exchange, through tiny pores in the leaf called stomata. The stomata also regulate transpiration, or the loss of water through the leaves. The glucose created in the leaves during photosynthesis is then used to make energy in a process called respiration, which occurs inside the mitochondria, a specific part of a plant cell. The tree uses the energy produced from glucose to fuel many of its physiological processes.