How does a tree absorb water and how does water transport work?

Water transport in trees: essential for survival and growth

 

Trees are impressive organisms capable of transporting water from the depths of the soil to their highest leafy tops.

This process is crucial for their survival, growth, development and conservation.

Water transport in trees occurs through a specialized system called the xylem. That extends from the roots to the leaves. In this article we explore how water transport works in trees, what the water is for, and the impact of drought stress on them physiological effect.

 

 

The water transport mechanism

 

The transport of water in trees starts at the roots, where water is absorbed from the soil via osmosis.

This water moves through the root cells and eventually reaches them xylem, the vascular tissue responsible for the upward transport of water and dissolved minerals. Once in it xylem water is transported through the stem to the leaves by a process known as transpiration.

The driving force behind the transpiration flow is the evaporation of water from the leaves, a process called transpiration. When water evaporates from the small openings in the leaves, known as stomata, a negative pressure is created within them xylem, which sucks water up from the roots. This process is enhanced by the cohesion and adhesion of water molecules, forming a continuous water column throughout the tree.

 

 

The functions of water in trees

 

Water fulfills several essential functions in trees. It is an important solvent that transports minerals and nutrients from the soil to different parts of the tree. These nutrients are crucial for the photosynthesis, the process by which trees convert carbon dioxide from the air into oxygen and glucose, the energy source for the tree.

In addition, water plays an important role in regulating the temperature of the tree. Transpiration allows a tree to lose excess heat, which is essential for maintaining an optimal temperature for enzymatic activities and metabolic processes.

 

 

Drought stress and the impact on trees

 

Drought stress occurs when the availability of water in the soil is insufficient to meet the tree's needs. This can lead to a series of physiological and structural changes that affect the health and growth of the tree. An immediate response to drought stress is the closure of stomata to reduce water loss through transpiration. While this saves water, it also limits the absorption of carbon dioxide, which... photosynthesis and growth decreases.

Prolonged drought stress can lead to reduced water conduction in the xylem, degradation of photosynthetic pigments, and ultimately to leaf loss and reduced growth rates. In extreme cases, drought stress can lead to the death of the tree.

 

 

How does a tree absorb water and what happens to the water?

 

Trees absorb water from the soil through their roots. This process begins when water, containing minerals and nutrients dissolved from the soil, is absorbed by the root hairs. Root hairs are thin projections of the root's epidermal cells, which increase the surface area available for water absorption.

 

 

Water absorption and transport

 

Osmosis

Water moves into root cells by osmosis, a process in which water molecules move from an area of ​​low concentration of solutes to an area of ​​high concentration, through a semipermeable membrane.

 

Root pressure

In some cases, a process called root pressure contributes to pushing water up through the roots and into the trunk, although this mechanism plays a relatively minor role in most large trees.

 

Capillary action and adhesion

Water molecules attach to the thin tubes (wood vessels or tracheids) inside the roots, trunk and branches (the xylem) and pull each other up by capillary action.

 

Cohesion and perspiration

Water molecules are cohesive, meaning they stick together, forming a continuous column of water molecules throughout the plant, from the roots to the leaves. When water evaporates from the leaves (transpiration), it creates a negative pressure that helps draw more water up from the roots.

 

 

Use of water in and through the tree

 

Once the water is in the tree, it is used for various life processes

 

Photosynthesis

In the leaves, water is used in the process of photosynthesis, where carbon dioxide from the air is converted into glucose and oxygen. Water is split (photolysis) to provide electrons for the photosynthetic reaction chain, and oxygen is released as a byproduct.

 

Transport of nutrients

Water transports dissolved minerals and nutrients from the roots to other parts of the tree.

 

Cell structures and functions

Water is essential for maintaining cell structures and facilitating various biochemical and physiological functions within the cell.

 

Temperature regulation

Perspiration and evapotranspiration also help regulate the tree's temperature through evaporative cooling.

 

Water loss

Most of the water a tree absorbs is lost through transpiration through the leaves. This is an essential process that contributes to the movement of water through the tree and helps regulate temperature and gas exchange. Only a small part of the absorbed water is used for growth and metabolic processes.

In summary, water uptake and use in trees is a complex process that is essential for their survival and growth, and plays a crucial role in the Earth's overall water cycle.

 

 

Compromises in the structure of the wood

 

The need for efficient water transport involves certain structural compromises. For example, wide wood vessels offer little resistance to water transport. But these xylem vessels are more sensitive to cavitation, in other words the drawing of an air bubble into the water column that is present in the xylem vessels.

 

 

Xylem structure

 

The need to transport large amounts of water has led to the evolution of wide xylem vessels, which increases the efficiency of water movement.

Fewer pipe losses can be said. Compare it with a garden hose with a larger diameter, which can also accommodate more water.

But wider xylem vessels also make the tree more vulnerable to cavitation. This is the formation of air bubbles in the xylem vessels, especially in dry and warm conditions, which interrupts the water column and prevents water transport.

Sometimes it may be that at night when there is no or very little evaporation through the leaves, but there is still root pressure, the air bubbles are dissolved again in the water column. Then the wood barrel has not lost definition.

Note that in a dry and hot summer, also due to the significantly lowered groundwater table, more wood vessels are permanently lost due to irreversible cavitation. And because there is less thickness growth during such a summer, more wood vessels are lost than are created. Since wood barrels only last about ten years, in terms of functional water transport, there is a net loss of wood barrels.

It goes without saying that if there are several extremely hot and dry summers in succession, trees can die as a result. Because water transport has broken down.

 

 

Wood density

 

Trees with fast growth and a large xylem vessel often have less dense wood, which in turn provides less structural support.

 

 

Differences between ring-pore and diffuse-pore wood

 

The structure of xylem vessels in trees varies between ring-pore and diffuse-pore wood, which influences water transport.

 

Ring-pore wood

In ring-pore trees (such as oak and ash), the xylem vessels are large at the beginning of the growing season and become smaller towards the end of the season. This ensures efficient water transport during rapid spring growth, but makes the trees more sensitive to drought and cavitation.

 

Diffuse-spored wood

In diffuse-spored trees (such as beech and maple), the xylem vessels are more evenly distributed throughout the annual ring, which provides more consistent water distribution throughout the year, but may be less efficient during peak periods of water need.

 

Both types of wood have developed adaptations to meet their specific water transport needs. And to cope with the challenges of their environment, resulting in differences in wood structure, density, and vulnerability to environmental stress.

 

 

Difference between hardwood and coniferous wood

 

Hardwoods are usually constructed from barrels, a series of barrel members (barrel elements) arranged end-to-end, with the ends partially or completely broken off to form a long, continuous tube. Where coniferous wood usually consists of tracheids, elongated cells with tapered ends, which individually transport water through the pits (small pores) in their cell walls.

Wood vessels have the disadvantage that they are more sensitive and vulnerable to cavitation compared to tracheids. Wood consisting of trachides and faced with irreversible cavitation has more deflection options within its structure to still transport water. Because the elongated tapered cells are in contact with several adjacent cells and a way can still be found to transport the water. Unlike wood barrels, if they are lost, there are no escape options to adjacent wood barrels or “tubes” that are almost hermetically sealed from each other.

Wood vessels offer faster transport of water and are therefore advantageous in environments where water is plentiful, while tracheids provide a more robust solution in more challenging environments, with the added benefit of increased structural strength.

 

 

Conclusion

 

Water is essential for the survival, growth, development and maintenance of trees. The water transport via the xylem and the resulting transpiration flow and evapotranspiration are crucial processes that not only transport water and nutrients through the tree, but also aid in their temperature regulation.

Drought stress poses a serious threat to these processes, with potentially far-reaching consequences for tree health and functionality. Understanding these dynamics is essential for managing forest areas and conserving our valuable tree populations.