Exploring the basics of how trees grow and what they need.
The importance of trees for ecosystems and civilization cannot be overstated. The life of trees is immensely complex and could easily fill many books, so in this article we're going to just touch on the basics of some of Earth's most important life forms.
What is a Tree?
A tree is a perennial plant characterized by its elongated stem, or trunk, which supports branches and leaves in most species. Unlike other types of plants, trees typically grow to a considerable height and can live for many years, even centuries or millennia in some cases.
Anatomy of a Tree
Trees are made up of 3 major parts: roots, trunks, and branches and leaves. The roots are often below ground, anchoring the tree to the ground and absorbing water and minerals from the soil.
The trunk serves as the main support and transports nutrients and water between the roots and leaves. The branches extend from the trunk and hold the leaves, which are the primary sites of photosynthesis, converting sunlight into the energy needed for growth and survival.
Types of Trees
Trees can be classified in many different ways. The taxonomy of trees is complicated and is beyond the scope of this article. Some common categories of trees include evergreen vs deciduous, gymnosperms vs angiosperms, conifers vs broadleaf, etc. Since trees have evolved to fill a plethora of ecosystem niches, many classifications overlap and can be a bit fuzzy.
How Seeds Germinate
Germination is the process by which a plant emerges from a seed. Seeds are little packets that contain tiny embryonic plants and a nutrient-rich food supply called endosperm to fuel the baby plant's growth during germination. With their protective coats, seeds can survive extreme conditions, springing to life once the right conditions for germination arise.
Seeds wait in dormancy until the right mix of water, oxygen, and temperature triggers germination. The water rehydrates the cells in the seed and triggers metabolic processes. During this phase, water and oxygen enter through the seed coat, causing the embryo to expand and break the seed coat. The root (radicle) appears first and emerges downward, oriented by gravity. Next, the shoot (plumule) with leaves and stem emerge upward towards the surface of the soil.
Sunlight warms the soil, aiding germination, but in most cases, it is not required for seed germination. As the embryo grows, it consumes the endosperm to fuel growth until a small leaf pops out above ground. At that point, the baby plant can transition to producing its own food through photosynthesis using sunlight.
How Trees Grow
After the seed has germinated and turned into a seedling, its growth comes from the process of photosynthesis. Photosynthesis is the process by which plants use sunlight to convert carbon dioxide and water into carbohydrates. The mass of a tree comes from the carbon in the air, not from the ground. If it came from the ground, trees would all be in holes.
The leaves of the trees collect solar energy to drive the photosynthetic process, and the roots absorb water and nutrients from the ground.
Most of a tree trunk is dead tissue and serves only to support the weight of the tree crown. The outside layers of the tree trunk are the only living portion. A layer called the cambium sits between the bark and the wood of the trunk and serves to produce new bark and new wood in response to hormones (auxins) that pass down through the phloem with food from the leaves.
The band of tissue outside of the cambium is the phloem, which transports the sugars created from photosynthesis from the leaves to the roots. Dead phloem tissue becomes the bark of a tree.
The band of tissue just inside the cambium is the xylem, which transports water from the roots to the crown. Dead xylem tissue forms the heartwood of the tree.
Trees only grow from the tips of the branches (apical meristems), the tips of the roots (root apical meristems), and the cambium.
What Trees Need to Survive
Trees need carbon dioxide, water, light, and nutrients to survive and grow. While the first three are obvious, the nutrients trees need are less so. Trees require a variety of minerals. Some are needed in larger quantities and are called macronutrients, and others in smaller quantities called micronutrients. Each serves a unique role in tree growth, development, and overall health. Organic and synthetic fertilizers often state their N-P-K percentages, but there are many other nutrients vital for tree health.
Macronutrients
Nitrogen (N): Vital for leaf growth and green color, as it's a major component of chlorophyll, the compound plants use in photosynthesis.
Phosphorus (P): Supports the development of roots, flowers, and seeds. It plays a key role in energy transfer within the plant, helping to convert sunlight into chemical energy.
Potassium (K): Crucial for overall plant health, it aids in photosynthesis, fruit quality, and disease resistance. It also regulates stomatal opening, affecting water use.
Calcium (Ca): Part of the cell wall structure, it contributes to plant structure and helps in nutrient transport and signal transduction within the plant cells.
Magnesium (Mg): A central component of chlorophyll, necessary for photosynthesis. It also plays a role in enzyme activation and plant metabolism.
Sulfur (S): Important for the formation of proteins, it is involved in the synthesis of oils and contributes to the plant’s disease resistance.
Micronutrients
Iron (Fe): Essential for the synthesis of chlorophyll and acts as an oxygen carrier, supporting cellular respiration and photosynthesis.
Boron (B): Affects cell wall formation and is involved in the metabolism of carbohydrates and proteins, as well as pollen growth and seed production.
Chlorine (Cl): Involved in osmosis and ionic balance, it also plays a role in photosynthesis.
Manganese (Mn): Participates in chlorophyll production, metabolism, and the activation of certain enzymes.
Zinc (Zn): Important for the synthesis of chlorophyll, carbohydrates, and starches. It also aids in stem growth and leaf formation.
Copper (Cu): Integral to photosynthesis, enzyme activation, and metabolism. It also plays a role in lignin synthesis, strengthening the tree's structural components.
Molybdenum (Mo): Necessary for nitrogen fixation and the assimilation of nitrates into amino acids within the plant.
Nickel (Ni): Required in very small amounts, it's important for iron absorption, nitrogen metabolism, and the activation of certain enzymes.
These nutrients are critical for the tree’s ability to perform photosynthesis, grow, reproduce, and resist diseases and pests. A deficiency or imbalance in any of these nutrients can lead to poor health and increased susceptibility to diseases.
Another thing trees need to survive is oxygen at their roots. The presence of oxygen in the soil is crucial for the process of respiration in tree roots. During respiration, root cells consume oxygen to break down glucose that’s been transported from the leaves. Glucose is transformed into cellular energy (called adenosine triphosphate or ATP) that’s used to drive metabolic processes, mainly water and nutrient uptake. Without oxygen, respiration does not take place. Oxygen is the final electron acceptor in aerobic respiration which is essential for transforming glucose in ATP.
Poorly aerated or waterlogged soils can suffocate roots by limiting their oxygen supply. This is why good soil drainage and proper aeration are important for tree health, allowing gases to exchange between the soil and the atmosphere. In response to low-oxygen environments, some trees can develop adaptations, such as specialized root structures, to improve oxygen intake.
Trees Are Far More Complex
We've covered the very basic anatomy and function of trees. However, trees have a myriad of complexities and abilities that have only recently been discovered and many more that have yet to be. A future article will cover some of these intricacies, from how they communicate and share resources to how they form symbiotic relationships with other organisms and fight pests.
Questions for you:
What's your favorite tree?
What did you learn about trees in this article?
What else do you want to know about trees?