As with all living things, trees have evolved over time to fit with their environment, adapting their physical shape and internal functioning to maximise their success within their surroundings. Their basic body plan, leaf shape, type of bark, reproduction and all the other components of their life strategy have evolved over millions of years to help trees survive in their particular environment.
The overall shape of a tree is governed by factors such as the need to display the leaves without self-shading, aiding pollination and coping with specific environmental conditions. For example, to survive in colder climates and at high altitudes, conifers are cone-shaped (excurrent) with sloping branches to help shed snow easily. Their conical shape also helps them to absorb light from the sun when it is low on the horizon. Conversely, broadleaved trees have wide domed crowns enabling them to gather light from cloudy temperate climates where the light is soft and diffuse.
Leaf Shape and Size
Leaf shape and size can vary considerably across different tree species, but all leaves are there to photosynthesise and produce food for the tree. Leaves need to be able to absorb the most amount of light and carbon dioxide while retaining moisture within the tree. Variation in leaf shape is influenced by the need to perform these functions and also by factors such as climate, temperature, day length, water availability, nutrition and predators.
Large leaves are good for catching light, and because of their large surface area, they also hold a layer of warmer air close to them which provides additional insulation. However, this extra warmth can lead to water loss causing the leaves to burn in direct sunlight. Therefore, large leaves are characteristic of shaded environments or areas where there is a low risk of water loss, such as in humid tropical forests. The large leaves of the banana tree are a good example.
Small leaves with a small surface area tend to stay cool from air movement alone and are less at risk of losing water. Small leaves are therefore more common in areas where water is scarce, such as in temperate environments where rainfall is seasonal. Lobes and teeth on the edges of leaves are a further adaptation to prevent water loss because they help to create more movement to keep the leaf cool.
Very small leaves, such as the needle-like leaves of conifers, evolve as an adaptation to harsh conditions. A needle is simply a compact version of a simple leaf, but its small size ensures minimum water loss, and its thin shape enables it to shed snow easily.
Trees cannot move when conditions become difficult or when they are under attack, so they have evolved ways to protect themselves. The outer bark of a tree is a tough, weatherproof layer providing protection from the elements as well as from disease, insect invasion or damage from mammals. The bark stops air from entering the living cells of the tree and helps retain moisture.
The bark and leaves of trees often contain chemical defences, making them less palatable to feeding insects, and some may be toxic. Many of our British trees such as oak, birch, willow and alder have bark and leaves containing tannins which make them hard to digest for many insects and animals. Trees also produce substances such as resins and gums to help isolate and block infections or quickly heal wounds, and some trees grow thorns, prickles or spines in an effort to protect their leaves and seeds from being eaten.
|Years ago (millions)||Period||Evolution|
|443 - 417||Silurian||The earth’s climate stabilises and the first vascular plants evolve. Coral reefs and fish evolve.|
|417 - 354||Denovian||The first seed-producing plants and trees evolve and early forests are formed.|
|354 - 290||Carboniferous||The land is dominated with lush forests. The first four-limbed land animals appear (tetrapods).|
|290 – 248||Permian||Trees we recognise today begin to evolve such as the monkey puzzles, ginkgo and cycads. Reptiles evolve.|
|248 – 206||Triassic||At this stage of evolution all the continents are joined together in one land mass called Pangaea. Archosaurs evolve; the immediate ancestors of the dinosaurs.|
|206 – 144||Jurassic||Dinosaurs dominate the land and the first pine trees appear including the Wollami pine which has survived to the present day.|
|144 – 65||Cretaceous||Flowering plants evolve and the first hardwood trees appear including oaks, maples, willows, laurels and magnolias.|
|65 – 1.8||Tertiary||The climate warms and hardwood trees begin to dominate, displacing conifers. Grasslands evolve along with the first horses and elephants.|
|1.8 - Present Day||Quarternary||Modern humans evolve and many tree species evolve that flourish today including our native beech, ash, hazel birch and alder.|