Plant FAQs (Frequently asked questions)

Q.1 Why are plants green? Click the link to see Bot's answer.

Q.2 How do copper beech trees photosynthesise?

Answer: Chlorophyll is the main photosynthetic pigment of plants, and chlorophyll is green, which gives
foliage its green colour. (
Photosynthesis is the process whereby plants use the energy in sunlight to
manufacture sugars, fats, amino acids and all the building blocks and fuels that the plant needs). When
chlorophyll absorbs light, it passes the energy on to enzymes that generate fuel and building blocks for the
plant. However, chlorophyll by itself can only absorb a narrow range of colours of light - it absorbs blue
and some red. This wastes much of the green and yellow light. However, plants contain
that can absorb some of this other light (including some of the green light) and then pass the
energy onto chlorophyll. Thus, these accessory pigments broaden the range of colours of light that a plant
can use for photosynthesis. These accessory pigments include the
carotenoids, so-called because of their
orange-red colour. Carotenoids colour the fleshy tap root of carrots orange. The copper beech tree has
purplish-red or bronze-coloured foliage because it has so many red carotenoids that they mask the green
chlorophyll. However, the mechanism of photosynthesis is ultimately the same as in green plants, using the
same types of green chlorophyll.

Q.3 How tall is the tallest tree and why don't trees grow any taller?

An average oak tree is about 30 metres (90 feet) tall, and the tallest about 40 metres. The tallest tree in
the British Isles is a Douglas Fir around 65 metres. The tallest giant sequoia reaches 94 metres and the
tallest coastal redwood, the tallest tree of all, is 112.2 metres (368 feet, as of October 1996) tall. However,
Eucalyptus regnans tree in Australia was once measured at 132.6 metres (435feet) and was estimated
to be over 150 metres at its tallest. Reports of Douglas firs over 140 metres tall also exist. Trees tend to
grow taller and thinner when growing in dense forests or woods, as each tree is competing for light (it is
bad news for most trees if their neighbours grow taller and cast them into shade, though some shrubs are
adapted to grow in low light levels). Also, trees in dense groups tend to shelter one another from the wind
and so they can afford to be taller and thinner, whereas trees growing in isolation tend to grow shorter but
thicker, so that they are more resistant to high winds. Oak trees growing on dry and exposed slopes grow
slowly and so are stunted. Dwarf birch and dwarf willow trees that grow in harsh cold climates, close to the
northernmost limit of trees, are dwarfed, indeed dwarf willows also grow almost horizontal, thus avoiding
the harsh winds which not only could throw or break a tall tree, but dry the leaves, whilst the frozen soil
severely limits water availability.

Availability of water is one limiting factor for tree height. Coast redwoods, for example, may reach 100 m in
damp soils, moistened by frequent fogs, but may be only 30 metres tall on drier soils. However, the
maximum height is very dependent on tree species. Mechanics impose limiting factors - for example, a
beech tree tends to have a cylindrical trunk and if it grew too tall would be top heavy, so beech trees are
restricted to around 30 metres as are oaks, but redwoods are designed to taper towards their top, so they
are broad at the base and taper almost to a point at the top. This tapering reduces the stress on the lower
trunk when winds bend the tree. The wood of redwoods is also light (low in density) to reduce its weight.
Thus, some trees have genetically determined designs that enable them to grow taller.

There comes a point, however, when the tallest of trees hit an ultimate limit. This limit is thought to be
around 150 metres. This limit probably exists since it gets harder and harder to draw (suck) water to the
top of the tree, without the water column breaking (cavitating) under tension and blocking the vessel.
Redwoods are also vulnerable to another hazard that affects taller structures more than shorter ones -
lightning. Lightning has been seen to devastate redwoods, as their wood tends to shatter into tiny cubes
when struck. Sometimes the topmost few metres can be entirely destroyed, sometimes a middle portion
(causing the top to plummet and split the base) and rarely the whole tree is turned into match wood!

Finally it should be noted that some trees don't need to be very tall as they avoid competition. The birch
tree is a medium-sized tree that grows quickly and dies after 80-100 years or so. It is a pioneer species, it
takes advantages of openings in woodland canopies and grows quickly, scatters its seed, and then dies as
slower growing oaks and beeches and ash trees overtake it. Birch trees can also grow in places where few
other trees grow well - on dry slopes in cold climates, such as the Caledonian Forest in Scotalnd, where
the birch grows alongside aspens and Scot's pines.

Q.4 How thick is the thickest tree trunk?

The thickest British oak is the Fredsville Oak in Kent (only 24 metres tall but 3.6 metres wide). The
Fortingall Yew in Scotland is now broken into two living fragments, but was once 5.4 metres in diameter.
The thickest species is the African baobab tree, with its enormous bottle-shaped tree that stores water.
Baobabs frequently reach 10 metres in diameter. However, the thickest individual recorded was a
pollarded European chestnut with a diameter of 57.9 metres! Pollarding involves cutting the top off of a
tree every few years or so, to harvest the wood. This damage stimulates the tree to grow thicker - since if it
is being damaged by wind this will help it resist and it also puts out more shoots, so if animals are grazing
it, some of these shoots should survive. Ancient pollards can be truly thick and fabulous to look at with
their many twisted branches and gnarled trunks. A Montezuma Cypress (Taxodium mucronatum) is
recorded with a diameter of 11.42 metres, but this still does not rival the pollarded European chestnut. The
diagrams below show the thicknesses of various tree trunks, drawn to scale - click on the thumbnails to
enlarge. (The scale shows diameter, the figures give girth (circumference)).
Q.5 How old is the oldest tree?

This is a controversial question. The oldest living trees that have had their age verified by measurements,
are the bristle-cone pines (
Pinus aristata) in California and Nevada, with measured lifespans up to 4900
years, and over 5000 years possible. However, yes trees are extremely long-lived, but their trunks hollow
with age making it impossible to age them by counting their rings! However, evidence suggests that the
oldest yew trees may be over 5000 years old. The oldest is probably the Fortingall Yew in Perthshire,
Scotland, which could be anywhere between 5000 and 10 000 years old (though 5000 is considered more
likely). Recent opinion has even lowered the age of this specimen to 2000 years - nobody is quite sure!

Q.6 Which is the largest tree?

Height and trunk girth are two ways to measure the size of the tree, however, the largest is really the
heaviest. The heaviest tree with a single trunk is the German Sherman tree which is a giant sequoia
Sequoiadendron giganteum). General Sherman (in Sequoia National Park) is 83.8 metres and 11 metres
at the base, making its weight something like 2030 tonnes!