Quercus (Oak Trees)

Pov-ray model of oak tree

Although rather small for a mature oak, this tree had the appearance of being very old. Indeed, on certain soils or in unfavourable climes, oaks will grow slowly and develop forms considerably more stunted than this.

Click to explore a virtual oak wood in
Spring.

Right: a close-up view without the mist
(click image to enlarge).
Pov-Ray model of an oak tree with fog
Old oak pollard

Above: A drawing of an old oak pollard in Staverton Park, Suffolk, England - taken from a photo by Gareth Lovett Jones appearing in the beautiful book: The Wildwood, In search of Britain's ancient forests, Gareth Lovett Jones and Richard Mabey (Aurum press). To the Alchemists Nature was represented by a green dragon, breathing the fire within itself. Can you see how the tree above may be likened to a dragon?

Quercus robur (the Pedunculate or English Oak) and Quercus petraea (Sessile or Durmast Oak) are the two
species of oak native to the British isles. We shall focus on these two examples here.

Wood

pores visible with a hand-lens in a cross-section) with multiseriate rays (rays more than one cell wide) 0.3 to 0.55
mm wide and 2.5 cm deep and also uniseriate rays (one cell wide). The sapwood is white, the heartwood golden
brown and very strong.

Leaves

Q. robur: obovate (egg-shaped/oval with narrower end at the base and flat) widest well above the middle and with
deep, irregular lobes (3-6 lobes on each side) with some veins running to the sinuses (gaps) between the lobes.
oak leaf
Anatomy and physiology of leaves.

The woodland
canopy.
The petiole (leaf stalk) is very short (2 to 7 mm long) and auricles are
present (small flaps of leaf blade either side of the petiole which are
not joined to it) and the leaf-base is narrow and cordate
(heart-shaped). The leaf is glabrous (hairless) except for a few
inconspicuous simple (unbranched) hairs on the lower lamina (leaf
blade) and midrib.

Left: which species of oak did this leaf come from?

The lobing is not particularly deep, though the auricles at the leaf
base are quite prominent and some veins run to the spaces between
the lobes, so this is likely a leaf from
Q. robur.

The differences are not always clear-cut, and many oaks are hybrids.
also, leaf morphology varies according to the amount of sunlight the
leaf is exposed to. In
Q. robur, a dry atmosphere tends to produce
paler, duller and more deeply lobed leaves with more prominent veins
beneath.
Q. petraea: ovate leaves (oval and flat) with the widest point more-or-less in the middle, but shade leaves tending to be
more obovate. The lobes are shallow and regular and number 5-8 per side and there are no veins to the sinuses. The
petiole is longer (13-25 mm) and weakly auricled and often tapering gradually to attach to the petiole. Long, clustered
hairs are clearly visible on the lower midrib and in the axils of veins. The leaf-blade (lamina) also has tiny stellate
(star-shaped) hairs and the lower (abaxial) surface is often downy. The leaves tend to be larger than those of
Q. robur,
darker green and glossy on their upper surface.

Flowers

The flowers appear around mid-Spring (around may time) after the breaking of bud dormancy. Separate male and
female catkins occur on the same tree. The male catkins are pale green and the less conspicuous green
wind-pollinated female flowers occur in smaller groups of 2 or 3, and have longer stalks in
Q. robur, the stalks of the
female flowers of
Q. petraea sometimes being absent.
Acorn diagram
Acorn diagram - section
The acorns of Q. robur are usually borne on longer hairless stalks (pedubcles) 2 to 9 cm in length and the acorns
may be single or in clusters. The acorns of
Q. petraea either lack stalks or are borne on shorter stalks 3-4 cm long
which bear clustered hairs. Acorn fall occurs in early autumn (Sept / Oct). The mass of individual acorns varies up to
about 8.8 g, with the minimum viable mass about 0.5 g. Typically about 20-30% of acorns are insect-damaged,
especially by acorn weevils. The smallest acorns are abortive and contain no embryo.

Bark

The grey-brown bark of Q. robur is thick, firm and deeply fissured to form elongated blocks/scales. The bark of Q.
petraea
is thinner and smoother with shallower fissures and in shorter blocks.

Growth Form (Habit)

The main trunk of Q. robur is short as it tends to end in large branches such that the trunk disappears into the crown.
Branching is irregular and the leaves and twigs are clustered, resulting in quite an open crown. In
Q. petraea the Q.
robur
regular and the  is short as it tends to end in large branches such that the trunk disappears into the crown.
foliage evenly spread to produce a dense crown. Mature oaks typically reach 30-40 metres in height.   ('Wiseman's
('Wiseman's Wood'?) in Dartmoor is an ancient Wood'?) in Dartmoor is an ancient
Q. roburQ. robur wood growing on
high, rocky ground  wood growing on high, rocky ground unsuitable as pasture. Under these harsh conditions the
oaks are stunted and extremely slow growing (and appear to still be getting gradually taller) and the tallest trees
here, despite being many centuries old, are only 5 metres tall! Their leaf-buds open much later in the year than
lowland oaks and their branches grow very long sideways from the tree endlessly angled, twisted, raked, interlocked,
and reach quite as much downward as upward'  and the trees appear writhing and convulsed, reminiscent of bonsai
trees (John Fowles). The trunk of
Q. robur is often buttressed.

Oak trees live as long as 1000 years or so, with pollards tending to live longer. A rough rule of thumb is that an oak
takes 300 years to fully mature, endures optimum health for a further 300 years and then declines over 300 years.
However, repeated droughts can shorten the life span considerably. Old (and drought-damaged) trees have
so-called 'stag's-head' crowns, with a mixture of living and dead branches, with the dead branches, stripped of bark,
looking like the antlers of a deer stag. Oak trees first produce fruit at about 30 to 50 years of age.

Root Systems

In the germling, the tap root lengthens by 3 to 7 mm per day and is 20 to 30 cm long by the end of the first year.
Only later on do radial or lateral roots (growing sideways) take over and by 50 years of age the
lateral roots form
the main root system and they put out deeper vertical
sinker roots to collect water from the water table in times of
drought.  The roots are 10 to 50 cm deep but spread up to 18 metres from the trunk by the age of 160 years.
Mycorrhiza are present with the fungus mycelium clothing the outside of the root (ectotrophic) and forming a Hartig
net.

Habitat

Q. robur is primarily a lowland oak, preferring more basic and nutrient-rich soils such as wet and heavy alluvial clays.
In contrast,
Q. petraea is more of a highland oak, preferring well-drained and acidic soils, such as sands, gravels,
granites, shales, sandstones and schists.
Pov-ray model of an oak tree
Pov-Ray oak model close-up without fog
Acorns

The acorn is the fruit of the acorn and is a nut. A nut is a type of achene with a hard and woody pericarp or fruit wall
and occurs in many other tree species, including chestnuts, beech and hazel. An achene is a type of dry fruit which is
indehiscent (does not rupture when dry like a pea pod) formed from a single carpel enclosing a single seed. The acorn
is derived from a trilocular ovary (an ovary with three compartments) with two ovules in each locule, but almost always
only one ovule develops into a mature seed. The acorns of
Q. robur are pale fawn or olive-green with longitudinal
stripes and larger (~3.5 g). The acorn of
Q. petraea is usually smaller (~2.5 g), rounder and dark brown with no stripes.
oak female flower in L.S.
oak female flower in T.S.
Above: left, L.S. (longitudinal section) of a female flower of Quercus; right, T.S. (transverse section) of female flower of
Quercus. The perianth (petals and sepals) are thought to be sepals only, numbering 5 to 7 (forming the calyx, K). The
three carpels (collectively the gynaecium) are fused and inferior (attached lower on the receptacle, or swollen end of
the flower stalk, than the stamens). The male flower has 5 to 7 more open sepals, no gynaecium and 6-12 anthers in
Quercus robur: K(5-7) C 0 A 6-12 G 0.

Mature oak trees have large globular crowns, that of Quercus robur being wider than that of Quercus petraea. The Florida live oak, Quercus virbiniana, is a majestic tree whose naturally sweeping branches made them ideal shapes for shipbuiding. Most oaks require training of their branches and trunks to achieve the right shapes. Old oak trees may lose vitality in parts of their crown, especially after several years of drought, with dead whitish branches, dead and stripped of bark, protruding from the crown like 'antlers' to form a so-called 'stag's-head tree'. This is beginning to happen to the tree in this picture.

Oak tree: Pov-Ray model
Oak tree: Pov-Ray model
Oak tree: Pov-Ray model

Another Pov-Ray model

Oak tree: Pov-Ray model

Article updated:
28 Dec 2015
14 May 2019
18 May 2019

oak tree, Pov-Ray model
oak tree

Above: Quercus robur. Plants are inspirational to both scientists and artists alike, so before delving into the science of oak trees, we shall ponder their aesthetic appeal. Below are some 3D computer model oak trees generated in Pov-Ray with TomTree. I found creating these graphics both artistic and scientifically relevant. They are produced by tweaking a long list of parameters (with some randomness thrown in by Pov-Ray's pseudo-random number generator). I think I obtained a degree of success, though it should be remembered that their are some 600 species of oak (Quercus) worldwide and each species differs to some extent in its form.

Oak tree: Quercus robur

A study of the form of a Pedunculate Oak (Quercus robur).

Oak tree: Quercus robur
Oak tree: Quercus robur
Oak tree: Quercus robur
Oak tree: Quercus robur