A Beech Wood in Autumn
The beech tree (Fagus sylvatica) forms grand grey columns covered in smooth grey bark, which looks black when wet, but is
often patched here and there with sheets of smooth green algae which rub off on one's fingers as a light green powder. This
wood is in the Likey Hills, England, and borders a deciduous mixed woodland of predominantly oak, hazel and birch, but on the
drier steep slopes, beech dominates. Some of the slopes are dominated instead by non-native conifers. In autumn, beech leaves
turn a spectacular orange-red or golden-brown before falling from the tree to carpet the woodland floor. These leaves contain
chemicals noxious to most plants and coupled with the dense summer canopies that intercept most of the sunlight, there is little
undergrowth in beech woods.
Above left: an unusually stout birch tree almost passes for a beech at first glance!
Walking amongst beech trees, with their great column-like statures, reminds many (including myself) of walking in a cathedral - a
natural cathedral. The trees often appear to be sentinels, guarding something, but quite benevolent nevertheless. Some
undergrowth can be seen, including a holly bush in the bottom left corner above. Two pines are also visible in the above photo.
Beech trees are shallow rooting and so suited to the thin soils on slopes. Their roots spread laterally just beneath the soil
surface. Wood mice and grey squirrels feed upon the beech mast (beech nuts are also quite edible to humans and taste a bit
like almonds). Grey squirrels, however, apparently damage the trees by stripping away the bark, presumably to reach the
sugary phloem beneath the outer bark. (They may also be after key minerals such as calcium, perhaps by gnawing the
sapwood).
In the two pictures above, young birch trees and young pines can be seen behind the beech tree in the foreground.
The leaves of beech trees, in full summer, are arranged in such a way that very little light and rain are filtered between them;
leaving the woodland floor dark and dry. In the Spring, however, early flowering herbaceous plants such as bluebells and wood
anemone (see
woodland flowers) put out their leaves and flower before the beech leaves open fully. Bird's-nest orchid (Neottia
nidus-avis
) is a  pale yellow-white plant lacking chlorophyll (it has a very small amount, but no significant photosynthesis
apparently occurs) and is well-adapted to life on heavily shaded beechwood floors. It forms an association with a fungus,
Sebacina, which is found  in the three cortical cell layers beneath the epidermis of the roots. This fungus forms mycorrhizae
(ectomycorrhizae) with beech-tree roots. The fungus exchanges mineral salts it has scavenged from the soil for carbohydrates
provided by the tree and made by photosynthesis. The bird's-nest orchid is
parasitic on the fungus, drawing off nutrients whilst
giving nothing in return. To see more fungi that are associated with beechwoods visit:
http://www.naturescalendar.org.uk/wildlife/factfiles/beech_woodland_fungi.htm
Grey Squirrel
Above: a Grey Squirrel (Sciurus carolinensis) - photo credit:
http://commons.wikimedia.org/wiki/File:Eastern_Grey_Squirrel_Beacon_Hill_Park.jpg
Leaf senescence

The autumn colours of falling leaves are due mainly to the following accessory pigments:  
carotenoids (orange), xanthophylls (yellow) and anthocyanins (red), reddish plant
pigments. Yellows and oranges arise when a leaf breaks down its chlorophyll for recycling by the
plant, unmasking the carotenoids and xanthophylls normally present but hidden by the green of
the chlorophyll. These pigments function as photoprotectants, protecting
photosystem II from
damage by access light energy.

Browns are due to the oxidation of phenolic compounds and are an indication of cell death.
Reds occur when a senescing leaf manufactures new anthocyanin pigments. As leaves age they
become more susceptible to light damage as their photosynthetic efficiency declines (meaning
that light energy not harnessed has to be dissipated by other routes, leading to cell damage
unless absorbed by accessory pigments). This additional anthocyanin synthesis protects the
leaf by absorbing the excess energy and hence prolongs its useful life.

Early autumn leaf senescence in red maple is associated with high leaf anthocyanin content.
This may seem paradoxical, after all the anthocyanins should prolong the leaf's life. Indeed they
do and they are manufactured by stressed leaves, which might otherwise fall even sooner.
Wounding and other stresses, including cool temperatures and high light levels increase
anthocyanin synthesis in aging leaves. Low nitrogen and phosphorous levels have the same
effect. In the Red Maple (
Acer rubrum)  partial shading of leaves by other leaves results in
considerable variation in senescing leaf colour: shaded leaves are more likely to be yellow or
orange, exposed leaves red.

Anthocyanins act as a sunscreen and as antioxidants and possibly increase the time for efficient
nutrient reabsorption from an aging leaf (correlating with delayed leaf abscission) Cool, sunny
days, low phosphorus and acidic pH will increase anthocyanin synthesis.

Gibberellic acid (GA) is a phytohormone (plant hormone) which delays senescence.
Application of GA to leaf discs of various plants, including the dock
Rumex crispus, delayed
senescence.
Cytokinins, another group of phytohormones, also delay senescence and when
applied to yellow leaves may result in re-greening due to reassembly of senescing plastids.
Conversely, some phytohormones increase senescence, such as
abscisic acid (ABA).

Bibliography

Anderson and Ryser, 2015. Early Autumn Senescence in Red Maple (Acer rubrum L.) Is
Associated with High Leaf Anthocyanin Content.
Plants 4: 505-522.

Rapp, Ransbotyn and Grafi. 2015. Senescence Meets Dedifferentiation. Plants 4: 356-368.
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