The autumn, or fall, is one of the delights of living in the northern hemisphere. Temperate, or deciduous, forests still cover much of North America and northern Europe. In autumn forests of Maple, Oak, Aspen and Birch collectively change colour and loose their leaves. How and why does this process work?

The fall in Eastern Vermont. Source: Wikipedia

The cause of leaf drop varies between species.  It can be driven by changes in temperature, daylight hours, cabron dioxide level and water supply. For  most species the falling autumn temperature triggers the change.  The loss of leaves reduces the water requirement during the winter which may be vital when the ground freezes.  In the following spring an exuberant new growth is required to replace the lost leaves.

 Leaf drop takes place at the base of the leaf stem.  Here, a specialist layer of cells, known as the abscission layer, has the ability to disconnect from the neighbouring layer cells.  This instruction comes from a plant hormone called auxin.  In summer the supply of auxin through the abscission layer is in equilibrium.  In autumn, or in times of stress, the flow of auxin from the leaf decreases, the abscission layer shuts off and the leaf begins to die.  

 At the same time, as the days shorten and the nights grow cool, deciduous trees produce less chlorophyll. Chlorophyl is the strong pigment that makes leaves look green.  When it is less prevalent we see other colours.  Yellow, brown and orange colours are associated with caretenoids.  Red and purple colours are associated with anthocyanamins which form from sugars trapped within the leaves.  

 Cross section of a leaf showing color changes. Source: Wikipedia

 Aspen trees are notes for their autumn yellows. Source: Wikipedia

 Carotenoids are the dominant pigment in about 15-30% of tree species. Vivid yellow colours are associated with hickories, ash, poplars,aspen, birch, sycamore and alder. 

Maple trees are noted for their red colours in autumn. Source: Wikipedia

Anthocyanamins colour the leaves of about 10% of all species found in the temperate forest.  These include maples, oaks, dogwoods and cherry trees.  In specific areas the concentration of species with red colouration may be much higher.  Some 70% of the species found in Maine produce anthocyanamins.  The chemistry behind the formation of anthocyanamins is complex.  Bright sunlight is a requirement, which fits in with the folklore that an Indian summer will produce spectacular colour in the trees.  The deepest colours are produced when the colours from the carotenoids combine with those of the anthocyanamins. Purple is associated with more alkaline conditions.

Biologists do not know the reason behind the strong autumn colours.  During autumn the tree is taking nutrient from the leaves for storage into the trunk and roots.  One thought is that the anthocyanamins help in that process.  The pigments could protect the leaves from the harmful effect of low temperature light and photo-oxidation until nutrients have been reabsorbed.  Another theory, less popular among scientists,  suggests that the red colour is a warning to insects, advertising that the tree has good chemical defences, please go elsewhere.  This is necessary because during the period in which nutrients are being reabsorbed the tree is particularly vulnerable to sap loving insects.

New England is famous for the fall.  There are a number of reasons why the fall is more spectacular in North America compared to Europe.  North America has more deciduous tree species than Europe.  This is a product of geography. During the ice ages American species were able to retreat along the north south ranging mountain which were not available in Europe.  North America has greater expanses of untouched forest and has a climate suitable for the production of anthocyanamins.

A second mystery questions why the autumn red colours are prevalent in New England and the Far East but not Europe.  Recent work by Prof. Lev-Yadun and Prof. Holopainen explains this using the insect deterent hypothesis.  North – south mountain ranges are found in North America and Asia.  Insect pests migrated with the trees as they migrated in response to changing climatic conditions leading to a long period of co-evolution. This did not happen in Europe because the East-west facing mountain chain led to the loss of both insect and tree species leading to a shorter period of co-dependence.