First off, you may have noticed that my field journal has been unattended for a few months - my bad! I don't like making excuses, but in this particular case I will blame it on my day job (or rather a specific project I've been working on since the end of the summer), which completely (and quite unhealthily) took over my life! I will blog about the project in the new year once it's been peer reviewed and published (hope you don't mind a bit of mystery?).

Now, onto today's post!

This time of year, I often spend some time reflecting on the successes (and failures) of the past 12 months. Have I achieved everything I set out to do? Have I been true to my principles? Am I any closer to achieving my dreams? (I admit such fundamental self-examination often requires a glass of Gran Marnier or Glayva!) I dare say an equally profound retrospective begins to happen in nature during the autumn months and comes to its full glory in winter.

Below are but a few of my favourite shots of the English countryside taken during the coldest seasons of the year. Trees, both deciduous and evergreen, are so beautiful this time of year, don't you agree?

Autumnal woodland canopy, Virginia Water Lake, Surrey

Leaves of the European beech (Fagus sylvatica) in early autumn

Richmond Park, Surrey

Park Hall Country Park, Staffordshire

Arable landscape in Norbury Park, Surrey Hills Area of Outstanding Natural Beauty, Surrey

So today, my friends, we are ramping up science! The way I look at it - some basic biology or in this case plant phenology (the study of a plant's response to seasonal and climatic changes in its environment) has never hurt anyone, so let's crack on!

Those of us living in the Northern Hemisphere observe year after year a seasonal event known in scientific circles as senescence (Latin for growing old or being in decline) or the greening-down of nature; first we notice the rich colours of autumn foliage, followed by the falling leaves and finally clusters of bare trees appear almost lifeless against the stark winter landscape. To our eyes, there is little excitement in this phase of a tree's qualitative cycle. But don't be fooled, my friends, because as you'll see appearances can be deceiving.

Tree dormancy

During their life cycles, trees are exposed to periods of stress caused by low or high temperatures, drought or other environmental factors. In the course of evolution, complex defence mechanism have developed for protection against stresses. One such mechanism is dormancy, which refers to the inability of an otherwise viable seed, whole plant or meristem (a bud, apex, etc) to grow.

Leaf abscission: As daylight becomes shorter, temperatures cooler and water harder to get, trees begin to shut down food production in preparation for dormancy. The first sign of dormancy is the loss of leaves in deciduous trees; it is an essential part of winter cold survival. Trees don't make food in the winter, so they have no use for masses of leaves that would require energy to maintain them. When it's time for leaf fall, a hormone called abscisic acid (ABA) is produced in terminal buds (the part at the tip of the stem that connects to the leaf). The terminal bud is where the leaf breaks off when it falls, so when ABA gathers there, it signals the leaf to break off.

Impeded cell growth: ABA is a plant hormone that also suspends growth, preventing cells from dividing (see table below). This is something that occurs in both deciduous and evergreen trees. The tree's metabolism also slows down during dormancy, and this is part of why cell growth is impeded.

Acclimation: The first cold days of autumn are the environmental cue for trees to develop resistance to below-freezing temperatures. When trees are growing, the cells cannot withstand below-freezing temperatures because ice crystals forming inside the cells tear membranes and destroy the cells. As the cells become increasingly resistant to subfreezing temperatures, their water content is reduced and the structure of the cell's contents changes. The result is that the water is in such small, separate compartments that it can be supercooled to temperatures well below freezing without forming ice crystals, and thus without damaging the cell's membrane. 

Storage of photosynthate: Another aspect of preparation for winter is the storage of photosynthate - mostly in the form of starch - for the following spring's shoot growth. Storage typically occurs in parenchymatous cells (a soft cellular tissue) in the roots or stems, or, for evergreen species, leaves. Deciduous trees in the spring are completely dependent on stored photosynthate until the leaves become self-sufficient. 

Overview of plant hormones

Focus on leaf senescence and abscission

During its life, a tree will shed many of its organs, including leaves, fruit, petals, buds, bud scales and bark. Abscission is involved in the loss of leaves, fruit, flowers and floral parts, whereas general attrition is responsible for the detachment of dead or dying tissues such as bark, old branches and roots.

Abscission is an active metabolic process resulting in the weakening of anything from 1 to 20 rows of cells in genetically determined abscission zones. The process itself is very precisely controlled by a change in the balance of plant hormones, ethylene and auxin (see above table); a higher proportion of ethylene causes the cells to produce enzymes that digest the cellulose of cell walls. A layer of cork forms a leaf scar that protects the plant.

An example of maple leaf abscission

How does abscission fit into a deciduous leaf's life cycle, you may ask?

1. First, a leaf is produced by the meristem. 
2. It may then spend a winter inside a bud, but it eventually grows out to form a stalk and a blade.
3. Once growth stops, senescence begins as the leaf ages. Leaves on evergreen trees may live more than a year, but they also age and eventually die. 
4. For deciduous trees, as autumn approaches the green chlorophyll pigment in the leaves begins to break down and other yellow, red and brown pigments which had been masked by the chlorophyll produce the colours of autumn foliage. 
5. The final stage of the leaf life cycle is leaf fall or abscission. Most leaves do not simply fall off. They break off through a special abscission zone at the base of the leaf stalk. The abscission zone is not present in the summer, however, in the autumn, as the growth-regulator production by the leaf blade begins to decrease, the zone forms. There are localised cell divisions to form the zone and then the cells in the zone actually begin to separate from each other. As this process continues, the action of the leaf blowing in the wind or just its own weight completes the separation between the cells, and the leaf falls. The timing of leaf fall is dependent on the timing of the formation of the abscission zone.

So next time you walk past a senescent tree, give it a hug (or a warm thought at least). Remember that this living organism makes a lot of sacrifices to stay alive until the arrival of spring.

Recommended reading

• Addicott, F.T. (1982) Abscission. Univ of California Press. Available via Google Books. 
• Fuller Wilson, B. (1984) The Growing Tree. Univ of Massachusetts Press. Available via Google Books.
• Pessarakli, M. (2001) Handbook of Plant and Crop Physiology. Edition 2, illustrated, revised. CRC Press. Available via Google Books (Chapters 8-10).