Deciduous - a bit of a strange word. It means to shed it's leaves annually - typically during the Autumn / Winter time. So the next time you see a tree in the cold season with no leaves you know it's Deciduous. The opposite of Deciduous is Evergreen. So the Evergreen trees are the ones that have leaves (usually needles) all year round. They typically have cones as well. There are more types of Deciduous trees than Evergreen across the world. There are only five native Evergreens in the UK and they are Box, Holly, Juniper, Scots Pine and Yew. There are around fifty different types of Deciduous tree that are native to the UK including Ash, Alder, Beech, Hazel, Lime, Oak and Willow.

Hornbeam in October 2016 - just before it's leaves were starting to drop off.

Hornbeam in February 2017. All of the leaves have been shed now.

So the real question is why do we have Deciduous and Evergreen trees? Typically it is down to the seasons which are heavily influenced by temperature and climate. In the UK we have four seasons, but some countries only have two. Singapore is one of them. Countries that are on or near the equator typically receive more heat and sunlight. So they have Summer and Winter (dry /wet) seasons whereas we have the Spring, Summer, Autumn and Winter. Leaves make a lot of energy for the tree but also use up some energy. So during the colder seasons where there is less sunlight the Deciduous trees cleverly withdraw any useful components from the leaves and then shed them. Nothing gets wasted in nature. For us in Exeter we are part of the South West so we tend to have more mild winters.

A London Plane during January 2017. Nearly all of the leaves have been shed and some of the fruit remains

Wild Cherry during October 2016. The leaves are beginning to shed in preparation for winter

Scotland on the other hand have very harsh winters which explains why they have so many Pine trees and Birch trees. We know that Pine trees are Evergreen and are well suited for cold weather. Birch trees are Deciduous but they are very adapted for the cold. They have very thin papery white bark which reflects the light. This might seem a bit odd as you'd think you would want to absorb any sunlight during the winter to keep warm, but it's the temperature changes that can be a problem. If you think it's cold during the day you can only imagine what it's like during the night. So by reflecting the light from the sun the overall temperature of the tree is much the same. Rapid temperature changes can be just as lethal as ice crystals forming within the living cells of the tree.

A Maidenhair tree in January 2017

A Maidenhair tree in August 2016

Also, the leaves for Deciduous trees tend to be quite big so they are often referred to as Broad-leaf-Deciduous.  If you imagine a heavy snowfall in winter and a Deciduous tree keeping all of it's large leaves - not only would it look a bit odd but would be very bad for the tree. The weight of all of the snow on the leaves would likely lead to breaks and fractures within the branches. Ice crystals are more likely to form in the larger leaves which can be lethal for the tree. The Deciduous trees essentially go into a dormant state during the Autumn and Winter to help conserve energy. All of the old leaves will eventually rot down into the ground and the tree can re-use any left over materials. ​Another key reason is water. Leaves use up a lot of water, mostly for photosynthesis. Given that there is less sunlight in the Autumn and Winter it makes sense to ditch the leaves and save the energy for later. This is also why the leaves start turning the lovely golden yellow, orange and red colours during the Autumn. The chlorophyll in the leaves is what gives them their vibrant green colours. Without chlorophyll other pigments in the leaves can be seen.

Evaporation on a Deciduous tree during January 2017

Hornbeam tree in the rain during April 2017

We know that water for trees is vital for survival, but equally it can be fatal. Water in it's liquid form is fine. It gets collected from the roots and sent upwards to the rest of the tree and so on. But when it gets cold water can quickly change from the normal and safe liquid form to a solid form which causes massive problems. What we see as pretty snow-flakes and frost covered fields is a nightmare for our trees, for all plants in fact. Ice crystals are the killer for plants as they can shred through the living cells which can lead to a swift end of the plant. Also, with no active flow of water the aerial parts of the tree will soon die. Luckily our trees have a few tricks up their branches so to speak. They increase the amount of sugars which act as natural anti-freeze. They also have special proteins which work as an anti-freeze. For our Evergreen friends their leaves are covered in a waxy substance called Cutin which helps prevent water loss as well as reducing severe ice damage. Deciduous trees don't use these techniques as they shed all of their leaves before the really bad weather sets in. There are pro's and cons to both methods, but as mentioned earlier a lot is down to the climate and seasons.

Norway Maple leaves during November 2016.

Wild Cherry leaves during October 2016.

FACTS & INFO

• An adult Pedunculate Oak can have up to 700,000 leaves
• Around 70% of the nutrients of the fallen leaves are re-absorbed by the tree
• Chlorophyll is what gives the leaves their green colour and is the key component for Photosynthesis
• Carotene is another pigment which gives leaves their yellow / orange / brown during the Autumn
• Anthocyanin is yet another pigment which gives leaves their red colour during the Autumn
• During the Autumn our deciduous trees and shrubs withdraw these pigments to save energy
• On average a mature oak tree can draw up to 50 gallons of water a day
• Clever AFP (Antifreeze Proteins) means our trees can survive sub-zero temperatures
• Sugars within the tree also help as a natural anti-freeze
• There are around 3 billion trees in the UK which is around 47 trees per person
• Scotland has around 56% of the UK's trees. England is 34% and Wales 10%. These figures were from a report in 2011 by the Woodland Trust. That's about 1.7 billion in Scotland, 1 billion for England and 0.3 billion for Wales
• Most of our native trees in the UK are deciduous

SCIENCE STUFF

• Abscission - the process of how deciduous get rid of their leaves. It may surprise you but leaves don't just fall off because it's cold or winter - they are very slowly 'cut' off the tree
  
• AFP - antifreeze proteins. Turns out it's not just plants that have them! Wikipedia covers this well

• Anthocyanin -a red pigment in the leaves
• ​Carotene - a yellow / orange pigment in the leaves
• Chlorophyll - the green pigment in the leaves
  
• Conifers - a group of trees that are mostly Evergreen. They are gymnosperms (cone-bearing seed plants)
• Cutin - a waxy water-repellent substance in the cuticle of plants. An insoluble mixture containing waxes, fatty acids, soaps, and resinous material
• Deciduous - a tree or shrub that sheds it leaves annually, typically in the winter
• Evergreen - a plant that retains green leaves throughout the year
• Photosynthesis - typically done within the leaves. A clever process of using light energy from the sun and changing it into chemical energy which is used by the plant. It requires carbon dioxide and water to make this work. Oxygen is released as a waste product
• Sugar - this is made during Photosynthesis (amongst other things). It is very important and converted into other useful components such as starch or cellulose. Sugar acts a natural anti-freeze