Deciduous or Coniferous? Or Deciduous Coniferous?

If you think about it, trees dropping their leaves is a bit of a strange move. After all, the leaves are the site of photosynthesis and the main feature of the tree where energy production takes place.

So why do deciduous trees drop their leaves?

Not only is it a time for the trees to lose their old leaves and give way to new leaves in spring, but there are other reasons to.

Before dropping its leaves a deciduous tree will pull all of the moisture and nutrients from the leaves and move it into the trunk to store for winter, giving its winter store an extra boost. This extra moisture is stored deep within the trunk, with many species having an ‘anti-freeze’ type chemical to prevent this moisture from freezing and risking the structural integrity of the tree. The winter store of water and nutrients can be enough to sustain a tree through its dormant months when growing in its optimum environment. 

Another reason for deciduous trees to drop their leaves is an act of storm protection. By dropping their leaves the tree reduces its surface area and so reduces the surface that the wind can act on pushing the tree in a storm. A tree without leaves allows the wind to pass right through, with little wind resistance the tree is safer as this action limits the potential damage when a storm hits, and as long as the roots are healthy and the soil is not water logged this action greatly reduces the chance of the tree being blown over completely. 

As the leaves fall to the ground, they act as extra nutrients to the soil. This in turn helps to sustain the tree by boosting the soil nutrient content over time as the leaf litter breaks down.

The leaves of deciduous trees are also very vulnerable in structure. Deciduous leaves do not have the strong waxy layer found on many evergreen trees. Without this thick waxy layer the leaves are at risk through the colder months of damage from freezing and thawing. The freezing and thawing would create swelling within the leaves meaning that they would be at risk of structural damage leaving the tree without strong healthy leaves when the longer days do come. But by losing its leaves, storing what water and nutrients it can in the trunk, the tree can fuel new leaf growth. The deciduous tree can then enter the spring months with young leaves ready for efficient photosynthesis and energy production to fuel the next stage of the tree’s lifecycle. 

So why can evergreen leaves/needles survive the winter, but deciduous leaves can’t?

There are many adaptations in the evergreen shrubs and trees meaning that their leaves and needles cope with and survive with the harsh and unpredictable winter conditions. 

Most evergreens have needles rather than leaf shaped structures. The needle’s shape has a reduced surface area. This does mean that there is less surface area for photosynthesis – which is why there are so many of them – but this is also a positive as it means less surface area for water loss. The needles also have a low surface area compared to their volume, this means that they can retain water far more effectively compared to the flat large areas of the deciduous leaves. 

The thick waxy layer of the needles not only helps to prevent water loss but also adds a protective layer to the structure helping to protect the needles from the harsh winds and frost of winter. 

The stomata on the leaves – the important site of gaseous exchange – are positioned in the sunken pits of the evergreen’s needles. This adaptation gives extra protection from the harsh winter conditions to this important part of a leaf structure that is far more vulnerable on the deciduous leaf structure. 

These adaptations sound great, they help the tree to keep hold of its leaves/ needles, allowing it to photosynthesise all year round. This allows the tree a constant energy production facility all year round. Perfect….right? Well, not completely. These adaptations that protect the needles from the harsh conditions of winter also mean that they are less effective for photosynthesis. 

The deciduous trees are energy producing power packs through the spring and summer producing as much as they can for growth, reproduction and survival. Deciduous leaves are highly efficient and effective for photosynthesis with their large surface area, high levels of chlorophyl and do not have a thick waxy layer. They can produce a large amount of energy in a shorter amount of time, and then have the capability to store some nutrients to sustain them through their dormant months.   

Whereas their evergreen counterparts can photosynthesise all year round but just on a constant amount that is determined by daylight hours which of course are so much lower in winter, but they have no way to increase their photosynthesising capability through the summer months. But they are still able to sustain themselves all year with well protected needles. 

Each type of tree has incredible adaptations. But there is also a sub-section referred to as deciduous conifers, such as the Larch, Dawn Redwood and Giant Sequoia – so though they have needles, not every conifer is an evergreen – and not every evergreen is a conifer. These trees do have needles and some form cones – but through the Autumn their needles change colour and fall like the leaves of the deciduous broadleaves. 

So does that mean that they have more efficient needles so they can photosynthesise at a more efficient rate to store extra nutrients through the winter like a deciduous tree? It seems to come down to the amount, the amount of needles seems to be far greater on deciduous conifers compared to their evergreen counterparts when growing in their optimal environment. So as much as the reduced surface area of the needles can limit its photosynthesis capabilities, if you have far more of them then that can increase the photosynthesising capability of a tree. 

The deciduous conifers generally grow in regions with reduced daylight hours through the winter, this means the opportunity for efficient photosynthesis is reduced anyway. So by dropping their needles the deciduous coniferous trees are conserving energy, not needing to sustain the needles through the darker winter months.

Each tree has incredible adaptations helping it to thrive, not just survive through the different seasons as they grow through the years and generate enough energy to sustain themselves through each life cycle – all while providing homes and habitats for wildlife and converting carbon into oxygen for our benefit as well as theirs. Trees truly are awesome!