How do trees survive the winter?

Whilst we may put food out for the birds and marvel at how creatures survive the snow and icy conditions that winter can bring, it’s easy to forget about the trees, stoically and steadfastly facing whatever weather comes.

Different species of trees have evolved a myriad of techniques for surviving freezing conditions and winter storms, both structurally and at the cellular level. Much of this still remains a mystery but it’s fascinating to wonder and research the latest science to discover a little bit about what’s going on beneath the bark.

One of the biggest risks of winter is water inside the living tissues freezing, with the force of sharp edged ice crystals puncturing and damaging cells. The main strategy to combat this is to reduce the water content in the wood and the flow of sap, by reducing activity and going into hibernation. Trees prepare for winter well in advance, with each species and even each individual having a slightly different reaction depending on altitude, health,  exposure and so on, in response to shortening day lengths. It’s a trade-off between storing more food away in the last moments of summer and not being caught out by the first heavy frosts!

Deciduous trees drop their leaves and slow or stop growth, reducing water content in their branches. They also increase the concentration of sugars in branch and root tissues, making their own cellular antifreeze. The dropping of leaves means deciduous trees can cope better with high winds and heavy snow fall, without leaves to act as a large sail in wind, branches can bend and flex, hopefully without too much damage and the snow has nowhere to land.

Conifers such as our native Scots Pine which keep their needles main strategy is to fill their cells with a concentrated sugar solution – which acts as a antifreeze. They also reduce the moisture content in their wood, slowing or stopping growth and coat their needles with wax to reduce transpiration. This has the advantage that come the spring they can start photosynthesising quicker than the deciduous trees.

Delving a little deeper into what is going on at a cellular level…It is not just the overall water content in living tissues, but how and where that water is stored. Water freezing between the cells is less of a problem than inside the cells, as the cells are protected by their strong cell wall. As water freezes between the cells it draws water out. The large vacuole which contains mostly water is replaced with numerous small vacuoles which store sugars, proteins and fats. Less water also means the plasma membrane which holds the cell organs and fluids can shrink away from the cell wall…and away from the danger of pointy ice crystals even if they did burst through the cell wall ! Clever stuff…

We’ll explore what happens in the spring in a future blog post!

Ali Murfitt

Site and Community Officer.

References:

The Hidden Life of Trees. Peter Wohlleben

//www.ncbi.nlm.nih.gov/pmc/articles/PMC3669742/