Autumn to winter

Deciduous trees continue shedding their leaves and stop drawingwater from the ground before going into dormancy for winter. When sheddingleaves, they form a layer of scar tissue called the abscission zone, forprotection from external invaders. Abscission is the technical term forleaf-shedding.

Keeping their water from freezing is a priority for these trees,as it can be fatal. Deciduous trees have different techniques to achieve this.

One is supercooling, which lowers the freezing point of water byproducing a hormone that acts as an antifreeze. Another is extracellularfreezing, which protects the cell tissue by freezing the outer cell layer.

Conifers don’t go into dormancy, only reducing the amount of waterdrawn from the ground. They continue growing new branches and leaves (needles)but at a slower rate. Their needles also retain water better than ‘normal’leaves due to a lesser surface area and a waxy outer layer that limitstranspiration- and evaporation-based water loss.

There are exceptions – deciduous conifers are conifers that shedtheir needles. Exceptions apart, this makes it simple to differentiate betweenthe two. Deciduous conifers exist in five genera: Larix (larch), Pseudolarix(golden larch) and Metasequoia (dawn redwood), plus Glyptostrobus(cypress) and Taxodium (yew).

Regardless of tree type, winter brings a host of potential threatsto trees, especially those in harsher environments. For example, storms canbreak branches, exposing trees to pathogens. Equally, snow and ice can collect,creating enough mass to do the same. Dry air is a less well-known winter threattoo, making it difficult for trees to retain water.

Other threats include frost crack, caused by expansion andcontraction of water in the trunk. This risk is increased after a suddentemperature drop. Extreme cold can also make a tree ‘explode’ when freezing sapexpands. This occurred in Texas in 2022 during Storm Landon, creating “soundslike gunshots going off”.