Autumn to winter

Deciduous trees continue shedding their leaves and stop drawing water from the ground before going into dormancy for winter. When shedding leaves, they form a layer of scar tissue called the abscission zone, for protection from external invaders. Abscission is the technical term for leaf-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 water drawn 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 limits transpiration- and evaporation-based water loss.

There are exceptions – deciduous conifers are conifers that shedtheir needles. Exceptions apart, this makes it simple to differentiate between the 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 threats to trees, especially those in harsher environments. For example, storms can break 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 threat too, making it difficult for trees to retain water.

Other threats include frost crack, caused by expansion and contraction of water in the trunk. This risk is increased after a sudden temperature drop. Extreme cold can also make a tree ‘explode’ when freezing sap expands. This occurred in Texas in 2022 during Storm Landon, creating “sounds like gunshots going off”.