When leaves fall in the autumn, the tree (or shrub, or vine) no longer has to supply water to those leaves, and this helps to keep deciduous trees from drying up when the roots cannot absorb water from the frozen ground. Senescence is the term given to the steps that lead up to leaf dropping and it’s an ordered and carefully controlled set of steps designed to salvage many essential elements from leaves before they die. These are then recycled back to developing leaves the following spring. Once senescence begins it can’t be reversed, so the plant has strict controls over the whole process, enforced by a number of hormones. Senescence includes the breakdown of chlorophyll in the leaf, and weakening of the cell walls in the ‘abscission zone’ near the base of the leaf stalk.
As the minerals are released from their bound form in proteins and chlorophyll in the leaves, and transferred back to the stem, the breakdown of the green colored chlorophyll causes other pigments to show through, hence the change in colors of the leaves. Since mineral relocation is in progress when the leaves change color we can help the plants conserve nutrients by not removing dying leaves for a few days. This applies to any leaf that is dying, not just autumn leaves. Once the nutrients have been passed back to the stem the abscission zone cuts the leaf off. The weight of the leaf, with the help of the wind causes the dying leaf to fall.
When temperatures drop below freezing, the abscission zone hardens more rapidly, causing some trees to lose every leaf to the freeze all at once. Without the push from the freezing temperatures more of the minerals in the leaves would have been absorbed back into the plant, but the trees can take a small loss of these nutrients if they are healthy.
On the other hand, an early killing freeze injures all tissues including the abscission zone. When severe enough, the cells of the abscission zone are killed, preventing normal dropping of the leaves, and the dead leaves may hang on the tree throughout the winter. This is what happened in my garden when we had the subzero temperatures before Thanksgiving. It came too early and too suddenly, and not even some of the native willows were ready for it. Eventually wind and snow will force those dead leaves to drop, but in the meantime there is a danger of branches breaking if we get a wet, heavy snow, where the added weight held by the clinging leaves can be a problem. Keep an eye on your branches when we get those wet snows, and be prepared to wade out there and shake some of it loose should it get too heavy. These trees also missed out on a lot of nutrient recycling, and will start the spring with less reserves. Gosh you have to be tough to survive this climate!