Why do some trees have leaves and others don’t?

Answer

Dear Neil,

That’s a very good question, one that scientists have already done a lot of research on. We don’t know everything yet, but we already know a lot:

Every tree should try to develop the leaves quickly in the spring, but without the risk of frost damage. Since some leaves are more sensitive to this, not every tree species has the same optimal moment. Since a tree does not have a calendar, the tree responds to a number of signals to “know” whether it is spring. Depending on the species, trees respond to one or more of the following signals:

* number of cold days (frost) that have passed (the winter must be over because a warm December day is really too early)

* number of warm days

* day length (those are trees that form leaves fairly constantly on the same date)

Often it is mainly the temperature of the soil that counts and that differs from the air temperature, sandy soils also warm up faster than clay soils. That is why there are also differences within a species. As far as the age of the tree is concerned, the height is of great importance: the signals for leaves to bud are produced in the root (a kind of hormones) and reach a small tree, which reach the branches more quickly. On top of that comes a considerable amount of genetic variation: a seed from an Italian beech grows into a tree that actually expects the Italian climate and is therefore programmed differently than our native beech trees. Trees in gardens often come from other countries and show more variation than the trees in our natural forests.