Suddenly deciduous

Last night it froze; the low temperature was 25 F.  Up until now only the lightest of frosts had happended, so lots of trees still have green leaves.  Under these circumstances a number of tree species will drop their leaves all at once including hackberry and black walnut, and most definitely ginkgo.  What causes tree leaves to drop is an abcission layer, an anatomically weak zone that forms in the fall at the base of the leaf stalk.  Trees like ginkgo still hang on to their leaves until a freeze, and then, perhaps as a result of ice crystal formation, the weak connection is broken and as the temperature goes up a few degrees, all the leaves fall at once.  TPP sort of wishes other trees did the same because then you'd be done with it, but some of our big trees hang on to leaves so tenaciously you end up raking in the spring.  The areas under the suddenly deciduous trees were just a carpet of dropped leaves this morning, and most people won't even notice. And leaves were still falling like rain.  Most other trees drop leaves more gradually as the abcission zone matures.  All trees drop leaves, but when they drop them all seasonally, they are deciduous.  Otherwise they are evergreen, which is not synonymous with conifers because some are deciduous.