Hah, botany lecture alert… In the context of deciduous trees, dropping the leaves is an active process. The tree shuts down normal leaf function (photosynthesis) with a combination of environmental cues and hormone signals and starts breaking down subcellular components and extracting nutrients from the leaves to be dropped. Hence the color change in the fall as chlorophylls are broken down and cartonoids and other pigments are starting to show through. Programmed cell death is involved but I think metabolism still continues for a while after photosynthesis stops, breaking down the chloroplasts and loading nutrients into the phloem to transport valuable resources such as sugars, nitrogen, and phosphorous back into the tree and down to the roots.
The leaves aren’t just dying, they are actively shed. The tree cuts the leaf off by forming a cork layer at its base that eventually becomes a leaf scar and part of the bark. By doing this, the tree protects itself from injury. On the leaf side of this, a separation layer forms that consists of cells undergoing changes such as digestion of their cells walls to direct exactly where the leaf will break off. The cork and abscission layers seem to be built from the outside in with the last part to be cut off being the vascular bundle. So I would say the point of death for the leaf would be once those layers are completed and the leaf has been cut off entirely from the tree’s vascular system. It’s then a matter of time (and wind and rain) before the leaf separates completely and drops off.
Here’s a picture of one of the microscope slides of a maple leaf abscission zone that we use in our botany labs with a couple of labels to explain what everything is. It’s a longitudinal section (top to bottom) through the connection between the stem and leaf. Note how the separation layer forms exactly at the point between the bud for next year, which will stay on the tree and grow out again in spring, and the leaf, which will get dropped.
