It’s called “fall” for a reason: leaves fall from the trees on a seemingly regular annual schedule. But it is neither consistent nor comprehensive as some leaves wither but do not fall off, and leaves do not always drop at the same time each year.
Everyone pretty much understands that the lack of chlorophyll causes leaves to turn different colors throughout the fall. Once the pretty display is over, however, few give any thought as to why the leaves actually fall off the plant, other than the notion to “get those leaves off my lawn,” and, furthermore, that this leaf drop begins the process for new leaves to emerge the following spring.
Why Leaves Fall
The technical term for the process of leaf color change and shedding is “leaf senescence”. Leaf senescence is marked primarily by a degradation in chlorophyll levels and in the resulting reduction of the plant hormone known as auxin. As the diagram above shows, reduced auxin increases ethylene sensitivity at the base of the leaf, causing multiple cell changes that eventually lead to leaf drop.
In deciduous trees and shrubs, the leaf separates from the plant at the “abscission layer” (or the abscission zone), located at the base of the leaf stem (aka the petiole). The biochemical changes described above lead to changes in cell walls throughout the leaf stem. On the twig side, the abscission layer hardens as suberin and lignin seal the wound, which protects the auxiliary bud from diseases and water loss.
This is also the reason that leaves fall in late summer due to heat and drought stresses, but with active and high auxin levels, the auxiliary buds tend to leaf out. These are truly fascinating molecular mechanisms in plants!
Why Leaves Don’t Fall
Leaves that tend to hang on to their twigs are known as “marcescent leaves”. In marcescence, the abscission zone does not develop until the leaf bud breaks in the spring. It is unclear why some plants do not drop leaves, but it is most likely due either to protect the leaf bud from drying out in winter winds, or to delay a source of nutrients into the spring (leaves provide compost and compost feeds the plants).
Plants of our area that typically exhibit marcesence include:
- American beech (Fagus grandifolia)
- Many oak species (Quercus) especially Pin Oak (Quercus palustris)
- Ornamental Pear (Pyrus calleryana)
- Hornbeam species (Carpinus)
A combination of unusual weather can also cause marcescence. In the last few years an abnormally warm October delayed the complete process of leaf senescence and prevented the abscission layer from fully developing. This was immediately followed by abrupt and extremely cold temperatures in early November, which froze and killed cells in the not yet fully developed abscission layer.
Eventually, all leaves will drop either from winter winds, snow and ice or once biochemical conditions are right for the auxiliary buds to emerge.