Why do some plants lose all their leaves at about the same time?
Show answer & explanation
Answer: A seasonal signal cuts off every leaf stem, so all the leaves fall together.
A sudden freeze kills every leaf stem, so all the leaves fall together. — Wrong. Cold weather can set the process in motion, so the temperature drop is not irrelevant. But leaves do not simply die and fall off in a freeze; the tree actively forms an abscission layer at each leaf base, sealing and cutting off the tissue. The timing comes from shortening days, not from a single sudden freeze.
A seasonal signal cuts off every leaf stem, so all the leaves fall together. ✓ — Correct! As days shorten, trees form an abscission layer at the base of each leaf. Special cells there cork over and disconnect the vascular tissue, while the tree reabsorbs nutrients and chlorophyll breaks down. The layer then weakens, so all the leaves separate around the same time because they respond to the same seasonal signal.
A fungal infection rots every leaf stem, so all the leaves fall together. — Wrong. Disease can make leaves drop, so this is close to a real possibility. But healthy deciduous trees lose all their leaves together on a seasonal schedule, and their leaf stems are not rotted by an infection. The coordinated drop is a controlled strategy for winter, not a symptom of disease.