On my dresser, I have a set of hair-ribbons that I haven’t used since I cut my long hair into a short bob. Unused for now, but not discarded, since they might have some other value I can’t foresee.
Similarly, some deciduous trees hang on to their dead leaves, a phenomenon called marcesence.
During the growing season, trees transport water and nutrients through their vascular system, akin to our blood vessels. In the fall, most trees close off this circulation to form an abscission layer, which causes its dead leaves to fall. But the abscission layer in marcescent trees doesn’t form till spring when their leaves finally fall to the ground.
Oak, beech and witch hazel trees are marcescent. And many palm trees retain collars of dead fronds, creating short skirts of leaves that persist for years.
The reasons for marcescence are not clear, but theories abound. The presence of dead leaves might protect leaf buds during the harsh, dry conditions of winter. Marcescent trees may also benefit from the shed leaves that arrive and decompose in the spring, just when they’re needed for new roots and leaves.
Those remnant dead leaf surfaces trap snow during the winter months, providing trees with more water in the spring, just as bigger snowpacks in our mountains provide more water in our valley.
Finally, a marcescent tree may deter feeding of deer, who eat its buds. Rummaging around dead leaves would make for louder feeding noises. So to avoid rousing the attention of predators, deer might choose to find food from non-marcescent trees.
Whatever the reason, you'll see the marcescent Gambel oak in the Wasatch Mountains holding onto their dead leaves for a while — like the hair ribbons on my dresser — because they might have some other values we can’t foresee.