Abscission Decision

The raking of leaves this fall started months ago as the leaf was being formed.  A specialized zone of cells developed at the base of the leaf, the abscission zone, which would allow the leaf to be shed when no longer necessary.  In response to certain plant hormones,auxin and ethylene, the mature leaf separates  as light and temperature conditions change.  The abscission zone then seals the leaf scar preventing further damage.  The first growing edge of the leaf are cells preparing for the leaf's demise.

Abscission, the shedding of parts is common, fruits and flowers are separated.  Deer shed antlers after rutting season.  Leaf fall is the visible abscission event.  The variation is striking.  Some trees lose leaves early, some all at once, some not until spring.  The evergreens lose their leaves at a slower constant rate all year.  There seems to be a premium on diversity.

There is an interest in the exact biochemistry of the process (second figure).  The interest is in part due to civilization depending on abscission.  The wild wheat seeds 10,000 years ago, had weak abscission zones.  The seeds blew with the wind like dandelions.  Over time  the early farmers selected varieties of wheat with strong abscission zones.  The seeds stayed more firmly attached and were easier to harvest.  Without strong abscission zones there would be no modern culture, no Ipads.

The moral is to plan ahead for the shedding of unnecessary parts, clean out the attic.

The next December the remaining leaves still on the trees seem to be oak or beech.  The first photo is the oak tree at the end of Wagner, progeny of the famous Wye Oak. Leaves are mostly still intact.  The second picture is the beech along Cloverlea, though there are a number of small beeches in the woods with leaves still on the branches.  Trees make some complex decisions, maybe the leaves will provide nutrition in the spring. At least there is a little added color during the winter.