Abscission Zones  - Abscission zones form at the base of leaf stems and where mycorrhizae are attached to non-woody roots and where non-woody roots are attached to woody roots.  The processes with the roots are different then that of the stems.  With non-woody roots and mycorrhizae the abscission zone forms first and then the non-woody root or the mycorrhizae die and are digested by soil microorganisms.  When mycorrhizae are killed before the zone forms, then it does not form, and becomes an infection court. The leaf on the other hand, first dies and then the abscission zone forms and the leaf is shed.  Some trees like young beech and pin oak do not form the abscission zone and therefore the trees hold onto the brown leaves over the winter.  We do not know why this is.  If you remove the leaf stem of the latter you will see the green tissues where the zone did not form.  Click here for some pictures

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