Imperfect metabolism

[DJHicks%Faculty%MC at MANCHESTER.EDU]

A previous poster asked about retention of dead leaves (marcescence) in beech 
and red oak.  In one sense, this indeed must be due to low metabolism in the 
fall, since the lysis of cell walls in the abscission layer is incomplete.  
But there is also a "why" question here.  I haven't seen anything published 
which supports an adaptive role for marcescence, but I can think of a couple 
of possibilities:

1. Perhaps the dead leaf bases provide seals against entry of pathogens or 
loss of water.

2. Retention of dead leaves might influence nutrient cycling.  Since 
significant redistribution of fallen leaves is known to occur in some 
circumstances, perhaps it increases the likelihood of nutrients being 
REcycled to the tree producing the leaves.

Nonadaptive explanations are possible, too.

1. Oak is ring-porous and has large vessels, so perhaps the vessels cavitate 
particularly early in the fall, and the abscission layer is forced to become 
inactive until production of the new annual ring in the spring.  (Beech is 
diffuse-porous, however.)

2. Some families and genera seem to be more prone to marcescence than others, 
so perhaps it is a phylogenetic "heritage" of no current adaptive 
significance.  Oak and beech are both Fagaceae, a family with many evergreen 
or semi-evergreen taxa.

David J. Hicks				djhicks at manchester.edu
Biology Dept., Manchester College