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Dallas Morning News - January 25, 2018


EVERGREEN, SEMI-EVERGREEN, MARCESCENSE AND CROSS BREEDING OF OAKS




Red oak with marcescence


Evergreen trees and plants in general are those that keep their green leaves year round. Even they drop their leaves but it’s usually in the spring. Deciduous plant are perennials that loose all their leaves at one time usually in the fall, but there are two variations of this condition that are interesting to understand.



Red oak without marcescence


The first is semi- deciduous. Trees with this factor hold some green leaves throughout the winter releasing the dead leaves over a period of time. The fall color of these trees is spotty and inconsistent. Water oak, willow oak, Canby oak and others are in this category.



Caddo maple with marcescense


The second is called marcescence. It literally means “withering but persistent.” In trees it means that the leaves turn brown in the fall but stay on the tree through the winter and release in the spring when bud swelling of the upcoming new growth pushes the old leaves off.



Semi-evergreen canby oak on left. Evergreen live oak on right.


Deciduous trees form an “abscission zone” at the base of each leaf stem that allows leaves to fall off the twigs. Trees such as some red oaks, post oaks, pin oaks, white oaks, Caddo maples and others don’t allow this to happen until closer to spring.



Semi-evergreen canby oak foliage


Scientists are a little baffled. Are the dead leaves clinging to their trees. Or - are the trees are clinging to their dead leaves. Theories for this clinging range from deer browse protection, to delay of leaf decomposition once on the ground, to plant immaturity since marcescence seems to be more common on younger trees.



Better shot of canby oak in winter


Some experts suggest that marcescence helps trees growing on dry, infertile site, thinking that holding leaves until spring could be slowing the decomposition of leaves and that dropping them in spring puts organic material on the soil when it is most needed. Even small amounts at the right time could shift the competitive advantage toward these trees on poor sites.



Better shot of canby oak in winter


Others suggest that retained leaves, especially on young trees and the lower branches on bigger trees, is an effective means of trapping rain or snow for more moisture at the base of the trees come spring. Still others have suggested that persistent leaves might provide some frost protection for buds and new twigs over winter. And at least one study suggests that marcescent foliage could be a deterrent to browsing by deer and other wildlife. Buds hidden by dead leaves are protected and live to become new growth in the spring.

The bad news is that the trees have conspired to give us leaves on the ground to manage year round. I think the interest is worth it.
 

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