Oak Trees- why won't the leaves fall?

[mdvaden]

One reason I don't like Pin Oak is that they don't drop leaves in autumn.

I like Live Oak. We had those during our short 8 month experiment living in Georgia.

Up here in Oregon, I was noticing how 1/2 our purple beech leaves were still clinging on this week. Seems that's not the norm. But we just planted them last summer.

[bewildered_nmsu]

Shumard oak (quercus shumardii) is a deciduous oak that retains its leaves until spring. Though they stay on the tree, after the first frost they will turn brown and die similar to a Beech tree.

[RagweedFarmer]

I saw this site while researching the leaf drop patterns of deciduous trees, an interesting subject. I have a very plausible reason why oaks and beeches retain their leaves, one I have never read anywhere. They retain the leaves to keep them from burying their nuts. Actually, they very rarely retain any leaves above the twenty or so foot level. So there is a pattern where the taller trees have leaves on the bottom, and are bare on top. The shortest oaks and beeches retain all of their leaves. At higher levels the wind is able to blow the leaves far enough away so they won't cover the nuts.

Throughout evolution it was easier for squirrels and other nut spreading animals to find nuts that were not covered. Both oaks and beeches have a fairly hard wood compared to say apple. The oaks are the hardest because their leaf shape retains more ice and snow. The beeches’ pointy downward facing leaves shed ice and snow better than the oaks so they do not need to have as hard a wood. it's still pretty hard, but not as hard as the oaks.

Fruit bearing trees shed their leaves and have a softer wood. During the winter there are plenty of acorns still under the oak trees, but any fruit is long gone or at least spoiled so there is no advantage to fruit trees retaining leaves. Trees like maple that spread their seeds through the air also drop their leaves and have a soft wood.

I live in Massachusetts and I have not seen any black walnuts or horse chestnuts in over 20 years. I did find one short statement on the web that mentioned autumn leaf retention for black walnut, but could not find any such pertinent info on horse chestnuts. I do wonder though if they have a similar leaf retention pattern as the oaks and the beeches.

Another evolutionary observation worth sharing is that weeping willow has a very soft fragile wood. To compensate for this weakness it has very small leaves that swing rather freely on ropelike stems. If it had rigidly fixed broad leaves its branches would surely break.

One evolutionary observation in trees that I have puzzled over is why do pine trees drop the pine cones on the lower branches in the fall, yet retain the ones on the upper branches until spring?

[bewildered_nmsu]

It's funny that you mention that on the pine trees. Just the other day I noticed that my neighbors Italian stone pine retains it's upper cones through the winter, while dropping its lower cones in the Fall. Perhaps to ensure that there are fresh seeds stratifying on the ground in Fall and Spring?

[plants4us]

For reasons that are unclear, some species of oak trees do not lose their leaves over winter. Leaves detach (senesce) in the fall because of the formation of an abscission zone where an abscission layer (separation layer) forms at the base of the petiole (leaf stem). The process is usually associated with a decline in auxin "hormone" levels. Peptic acid in the cells of the abscission layer convert to pectin resulting in weakening of the zone via cell death and formation of a callus. Abscisic acid may also promote "senescence" of the leaf. In the case of the oak trees, it is believed that the cells of the abscission layer do not completely die so the leaves remain attached until the following spring when new growth begins.