Sunday, September 7, 2008

Student Post, Cottonwood

The Eastern Cottonwood, Populus deltoides. Family Salicaceae (Willow Family)

I grew up with this tree in the South, specifically along the sandy riparian creek banks of the Ozark Mountains, in northwest Arkansas. As many of the adult trees were harvested for timber, it became apparent that no new saplings were growing to replace these old stands of towering trees. This caused me to do a little research to see why.

I began to search the internet for plausible reasons. This is what I learned about the culture of the tree.

Cottonwood trees are one of the largest North American hardwood trees, exceptionally tolerant of flooding, erosion and flood deposits filling around the trunk.

Cottonwoods are widely grown for timber production, providing a large crop of wood within 10-30 years. The wood is coarse and of fairly low value, used for pallet boxes, shipping crates and similar. Cottonwood is one of the poorest woods to use as wood fuel. It does not dry well and rots quickly.

There are three species of Cottonwood: Eastern Cottonwood Populus deltoides; Fremont Cottonwood, Populus fremontii and Black Poplar, Populus nigra. Many of the cottonwoods grown commercially are the hybrid between Eastern Cottonwood and Black Poplar, Populus × canadensis, Hybrid Black Poplar or Carolina Poplar.

The bark is silvery-white, smooth or lightly fissured when young, becoming dark gray and deeply fissured on old trees. The twigs are grayish-yellow, stout, with large triangular leaf scars.

The leaves are large, deltoid (triangular), with a truncated (flattened) base. This flattened petiole causes a distinct leaf motion in the wind. The leaves are dark green in the summer and turn yellow in the fall. The leaves are green on both sides.

Cottonwood are dioecious; the male pollen catkins are reddish-purple; the female catkins are green, which split open to release the numerous small seeds attached to cotton-like strands.

Cottonwood trees are susceptible to the Cottonwood Borer (Plectrodera scalator Fabricius). Note, this beetle is native and not the same as the Asian Longhorn Beatle.

The adult beetles appear in midsummer, feed on leaves and then descend to the base of the tree where the female deposits eggs into small pits gnawed in the bark. Eggs hatch and the larvae bore downward in the inner bark, entering a large root by autumn. Pupation occurs in the gallery from April to June. The new adults chew exit holes and emerge through the soil. Larvae complete development in 1 – 2 years.

My research could not confirm that the beetles were the cause for the disappearing trees. While beetles can cause severe damage to young trees and can severely weaken mature tree trunks to the point of breaking off during high winds, they do not completely wipe out stands of trees.

As with most insect infestations, healthy trees can withstand attacks from insects better than unhealthy trees. Therefore, proper site selection, irrigation and fertilization are important steps to reduce injury. In the instance of the trees along the sandy banks of the Ozarks, the site may have been unfavorable in combination with the appearance of the beetles. It would be great to reintroduce the trees once further study can determine the cause for the decline.

This post is by Paul Harness, Photos by Kevin Waltermire

No comments: