Flora and Fauna News

Chinese Elm
Bonanza

PHOENIX, Az. ----- Unlike American Elm and Slippery Elm of the East and Midwest, Chinese Elm (Ulmus parvifolia) waits until early December to shed its autumn leaves. Then by mid December its crop of seed is ripe. The seed looks every bit like typical elm seeds, bearing a flattish seed set at the center of a papery disc. Breezes catch the seeds and carry them away to be sewn elsewhere.

The majority of landscape trees in the urban environments of the Desert Southwest are exotic. That is, they originated in another part of the world, and are here because people have transported their seeds and propagated them in cultivation. A few have escaped cultivation and can now be found growing in the wild, much to the dismay of wildlife ecologists. There they often become vigorous weeds crowding out native plants. One reason they do so well is that they exist without the cadres of beetles, butterflies, moths, bugs, rusts and diseases that naturally live upon them in their homelands.

Not many people think of plants as having a budget, but they too must wisely allocate limited resources in the form of energy and minerals to various activities. Of the total budget, a portion goes to new growth, another goes for chlorophyll and leaf production, while another goes to root growth and maintenance. Transportation is another budgetary demand, with energy needed to move minerals and food between the leaf system and the root system. With so many resources invested, plants also require a good defense system. Without defenses the plant would be vulnerable to plant eaters of all kinds, especially insects. And what form is this defense? Toxic chemicals. These slow the munching hordes down, but come with a production cost. In other words a certain percentage of the plant's annual budget goes into making these defensive chemicals. But a feed back mechanism helps the plant adjust a bit the chemical production to meet the threat. If there are larvae chewing the leaves, they are stimulated to produce more bitter chemicals. If there are virtually no bugs chewing away, the plant reduces the defense expenditures, freeing resources for growth, or better yet, reproduction.

This often results in a significant bonus for an exotic growing in a land without the insects that can eat it. Not only is less tissue lost to the direct feeding by herbivores, but there is more left over from defense reductions. This explains why exotic weeds and trees often are able to out compete native plants. But it also partially explains why trees like Chinese Elm produce such huge crops of seeds when cultivated here.

Luckily, in this case the seeds do help provide for wildlife, namely seed-eating birds. Flocks of house finches can be found feasting within the Chinese Elm crowns. Others such as Rock and Mourning Doves prefer those on the ground that have been threshed by passing feet or tires. Thus, at this time of year at least, the Chinese Elm gives something back to the Sonoran Desert urban environment. Through much of the remainder of the year however, birds such as Verdins, gnatcatchers and warblers will find little sustenance within the crown of these otherwise vary attractive shade trees. They seek insects to eat, and so must instead forage in the native trees.

Leaves, and Fruit/Seeds of Chinese Elm.