Invaders
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Tamarisk, saltcedar, athel tree (Tamarix spp.)

What is it?

Tamarisk is a general term for several species of Old World shrubs and trees in the genus Tamarix with scalelike leaves on very thin terminal twigs.

Saltcedars are large shrubs or small trees 8-16 feet tall and usually less wide. They have tiny, triangular, scale-like leaves that are winter-deciduous. The flowers are pink to near-white, densely crowded along branched terminal spikes; they appear from January to October. Fruit and seeds are tiny, brown, inconspicuous. The several species introduced to North America have been labeled Tamarix chinensis (our current choice for southern Arizona plants), T. ramosissima, T. pentandra, T. parviflora, and T. gallica. They are very similar in appearance and are hybridizing, so distinguishing among them is difficult. Apparently the hybrid populations are the most invasive.

Young saltcedar (Tamarix cf. chinensis) in flower. Photo: Steve Dewey, Utah State University; www.forestryimages.org

Tamarix chinensis branch with flowers. The tiny triangular leaves stick out from the stems, giving the twigs a prickly feel. Photo: T.R. Van Devender

Athel tree (Tamarix aphylla, also called saltcedar) is a large evergreen tree to 50 feet tall and wide with virtually no leaves (reduced to tiny scales) and inconspicuous whitish flowers. It was long thought to be sterile and therefore at most mildly invasive, but it is reproducing from seed in some localities. It has been doing so at Lake Mead for 30 years, and is hybridizing with the deciduous saltcedars.

A young-mature athel tree at Lake Mead. Photo: Elizabeth Powell Closeup of athel tree twigs showing the tiny scalelike leaves and small white flowers. Photo: Mark Dimmitt; inset: Elizabeth Powell

Australian pines (Casuarina spp.) could be confused with tamarisks. They have similar thin branches and scale-like leaves. Casuarinas don't seem to be invasive in the southwestern U.S., where they are found mostly in landscaped areas.

Why is it a Threat?

Tamarisks are extremely invasive in riparian communities, often nearly completely replacing native vegetation with impenetrable thickets. They are extremely competitive against native vegetation because they are aggressive usurpers of water. They also sequester salt in their foliage, and where flooding does not flush out soil salts the leaf litter increases the salinity of soil surfaces. Dense stands of saltcedars support lower biodiversity than the natural communities they displace.

This stretch of Tonto Creek above Roosevelt Lake in Arizona is choked with Tamarix chinensis. Most of the green on the valley floor is this saltdecar. Photo: Mark Dimmitt Several miles of Greene Wash south of Casa Grande, AZ is dominated by athel trees. These may have propagated by pieces of branches that wash downstream in floods and take root, or they may be seeding. Natural washes in this region are vegetated by desert ironwood trees (Olneya tesota) and blue palo verde (Parkinsonia florida). Photo: Mark Dimmitt

Distribution

Tamarisks are almost throughout the Southwest below about 6000 feet elevation. In Arizona they are widespread, especially south of the Mogollon Rim and in the Grand Canyon.

Habitat

Tamarisks occur mostly on low ground where water collects. They are most abundant in riparian habitats, both natural and artificial, often in extensive pure stands. They are less common in drier places. They thrive in alkaline and saline soils.

History

Various species of tamarisks were in cultivation in the United States in the early 1800s. The National Arboretum released T. pentandra in 1870. Tamarisk was first noticed to be escaping from cultivation in 1880, and by the end of the century it was common along southern Arizona and Texas rivers.

What can be Done

There are teams of "tammywackers" in several states that are making progress in local areas. Specially modified jacks are used to grip and pull trees up by their roots. Cutting and treating the stumps with herbicides gives good control. Aerial spraying is being used in large areas where tamarisks have completely taken over. In areas too large or too remote for manual control, flood management can reduce the dominance of saltcedars. Periodic floods prevent saltcedars from forming dense thickets, permitting native trees and substory plants to establish and maintain codominance.

Links

Earlham College invasive species site, saltcedar page

The Nature Conservancy/UC Davis saltcedar ecology and control discussion

U.S. Government bibliography on saltcedar (links to many sites)

References

Gaskin, J.F. and P.B. Shafroth. 2005. Hybridization of Tamarix ramosissima and T. chinensis (saltcedars) with T. aphylla (athel) (Tamaricaceae) in the southwestern USA determined from DNA sequence data. Madroño 52(1):1-10.