BOARD01713

• Board Meeting Date: 7/28/2003
• Description: CF Section - Tamarisk Control and Eradication Update
• Board Meetings - Doc Type: Memo

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

Impact of Tamarisk Infestation on the
Water Resources of Colorado
May 3D, 2003

stream banks from Mexico to Canada. Tamarisk is now believed to cover anywhere between 1.0
and 1.5 million acres ofland in the western U.S. and may be as high as 2 million acres
(Zimmerman 1997). The severe impacts on riparian systems that this infestation causes
throughout the West include (Carpenter 1998, DeLoach 1997):

);> Tamarisk populations develop into dense thickets, with as many as 3,000 plants per acre that
can rapidly displace all native vegetation (e.g., cottonwoods and willows).

);> As a phreatophyte, tamarisk invades riparian areas, leading to extensive degradation of
habitat and loss of biodiversity in the stream corridor.

);> Excess salts drawn from the groundwater by tamarisk are excreted through leaf glands and
are deposited on the ground with the leaf litter. This increases soil salinity to Ie vels that kill
saline intolerant willows and other plants and prevents the germination of many native plants.

);> Tamarisk seeds and leaves lack nutrients and are of little value to wildlife and livestock.

>> Leaf litter from tamarisk tends to increase the frequency and intensity of wildfires which tend
to kill native cottonwood and willows but not tamarisk.

);> Dense stands on stream banks may gradually cause narrowing of the channel and an increase
in flooding. Channel narrowing along with tamarisk- induced stabilization of stream banks,
bars, and islands lead to changes in stream morphology, which can impact habitat for
endangered fish.

);> Dense stands affect livestock by reducing fomge and preventing access to surface water.

);> Aesthetic values of the stream corridor are degraded, and access to streams for recreation
(e.g., boating, fishing, hunting, bird watching) is lost.

While each of these points is important to one or more constituencies, the single most critical
problem is that tamarisk uses significantly more water than native vegetation that it displaces.
This non-beneficial user of the West's limited water resources dries up springs, wetlands, and
riparian areas by lowering water tables (Carpenter 1998, DeLoach 1997, Weeks 1987). As
tamarisk mO\es into adjacent upland habitats through the aid of its deep root system (up to 100
feet (Baum 1978)), it consumes even more water as it replaces the native grass/sagebrush/rabbit
brush communities (DeLoach 2002). Zaveta (2000) demonstrates that a program of tamarisk
control and revegetation would have clear economic, social, and ecological benefits. The
National lnvasives Species Council has identified tamarisk as one of its primary targets, the
Colorado Department of Agriculture has listed it on its noxious weed list, and Colorado
Governor Bill Owens has issued an Executive Order to control tamarisk on public lands within
ten years (see Appendix A).

Tamarisk is the primary non- native phreatophyte of concern in Colorado and thus has the
dubious distinction as the "poster child" of non-native plants impacting the riparian zone of the

4