BOARD01713

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

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Impact of Tamarisk Infestation on the
Water Resources of Colorado
May 30, 2003

Why these discrepancies? The likely reason for this information gap is two-fold. Many county
weed managers serve many different functions and simply did not have the time to develop an
inventory and/or tamarisk and Russian-olive were not recognized as a problem. Based on a good
approximation of the inventory using anecdotal information (e.g., personal observations,
discussions with weed managers) and the more detailed information that the Corps of Engineers
has developed, a reasonable approximation of the acreage that is currently infested with tamarisk
and Russian-olive is 50% higher than that reported. Thus, the estimated infestation used within
this report is 42,000 acres for tamarisk and 15,000 for Russian-olive, or a total of 57,000 acres.
Infestations occur on every major river drainage in the state with the exception of the North
Platte.

Another question is the rate of invasiveness that will likely occur into the future. Zimmerman
(1997) estimates that the rate of spread could be as high as 5 percent per year. This seems high
for Colorado where the higher elevations do not offer tamarisk much competitive advantage.
However, there are river systems, particularly the South Platte and Republican and the tributaries
to the Arkansas in which tamarisk has a foothold and could spread rapidly. Figure 3 presents the
predicted acreage occupied by tamarisk and Russian-olive over the next 50 years based on a
modest growth rate of 1 percent per year and a moderate growth rate of 2.5 percent per year.
What is notable about this information is that tamarisk and Russian-olive infestations are
predicted to jump to between 90,000 acres and 200,000 acres during this 50 year time frame if m
active control program is put into place. These are considered to be conservative estimates.

Photographs of typical tamarisk infestations are presented in Figures 4 through 11.

Research Need - A more accurate means of inventorying is required to identify
location, density, and mass of tamarisk and Russian-olive in comparison to native
species. The survey should rely on the physiological characteristics of the non-
native plants; previous studies; historic records; and personal knowledge from
county weed managers, state weed coordinator, state agriculture specialists, federal
weed managers, university researchers, etc. Based on the historic progression of
tamarisk over the past 100 years and the physiological constraints of tamarisk and
other non-native phreatophytes, future areas of infestation should be predicted for the
next 50 years.

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