During our three days on the San Juan River, it was almost impossible not to notice the abundant tamarisk growing alongside the river. Our guide Bret informed us of the negative impact this invasive species is having on the riparian or river corridor ecosystem. Tamarisk, or salt cedar, is a deciduous shrub or tree that was introduced to the western United States in the early 19th century as an ornamental and later for windbreak and erosion control purposes (1). By the mid 1900’s the plant had become a well known problem in western regions of the U.S.
Tamarisk has a natural tendency to grow into very dense thickets, at times as many as 3000 plants per acre, which prevent the expansion of native vegetation (1). Native to Central Asia and the Mediterranean, this plant species has an “extensive root system well suited to the hot, arid climates and alkaline soils common in the western United States” (1). This specific species of plant mines into the watershed, monopolizing the water supply, and enabling tamarisk to thrive while native vegetation perishes. Due to the depth of its far-reaching root system, tamarisks draw more salts from the groundwater than native vegetation which is then excreted through the leaves and deposited into the soil once they fall off the plant. This process causes an increase in soil salinity which in turn prevents the germination of many native plants, allowing tamarisk to take over. “Because tamarisk stands develop into dense thickets, sediment accumulates in their extensive root systems and promotes further tamarisk growth, “ (1, p.1). From the rafts, we observed that tamarisk dominated the shoreline with no other flora competing for even the smallest space.
In addition to out-competing other native plant species, the spread of tamarisk causes other problems as well. The dense habitat of the plant limits recreational access to the river. The dense growth also causes a fire hazard and allows fires to spread where once they weren’t such an influence on the ecosystem. Another impact is due to their accumulation of soil, tamarisks cause river and stream channels to gradual narrow and flooding increases (1, p.1). Yet still another negative consequence is, “conversion to tamarisk typically coincides with reduction or complete loss of bird species strongly associated with cottonwood-willow habitats http://bulk.resource.org/gpo.gov/register/1995/1995_10708.pdf. If ecological reasons aren’t bad enough, “cottonwoods and willows are culturally valuable to tribes such as the Hopi and the Navajo. Roots, branches, and logs are used in baskets, kachinas, and structures” (http://www.gcrg.org/bqr/13-3/aliens.html).
As I stared at the river bank, I wondered what the area looked like 50 and 100 years ago, before the takeover of tamarisk? Bret pointed out the huge flood plain in the upper part of the river that once existed before dams upstream started controlling the water flow. Bret showed us old river channels that were still lined by the native cottonwoods. Camping under cottonwood trees in particular is ideal as the trees cast a cool shadow from the hot, high desert sun.
For all of the above reasons, the prevailing thinking has been to eradicate tamarisks and restore once native vegetation. Mechanical, chemical, and biological eradication efforts have all been experimented with to prevent further spreading of tamarisks. There have been attempts to reverse the damage that has already occurred and return river corridors in the West to their original state. Mechanical removal efforts are methods by which the shrub or tree is cut or mowed, however this is rarely effective as re-growth is high. Burning is not a viable option either as tamarisk can recover far quicker from fire damage than native vegetation can because it sprouts vigorously from the root crown. (2) The abundance of leaf litter from tamarisk raises the threat of wildfires which actually stimulates its growth, but destroys native vegetation such as cottonwoods and willows.
Chemical methods involve cutting the stump of a tamarisk two inches above the soil surface and treating it with an herbicide immediately. When the bark is dry, another herbicide can be applied near the base of the plant. In the fall months, herbicides may be sprayed on the foliage, however re-growth is common following these methods and re-treatment must be applied in order to kill the shrub (2).
Biological eradication methods involve the use living organisms in order to suppress the growth of the tamarisk species. Diorhabda elongata, or the “tamarisk leaf beetle,” has been tested since 1992 and approval of field testing was granted in 1999. In 2001, beetles were released into the wild. This biological control is still relatively “new” and the overall effect it will have on the suppression of tamarisk growth is still being observed and studied. Beetles control tamarisk by feeding on the plants in massive hoards, completely defoliating the plant which prevents photosynthesize and storage of food in root systems. Repetition of this process over the course of years causes the roots to diminish in size to such a degree that they can no longer support the plant. Introduction of a foreign organism, like beetles, into an ecosystem has raised concerns. Years of testing, prior to being released into the wild, has shown that the beetles prefer the species of tamarisk over other native flora (2).
Not all scientists saccept that tamarisks need to be removed everywhere in the West. Biologists studying the endangered willow flycatchers discovered a significant amount of the remaining 400 pairs of birds now nest in tamarisks- indicating how long the plants have been around (3) “The attraction is proximity to water, shade and a branching structure that apparently reminds the birds of the native willow trees they historically nested in.” So, if enough beetles were released to eat, “… all of the tamarisk, the reasoning went, an important chunk of nesting habitat would disappear, pushing the bird closer to extinction.” This argument is found in a fascinating article entitled, “Tackling tamarisk” a feature story- an ecologist, Bob Ohmard, of Arizona State University, who did “pioneering studies in the 1970s showing that tamarisk supports less animal life - from insects to birds and mammals - than native vegetation. …Ohmart argues that tamarisk has merely taken advantage of our sick, static river systems, and that removal of it will leave next to no habitat for the willow flycatcher and other riparian-dependent species. Dams and water diversions have permanently altered river corridors in the West, he says, rendering many inhospitable to native cottonwoods and willows. ‘If you wipe out saltcedar, what will replace it? Not much,’ says Ohmart. ‘I'd rather see a monoculture of saltcedar than bare dirt. At least it holds in soil and provides a little habitat. We're damn lucky tamarisk came along when it did.’
Backing up Ohmart is Bertin Anderson, an ornithologist turned soil scientist based in Blythe, Calif.. "Removing tamarisk and replacing it with native vegetation isn't going to happen on any major river in the Southwest," says Anderson, who has spent the last 25 years restoring native vegetation to sites along the lower Colorado River for the Bureau of Reclamation, various Indian tribes and other clients. Rivers and their banks have become saltier and drier since irrigated agriculture and dams came in more than 50 years ago, he says, and these conditions favor the hardier salt cedar over the cottonwoods and willows. Anderson says that dams have prevented the nurturing floods that wash salts out of the soil and moisten it to sustain native plants. “ Anderson says he analyzed a large portion of the Southwest's rivers, looking at soil conditions, and found that 75 percent of the area is no longer suitable for natives. ‘But a large percentage of the area is still suitable for tamarisk,’ he says. ‘In some places, the conditions are so bad that even tamarisk can't survive.’ Ohmart's and Anderson's viewpoints have elicited visceral reactions from those involved with tamarisk control. “I don't want to damage the willow flycatcher, but it's clear that the willow flycatcher has been hurt by saltcedar," says DeLoach. "There are 50 other endangered riparian species that could benefit from biocontrol of saltcedar. The Fish and Wildlife Service needs to look at the benefits to the whole ecosystem, not just the willow flycatcher. You want to manage for flux."
It seemed like our PSU group was in such a rural, unpopulated area of Utah, and yet once we were told about the invasive tamarisks, the impact of humans became clearly visible. A quick glance to either shoreline along our three day trip down the river between Bluff and Mexican Hat on the San Juan provided first hand views of this overpowering plant species. Only upon researching the topic did the controversial, complicated, conservation issues facing the West regarding this plant surface. We gained a new appreciation for the longterm consequences of invasive species- a global issue of concern.
Citations
1.) http://www.tamariskcoalition.org/tamariskcoalition/index.html
