Weed invasions often caused by human impacts
summit daily news
COLORADO SPRINGS ” In the bad old days, the post office featured most-wanted pictures of horse thieves and cattle rustlers. But this is the new West, and images of scruffy outlaws have been replaced by posters of a new public menace – invasive weeds that threaten the ecological well-being of the land.
But how serious is that threat, and what is the best way to address it?
Those questions were raised Monday at the State of the Rockies conference by Anna Sher, research director at the Denver Botanical Gardens. Sher, also a professor at the University of Denver, has developed models for predicting invasion impacts and restoration success and studied tamarisk with Fullbright Scholarship funding in the Middle East.
For years, Sher has focused on scrutinizing and understanding the environmental threat posed by run-amok plant life, with a special eye toward tamarisk, identified as huge threat to the region’s water supplies and biodiversity.
The questions are relevant to Summit County, where governments have committed considerable resources to controlling unwanted plants during the past few years. Friends of the Eagles Nest Wilderness, for example, is about to launch a noxious weed mitigation program with grant funding from the Rocky Mountain Elk Foundation.
Simply removing the unwanted weeks without addressing the underlying ecological reasons for the “invasion” is unlikely to resolve the problem, Sher said.
“Restoration of functioning ecosystems should be the goal,” Sher said. “This is why creating state weed lists is still an important strategy, but we must also consider the … lists as indicators of larger problems that may need to be addressed. Effective weed control will necessarily be an ecosystem approach,” she explained.
The results of Sher’s research challenge some of the conventional weed wisdom and have led her to some conclusions that may be somewhat surprising and help provide a broader context for weed control efforts.
Sher doesn’t challenge the assumption that invasive plants pose a huge threat to ecosystem integrity and biological diversity, calling the invasive plants the “strip malls of nature.” In some cases, the unwanted plants are clearly crowding out desirable native species. But she said she hopes her work will help shift focus away from the species themselves and toward the underlying issue: Ecosystem disturbance.
In other words, invasive plants and noxious weeds didn’t just arrive from outer space and start growing in random places. In many, if not all cases, human activities like development, road-building, stream channelization and water diversions led directly to the profusion of unwanted plants. Understanding that element of the weed invasion will help lead to more effective management and control strategies, and a more cost-effective way to apply limited financial resources, she said.
The tamarisk is a prime example, and Sher spent a considerable amount of time explaining the ecology of that plant, which was intentionally imported into this country as a way to stabilize stream banks. After studying tamarisk, also known as salt cedar, in its native Middle East habitat, Sher has come to the conclusion that dam-building, stream-channelization and changes in the natural flood regime have directly contributed to the spread of tamarisk in the Colorado River drainage and the rest of the country.
In other cases, tamarisk has spread into relatively empty ecological niches – desert environments too harsh for most native plants. This phenomenon has been documented photographically, by comparing images of dry desert washes from 100 years ago with up-to-date pictures showing the growth of tamarisk in those areas.
Sher advocated managing tamarisk with water management regimes that mimic natural cycles, as well as with fire.
“If we manage with flooding, the natives will win out eventually,” she said. Without that flood cycle, there is no “safe place” for native plants to gain a toehold, she explained.
And fire should also be cautiously used. Timing is everything, she said, explaining that, if the fires burn at a time when its advantageous to native seeds, it can help promote the growth of riparian vegetation like cottonwoods and willows that will out-compete tamarisk in the long run.
In both cases, the research suggests that the invasive tamarisks are not highly competitive as seedlings. Given half a chance, the broadleaf cottonwoods can hold their own against tamarisk.
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