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Why are the Trees Brown?

July 1st, 2021
Why are the Trees Brown?
A note from Ben Bloodworth, Tamarisk Beetle Program Coordinator with RiversEdge West
 
Whether you’ve been on the river, up in the Redlands, or out on G Road, you’ve likely seen large “dying” brown shrubs/trees that were bright green just a few weeks ago. These are tamarisk trees, or salt cedars, and the phenomenon is currently so pervasive here in the valley that folks are starting to wonder about it on social media. So, before you blame anyone for over-spraying, or the hot, dry air of June, let’s chat about the millions of little beetles that live in the valley with us.  
Tamarisk is an invasive plant from Asia and the Mediterranean that occupies many Western waterways in the U.S. For various reasons, the plant was enough of an environmental and economic impact that the U.S. Department of Agriculture (USDA) decided to research and release a biological control (biocontrol) agent to address the issue. Biocontrols are natural “predators” that feed on plants in their native ranges but are not found in the U.S. In 1987 the USDA began a program to find natural enemies of tamarisk and see if they could live and feed on tamarisk in North America without feeding on anything else. Out of 300 organisms that feed exclusively on tamarisk, the tamarisk beetle proved to be the most successful. Since tamarisk beetles survive on tamarisk as their only food source, they were never intended to eliminate the invasive plant. Rather, they were released to help control the spread of the plant and reduce the amount that land managers must remove by other, more costly means. Prior to being released, tamarisk beetles were tested on native species and agricultural crops to ensure they would not eat any non-target species and, in fact, they will only eat tamarisk. After more than a decade of successful testing and field trials, tamarisk beetles were released on the Colorado River near Moab in 2001 and moved up the river into the Grand Valley a few years later. RiversEdge West has been monitoring the movement and impacts of the tamarisk beetle since 2007, but we have never been involved in releasing the beetle.
Tamarisk beetles are green with yellowish stripes and are similar in size to a ladybug. They defoliate tamarisk trees and large populations can do so very quickly, turning large swaths of green trees to an orangey-brown in a matter of weeks. The beetles’ life cycle is about five or six weeks and they spend most of that time as a small black caterpillar-looking larvae that does most of the feeding/damage. You will see three to four generations of beetles here in the valley each summer before they go into diapause (like hibernation) beneath the trees for the winter. The populations ebb and flow with available tamarisk and some years we have almost no tamarisk beetles anywhere. As you may have noticed by all the brown tamarisk, this is not one of those years!
Oftentimes, people are worried that the green vegetation on the riverbanks suddenly becoming brown means that the risk for wildfire has greatly increased. But this is not the case. While there may be a brief period of increased ignitability when the dead leaves are hanging on the trees, this only lasts until the leaves are dropped (usually a month or so). The dead branches of tamarisk after defoliation are not nearly as flammable as the green vegetation. Unlike our native riparian species that have historically stopped fires, tamarisk promotes wildfires and flourishes when burned. Due to its structure and biomass, green tamarisk is literally explosive when it burns and grows back immediately after a fire. Flame lengths of over 130’ have been documented in tamarisk stands and experiments have shown that trees repeatedly defoliated by the beetle, and now having dead branches instead of a green canopy, burn with flame lengths of less than 25’. So, while it is hard to wrap our brains around green trees burning better than dead trees, this is definitely the case for tamarisk.
The brown trees may not be pretty, but they do provide for opportunities to restore native vegetation alongside our rivers. If you would like to learn more about what you can do to help restore riverbank vegetation in your area, visit our website and take a look at the ways watersheds throughout the Southwest are returning their rivers to native green vegetation that is resistant to wildfires.
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RiversEdge West is a Grand Junction-based nonprofit dedicated to the restoration of riparian (riverside) ecosystems for the benefit of both fish and wildlife as well as the economic, social, and cultural well-being of communities in the American West. RiversEdge West does not release the tamarisk beetle, rather, its Tamarisk Beetle Monitoring Program maps the tamarisk beetle’s movement and connects land managers with information on the beetle and its potential impacts.
 
To learn more, visit tamarisk beetle page or contact Ben Bloodworth, Tamarisk Beetle Program Coordinator, at bbloodworth@riversedgewest.org.
Figure 1 Adult tamarisk beetle
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