Watershed Stewardship Guides

What is an invasive species?

Interestingly enough, there is not a single, standardized definition of “invasive species.” Most of the time, though, the term refers to a species that has been introduced to an ecosystem by human beings–intentionally or unintentionally–that is able to take advantage of certain ecological conditions and outcompete native species. This typically occurs in “disturbed” ecosystems or areas, where the adaptations of native species are no longer aligned with habitat conditions. The consequence of any “species invasion” or infestation is that native species, the relationships between native species, and the usual functions of an ecosystem, can be destroyed and replaced by a single dominant species. As a general rule, more diversity is better; so when there’s a single dominant species, especially if it’s not native, the system is much less productive and resilient.

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What are some common invasive tree species in Santa Fe?

The two most common invasive trees in Santa Fe are Siberian elm Ulmus pumila and Russian olive Elaeagnus angustifolia, followed by Tree of Heaven Ailanthus altissima and, more rarely, tamarisk or saltcedar Tamarix spp. All of these trees originate in the Central Asian steppes, which have a similar climate to Northern New Mexico. They were therefore intentionally introduced to New Mexico by American settlers to provide shade and to control soil erosion. They can all indeed grow into beautiful trees, but the problem is they now mostly grow where we don’t want them!

The introduction of these four decorative and drought-resistant trees occurred at about the same time that drastic ecological changes were happening throughout New Mexico, such as overgrazing, gully erosion, and stream channelization. These ecological impacts have made it more difficult for native species like cottonwoods and willows to reproduce, and gave nonnative species like Siberian elm an extra advantage.

As you can see, then, many times invasive species are a symptom of ecological disturbance, rather than a primary cause.

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What are the benefits of invasive species?

Again, it’s always complicated! Sometimes, though, an ecosystem has been so altered that native species can no longer survive without ongoing human intervention. Depending on the management goals for an area, then, it can make sense to keep invasive species around while trying to prevent them from spreading. Here are some general examples of benefits invasive tree species provide in Santa Fe:

• Survive where other plants can’t
• Shade, heat island mitigation
• Soil cover, reduce evaporation from soil and surface water
• Greenspace
• Wildlife habitat and food (even endangered native wildlife)
• Carbon sequestration
• Wind and sound barrier
• Clean air and stormwater
• Reduce soil erosion (though often not as much as native plants)
• Help our watershed be more forested

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How are invasive tree species harmful?

• Outcompete native species
• Provide often lower quality forage for wildlife
• Damage infrastructure
• Vulnerable to pest outbreaks
• Can exacerbate soil erosion
• Diminish biodiversity

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Do invasive species use more water?

Studies have been inconclusive about how much water invasive species use compared to native species. The most indicative factor of how much water a tree releases through evapotranspiration is the size of its leaf, but this can vary with every individual, climate and weather patterns, and many other factors.

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What are the risks of invasive species removal?

Because the presence of invasive species is usually an indicator of disturbed soil, removing or killing the trees (or other invasive weeds) often does not solve the problem on its own. It can even make matters worse! Various kinds of herbicides are often used on cut stumps or sprayed over an area, but we still don’t know the full ecological impact of these chemicals. Since many of our invasive trees are in riparian (river and arroyo) areas, there is an added risk of water contamination when applying herbicide. Without applying herbicide, however, invasive trees tend to just grow back due to their rhizomatous roots. This method makes for nearly infinite maintenance!

The other option is pulling up the entire tree–roots and all. This method indeed has less risk of contaminating soil and water with chemicals, but the soil itself is horribly disturbed in the process. Pulling makes soil more vulnerable to erosion, drying, and recolonization by yet more invasive plant species.

Regardless of the method, if there is not a plan for replanting or natural reestablishment of native plants, removing invasive trees (and other plants) can result in a total sterilization of the soil and near-total loss of ecological function. If the soil remains bare and unprotected, it will lose its moisture in the hot sun and become uninhabitable to plants and soil microbes. As a result, it can be carried away by wind and water erosion, leaving the landscape less habitable still. Furthermore, as soil erodes surface water becomes more turbid (muddy) and uninhabitable for sensitive animal species.

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What are the benefits of invasive species removal?

If there is a good reason to remove the invasive species and a planting or restoration plan is in place, it can be extremely beneficial to remove some or all invasive species in a given area. Especially when there are only a few individuals in an otherwise native-dominant area. Removing invasive species provides the opportunity for native species to reestablish, and for some level of ecological function to resume. Even leaving older trees and consistently removing younger saplings and shoots can be a great way to give native species an advantage to grow in the understory, and eventually replace the canopy under the right conditions.

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So what do we do?

Once again, it’s complicated; it depends! No two sites, contexts, or management plans are the same, so the “answer” of how to deal with invasive species will also change accordingly. If you are thinking about invasive species on your property, we recommend consulting an ecologist or qualified arborist. In general, though, here are some questions that can help provide a more nuanced perspective of what management approaches might be appropriate in which contexts:

• What are the management goals?
  • To reestablish native species?
  • To create shade and greenspace?
  • To protect infrastructure?
  • Nature preserve?
  • Urban park?
• What resources are available for the project?
  • How much money, staff or volunteer time, water, etc. is available? For how long?
  • Will the project be able to sustain itself after a certain time?
  • Is monitoring and adaptive management possible?
• What are the ecological contexts of the site?
  • How much water is available to plants?
  • How disturbed is the soil?
  • Can other plants survive if the invasive species are removed?
• What are the desired ecological services provided by the site?
  • Wildlife habitat and corridors?
  • Carbon sequestration?
  • Scenery and greenspace?
  • Erosion prevention?