2021 APRIL 10 ISSUE: WHAT TREE SPECIES SHOULD REPLACE COTTONWOODS?

Revised guidelines for managing the Corrales Bosque Preserve were approved by the Village Council at its March 23 session. As explained by Corrales Bosque Advisory Commission Chair Joan Hashimoto, the guidance marks the first official recognition that since the preserve’s cottonwood trees are dying out, replacement species should be encouraged. “The question facing us is do we want another species of tree in the bosque,” she explained after a councillor asked whether it would be possible or prudent to replant cottonwoods as they die out.

“That is the issue everyone is grappling with right now,” she replied. Hashimoto was responding to questions following her power point presentation on revisions to policies for managing the preserve. She said Santa Ana Pueblo is exploring alternatives to maintain a forest canopy along the middle Rio Grande, including experiments with boxelder maples. “We should probably go and take a look at how they’re doing. And The Nature Conservancy has come out with a list of trees that they think will do well for the next 100 years, and we should take a look at those.”

Hashimoto told the mayor and council that a die-off of the cottonwoods can be expected over the next ten years. As has been explained in Corrales Comment often during the past 20 years, the cottonwoods’ desperate situation is due to erosion of the riverbed, which means the ever-deepening channel carries more water, precluding the over-bank flooding necessary for cottonwood seeds to germinate on the bosque floor. (See Corrales Comment Vol. XXXVIII No.12 September 7, 2018 “Rio Grande’s High Flows Bring Hope For Cottonwoods.”)

“There is no consensus as to what kind of tall canopy tree would do well here in the next 100 years.” Councillor Zach Burkett asked Hashimoto how many years it might take for the preserve’s cottonwoods to recover. She replied: “We would need a lot of bank-lowering to achieve that. If you go to the river’s edge and look down at the river bed, let’s say at the siphon or at Dixon Road… usually you can see that there is no way the trees’ roots can be down in the water. So we would have to have lots more Army Corps of Engineers projects to remove tons of dirt at the river’s edge for us to have good cottonwood growth.”

She referred to the technique used by the Corps in 2012 to lower the bank so that available river water could flow onto the floor of the bosque where cottonwood seeds could regenerate. But even if the soil is sufficiently moist, for long enough duration, for seedlings to sprout, the future trees might still be unable to thrive since their roots likely would not reach ground water reliably.

The chronic lack of over-bank flooding into the preserve has been blamed on unintended consequences of dams built on tributaries to the Rio Grande, especially Cochiti Dam, which have dramatically reduced silt flowing into the river; the relatively clean water flowing past Corrales has picked up tons of sediment from the river bed, incising a channel.

The following topics are covered in the new guidelines: the long-established mission of the preserve; animal and plant life; environment and pollution; the Corrales Fire and Police Departments; recreation; research and education; and interagency cooperation.

Guidelines for recreation include:
• Manage as a natural area for passive, non-motorized and non-consumptive slow-speed recreational activities including walking, running, horse riding, bicycling, photography and bird-watching;
• Prohibit infrastructure development contrary to natural and protected conditions, including paving or resurfacing of roads and trails;
• Discourage creation of new trails through the forest and down levee road slopes.
Under the topic Environment and Pollution, the bullet points are:
• Reduce non-point source pollution by encouraging pet waste pick up;
• Minimize use of pesticides and herbicides;
• Prefer mechanical treatment of invasive plant species over chemical treatments;
• Prioritize bank-lowering and backwater creation projects and maintain current backwater channel functionality.
Under the topic Animal and Plant Life, the guidelines read:
• Protect and promote a variety of habitats, including a mix of tree and shrub heights and species, grass meadows, younger willow stands, and a mix of water channel types in order to maintain a biologically diverse ecosystem of wildlife populations and plant communities;
• Mitigate detrimental effects of invasive plants by removal when appropriate;
• Consider introduction of canopy tree species adapted to changing climatic and hydrologic conditions;
• Maximize use of native plants over non-natives for any projects.

The guidelines call for the fire department to “conduct fire risk mitigation efforts such as shaded fuelbreak maintenance and targeted removal of invasive species and dead-and-down vegetation without compromising other preserve management principles and guidelines.”