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Why researchers want to know about every single tree lining Denver’s High Line Canal

A long-term study along the 71-mile recreation lifeline seeks dozens of pinpoint facts on each new tree, as climate change, drought and pests loom.

Tiffany Gentry, a research assistant at Denver Botanic Gardens and graduate student at CU Denver, prepares to take a soil sample on the High Line Canal on June 30, 2022. (John Leyba, Special to The Colorado Sun)
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Saving 71 miles of the High Line Canal starts with the 48 inches between the spherical densiometer in Alissa Iverson’s hand and the catalpa tree fighting to take hold on the stream bank. 

A densiometer is as Harry Potter-like as it sounds, using mirror reflections to estimate the amount of sunlight received by a newly established sapling hoping to survive amid 120-year-old cottonwoods. 

“Two, four, six, eight, 10, 12 … ,” Iverson counts, ending at 66 of 96 reflective dots that show a clear path to blue sky. Enough light to give the young catalpa a puncher’s chance amid drought, climate change, trunk-boring pests and doddering plains cottonwoods

For weeks now, Iverson and the team of budding arborists she is part of have scoured the banks of the High Line in this summer’s entry in a long-term study. They take precise measurements of hundreds of recently planted trees in order to help plan the future of a recreation lifeline that winds within a mile of 350,000 people.

TODAY’S UNDERWRITER

Height, latitude and longitude, distance from water’s edge, distance from jogging path, bug damage, canopy diameter. All eventually plugged into iPads for what they hope to be an open-source database built for the long haul of climate and botany research. 

Denver Water once used the canal to deliver precious irrigation to farmers and towns northeast of where the South Platte emerges from Waterton Canyon. Now there are less wasteful ways for that water to get where it needs to go, and the canal bottom is frequently dry. 

Yet recreational uses by hundreds of thousands of visitors are more popular than ever, under a ribbon of green canopy at once stitching together and transforming metro Denver’s high-desert culture. 

Research assistant and grad student Alissa Iverson uses a spherical densiometer to determine whether young trees are getting adequate sunlight to thrive under the dense cottonwood canopy on the High Line Canal. (John Leyba, Special to The Colorado Sun)

“We need to think forward over time. This is now a greenway,” said Christina Alba, assistant research scientist at Denver Botanic Gardens and co-leader of the High Line Canal surveying project. “So how can we take it from its original use as an irrigation corridor, which is being decommissioned for that use, and reimagine it going forward? Because the public uses it, and needs it and depends on it.” 

Earlier crews counted more than 23,000 mature trees along the dozens of miles of canal. Nearly half are cottonwoods, native to any flowing stream in Colorado but foreign to the dry parts of metro Denver until the High Line was dug and filled in the 1880s. 

Many of the oldest cottonwoods are nearing the end of their lifespan, including the half-dessicated specimen blocking part of the newborn catalpa’s sun. Denver Water helped seed fund a conservation group to add in thousands of new trees, and track their performance in the coming decades. 

First, of course, the new and old trees must have water. A consortium of agencies and nonprofits are rebuilding sections of the canal to create natural filtration of storm runoff and using the runoff to help wet the canal ecosystem when Denver Water can’t fill the artificial river. 

Chrissy Alba, assistant research scientist in the research and conservation department at Denver Botanic Gardens, measures a tree along the High Line Canal. (John Leyba, Special to The Colorado Sun)

The surveying crews, co-led by Alba from the botanic gardens and University of Colorado Denver associate biology professor Laurel Hartley, make field calls to give the High Line’s new trees their first annual checkups. 

On a late June day, behind the Eisenhower Recreation Center in southeast Denver, the leaders and their grad and undergrad students unholstered the tools of their trade.

Every new tree has the equivalent of a radio frequency collar. Not that the cottonwoods are migrating. But exact geographic location can be synthesized over time with temperature, precipitation and dozens of other factors to eventually write a deep narrative of the High Line environment.

Tiffany Gentry unsheathed a serrated dagger to take a wedge of soil from beneath the catalpa and work it between her hands. How long a rolled “ribbon” of soil gets before it breaks off, like a cigarette ash, tells much about the amount of silt vs. sand in the mix. 

Gentry then pours water into her hand, creating a mud both silky and gritty, indicating the soil behind the rec center features a just-right loam. 

Summer Merrell tries a moisture meter beneath a Shumard oak. Soil moisture begins to create viable conditions for many trees when it’s about 30 or above. On the land nurturing this young oak, Merrell couldn’t even shove the sensor point underground, a sign of overbaking likely to go in the books like a big zero.

Layla Blair, an undergraduate research assistant at CU Denver, uses a compact handheld clinometer for measuring heights, slopes, and angles during research along the High Line Canal. (John Leyba, Special to The Colorado Sun)

Hartley tosses a rectangle onto the ground under another sapling. Officially, it’s a Daubenmire frame. Unofficially, it’s a grid of thrown-together PVC tubing, slightly bigger than a legal pad. She takes note of all the plant life inside the grid, to judge whether invasive grasses are surrounding the tree and choking off the water supply. 

Iverson uses a clinometer and some basic trigonometry to judge the height of one of last year’s plantings. Kristina Parenko and Layla Blair stretch a measuring tape to precisely locate the tree between the canal bottom and the top of the berm where walkers are striding by and botany-gawking. 

For Alba, the co-lead, the trickiest part of the research is planning a useful long-term study when climate, drought, water supplies and municipal budgets change so quickly. 

Summer Merrell, research assistant and undergrad at CU Denver, uses a soil moisture meter to take a reading from ground near a northern catalpa tree. (John Leyba, Special to the Colorado Sun)

“You need to tease apart all these different stressors, and their effects on the trees takes a long time, and are not always in sync with the speed at which decisions need to be made,” Alba said. 

Amid such uncertainty, the fallback position in urban botany is usually a good bet: “A diverse plant palette.” Foresters, as do any crop farmers, insulate themselves from  the future by avoiding obviously dangerous outcomes like a canopy where every other tree is a century-old cottonwood. 

Front Range cities bragging on their leafiness have yet to feel the full destruction of the emerald ash borer, which edges closer and closer to decimating the green ash that dominated Colorado planting in the 1980s and 90s, Alba noted. The thousands of new High Line trees will eventually include a risk-averse mix of honeylocust, white oak, hackberry, catalpa, boxelder and yes, to the dismay of allergy sufferers everywhere, more thirsty cottonwoods

No one knows just what’s coming, Hartley put it, trying another tack. “It’s not quite a law, but a principle within ecology, that diversity allows for resistance to change.” 

While the students pack away their instruments and list the field hazards of poison ivy, carrying 3 liters of water every day, and falling down the embankments, Hartley is pondering how to make the High Line study a stable institution. The student assistance has been crucial, but volunteers and grants can be erratic. They would like to make the longitudinal High Line tree research an official UCD lab course. 

“If you’re a scientist, it’s hard to get money to do the same thing over and over again,” Hartley said.

By early July, each new High Line tree had finished a pediatric check. Alba and Hartley are on to planning next year’s survey while trying not to worry about the coming months of hail, 100-degree days, toppling cottonwoods and errant lawnmowers. 

Their first full survey after the planting push began left them surprised at their own optimism. 

“There were more trees in good condition than I expected,” Hartley said. “I thought more would have died or just not made it that first year. Because life is hard.”


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