A recent study reveals what some local gardeners say they see firsthand — Albuquerque is losing its trees.
The study, published in the journal Urban Forestry & Urban Greening in February, compared tree cover in aerial photographs taken in 20 cities across the country over several years and found that urban tree cover is declining by about 4 million trees per year.
Nearly all cities showed significant declines in tree cover and increases in impervious cover, such as buildings and sidewalks, but Albuquerque showed one of the biggest tree drops.
Here, we lost 2.7 percent of the city’s total tree canopy from 2006 to 2009. Only Houston and New Orleans, where researchers expected fewer trees due to Hurricane Katrina, showed greater tree losses.
“(In Albuquerque) you’re losing a lot more than you gain,” says David J. Nowak, one of the study authors and a project leader for the U.S. Forest Service based at the Northern Research Station in Syracuse, N.Y.
Local experts say a shift toward xeric landscapes that has triggered water and heat stress on trees, as well as poor planting practices, may be responsible for the decline, but a new “tree-bate” program introduced this spring aims to help homeowners and businesses.
The “tree-bate” program, through the Albuquerque Bernalillo County Water Utility Authority, offers rebates for professional tree services to maintain existing trees.
Nowak points out that Albuquerque did start with one of the largest tree covers, at almost 41 percent. Denver had less than 10 percent tree cover while Atlanta had nearly 54 percent.
The study compared images from 1,000 random points throughout each city, Nowak says. In Albuquerque, unlike other cities, a shrub category was included in the tree tally.
Only one city, Syracuse, had an increase in tree cover but Nowak says that was largely due to an invasive shrub.
Joran Viers, agriculture agent for the Bernalillo County Cooperative Extension, says he is not surprised by the study findings.
Much of the current urban tree canopy was planted 30 or more years ago with high-water trees like sycamore and ash, he says. Gardeners were encouraged to water extensively given the belief that the city was perched on an aquifer the size of a “submerged Lake Superior,” he says. The thinking was “water like mad and make everything green,” he says.
Judy Nickell, an arborist and former gardening columnist for the Albuquerque Tribune, has spent many years photographing and researching city trees. Tree loss is “obvious,” she says.
She traces urban tree growth to when residents were encouraged by Mayor Clyde Tingley to plant elms to cool the city. Gardeners were encouraged with lower water rates in summer, she says.
By the 1990s, policies had shifted dramatically, she says. Now people pay penalties for excessive summer water use.
In the early 1990s, new estimates showed that the aquifer was much smaller than previously thought, Viers says. As homeowners switched to xeric landscapes, they converted watered turf to unwatered landscapes but often left mature trees in place.
Trees had to adapt to more heat and less water. The problem is, Viers says, trees are big plants that need a lot of water.
Tree ring analyses show that the 1980s and 1990s were also some of the wettest for Albuquerque, he adds.
Since the 1990s, “it effectively got hotter and drier and we had trees that were all mature already, and it wasn’t too much of a push to put them into decline,” he says.
In Nickell’s travels around town, she regularly spots dead and dying trees, which she attributes to policies that discourage watering. Dying trees lead to less shade for homes, she says.
“The more you remove your trees, the more you’re going to need your air conditioner,” she says.
Reversing the trend does not just mean planting any tree. Choosing the right tree for soil, planting and watering properly are all important, Viers says.
Chinese Pistache and Desert Willow, for example, are well-suited to xeric landscapes but don’t provide much shade, he says.
“If you want a really big tree you’ve got to give it lots of water,” Viers says.
People also tend to plant trees too deep, he says. Roots actually form a broad but shallow system. When planting a tree, root tops should “show a little shoulder,” he says.
Drip systems work only if they add more water as trees grow, he says.
Trees are also increasingly vulnerable to root problems. About 80 percent of container-grown trees have “fairly serious root architecture problems,” Viers says. If a tree has many circling roots, he recommends choosing another tree or cutting roots before they start to turn.
Nowak hopes study results will trigger discussion about what kind of tree cover cities want. He hopes cities will measure tree cover and question whether they are planting the right species.
“If you don’t take stock of what you have, eventually it will slip away,” he says.
While trees cost money and water to maintain, they reduce heat, increase cooling through water evaporation, remove carbon and air pollution and cut energy use, he says. They also offer psychological benefits.
So far, interest in the Albuquerque Bernalillo County Water Utility Authority’s Tree-Bate program has been high, says Katherine Yuhas, water conservation officer with the water authority.
The utility is concerned about losing trees because they cool the city, create shade for plants and reduce swamp cooler use, she says.
“If our customers are planting, we want them to use xeric trees but this is about maintaining what you already have,” she says.