Matthew Gonser, manager of the city’s coastal and water program, knows that trees on Oahu are a key line of defense to offset the effects of global warming. They provide shade, draw in both heat and carbon dioxide from the air, help soak up the heavier downpours that are becoming more typical with climate change.

If only everyone realized that, he said.

“Despite what people say, there’s no such thing as a rubbish tree,” said Gonser, who is part of the city’s new Office of Climate Change, Sustainability and Resiliency.

Saturday was the official observation of Arbor Day here — which, for more than a century, has been held in the beginning of rainy season in Hawaii, seen as a more optimal time to plant than when mainlanders celebrate Arbor Day in the spring.

In fact, any old day could be Arbor Day here, as Hawaii has a year-round growing season. As a result, many of the trees that take root in a tropical climate are the type that produce piles of leaves, pods and flowers that need raking, and not just in the fall. Some grow fast, fed by aggressive root systems that seem to get in the way.

So yes, Gonser has heard people who, in the abstract, adore trees. But a particular one that blocks their view or adds “rubbish” to their weekend yardwork? Maybe that can go.

And as trees are devalued in green Hawaii, for whatever reason, there is a cumulative effect: The canopy of trees that shade developed parts of Oahu is shrinking. The city, and an enlarging network of nonprofit groups concerned about that, are working to reverse that trend.

There are two goals, said Josh Stanbro, the city’s chief resiliency officer: Plant 100,000 trees on Oahu by 2025, and increase the tree canopy from 25 percent to 35 percent by 2035.

This is a project not exclusively an official city task or enlisting only the nonprofits, but one that will involve everyone. This weekend, the city resiliency office launched a web-based app (www.resilientoahu.org/urbanforest) enabling the tracking of every new tree planted.

Research spurs action

One of its principal champions will be a new nonprofit, Trees for Honolulu’s Future (TFHF), first organized by Tom Dinell, emeritus professor in the Department of Urban and Regional Planning at the University of Hawaii at Manoa, formerly the department’s chairman.

Dinell said he was spurred by what he learned while co-chairing a committee the city formed after joining the World Health Organization Global Network of Age-Friendly Cities.

“The best research is from Camilo Mora,” he said, referencing a UH geography professor. “One of the parts that impressed me was that there weren’t going to be any seniors on the streets in 10 years, with temperature change.

“So then I decided we really need to move on trees, which provide shade among multiple other benefits.”

More scary information comes from a pair of studies using satellite imagery, the first one released about eight years ago, said Wai Lee, executive director of Smart Trees Pacific, a nonprofit to support urban forestry.

In conjunction with the state’s Kaulunani Urban & Community Forestry Program, Smart Trees Pacific commissioned the Urban Tree Canopy Assessment studies, funded by the U.S. Forestry Service and performed by the University of Vermont Rubenstein School of Environment and Natural Resources.

It covered an area of about 250 square miles on Oahu, between Kalaeloa and Kaneohe, Lee said. To everyone’s surprise, he added, even with all its reputation for greenery, urban Oahu’s tree canopy coverage was only fair to middling.

And it got worse.

Branching out with partners

An updated study in 2013 showed the canopy had declined by about 4.8 percent, he said.

“That’s about 1.8 square miles of canopy,” Lee said. “And that raised the alarm: Why? The first reaction was why and where.”

Dinell had no trouble connecting with like-minded people such as Lee who shared an interest in the mission of TFHF. Prompted by the looming threat of climate change, its principal aim is to “encourage and facilitate the planning, planting, and maintenance of shade trees along streets, in parks, at schools, and in public places on Oahu.”

The board of directors includes Dinell, Lee and, among others: Lisa Marten, executive director and president of the nonprofit Healthy Climate Communities, which marshals school groups in reforesting projects as an educational initiative; and Roxanne Adams, director of building and grounds management at UH-Manoa, which is itself an arboretum.

One of its projects was to convene a conference this past March, “Growing Our Urban Forest,” an event subtitled “More than a conference — the beginning of a movement.”

It was a way to get disparate groups and government agents — each of which has a generally supportive but different perspective on trees — to learn from each other about ways to remove impediments to forestry, Adams said.

“We had planners talking about trees but they were just talking to planners, and we had arborists talking about trees but they were just talking to arborists,” she said.

“That was a great example of bringing different parts of the city together that don’t normally focus on barriers that they might be creating for each other, planting trees,” Marten added.

For example, she said, there are sometimes regulations that have the effect of discouraging new plantings by roadways and sidewalks, or of prompting removal of existing trees. Some of these rules deserve another look, she said.

Tree-planting and tracking

The group has drafted a strategic plan outlining a number of objectives topped by the goal of eliminating the annual loss in tree canopy, currently estimated at 1 percent. Part of the job lies in education of the public, including the publication of a guide to the comparative costs, attributes and requirements of trees to be planted.

The plan from here, Stanbro said, is to get an expanded update of the satellite tree-canopy study and to launch a process that will lead to a more intensive consideration of trees in the city’s rules and regulations. Oahu’s urban forestry will be a component in the Oahu climate-change strategy that should be released by next spring.

Armed with better baseline data, Gonser said, the city will be able to develop a more intentional and strategic plan for forestry and revisit design standards and various regulations. Even the website for tracking tree planting will be a strategically useful tool, he added.

“That’s what will tell us who are the advocates, who are the ones to support changes to rules and regulations,” he said.

Honolulu is only the latest city to turn to urban forestry, so it has models to emulate. Within the U.S., New York City has made advances in tracking and promoting tree-planting efforts.

Seeding youth involvement

And at the March conference, the keynote address was given by Ian Shears, manager of urban sustainability for the city of Melbourne, which, Stanbro said, is a standout in this revolution. The Australian city is divided into 10 precincts, with public workshops in each one ongoing as part of the public education on how to nurture new trees (melbourneurbanforestvisual.com.au).

Building an urban-forestry volunteer army for this city is part of the mission, Stanbro said; there already is a Citizen Forester program enlisting help in collecting tree data.

And for its part, Healthy Climate Communities has worked with schools on projects such as the Hamakua Marsh watershed restoration, enlisting students in planting trees such as milo and kamani and other native plants.

This is how advocates hope to grow new attitudes about trees and why they need human support. The children, Marten said, catch on quickly.

“Kids naturally value trees,” she said. “They get it. It’s somewhere along the way as we get older that people start thinking that the effort is humbug.”