Heat-stroke protection, cleaner air, better breathing, sounder sleep, stress relief, disease prevention and defense from depression – trees and greenery offer a host of health advantages. Planting more trees beautifies urban neighborhoods while helping residents feel better. It's not just about city spaces, however. Even in highly wooded areas, tree loss is tied to a rise in disease and higher mortality. Below, experts make the case for investing in trees for health's sake.

Trees cool down neighborhoods. Trees in U.S. cities play an important role in keeping temperatures down, says Rob McDonald, lead scientist for global cities at the Nature Conservancy. "The average reduction is 2 to 4 degrees Fahrenheit in the summer," McDonald says. In a heat wave, he says, that can make the difference between people, particularly the elderly, staying healthy or risking heat stroke. Trees form a canopy that shades sidewalks and pavement to keep them from getting too hot.

Trees fight air pollution. Scientists refer to "particulate matter concentration" – most people call it air pollution. Large-leaved trees have more surface area for leaves to filter noxious particles from the air, McDonald says. In addition, he says, certain tree species, like elms, have a filtering advantage: "They have hairy leaves and do better at getting particles to stick to them – and if they're stuck to leaves, then we're not inhaling them."

Trees and greenery may boost lifespan. In a large study following nearly 110,000 women over eight years, published in April in Environmental Health Perspectives, Harvard researchers found that women living in the greenes t areas had a 12 percent lower non-accidental death rate than women surrounded by the least vegetation. While cause and effect wasn't determined, the strongest associations were for reduced death rates from respiratory illness and cancer.

Satellite imagery was used to determine seasonal greenness in the 250-meter radius (about 275 yards) surrounding each participant's address. Greenness, or vegetation, was not limited to trees, says study author Peter James, a research associate in epidemiology at the Harvard University School of Public Health and an instructor of medicine at Brigham and Women's Hospital and Harvard Medical School. "It really encompasses anything that undergoes photosynthesis," he says. "It would capture trees, it would capture grass, it would capture shrubs."

It's not just about parks versus pavement, James adds, or manicured lawns versus open spaces. "This could be street trees or landscaping; it could be a vacant lot that was overgrown by weeds."

Trees ease depression and stress. In the Harvard study, mental health was likely the key factor tying vegetation density to lower death risk, James says. Green environments tend to boost physical activity and social connection and dampen depression. That, plus nature's known stress-reduction effects, lead to better physical health. "We've already shown through other research that vegetation can help mitigate the effects of climate change," he says. "These findings suggest there's a co-benefit of vegetation to improve health."

Tree loss hurts health. "You don't know what you've got till it's gone," Joni Mitchell sang in "Big Yellow Taxi," as she mourned the sacrifice of trees to build parking lots. Tree loss isn't always man-made. A forest-infesting pest called the emerald ash borer is wreaking ongoing havoc in wooded areas. The spread of the tree-killing beetle gave U.S. Forest Service scientists and others the opportunity for a "natural experiment" to show the effect of environmental changes on public health. In the 15-state area and 17-year period covered, more than 15,000 additional deaths from cardiovascular conditions such as heart attack and stroke occurred. The study, published in 2015 in the American Journal of Preventive Medicine, also found more than 6,000 additional deaths related to lower-respiratory disease. Wealthier counties showed the most serious health effects from emerald ash borer infestations, which continue to spread in the U.S.

Trees reduce asthma's toll. A couple of years ago, strategically placed sensors measured nitrogen dioxide levels in nearly 150 Portland, Oregon, neighborhoods. Produced by vehicle emissions, industrial processes and other sources, nitrogen dioxide is a big part of smog and one of the six leading air pollutants identified by the Environmental Protection Agency. In the study, conducted by Portland State University, tree cover had a significant effect on nitrogen dioxide levels and residents' respiratory health by area, according to the study model. For instance, in higher-tree areas, young kids were estimated to have avoided missing more than 7,000 school days annually because of asthma attacks. Emergency room visits due to asthma were reduced yearly by an estimated 54 visits among all people. Similarly, over a year, hospital stays in older adults were reduced by an estimated 46 stays, according to the study in the November 2014 issue of the journal Environmental Pollution.

Trees support better sleep. In an extensive national study including more than 255,000 adults, researchers examined the effect of the natural environment on sleep. Proximity to oceanfront or bodies of water, ambient temperature, heat, humidity and light exposure were all factored into the study, published in the September 2015 issue of Preventive Medicine. Green space in itself helped protect men of all ages and older adults from insufficient sleep.

Time to replenish. In October, the Nature Conservancy released a report on the impact tree planting could have on health among 245 cities throughout the globe. If $4 was spent per resident in the hottest, most-polluted cities in need of tree cover, tens of thousands of lives could be saved, the report concluded. "We're hoping people will get motivated from this to plant another tree in their backyard or [encourage] their city governments to plant more trees along the street," McDonald says. (You can zoom in on this interactive map to see where your city stands.)

Older neighborhoods with beautiful oaks along the street are making way for newer subdivisions with fewer trees, McDonald says. While many U.S. cities were active in tree-planting right after World War II, he says, now those trees are now aging and dying out. "Trees provide a lot of benefits and they're healthy," he says. "So why don't we invest a little more in this resource?"