What a tree is for.
Shade on a sidewalk, a city that is survivable in August, a planet that keeps its rain. The case for trees is usually made with adjectives. Here it is made with numbers, each one traceable, and with the figures for where you live first.
Or use my location (we think you're around Columbus, OH).
What one tree does for a house and a block
The benefits closest to home are the ones you can feel in July.
Shade matters more than air temperature. Surfaces under tree canopy can run 11 to 25 °C (20 to 45 °F) cooler than the same material in full sun, which is why a shaded sidewalk is walkable and a bare one is not.
Trees placed on the west and south of a house cut summer cooling energy; US studies put the saving between 7 and 47 percent depending on climate and placement.
US EPA, Reducing Urban Heat Islands: Compendium of Strategies, Trees and Vegetation chapter
A large tree moves a lot of water. A mature oak can transpire more than 40,000 gallons (150,000 liters) a year, water that would otherwise run off streets or sit in the ground.
USGS Water Science School, Evapotranspiration and the Water Cycle
Street trees are tied to how people feel. In Toronto, ten more trees on a city block was associated with health perception comparable to being seven years younger or earning $10,000 more a year.
Losing trees shows up in the death statistics. Across 15 US states, the arrival of the emerald ash borer and the loss of ash trees was associated with an additional 6,113 deaths from lower-respiratory illness and 15,080 from cardiovascular disease over 18 years.
Donovan et al. 2013, American Journal of Preventive Medicine
What a canopy does for a city, a watershed and a farm belt
At the scale of a city the effects stop being comfort and start being infrastructure.
US urban trees remove about 17.4 million tonnes of air pollution a year, with health benefits valued at $6.8 billion, including roughly 850 avoided deaths and 670,000 avoided cases of acute respiratory symptoms.
The US is losing urban tree cover: about 175,000 acres (71,000 hectares) a year between 2009 and 2014, replaced mostly by impervious surface, at an estimated cost of $96 million a year in lost benefits.
Nowak and Greenfield 2018, Urban Forestry and Urban Greening
Across 93 European cities, raising tree cover to 30 percent would have cooled summers by about 0.4 °C and prevented roughly a third of heat-attributable deaths in 2015, 2,644 of 6,700.
Land under urban trees in European cities runs 8 to 12 °C cooler than treeless urban land in Central Europe in summer, two to four times the cooling that grass gives.
Trees in fields are agriculture, not decoration. Maize grown under Faidherbia albida in African parklands yields two to three times more than maize in the open, because the tree drops nitrogen-rich leaves at planting time.
Mangroves protect more than 15 million people a year from flooding and avert $65 billion in property damage worldwide; coastal cities and farms sit behind them.
What forests do for everyone
The global numbers are large and worth stating plainly, with the arguments attached.
Forests cover 4.06 billion hectares, 31 percent of the world's land. The world lost a net 4.7 million hectares a year in the 2010s, down from 7.8 million a year in the 1990s, because planting and natural regrowth offset some of the clearing.
The world's forests take up about 2.4 billion tonnes of carbon a year, around a quarter of what people emit, and hold roughly 860 billion tonnes in trees and soil.
Forests are a net sink of about 7.6 billion tonnes of CO2 a year once clearing is subtracted, roughly 1.5 times the annual emissions of the United States.
Restoration potential is real and contested. A 2019 estimate found room for 0.9 billion hectares of extra canopy outside farms and cities; later responses argued it overstated carbon gains and risked treating grasslands and savannas as empty. Where you plant matters as much as how much.
Forests make rain downwind. Moisture transpired by trees falls again as precipitation over land; in the Amazon, Congo and East Asia, 30 to 50 percent of rainfall is recycled through vegetation.
Forests hold most of the world's land-based species: more than 80 percent of amphibians, 75 percent of birds and 68 percent of mammals that live on land.
Why planting is the easy part
A tree that dies in its second summer delivered none of the above. Maintenance is where the benefits are decided.
About half of newly planted urban trees do not survive their first decade or two; a review of 16 studies found annual mortality highest in the first five years, driven by water, soil and neglect rather than species.
Street trees live short lives: a synthesis of US studies found a mean life expectancy of 19 to 28 years for street trees, with a 3.5 to 5.1 percent annual mortality rate.
Watering is the whole job early on. Most municipal guidance converges on 15 to 20 gallons (55 to 75 liters) a week in the first two growing seasons, applied slowly at the root zone.
USDA Forest Service, Urban and Community Forestry guidance, summarized across city programs
The benefits scale with size. A large healthy tree provides many times the shade, interception and pollution removal of a sapling, so keeping a 40-year-old tree alive is worth more than planting a dozen new ones in its place.