# Why trees: local, regional, global

> The benefits of planting and keeping trees, with a source on every claim, and the figures for your own place when you add ?near=City, Region.

## 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. (US EPA, Using Trees and Vegetation to Reduce Heat Islands: https://www.epa.gov/heatislands/using-trees-and-vegetation-reduce-heat-islands)
- 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: https://www.epa.gov/heatislands/heat-island-compendium)
- 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: https://www.usgs.gov/special-topics/water-science-school/science/evapotranspiration-and-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. (Kardan et al. 2015, Scientific Reports: https://www.nature.com/articles/srep11610)
- 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: https://doi.org/10.1016/j.amepre.2012.09.066)

## 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. (Nowak et al. 2014, Environmental Pollution: https://doi.org/10.1016/j.envpol.2014.05.028)
- 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: https://doi.org/10.1016/j.ufug.2018.03.006)
- 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. (Iungman et al. 2023, The Lancet: https://doi.org/10.1016/S0140-6736(22)02585-5)
- 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. (Schwaab et al. 2021, Nature Communications: https://doi.org/10.1038/s41467-021-26768-w)
- 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. (Garrity et al. 2010, Food Security: https://doi.org/10.1007/s12571-010-0070-7)
- 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. (Menéndez et al. 2020, Scientific Reports: https://doi.org/10.1038/s41598-020-61136-6)

## 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. (FAO, Global Forest Resources Assessment 2020: https://www.fao.org/forest-resources-assessment/2020/en/)
- 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. (Pan et al. 2011, Science: https://doi.org/10.1126/science.1201609)
- 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. (Harris et al. 2021, Nature Climate Change: https://doi.org/10.1038/s41558-020-00976-6)
- 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. (Bastin et al. 2019, Science, and the published responses: https://doi.org/10.1126/science.aax0848)
- 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. (Keys et al. 2012, Biogeosciences: https://doi.org/10.5194/bg-9-733-2012)
- 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. (FAO and UNEP, The State of the World's Forests 2020: https://www.fao.org/state-of-forests/2020/en)

## 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. (Hilbert et al. 2019, Arboriculture and Urban Forestry: https://doi.org/10.48044/jauf.2019.008)
- 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. (Roman and Scatena 2011, Urban Forestry and Urban Greening: https://doi.org/10.1016/j.ufug.2011.05.008)
- 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: https://www.fs.usda.gov/managing-land/urban-forests)
- 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. (USDA Forest Service i-Tree methods: https://www.itreetools.org/)

## More global figures

- Investing about US$4 per urban resident a year in street trees across 245 of the world's largest cities could save tens of thousands of lives a year through cleaner, cooler air. (The Nature Conservancy, 'Planting Healthy Air', 2016: https://www.nature.org/en-us/what-we-do/our-insights/perspectives/how-urban-trees-can-save-lives)
- At least 1.11 billion hectares of primary forest remained in 2020, 81 million hectares less than in 1990. (FAO Global Forest Resources Assessment 2020, 2020: https://www.fao.org/interactive/forest-resources-assessment/2020/en/)
- About 726 million hectares of forest were inside protected areas in 2020, 191 million hectares more than in 1990. (FAO Global Forest Resources Assessment 2020, 2020: https://www.fao.org/interactive/forest-resources-assessment/2020/en/)
- Forests are home to about 80% of amphibian species, 75% of bird species and 68% of mammal species, plus 60,082 known tree species. (FAO & UNEP, The State of the World's Forests 2020, 2020: http://www.fao.org/interactive/state-of-forests/2020/en/)
- Forests are still being lost to deforestation at nearly 11 million hectares a year, while legally protected forests now cover one-fifth of all forest area (FRA 2025). (FAO Global Forest Resources Assessment 2025, 2025: https://openknowledge.fao.org/server/api/core/bitstreams/2dee6e93-1988-4659-aa89-30dd20b43b15/content/cd6709en.html)
- Tropical primary forest loss hit 6.7 million hectares in 2024, nearly double 2023 and equal to 18 soccer fields every minute. (World Resources Institute / Global Forest Watch (University of Maryland GLAD data), 2024: https://wri.org/news/release-global-forest-loss-shatters-records-2024-fueled-massive-fires)
- The world had about 4.06 billion hectares of forest in 2020, nearly one-third of all land and about 0.52 hectares per person. (FAO Global Forest Resources Assessment 2020, 2020: https://www.fao.org/interactive/forest-resources-assessment/2020/en/)
- The world lost a net 4.7 million hectares of forest a year in 2010-2020, down from 7.8 million hectares a year in the 1990s. (FAO Global Forest Resources Assessment 2020, 2020: https://www.fao.org/interactive/forest-resources-assessment/2020/en/)
- About 10 million hectares of forest were cleared each year in 2015-2020, down from 12 million hectares a year in 2010-2015. (FAO Global Forest Resources Assessment 2020, 2020: https://www.fao.org/interactive/forest-resources-assessment/2020/en/)
- An estimated 420 million hectares of forest were converted to other land uses between 1990 and 2020. (FAO & UNEP, The State of the World's Forests 2020, 2020: http://www.fao.org/interactive/state-of-forests/2020/en/)
- Earth has about 3.04 trillion trees, over 15 billion are cut down each year, and the total has fallen roughly 46% since the start of human civilization. (Crowther et al. 2015, Nature, 'Mapping tree density at a global scale', 2015: https://www.nature.com/articles/nature14967)
- For the first time on record, fire was the leading cause of tropical primary forest loss in 2024 (nearly 50% of it), and forest fires worldwide emitted 4.1 gigatonnes of greenhouse gases. (World Resources Institute / Global Forest Watch, 2024: https://wri.org/news/release-global-forest-loss-shatters-records-2024-fueled-massive-fires)
- The world's forests absorbed a steady 3.5 billion tonnes of carbon a year in the 2010s, equal to almost half of fossil-fuel emissions. (Pan et al. 2024, Nature, 'The enduring world forest carbon sink', 2024: https://www.nature.com/articles/s41586-024-07602-x)
- Tropical deforestation released 2.2 billion tonnes of carbon a year in 1990-2019, cancelling two-thirds of the benefit of the global forest carbon sink. (Pan et al. 2024, Nature, 2024: https://www.nature.com/articles/s41586-024-07602-x)
- About 15 million hectares of mangroves remain worldwide, storing 21 billion tonnes of CO2, and mangrove loss fell 44% between 2000 and 2020. (Global Mangrove Alliance, 2024: https://www.mangrovealliance.org/mangrove-forests/)
- Between 2001 and 2019 the world's forests were a net sink of 7.6 billion tonnes of CO2-equivalent a year: 15.6 billion tonnes removed minus 8.1 billion tonnes emitted from deforestation and disturbance. (Harris et al. 2021, Nature Climate Change, 2021: https://www.nature.com/articles/s41558-020-00976-6)
- Bastin et al. estimated room for 0.9 billion hectares of extra tree canopy outside farms and cities, which could store 205 gigatonnes of carbon. (Bastin et al. 2019, Science, 'The global tree restoration potential', 2019: https://europepmc.org/abstract/MED/31273120)
- Veldman et al. argued the 205-gigatonne restoration estimate was about five times too large because it inflated soil carbon gains, ignored warming from high-latitude trees and counted afforesting grasslands and savannas as restoration. (Veldman et al. 2019, Science, comment on 'The global tree restoration potential', 2019: https://europepmc.org/abstract/MED/31624179)
- Mangroves store on average 1,023 tonnes of carbon per hectare, 49-98% of it in deep soils, making them among the most carbon-rich forests in the tropics. (Donato et al. 2011, Nature Geoscience, 2011: https://www.nature.com/articles/ngeo1123)
- Across 293 European cities, land under urban trees is 8-12°C cooler than built-up land in Central Europe and 0-4°C cooler in Southern Europe in summer. (Schwaab et al. 2021, Nature Communications, 2021: https://www.nature.com/articles/s41467-021-26768-w)
- Treeless green spaces such as lawns cool cities 2-4 times less than urban trees do. (Schwaab et al. 2021, Nature Communications, 2021: https://www.nature.com/articles/s41467-021-26768-w)
- In Zambia, maize grown under 8- to 22-year-old Faidherbia trees yielded 7-12 times more than maize outside the canopy, with the biggest gap in the driest year. (Yengwe et al. 2018, European Journal of Agronomy, 2018: https://doi.org/10.1016/j.eja.2018.07.004)
- A meta-analysis found maize yields 150% higher and sorghum 73% higher under Faidherbia albida canopies than in the open, with soil carbon up 46%. (Sileshi 2016, Journal of Arid Environments (meta-analysis of Faidherbia albida), 2016: https://doi.org/10.1016/j.jaridenv.2016.03.002)
- Raising tree cover to 30% in 93 European cities would cool them by an average 0.4°C and prevent about 2,644 premature deaths a summer, one-third of heat-island deaths. (Iungman et al. 2023, The Lancet, 2023: https://europepmc.org/article/MED/36736334)
- Urban heat islands caused about 6,700 premature deaths across 93 European cities in summer 2015, 4.3% of all summer deaths. (Iungman et al. 2023, The Lancet, 2015: https://europepmc.org/article/MED/36736334)
- Across 56 studies, newly planted urban tree cohorts lose a median 4.4-6.5% of trees a year, with mortality highest in the first five years after planting. (Hilbert et al. 2019, Arboriculture & Urban Forestry, 'Urban Tree Mortality: A Literature Review', 2019: https://www.fs.usda.gov/nrs/pubs/jrnl/2019/nrs_2019_hilbert_001.pdf)
- A meta-analysis of 16 street tree studies found annual survival of 94.9-96.5%, meaning half of a planting cohort is dead within 13-20 years. (Roman & Scatena 2011, Urban Forestry & Urban Greening, 2011: https://earth.sas.upenn.edu/content/street-tree-survival-rates-meta-analysis-previous-studies-and-application-field-survey)
- Mangroves prevent more than US$65 billion in flood damage a year, and 15 million more people would be flooded annually if they were lost. (Menéndez et al. 2020, Scientific Reports, 2020: https://www.nature.com/articles/s41598-020-61136-6)
- One-third of Amazon rainfall originates within the basin itself, and two-thirds of that water was transpired by the forest's own trees. (Staal et al. 2018, Nature Climate Change, 2018: https://www.nature.com/articles/s41558-018-0177-y)
- Over more than 60% of tropical land, air that has passed over extensive forest in the previous days produces at least twice as much rain as air that passed over little vegetation. (Spracklen, Arnold & Taylor 2012, Nature, 2012: https://www.nature.com/articles/nature11390)

All facts as data: https://treerm.org/api/v1/facts