Urban forest

An urban forest is a forest, or a collection of trees, that grow within a city, town or a suburb. In a wider sense, it may include any kind of woody plant vegetation growing in and around human settlements. As opposed to a forest park, whose ecosystems are also inherited from wilderness leftovers, urban forests often lack amenities like public bathrooms, paved paths, or sometimes clear borders which are distinct features of parks. Care and management of urban forests is called urban forestry. Urban forests can be privately and publicly owned. Some municipal forests may be located outside of the town or city to which they belong.

Urban forests play an important role in ecology of human habitats in many ways. Aside from the beautification of the urban environment, they offer many benefits like impacting climate and the economy while providing shelter to wildlife and recreational area for city dwellers.

Definition

The concept of urban forest was born at the end of the 20th century, designating a forest or woodlands growing in an urban area. Peri-urban forest when it surrounds the city or its suburbs. It has emerged mainly in Canada and in cities with large wooded areas such as Brussels, Oslo, London, Berlin, Stuttgart, Stockholm and Zurich. This recent concept differs from the notion of "urban park" by giving more importance to naturalness, environments and the ecosystem services provided.

Urban forests are of great variety, but seem to be categorized into four main types. First, some are preserved remnants of the natural forest. These woodlands have often been redeveloped, such as the Bois de la Cambre (in Dutch: Ter Kamerenbos) in the heart of the city of Brussels. Others are the result of old woodlands present before urban growth (and then open to the public or not), sometimes partly for reasons of military strategy (such as the Bois de Boulogne around Vauban 's citadel in Lille), and often to provide hunting parks near places of power (such as the Bois de Boulogne and Bois de Vincennes on either side of Paris). They can also originate from replanted or artificially created woodlands, for example on wasteland, as an urban garden, as a compensatory measure, as an amenity site or to protect water resources (protection of catchment areas or groundwater recharge areas). Finally, they can be peri-urban forests, such as the Sonian Forest, a relic of ancient forest of 4,383 hectares which covers approximately half the surface area of the Brussels-Capital Region in Belgium. 

Examples

In many countries there is a growing understanding of the importance of the natural ecology in urban forests. There are numerous projects underway aimed at restoration and preservation of ecosystems, ranging from simple elimination of leaf-raking and elimination of invasive plants to full-blown reintroduction of original species and riparian ecosystems.

Some sources claim that the largest man-made urban forest in the world is located in Johannesburg in South Africa. The city is located in the highveld, a grassland biome typically lacking large numbers of trees, yet Johannesburg is still a very densely wooded city with reportedly 10 million artificially introduced trees and is rated as the city with the eighth highest tree coverage in the world.

Rio de Janeiro is also home to two of the vastest urban forests in the world, one of which is considered by some sources to be the largest one. Tijuca Forest is the most famous. It began as a restoration policy in 1844 to conserve the natural remnants of forest and replant in areas previously cleared for sugar and coffee. Despite the worldwide recognition of Tijuca Forest, another forest in the same city encompasses roughly three times the size of its more prominent neighbor: Pedra Branca State Park occupies 12,500 hectares (30,888 acres) of city land, against Tijuca's 3,953 hectares (9,768 acres). The larger metropolitan area encircles the forests which moderate the humid climate and provide sources of recreation for urban dwellers. Along with seven other smaller full protection conservation units in the city, they form an extensive natural area that contains the Transcarioca Trail, a 180-km footpath.

Sanjay Gandhi National Park in Mumbai, Maharashtra, India is also an example of an urban forest. It covers roughly around 20% area of the city. The forest is filled with many animals freely roaming around. It also has an important cultural site of ancient history situated in it known as the Kanheri caves. Nebraska National Forest is the largest man-made forest in the United States located in the state of Nebraska. It lies in several counties within the state and is a popular destination for campers year-round.

Several cities within the United States have also taken initiative investing in their urban forests to improve the well-being and economies of their communities. Some notable cities among them are Austin, Atlanta, Nashville, New York, Seattle, and Washington, D.C. New York, for example, has taken initiative to combat climate change by planting millions of trees around the city. In Austin, private companies are funding tree-planting campaigns for environmental and energy-saving purposes. Nashville has an organization known as the Alliance to Conserve Nashville's Highland Rim Forest serving as a catalyst to pool action from across numerous conservation nonprofits that include the Cumberland River Compact, Friends of Beaman Park, and the Tennessee Environmental Council.

Environmental impact

Urban forests play an important role in benefitting the environmental conditions of their respective cities. They moderate local climate, slowing wind and stormwater, and filter air and sunlight. They are critical in cooling the urban heat island effect, thus potentially reducing the number of unhealthful ozone days that plague major cities in peak summer months.

Air pollution reduction

As cities struggle to comply with air quality standards, trees can help to clean the air. The most serious pollutants in the urban atmosphere are ozone, nitrogen oxides (NOx), sulfuric oxides (SOx) and particulate pollution. Ground-level ozone, or smog, is created by chemical reactions between NOx and volatile organic compounds (VOCs) in the presence of sunlight. High temperatures increase the rate of this reaction. Vehicle emissions (especially diesel), and emissions from industrial facilities are the major sources of NOx. Vehicle emissions, industrial emissions, gasoline vapors, chemical solvents, trees and other plants are the major sources of VOCs. Particulate pollution, or particulate matter (PM10 and PM25), is made up of microscopic solids or liquid droplets that can be inhaled and retained in lung tissue causing serious health problems. Most particulate pollution begins as smoke or diesel soot and can cause serious health risk to people with heart and lung diseases and irritation to healthy citizens. Trees are an important, cost-effective solution to reducing pollution and improving air quality.

Trees reduce temperatures and smog

With an extensive and healthy urban forest air quality can be drastically improved. Trees help to lower air temperatures and the urban heat island effect in urban areas. This reduction of temperature not only lowers energy use, it also improves air quality, as the formation of ozone is dependent on temperature. Trees reduce temperature not only by directly shading: when there is a large number of trees it create a difference in temperatures between the area when they are located and the neighbor area. This creates a difference in atmospheric pressure between the two areas, which creates wind. This phenomenon is called urban breeze cycle if the forest is near the city and park breeze cycle if the forest is in the city. That wind helps to lower temperature in the city.

As temperatures climb, the formation of ozone increases.

Healthy urban forests decrease temperatures, and reduce the formation of ozone.

Large shade trees can reduce local ambient temperatures by 3 to 5 °C

Maximum mid-day temperature reductions due to trees range from 0.04 °C to 0.2 °C per 1% canopy cover increase.

In Sacramento County, California, it was estimated that doubling the canopy cover to five million trees would reduce summer temperatures by 3 degrees. This reduction in temperature would reduce peak ozone levels by as much as 7% and smoggy days by 50%.

Lower temperatures reduce emissions in parking lots

Temperature reduction from shade trees in parking lots lowers the amount of evaporative emissions from parked cars. Unshaded parking lots can be viewed as miniature heat islands, where temperatures can be even higher than surrounding areas. Tree canopies will reduce air temperatures significantly. Although the bulk of hydrocarbon emissions come from tailpipe exhaust, 16% of hydrocarbon emissions are from evaporative emissions that occur when the fuel delivery systems of parked vehicles are heated. These evaporative emissions and the exhaust emissions of the first few minutes of engine operation are sensitive to local microclimate. If cars are shaded in parking lots, evaporative emissions from fuel and volatilized plastics will be greatly reduced.

Cars parked in parking lots with 50% canopy cover emit 8% less through evaporative emissions than cars parked in parking lots with only 8% canopy cover.

Due to the positive effects trees have on reducing temperatures and evaporative emissions in parking lots, cities like Davis, California, have established parking lot ordinances that mandate 50% canopy cover over paved areas.

"Cold Start" emissions

The volatile components of asphalt pavement evaporate more slowly in shaded parking lots and streets. The shade not only reduces emissions, but reduces shrinking and cracking so that maintenance intervals can be lengthened. Less maintenance means less hot asphalt (fumes) and less heavy equipment (exhaust). The same principle applies to asphalt-based roofing.

Active pollutant removal

Trees also reduce pollution by actively removing it from the atmosphere. Leaf stomata, the pores on the leaf surface, take in polluting gases which are then absorbed by water inside the leaf. Some species of trees are more susceptible to the uptake of pollution, which can negatively affect plant growth. Ideally, trees should be selected that take in higher quantities of polluting gases and are resistant to the negative effects they can cause.

A study across the Chicago region determined that trees removed approximately 17 tonnes of carbon monoxide (CO), 93 tonnes of sulfur dioxide (SO2), 98 tonnes of nitrogen dioxide (NO2), and 210 tonnes of ozone (O3) in 1991.

Carbon sequestration

Urban forest managers are sometimes interested in the amount of carbon removed from the air and stored in their forest as wood in relation to the amount of carbon dioxide released into the atmosphere while running tree maintenance equipment powered by fossil fuels.

Interception of particulate matter

In addition to the uptake of harmful gases, trees act as filters intercepting airborne particles and reducing the amount of harmful particulate matter. The particles are captured by the surface area of the tree and its foliage. These particles temporarily rest on the surface of the tree, as they can be washed off by rainwater, blown off by high winds, or fall to the ground with a dropped leaf. Although trees are only a temporary host to particulate matter, if they did not exist, the temporarily housed particulate matter would remain airborne and harmful to humans. Increased tree cover will increase the amount of particulate matter intercepted from the air.

Large evergreen trees with dense foliage collect the most particulate matter.

The Chicago study determined that trees removed approximately 234 tonnes of particulate matter less than 10 micrometres (PM10) in 1991.

Large healthy trees greater than 75 cm in trunk diameter remove approximately 70 times more air pollution annually (1.4 kg/yr) than small healthy trees less than 10 cm in diameter (0.02 kg/yr).

Rainwater runoff reduction

Urban forests and trees help purify water sources by slowing down rain as it falls to the earth and help it soak into the soil, thereby naturally filtering out pollutants that can potentially enter water supply sources. They reduce storm water runoff and mitigate flood damage, protecting the surrounding rivers and lakes. Trees also help alleviate the load on "grey" infrastructure (such as sewers and drains) via evapotranspiration. Trees are ideally suited as their canopies can intercept water (and provide dense vegetation), whilst their roots can pump substantial amounts of water back into the atmosphere as water vapor, all with a relatively small footprint.

Urban wildlife

Trees in urban forests provide food and shelter for wildlife in cities. Birds and small mammals use trees as nesting sites, and reptiles use the shade they provide to keep cool in the hot summer months. Furthermore, trees provide shade necessary for shrubbery. Not only do urban forests protect land animals and plants, they also sustain fish and water animals that need shade and lower temperatures to survive. Wealthier neighborhoods often have more tree cover (both community-subsidized and on private property) which results in an accumulation of benefits on those sections of a city; a study of neighborhoods in Los Angeles found higher levels of bird diversity in historically richer sections of town, and larger populations of synanthropic birds in historically poorer sections of town.

Economic impacts

The economic benefits of trees and various other plants have been understood for a long time. Recently, more of these benefits are becoming quantified. Quantification of the economic benefits of trees helps justify public and private expenditures to maintain them. One of the most obvious examples of economic utility is the example of the deciduous tree planted on the south and west of a building (in the Northern Hemisphere), or north and east (in the Southern Hemisphere). The shade shelters and cools the building during the summer, but allows the sun to warm it in the winter after the leaves fall. The physical effects of trees—the shade (solar regulation), humidity control, wind control, erosion control, evaporative cooling, sound and visual screening, traffic control, pollution absorption and precipitation—all have economic benefits.

The primary purpose of these forests is not timber production, but they can be attributed a high amenity value, as indicated by the price of land around these woodlands. In addition, some forests, such as the Zurich Forest, are managed in such a way that in case of necessity (oil or gas crisis, war, etc.), they can nevertheless one day be exploited for their timber.

Energy and CO2 consumption

Urban forests contribute to the reduction of energy usage and CO2 emissions primarily through the indirect effects of an efficient forestry implementation. The shade provided by trees reduces the need for heating and cooling throughout the year. As a result, energy conservation is achieved which leads to a reduction of CO2 emissions by power plants. Computer models indicate that annual energy consumption can be reduced by 30 billion kWh using 100 million trees in U.S. urban areas. This energy consumption decrease equates to monetary savings of $2 billion. Additionally, the reduction of energy demand would reduce power plant CO2 emissions by 9 million tons per year.

In an ecological footprint or "repayment of an ecological debt " type approach, the urban forest can logically contribute to the local and direct compensation of certain CO2 emissions.(or equivalent).

For example, a study aimed to quantify carbon storage and carbon sequestration by some urban forests in the relatively industrial city of Hangzhou (China). Using urban forest inventory data, through equations based on biomass volume, and the calculation of annual increment and through net primary productivity (NPP) modeling, estimates of stored carbon were made. The total carbon stored by Hangzhou urban forests was thus estimated at 11.74 TgC/year (or about 30.25 tons of carbon per hectare on average). Carbon sequestration by urban forests was 1,328,166.55 t/year, or a sequestration of 1.66 tons of carbon per hectare per year. However, industrial CO 2 emissionswere 7 TgC/year for Hangzhou. In this case, urban forests appear to have sequestered annually 18.57% of the amount of carbon emitted by the combustion of fossil fuels by local industry, storing the equivalent of 1.75 times the annual amount of carbon emitted by the city's industrial uses. This rate of sequestered carbon could be further improved by appropriate management practices.

Another study concluded that in Shenyang, a highly industrialized city in northern China (Liaoning Province), 101 km2 of urban forest (5.76 million trees) stored about 337,000 tons of carbon per year (estimated value $13.88 million), with a carbon sequestration rate of 29,000 t/year (equivalent to $1.19 million and 3.02% of the city's annual carbon emissions from fossil fuel combustion). Carbon sequestration in this case could offset 0.26% of the city's annual carbon emissions, which is still modest; but the study also showed that tree species, soil, species composition, forest structure and age classes had a significant influence on the sequestration rate and that it could be improved through appropriate management.

In Gainesville, Florida, the urban forest was denser, and stored more carbon than in Miami-Dade, due to environmental conditions, but also to the pattern of urbanization (3.4% of CO2 emissionswere absorbed by the urban forest in Gainesville and 1.8% in Miami-Dade). CO2 sequestrationby these trees was however for the 2010s comparable to the results of existing CO2 reduction policies.

According to USDA Forest Service data for ten major US cities and the North American forest canopy, urban trees in the US currently store approximately 700 million tonnes of carbon. On average, this carbon storage in the US (for the early 2000s) was estimated at 25.1 tonnes of carbon per hectare (compared to 53.5 tC/ha in forest stands).

Even when the results are modest, in light of long-term objectives, the multiple ecosystem services provided by afforestation, costs, and community needs, the preservation of existing forests must be reconsidered by integrating their value for adaptation to climate change and the fight against climate change and for the restoration or conservation of other ecosystem services,, including the improvement of air quality or the decontamination of certain soils.

Water filtration

The stormwater retention provided by urban forests can provide monetary savings even in arid regions where water is expensive or watering conservation is enforced. One example of this can be seen in a study carried out over 40 years in Tucson, AZ, which analyzed the savings of stormwater management costs. Over this period, it was calculated that $600,000 in stormwater treatment costs were saved. It was also observed that the net water consumption was reduced when comparing the water required for irrigation against power plant water consumption due to the effects of urban forests on energy usage.

In another instance, New York City leaders in the late 1990s chose to pursue a natural landscape management instead of an expensive water treatment system to clean the Catskill/Delaware watershed. New Yorkers today enjoy some of the healthiest drinking water in the world.

Tourism and local business expansion

The USDA Guide notes on page 17 that "Businesses flourish, people linger and shop longer, apartments and office space rent quicker, tenants stay longer, property values increase, new business and industry is attracted" by trees.

Increases in property values

Urban forests have been linked to an increase in property value surrounding residents. An empirical study from Finland showed a 4.9% increase in property valuation when located just one kilometer closer to a forest. Another source claims this increase can range as high as 20%. The reduction of air, light, and noise pollution provided by forests is cause for the notable pricing differentials.

Sociological impacts

Residents, developers, town planners and local elected officials attribute environmental (water, air, soil, ecosystems), landscape, social and sometimes economic value to the urban forest (productivity is not what is sought, but the presence of an urban forest significantly increases the land value of neighboring areas).

Some studies suggest that the demand for urban forests (in the United States) has a certain elasticity depending on the cost of access to these forests and income. In the United States, a survey showed that cities and managers of urban forests also give them an increasing value as carbon sinks, possibly tradable within the framework of the carbon market.

In France, just by counting travel costs related to recreational visits, the non-market value of the forest was estimated by the IFN in 2006 at around 2 billion euros per year, a figure exceeding the annual value of the timber harvest (around 1.7 billion euros).

In certain contexts, trees and forests also have a landscape value as a visual hide or sound buffer, for example to hide a transport route, a factory, a quarry, etc. with an effectiveness that is still debated: the first studies and experiments on the propagation of sound in forests date from the 1940s and have never ceased,,,, in particular to predict or model the propagation of sound waves. Their sometimes contradictory results show the complexity of the physical problems involving the soil and the relief, hygrometry and wind, the diffusion effect of foliage, trunks and branches (and therefore their density), and the variety of contexts (season, weather and type of forest).

Community health impact

Urban forests offer many benefits to their surrounding communities. Removing pollutants and greenhouse gases from the air is one key reason why cities are adopting the practice. Removing pollutants from the air, urban forests can lower risks of asthma and lung cancer. Communities that rely on well-water may also see a positive change in water purity due to filtration. The amenities provided by the city of each urban forest varies. Some amenities include trails and pathways for walking or running, picnic tables, and bathrooms. These healthy spaces provide for the community a place to gather and live a more active lifestyle.

Mental health impact

Living near urban forests have shown positive impacts on mental health. As an experimental mental health intervention in the city of Philadelphia, trash was removed from vacant lots, some of them being selectively "greened" by plantings trees, grass, and installing small fences. Residents near the "greened" lots who had incomes below the poverty line reported a 68% decrease in feelings of depression, while residents with incomes above the poverty line reported a decrease of 41%. The Biophilia hypothesis argues that people are instinctively drawn to nature, while Attention Restoration Theory goes on to demonstrate tangible improvements in medical, academic and other outcomes, from access to nature. Proper planning and community involvement are important for the positive results to be realized.

Increased home values and incomes

In addition to providing economic benefits at the community level, trees also benefit individual homeowners. A tree on a home's landscape or around the house can increase the dollar value received for the home upon sale. According to one study, a tree planted in the front yard can increase a home's sale price by $7,130 and raise the sale prices of surrounding homes. Healthy urban forests also correlate with higher incomes. In communities that have thriving urban forests, there are higher incomes, higher numbers of jobs associated with those communities, and higher productivity of workers.

Scientific interest

Urban forests are often overcrowded and isolated, sometimes affected by biological invasions, but they are also less subject to pressure from logging and the impacts of agricultural drainage, agricultural fertilizers and pesticides, or hunting. They can therefore serve as models for studying the effects of these phenomena.

Akira Miyawaki has shown in Japan that certain ancient parks have value in terms of conserving genetic resources, including for native species which in the forest may have been replaced by varieties considered more productive or interesting at certain times.

Furthermore, due to their more urban and sometimes industrial context, they have been exposed more than non-urban forests to higher ambient temperatures (cf. urban heat bubble). On the other hand, the deposition of carbon dioxide, carbon monoxide, nitrogen and sometimes ozone has been greater there, and earlier than elsewhere (these forests have been exposed to them for several decades). For these reasons, the "response curve" to climate change and global changes in these urban or peri-urban forests may be several decades ahead of other forests in the same regions.

Biologists have suggested that studying remnants of more recently formed forests or woodlands along urbanization gradients (from the most urban to forested to rural) in different biogeographic contexts could provide valuable information on the impacts suffered by these forests, and on the current and future responses of forests to certain factors involved in global changes, as well as on their resilience capacities in the face of certain changes. Provided that artifacts induced by the small size, insularization, maintenance, and overcrowding of these intra- or peri-urban massifs are minimized, studying these forests along urban-rural gradients could help to make better predictions on the evolution of forest ecosystems at local scales.

In North America, it has been shown that the rates of removal by predatory birds, cats or other animals of seeds and eggs in birds' nests (natural or artificial) varied greatly according to the " edge effect " of the environment considered and along the existing gradient (from rural landscapes to urban woodlands), with a maximum of predation in the suburbs where the disappearance rates of these propagules were the highest (86% of eggs lost each day and 95% of seeds not found), more than in urban sites (eggs, 64%; seeds, 88%). The fact that urban woodlands are maintained seems to be important in this phenomenon: Thus, on the ground, exposed seeds and eggs suffered from higher removal rates than seeds or eggs covered by leaf litter (where it had not been removed by gardeners). However, some undiscovered seeds may have been carried off by animals (from ants to birds to squirrels) who buried them for food, which will not prevent some of them from germinating. On the other hand, the disappearance of an egg from the nest, even if the egg is not eaten, leaves no chance of survival for the embryo it contains.

The scientific literature suggests that egg predation rates are extremely high in cities and apparently even higher in suburbs, before rapidly dropping off as one moves away from cities. In woodlands, egg predation is always higher at the edges. Thus, small urban woodlands could act as ecological traps for some species.

Specific difficulties and problems

The health of urban trees is affected by various sources of stress: chronic deposition of part of the air, water and soil pollution, and exposure to urban ozone; disturbance and impoverishment of wildlife; vulnerability to exotic species (including invasive ones), eutrophication and overpredation (by dogs and cats in particular), with significant edge effects; pressure from human activities and due to frequentation (sometimes overfrequentation), on the soils in particular; or the presence of a more dehydrating micro-climate. Soils can also lose their natural horizons, often polluted and abnormally compacted or even frankly anoxic. They are rich in rubble and other artificial filling materials, often excessively stony and coarse-textured. In addition to their structural degradation, there is a loss of porosity (and consequently of aeration, drainage and moisture storage capacity, which is unfavorable to the good health of the roots). The abundance of lime, cement and plaster waste increases the carbonate level by producing an alkaline pH which limits the circulation and bioavailability of heavy metals but deprives plants of certain micronutrients or promotes a deficiency in phosphorus, organic matter and sometimes nitrogen (except at the base of trees or conversely, excess nitrates are frequent).

Urban forests are also victims of road pollution (lead, with a worrying increase in new pollutants lost through catalyzed exhausts, which lose platinum group metals), light pollution that can affect the periphery and sometimes the interior of urban woodlands, and even more so tree lines in the middle of cities, more frequent construction sites and earthworks in cities affecting root systems, and vehicles that frequently damage tree bark. Furthermore, cities are often areas that have been drained, and their sealing has disrupted the local water cycle, amplifying the effects of climatic hazards. Finally, urban trees located in areas at risk of fire (a risk that seems set to increase with climate change and agricultural water consumption in several regions of the world) are also threatened.

This explains why urban trees are more often victims of certain parasites and diseases which can reduce their life expectancy.

For these reasons, urban or peri-urban forests require precautions and appropriate management methods.

In 2022, a study of 3,129 tree and shrub species in 164 cities in 78 countries shows that 56% of these species are subject to excessive temperatures in at least one city and 65% to insufficient or excessive rainfall, with 17% and 25% being at risk of extinction for these reasons. In some cities such as Barcelona, Niamey and Singapore, all the species present are threatened. Projections for 2050 are particularly worrying.

Management, conservation and restoration

It has been shown in countries or areas where urbanization is relatively recent (in California and Nevada for example, based on the analysis of the structure and ecological quality of 118 plots located along a gradient from forest to city center) that the remnants of native forests, despite the loss of ecological integrity of the former forest, still contribute significantly to the maintenance of native species in urbanized landscapes, and that their conservation plays an important role in the protection of native forest ecosystems.

Their conservation requires improved planning, design, implementation, and maintenance practices. A major limitation in urban landscape research is the lack of accessibility to trees on private properties. However, they represent a large proportion of urban green spaces. Ecologists are often unprepared for this obstacle and therefore often exclude private land from empirical studies.

Many large cities have a department dedicated to trees, within the green spaces department.

While in the 19th century, urban parks were mainly planted with exotic species, for their landscape interest, with very artificial management of flower areas, lawns and water, recently, there has been a tendency to apply the principles of landscape ecology and differentiated management to these forests, seeking more and more to integrate them into a green framework (or ecological network).

Furthermore, it is difficult to find available and unfragmented land, while urban demography has been growing exponentially almost everywhere for over a century. This obstacle concerns, for example, the city and urban community of Nantes, which want to plant or allow to grow, as part of the local green network (greenways and ecological corridors), 1,500 hectares of "urban forest" in eight to ten years. 

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