There are many kinds of forests. People everywhere depend on wild forests to clean the air and water and keep wildlife alive. Trees and other plants growing in the city, planted and wild, create an urban forest. The quality of urban life is dependent on the success of trees and plants growing in the city. Residents only enjoy the urban forest's ecosystem services when the trees and plants in it are prosperous. Selecting and planting the right tree for a site meets this demand.

A successful urban tree does a lot of work for the city in which it grows. All who pass nearby appreciate the cooler summer temperatures its shade creates. We love the birdsong coming from the canopy. This might be part of what maintains both our mental and physical health when we function near and in an urban forest. The benefits supplied by the urban forest extend to the increased real-estate value of our homes, and the decreased crime. Airborne pollution is removed on the foliage of urban trees. They help to reduce and clean stormwater runoff. Urban trees sequester carbon in their annual growth rings. This regulating ecosystem service has a big impact.

Trees growing in the extreme ecosystem of the city face many obstacles. Urban soil is compacted and the area for roots to grow is severely restricted. Light is limited, but reflected heat is intense. The soil, often a byproduct of construction, has diminished microbial activity. Air quality is low, and pests and disease have few natural pressures working against them. Utility lines and passing trucks restrict tree height and width. Tree roots must plunge beneath the sidewalk without lifting it. Each of these is considered to create the right urban tree site match.

When preparing to plant an urban tree, first look at the soil. It’s common to find the soil in urban planting sites suffering severe compaction. This poor aeration results in decreased water drainage. These problems create conditions that suffocate tree roots and soil dwelling microorganisms. A useful treatment is complete removal and replacement of the soil in the planting hole. Some cities invest in engineered soils. These soils sustain root growth with high levels of aeration and improved drainage. But this is an expensive fix which many municipalities cannot afford. Free-draining soil brings oxygen down to plant roots and soil dwelling microorganisms. Some trees are more tolerant of low oxygen soil. Using these species increases chances of success in urban soil that remains compacted. Species that experience frequent flooding often also tolerate low oxygen soils. They will tolerate the conditions created by compacted urban soils. Metasequoia meets this need. It has deep plunging roots that thrive despite low oxygen in the soil.

Some trees deal with soils suffering chronic waterlogging with shallow root systems. Keeping the roots near the soil surface is how those species deal with low soil oxygen. It’s critical to consider root structure when matching a tree to the site. Sidewalks are an impediment to shallow-rooted species. Don't plant these species in small planting pits because their roots soon lift the paving. Shallow-rooted species in an urban forest create a dangerous situation for pedestrians. Trees with shallow roots are not a good fit for planting pits. Species with deep-growing roots are best for in-sidewalk openings. Oaks are an example.

A tree’s crown has the greatest visual impact but can have a large economic impact as well. Like the roots, branches soon interfere with the built infrastructure of the city. New utility lines are often buried, but overhead lines transfer right through the growing tree canopy. This creates a conflict dealt with by pruning, or neglect - either expensive. Unchecked lateral branch growth impacts building facades and roadways. The tree canopy brings shade to pedestrians, parked cars and nearby buildings. But physical impacts with any of these is a danger. Reduce this problem with variety selection, though it isn’t eliminated. There are varieties of many tree species that grow with narrow form, like Metasequoia. These fit well in streetside sidewalks without interfering with traffic. But they are the wrong choice when there are utility wires above. In that case, selections that grow with lower growth are more useful.

Light and heat are part of the considerations too. Heat reflected off buildings and windows can be intense. It damages tree growth and might even kill the exposed trunk. This problem seems more common with species adapted to the understory conditions of a forest, such as many maples. Using species like Chinese Pistache that tolerate extreme heat helps solve this problem.

Heat induced cracking is a problem in winter cold climates too. Frost cracking occurs when a frozen trunk – a condition of no concern itself – thaws and then rapidly freezes during the night. The reflected heat is the problem. The crack is disfiguring and introduces structural problems and decay. Some species are more tolerant of reflected heat. It’s important to select species adapted to the extremes of local weather conditions. Native or nearby native species hold within their genes adaptations to local extremes. City conditions are often much more severe than the pre-urban site. The city is a new harsher environment. And with projected climate warming, cities will become even hotter over the next 50 to 75 years. Use judgment when selecting trees.

The second protection realized from increased forest diversity comes from the work living trees do. Dead trees do not filter polluted air. But living trees offer humans well-being benefits, eco-benefits. They provide shade and habitat, mediate stormwater and runoff, and clean the air while removing excess carbon. The health benefits of the urban forest are well documented. Visual and physical interaction with the urban forest keep residents healthier. But the forest must be protected from disease and destructive insect pests to give us these benefits. When a tree dies, it's missed for decades. That impact multiplies when a monoculture dies as a result of a pest. The loss of a whole forest is catastrophic like a wildfire for three quarters of a century or more! Most residents will not live long enough to see the canopy restored. The city is changed with tree death. Urban forest biodiversity is the first and most effective defense in this struggle. It’s an opportunity for growing more tree species and more trees from seed.

Humans plant forests to fulfill many societal needs. It's been common practice that these forests are single species forests, monocultures. But forest monocultures do not create a healthy forest or a deep functioning ecosystem. A diverse urban forest is more resilient to pests and disease. This is even more true in large forests with many thousands or millions of trees growing near one another. Monocultures are unable to resist infestation by dominant pests or diseases because all the trees are the same kind. Many pine plantations around the world have been killed by bark beetles. In a monoculture the insects move from host tree to host tree whereas in a biodiverse forest it’s harder for the insects to find host trees. Most insects and diseases specialize to attack a specific class of hosts. Native trees are often more resilient to pest infestation or at least occur in mixed species forest stands. But when native trees grow in monocultures, they can suffer infestation too. It demonstrates that too many of the right trees make the site wrong.

Single-age monocultures also have greater storm susceptibility and provide less habitat to vertebrates. They are less filled with life. Forests with mixed deciduous and evergreen trees are more resilient to windstorms because of differences in the way wind interacts with their canopies. In planted forests it is useful to transition the forest canopy to a mixed age stand. Lots of variation in tree height and tree species increases storm resiliency. It also creates opportunities for tree-site match in a planted forest. Biodiversity of plants in a forest increases the biodiversity of animals in it. Mixed age stands further increase animal biodiversity.

Diversity in the forest is very useful. It builds resilience against climate changes. Forest diversity spreads climate risk across the forest. Increases in temperature and changes in rainfall have different effects on each species. Monocultures have all their exposure present; nothing is held in reserve. This creates a unique risk scenario that requires mitigation through diversity.

Forest biodiversity also has great implications for carbon sequestration. Diverse forests store twice as much carbon as single species forests. And this effect increases over time. Trees build their wood with carbon they remove from the air during photosynthesis. But the carbon isn’t confined to the wood of the tree.

Trees and the microorganisms around their roots move vast amounts of carbon into the soil. This serves an important role in local ecosystems and has a global impact for us. In fact, much more carbon is stored in the soils of forests than in the wood of the forests. This isn't true of every forest, however. Biodiverse forests sequester more carbon in the soil than monocultures. And forests of native trees sequester more than nonnative trees. A commitment to diversity in carbon sequestering forests is the best approach.

To have an impact on global climate change, trees need to store carbon securely in their wood and the soil for the long term. They must mature, grow their full life-span of 80, 150 years or more. But it isn’t enough to have planted trees just survive, even when it is for their full lifetime. People realize that biodiversity is import as an indicator of ecosystem health. And since we are part of the ecosystem people work hard to support and increase biodiversity. It will make us healthier. That work is going to be easier in the long term if it becomes self-replicating - returns to a wild state. Forest regeneration is important for maintaining biodiversity. Biodiverse forests hold more animal species and together they create a healthier ecosystem. That ecosystem captures more carbon. Monocultures do not become wild. Even planted native trees don't necessarily become wild. The right native trees must be matched to the project site for rewilding to happen. Right tree, right site, and over time the forest becomes regenerative and self-replicating.

Wild forests are the most productive carbon sequestering forests. Getting planted forests to return to the wild state must be the goal in carbon forestry projects. In carbon projects it is important to use native trees. Their ties to the environment create the right tree for the site match more than nonnative trees. The regenerative capacity of a biodiverse forest is greater. It will have greater species to site matching, and better ecosystem interactions. In biodiverse ecosystems the benefits increase overtime, and the planted forest becomes wild.