Urban Tree Mortality

Grow Fast, Die Young: The life cycle of an urban tree

Mike Davison, Revised October 12, 2020

Just a year ago, like many people, I thought all trees lived the typical life span of their species. Spoiler Alert: I was very naive back then. Urban trees ALWAYS grow faster and die younger than their rural counterparts. It is very important to understand the basics of tree mortality and include this in long term planning when managing urban forests. The reason for the difference in lifespan between urban trees and rural forest grown trees is complicated and the same story applies to all tree species.

Years of age have little meaning to a tree. Size equates to elderly in the world of trees. As a tree approaches a mature size, it slows its growth and starts to show signs of end of life. This is especially true if the tree grew to the large size quickly.

Maximum size is only achieved through slow growth. Slow growth causes lots of compacted growth rings creating a hard, dense wood. Dense wood has handles compression and stress forces better. It also resists fungus, insects, and bird damage better.

Healthy, slow growth needs a natural forest. Trees are made mostly from water and air with some traces of nutrients from the soil. Sunlight powers the process of creating sugars and fats from air and water. Because of this, heavy shade from dominate trees slows the growth of young trees. These trees must wait for the dominate trees to fail before they get a chance at more sun. Urban trees tend to be planted all at the same time so there is no competition for sunlight. Because they are spaced out, they don’t need to compete for water and nutrients.

Nutrients and harmful plant protection are provided by rotting wood and leaves from the tree. The healthiest trees have access to the best quality food. Nutrients processed from dead trees have everything the tree needs. These are broken down by insects and fungus into the perfect supplements. In addition, tree leaves will frequently contain toxins that inhibit harmful plants from growing nearby. In the urban environment, we tend to cart off the good stuff and mulch with usable, general-purpose wood-chips. The tree can still grow fast from this and other minerals in the soil, but it can lack some of the substances that contribute to the best possible health.

Mycorrhizal fungi in an old Scottland forest

Forests exhibit coordinated growth through mycorrhizal fungi networks. The extent that this takes place is is a somewhat recent discovery and is still not fully understood. Simply stated, in a forest, there are vast networks of fungus underground that find and process decaying matter and minerals that the trees and plants need, and exchange these for fats and sugars created by the trees and plants. These fungi do not have the ability to synthesize food on their own. This greatly expands the tree’s access to nutrients and filters out the toxins, but in exchange, the tree must trade for it. These vast fungal networks can consume quite a lot of the tree’s production, but at the same time, it stabilizes the health of all the trees in the area. The tree grows slower but far healthier. Even trees in full shade benefit from the exchanges. Dominate trees are sometimes called mother trees since they appear to nurse and influence all the nearby trees, but with a strict control of the slow growth of the young trees. These are very complex relationships between the various trees and plants and the fungal networks. Urban environments have some fungal networks but they are very limited because of the lack of rotting wood and the use of chemicals. The fast growing trees do not have the benefit of the fungi toxin filters

People tend to want urban trees to grow fast. Conditions that promote fast growth are encouraged, and of course, urban forests are created by people for their own enjoyment.

Urban hazards also contribute to tree mortality. Such hazards range from damage by cars, gardening equipment to improper pruning and irrigation. Trees that prefer occasional deep watering are planted in lawns and watered with frequent shallow watering. Mechanical damage and incorrect watering leads to increased risk of rot. The fast grown soft wood is particularly susceptible to disease and insect infestations, especially when the bark is damaged.

In a forest, a great deal of water can be stored in thick rotting mulch and the underground network of fungi and roots can greatly help trees through a drought, at least for a while. In an urban environment, good management includes deep watering of the trees which is especially important because the forest reserves are lacking. A failure to properly keep the tree hydrated, causes the tree to protect its water by reducing the water requirements. This is seen as slowing the flow of the sap and even dropping limbs that are not as efficient in producing sugars as other limbs. But the slowed sap flow, also leaves the tree defenseless against insects and rot. The dry wood cracks easier under stress and more wounds are exposed.

Poor pruning practices create multiple limbs to from the same point causing weak limb connections and structural failure later. Early in life, this may look like it causes the tree to attractively bush out, but the tree can be permanently damaged.

Typically, young urban trees under 5 years old, have a very high mortality rate. It is not uncommon to loose 25% of all young trees in the first 5 years. There are many reasons for this. Young trees are especially vulnerable to all the hazards previously discussed. In addition to these, they may not be the best species for the planting spot chosen. They are being transplanted to a different environment than they were initially raised. Poor planting and staking practices reduce the tree’s chance for survival. But mostly, young trees are not considered as valuable as mature trees and thus do not get proper care during these especially vulnerable years.

As urban trees mature, so do their conflicts with people. Otherwise healthy trees may be removed because their roots are damaging sidewalks, streets, and structures. Trees damaged by rot, insects, or storms, may be removes as the risks to people and property increase. Pruning errors from when the tree was young may increase limb failure risks. They may conflict with other trees or even loose their esthetic value in some cases, or maybe the place they are planted is wanted for another use. Especially tall trees that are over structures or peopled areas, become more costly to maintain and more of a risk if a failure were to occur. These all contribute to the reduced average life expectancy for urban trees.

Fully mature trees begin showing signs of declining health. The fast-grown, soft wood has difficulty in supporting the large tree. Stress fractures form, rot and insects are not fought off well, and the tree starts to fail. In storms, branches will start failing and eventually, the trunk or large parts of the canopy may snap. It is the job of good urban forest management to recognize these conditions and remove the tree before the risks become too great.

Sustaining Urban Forests

In a 2018 study, “Forty five states showed a net decline in (urban) tree cover with 23 states having statistically significant decreases.” (Urban Forestry & Urban Greening – US Forest Service, See link below) The primary cause for this is due to poor propagation management. Planting trees is not enough. Human intervention created the urban forest and human intervention is required to sustain it. An understanding is needed of how tree mortality affects the forest value, and a replacement strategy is needed to achieve and sustain those values.

The life expectancy of urban trees can be maximized with proper planning, management, and care. Proper soil testing and sapling selection is very important, as is irrigation planning. Corrective pruning is important while the tree is young and an appropriate pruning plan should be noted in the tree inventory. Early structural corrections have a large effect on the tree’s long-term health. As the tree grows, the irrigation plan changes. Most drought resistant trees suitable to Southern California, do best with occasional deep watering. However, even with the best possible care, an urban tree will not live as long as a rural forest tree. Even so, the management goal is to maintain the forest indefinitely, and therefore, the maintenance plan should not be limited by the life span of its trees.

Mortality planning is essential for propagating a successful urban forest. Landscape designs frequently overlook the need to plan for the trees’ end of life. Basically, what is the plan when a tree needs to be removed to keep the urban forest from declining? The answer to this can be complicated. In the early years, while the tree is trying to establish itself, the removed tree can be dug up and replaced. This is not so easy as the tree grows and the roots intertwine with other trees. It is common for the roots of street trees to grow into one large mass where it is not possible to remove the roots of one tree without damaging another, and with the trees in a neatly spaced row, planting a young tree where a mature tree was just removed is not practical. Cost has a lot to do with managing tree replacements. On embankments, it is far cheaper to flush-cut a tree stump and wait several years for the stump to decompose before that space is planted again. In the mean time, trees can be planted near by. Mortality issues are managed differently for a few potential types of urban forests. The two extremes of management practices are the crop-like Street Tree management practice, and the Simulated Forest management practice. Other practices are a hybrid of these two management approaches.

The Life Cycle of Street Trees

Street trees are what we sometimes call parkway trees. They are planted in small areas along streets and sidewalks. Planting street trees evenly spaced, is common practice, and a street lined with mature trees is always attractive. Street trees add value to the community, both as property value and as esthetic value. Maintaining that value requires planning. All long term urban forest management plans must include a way to deal with the life expectancy of these trees.

In Serrano Park, all of the streets were originally planted with trees evenly spaced in the parkways. Of these, the Brazilian pepper trees along Paseo Sombra and Paseo Tranquilo have been maintained by the HOA, and are now about 40 years old. Unfortunately, they have arrived at their end of useful life without ever having a long-term management plan in place.

Street trees are grown similar to crops. Some path lined trees are included in this management practice. In most other areas of the community, young trees can be planted to grow near mature trees so that as the mature tree reaches a point in its lifespan where it no longer is viable, the younger tree is ready to take its place. This is not practical with street trees and orderly planted path trees. These we plant in neat rows, grow them to maturity, and then replace and repeat, much like a crop.

Street trees are first planted all at the same time, evenly spaced and they grow quickly. The tree management is focused on fast growth. All of us want the look of a street lined with mature trees and the sooner the better. Unfortunately, the young trees usually have a high mortality rate due to various reasons as previously discussed. With this street tree plan, the higher mortality rate of young trees is managed through replacement of the failed trees. The young trees have not yet established large root systems so removing the stump and roots, fixing the soil, and replanting the tree is not too difficult. Since the young trees grow quickly, by 10 years into the planting cycle, all of the trees are of similar size again. The survival rates improve greatly, as the trees get established and fewer trees need to be replaced each year.

The trees continue to grow quickly to approach their mature size and then growth slows. The tree’s life time is more dependent on size and growth rate than years. A fast growing urban tree will not grow large and will not live as long as the same species tree in a rural environment. As the street trees approach mature size, they behave like old trees. “Live fast, die young” is the life cycle of an urban tree. As the tree matures, the risks of limb and trunk failures start to increase and mortality rates again rise. Replacing lost trees at this stage is difficult and the results are unsatisfactory. The soil is a mass of living and dying roots, making planting difficult and planting young trees at this time has poor yields. The early replaced trees catch up in size but gaps exist where old trees were lost too late in forest cycle to be replaced. As the risks increase and failures accelerate, the trees reach their end of useful life. At this point, the only thing to do, is to remove all the trees, clean out the soil of the dense mass of roots, replant the next crop of trees, and start the cycle again.

Managing a Simulated Forest

Trees in a natural forest range from seedlings to fully mature trees, somewhat randomly placed. The large trees shade and slow the growth of the younger trees. As the large trees fail, the medium sized trees quickly grow into the light. In this way, the forest is constantly being renewed. The same practice works in an urban environment with a simulated forest management plan. Several seedlings or young trees are planted near larger trees to create the forest effect. New plantings every 5 years or so, along with occasional thinning if needed, can maintain a constant range of maturing trees (even if mortality rates are high), ready to take over when a mature tree must be removed. Normally, the mature trees provide the supply of rotting wood to feed future generation so the urban version needs a good supply of chipped mulch. This is an effective and cost efficient management practice for small groves of trees.

In Serrano Park, we have quite a few areas where we grew mini-groves of large trees. Many of these trees are in low risk areas and can be allowed to grow to extreme heights. But, these trees are reaching maturity without young trees to take over when they are no longer viable. Growing several generations of young trees between these, will keep this mini-grove regenerated. Saving trees is important, but the goal of managing this area is to save the forest.

A Complete Management Approach

Rarely is it possible to manage the entire urban forest using a single technique for dealing with tree mortality. In some areas, each tree may need a slightly different plan. But usually, different areas can be broken up into smaller theme areas with a different plan for each theme area. Since all urban forest management plans require a tree inventory to be maintained, one of the fields in a tree’s data record should include a reference to the tree replacement plan that will be used and the expected tree lifespan. Good urban forest management requires a good understanding and planning for tree mortality.

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