What happens when big trees are replaced with small trees?

Some people claim that when mature street trees are cut down and replaced with young trees, the urban canopy is just as good as it was before. Others go further, claiming that leveling a whole forest and replanting the area is a smart move, because the young trees will absorb more carbon than the older trees would have. Is this good logic?


The second claim has some appeal to it. Surely small trees, which are actively growing, pull more carbon out of the air than mature trees that are not going to get any bigger. But new research shows that this is not true. Large trees can and do continue to absorb significant quantities of carbon from the air. It’s also important to note that if the cleared trees are burned, or otherwise disposed of in a way that doesn’t keep their carbon safely sequestered, the next generation of trees is just re-absorbing the same carbon that their predecessors were already doing a perfectly good job of storing. That’s clearly no victory.

What about the claim that cutting down a street tree and planting a new one results in no loss to the community? A basic understanding of what trees do shows that this cannot be correct. A young tree cannot filter as much air as a mature tree; it absorbs pollutants less effectively. A young tree cannot take up as much water as a mature tree; it mitigates flooding less effectively. A young tree does not cast as much shade as a mature tree; it moderates temperatures less effectively. A young tree cannot host as many bird nests or produce as much fruit as a mature tree; it provides habitat less effectively.

For all of these reasons, replacing a mature tree with a young tree means a significant loss of ecosystem services over the next several decades, until the new tree catches up to the size of the tree that had already been there. (For the same reasons, when city officials cut down a large tree, replace it with a tree that will never get more than about fifteen feet tall, and say it is just as good, that is also not true.)

Imagine a company that fired all of its employees and replaced them with new staff. Would that company continue to run successfully? Probably not; the loss of experience and institutional knowledge would be too disruptive.

To put it even more starkly, imagine a company that fired all of its employees and replaced them with children. It’s obvious that this company would struggle to continue providing high-quality products and services to its customers.

It’s no different when we destroy mature trees and replace them with young ones. It’s simply not realistic to expect an adolescent tree to do what a fully-grown one can do.

What happens when big trees are replaced with small trees?

Why do we need nature in every neighborhood?

People often toss around the words “appropriate” and “suitable” to describe where they think nature belongs. Nature is “appropriate” in city parks. Nature is “appropriate” in faraway wildlife refuges. Nature is “inappropriate” in people’s yards.

We should be asking these people why they think it’s “appropriate” to deprive others of a healthy living environment.

It’s a fact that some neighborhoods have more nature than others. In particular, affluent neighborhoods tend to have more street trees and more green space than poorer areas of the same town. Now, of course, there are lots of reasons why rich people are doing better, in all sorts of ways, than those who are less financially advantaged. There are also reasons why wealthy neighborhoods are greener: for example, those with money and status are more likely to demand that these kinds of amenities are created and preserved.

Presumably, the well-off would not be demanding more trees and parks if they thought these things were bad for them. But the more important point is that there are direct links between more greenery and being better off. Experts say that access to nature tends to move people towards healthier patterns in their exercise routines, transportation choices, and diets. Nature also reduces stress, moderates temperature, and combats air pollution. When people don’t have access to nature, they don’t have access to these important benefits either.

It’s fairly obvious that people who have nature right outside their front doors have more access to nature than people who have to travel some distance to experience healthy plants and plant communities. People who have more access to nature and all its benefits are more likely to actually receive those benefits.

The city of Madison recognized this when it said that destroying trees in some neighborhoods and not destroying trees in other neighborhoods would be unfair to the residents of the de-greened areas. It was exactly because of this unfairness that the city decided to work harder to protect trees.

In saying that it’s not fair for some people to have more trees and some people to have fewer trees, the city of Madison was expressing that trees have value. Nature has value. And so, to return to the original point – why would it be “inappropriate” to have valuable things on our property?

Nature belongs in every neighborhood – in the form of pocket parks, street trees, and natural yards – for the same reason that every neighborhood should have fire hydrants and nearby places of employment and access to public transportation. These things make our lives better. Anyone who says otherwise is not acting in your best interest.

Why do we need nature in every neighborhood?

What is green concrete?

People who are knowledgeable about the environmental importance of gardening choices sometimes refer to lawns and geometrical hedges as green concrete.

These types of plantings share certain visual similarities with regular concrete: they are flat, square-cornered, uniform in color, and unchanging. They also have some ecological similarities: like concrete, lawns are not very effective at absorbing water, cleaning the air, providing habitat for wildlife, moderating the local temperature, or performing other ecosystem services.

In fact, by some measures, lawns are worse for the environment than concrete. How is this possible? The answer is maintenance.

Concrete is environmentally damaging to produce and install. Once in place, however, it more or less just sits there until the end of its lifespan. A lawn, on the other hand, has little impact while it’s being planted, but then consumes a steady supply of water, fossil fuels, and toxic chemicals in an unproductive cycle of maintenance that can continue for many, many years.

It is worth considering: if you own a home for thirty years and mow the lawn every week, what will you have to show for it at the end? Will you look back with pride that for three decades, you prevented grass from growing? If the answers to these questions are unsatisfying, it may be time to consider alternative gardening practices.

What is green concrete?

How can one yard make a difference?

In some respects, sustainable gardening is what’s known as a collective action problem: a challenge in which many people need to work together to enact a solution, and in which the first few people to act incur high costs while receiving few, if any, benefits. These problems are difficult to solve because everyone needs to act, but no one wants to act first.

In the case of sustainable gardening, the first person on the block to plant native wildflowers, or not rake their leaves in the fall, or even just set their lawnmower blade to four inches high (it really is better for the grass) could end up getting criticized – or worse – by their neighbors, while the positive impact they’re having on the environment seems like hardly a drop in the bucket compared to the serious problems our planet is facing. Under such circumstances, who would want to be a trendsetter? Even if our neighbors nod politely at our efforts, isn’t one yard just too small to make a difference?

That Blogger had an opportunity to put these questions to Doug Tallamy, a professor of entomology at the University of Delaware and a well-known advocate of natural yards, just a few months before That Blog was launched. Here are his answers.

Can one yard make any difference to nature?

Less than one yard can make a difference, Tallamy said. Even a few flowers on a balcony or in a window box can provide a valuable foraging stop for butterflies and other pollinators. And a space the size of a bedroom can have real, measurable benefits for these small creatures. A 15-by-15-foot garden in the completely enclosed courtyard of the Department of Agriculture building in Dover, Delaware, became a nursery for no fewer than 150 monarch caterpillars, when employees in the building simply chose to use that space for milkweed rather than for grass.

What if we just don’t put nature in our yards?

“We no longer have the option of opting out,” Tallamy said bluntly. “Most people do not have viable habitat [in their yards], and we’re seeing a steady drain of species from our ecosystems.”

As those species vanish, the ecosystem services they provide disappear too. That’s a serious problem for us humans, Tallamy said. If we want to safeguard our own survival, we need to do something now. Once the species we exclude from our yards are extinct, we can’t bring them back.

Do enough people have nature in their yards?

No, Tallamy said. But we’re getting closer to the critical threshold of solving this collective action problem.

Sustainable gardening is “certainly not mainstream yet,” Tallamy said (speaking in early 2015), “but it’s headed in that direction. I’m optimistic. We have turned the corner much faster than I thought we would have.”

But seriously. One yard?

Tallamy turned the question on its head, pointing out that regardless of property lines, we don’t need to think in terms of just one yard. Our next-door neighbor has a yard. Their neighbor on the other side has a yard. There are yards all the way down the block and into the next town and across the country. We can invite all of those neighbors to join us in our efforts to garden with nature. We can encourage landlords of apartment buildings to landscape their grounds with native plants instead of with lawn. We can pressure our local officials to plant more street trees and spray fewer pesticides.

When we have the courage to be a trendsetter, we are not small. And we can make a difference.

How can one yard make a difference?

Does a lawn produce enough oxygen for a family of four?

Lawn service companies often say in their advertisements that a well-maintained, average-sized lawn produces enough oxygen for a family of four. A popular political fact-checking website might rate this claim as “mostly false.”

It is true that if you count up all the oxygen molecules that an average-sized lawn puts into the air, that number is approximately equal to the oxygen needs of four humans (at least, if the four humans sit very still, and avoid engaging in any activity that might increase their oxygen demands). However, doing the math this way assumes that the system of interest includes a lawn, four humans, and nothing else.

The companies that promote this claim surely intend for the system to also include themselves and their services. In other words, they mean for it to include lawnmowers. And it turns out that the amount of oxygen produced by an average-sized lawn is somewhat less than the amount of oxygen that is consumed by the internal combustion engine of a lawnmower in the course of cutting that lawn.

So, a more realistic calculation finds that lawns result in a net loss of oxygen from the atmosphere.

Even if the claim were completely true, the savvy consumer would ask themselves how much oxygen is produced by lawn alternatives. A prairie planting or forest garden produces more oxygen than a lawn of the same size, while sacrificing less of that oxygen to motorized maintenance equipment. A homeowner who wants to be sure their family has enough air to breathe would be better off going with a natural yard than a lawn.

But in the end, it is somewhat strange to evaluate landscaping options on the basis of how much oxygen they produce, since our planet is not suffering from a shortage of oxygen. Running out of breathable air is not a problem we are going to face in the foreseeable future. If a company is trying to promote lawns by claiming that lawns solve a problem that does not exist, we should wonder why this company doesn’t have anything better to say about its product.

Does a lawn produce enough oxygen for a family of four?

What is regenerativity?

We all know what sustainability is – living in such a way that we could continue to live that way indefinitely. Sustainability, though, is a zero-sum game, equivalent to spending exactly as much money as you earn. Sure, you could live on that budget indefinitely. But by doing so, you don’t put anything into savings to protect yourself against an emergency or to pass on to your children.

Some experts are now saying that we need regenerativity – a way of living that takes less than all of the sustainably-available resources, in order to build up our ecosystem savings account. For example, we need to plant enough trees not just to replace what we cut down, but to increase the size of forests. We need to take few enough fish from the ocean that those who are left can reproduce and increase their total population. That’s living regeneratively.

There are two main ways to live more regeneratively. Just like with our finances, we can decrease our expenses or we can increase our income. In ecosystem terms, we can use fewer resources – by reducing our energy consumption, eating lower on the food chain, and eliminating single-use disposable items from our daily lives – or we can mindfully help the Earth be more productive, by using compost to build soil, gardening with native plants that support pollinators, and taking care of trees to maximize their ability to clean air and water.

Last year, Earth Overshoot Day – the day on which we have used as many resources since January 1st as the Earth will produce in an entire year – was August 8th. This year, it fell on August 2nd.

If we were living sustainably, Earth Overshoot Day would be on December 31st every year. If we lived regeneratively, it would fall sometime in the next year. Our world’s resources would continually increase, allowing our children to live the same way we do and enjoy thriving ecosystems on a healthy planet.

What is regenerativity?

What’s new in natural yards? May 2017

Two weeks ago, the City of Madison made a change to how it will handle emerald ash borer (EAB).

Previously, the City had decided that it would not give any ash trees that were already unhealthy a treatment to protect them from EAB. What the City did not make clear to residents was that any tree located under a power line would be considered unhealthy, regardless of what condition it was actually in.

In thinking about the impact that this decision would have, the City realized that older neighborhoods in Madison, which have overhead power lines, stood to lose a lot of trees, while newer neighborhoods, in which the lines are underground, would be able to keep their trees. Recognizing the immense value of trees to nearby residents – due to trees’ ability to clean the air, reduce flooding, moderate temperatures, increase property values, and so on – the City concluded that it would not be fair for some neighborhoods to lose a lot of trees while others are able to keep their trees.

Based on this conclusion, Madison’s Common Council voted that ash trees under power lines should be treated to protect them from EAB, provided that they meet the City’s other requirements for treatment.

The full text of the resolution can be read here.

What’s new in natural yards? May 2017

What is this week?

Recognizing the importance of trees in our neighborhoods, the city of Madison has declared the coming week to be Arbor Week. The following resolution was adopted last month:


WHEREAS, the City of Madison has been a Tree City USA for 28 years; and

WHEREAS, children and youth living in greener neighborhoods are healthier; and

WHEREAS, trees give us oxygen, clean the air, and filter air pollutants; and

WHEREAS, trees in our neighborhoods increase property values; and

WHEREAS, the continued planting and care of trees in our city shall provide the same benefits for the present and future residents of Madison.

NOW, THEREFORE, BE IT RESOLVED that the Mayor of the City of Madison, Wisconsin, does hereby proclaim the week of April 30 through May 6, 2017 as ARBOR WEEK in the City of Madison, and urges everyone able to observe this week to plant trees and to participate in programs that the sponsors of Arbor Week may provide.

What is this week?

What is a tree worth?

There are several ways of answering this question.

First, as described in an earlier post, a mature tree can add thousands of dollars to the value of a property, in addition to saving the occupants money on utility bills.

Second, trees provide measurable value to a city through their role in absorbing and filtering stormwater, cleaning the air, and improving human health. New York City has calculated that its street trees are worth $122 million a year, and that every dollar spent on improving this urban forest generates a return of $5.60.

Third, a recent study found that adding ten trees to a city block produces benefits to the health of nearby residents that are equivalent to giving those residents an extra $10,000 a year of household income.

Urban trees have also been found to decrease childhood obesity, improve ADHD symptoms, deter crime, reduce traffic accidents, improve memory, speed recovery from illness, and even lower the rates of suicides and premature births.

Trees are usually the last thing to be considered in development projects, getting treated as nice-to-have amenities that are added at the end if there’s any money and space left. Experts on urban trees, however, say that the presence of trees in our neighborhoods is crucial to our wellbeing. Their true value, these experts say, is effectively incalculable.

What is a tree worth?

What is Earth Overshoot Day?

We all understand budgeting with money. If a household makes $50,000 in a year, then $50,000 is the most they should spend in a year. To spend more, they would have to overdraw their checking account or run up their credit card balance – practices that can quickly lead to financial disaster.

Ecosystem services need to be budgeted, too. The Earth “pays” us various resources, like food, wood, clean water, and breathable air. These resources continually replenish themselves, which is why we call them “renewable”.

However, even renewable resources only renew so fast. Each year, the Earth only generates a certain amount of the products and services we need. To budget wisely, we should make sure we aren’t using these services faster than the Earth can provide more of them.

Unfortunately, right now we’re not living within our ecosystem budget. Between last January 1st and today – August 8th – humans have collectively used as many resources as the Earth will produce in all of 2016. In other words, it’s taken us just over seven months to use up resources that our planet needs an entire year to generate.

That’s why today is called Earth Overshoot Day. But don’t mark your calendar for August 8th, 2017 – unlike other holidays, Earth Overshoot Day gets earlier every year. That is, we’re using up resources faster and faster, and wiping out the plants and animals that provide those resources, thus slowing down the rate at which the Earth can produce more of the things we need.

In 2015, Earth Overshoot Day fell on August 13th. As recently as 2009, it was in September. And in 1970, we just barely overspent our ecological income, using up a year’s worth of resources on December 23rd.

This isn’t a sustainable path. By taking less from the Earth, and putting back more, we can live within our ecological means.

What is Earth Overshoot Day?