Why do trees lose their leaves and branches?

The common understanding is that a tree’s leaves die at the end of the summer, and then fall off. This isn’t quite accurate. It’s more correct to say that the tree kills its leaves.

What’s really happening is that the tree builds a layer of cells inside the leaf’s stem to create a seal that blocks the flow of nutrients to the leaf. Imagine putting a tourniquet around your arm and leaving it there: eventually your arm would die and fall off, and that’s exactly what happens to the leaf.

A similar thing happens with branches. Humans, like most other animals, start their lives with a fixed number of limbs, and don’t get any more. For this reason, we’re very interested in keeping our arms and legs intact. But a tree can grow a new limb pretty much whenever and wherever it wants. When a branch isn’t pulling its weight, the tree will kill it – by creating an internal seal and starving it to death – and then grow a new branch that contributes more to the tree as a whole.

When a leaf or branch that has been killed in this way falls off, it doesn’t do the tree any harm. The tree wasn’t using that body part anymore, and the internal seal prevents any infection from entering the living parts of the tree. In contrast, when a leaf or branch is broken off suddenly – perhaps due to a hungry herbivore, pruning, or a powerful windstorm – the tree loses a healthy, productive body part, and acquires a wound through which insects or fungal infections can easily enter. Because trees don’t do anything very quickly, it can take them years to seal even a relatively small wound – more than enough time for a serious infection to set in.

It’s therefore inaccurate to say that cutting off a tree’s limb doesn’t hurt the tree. Of course, it doesn’t hurt the tree the same way that it would hurt a human, because the tree can regenerate. But the loss of a productively photosynthesizing branch, coupled with the energy demands of sealing a wound and growing a replacement branch, can put serious stress on a tree. While pruning may be desired for other reasons, the idea that pruning is good for a tree’s health just doesn’t cut the mustard.

Why do trees lose their leaves and branches?

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 is a tree spade?

A tree spade is a piece of heavy equipment designed to dig up and move large trees.

Small trees can be dug up and moved by hand, but what is a property owner to do when a mature tree is growing into power lines, standing in the way of a construction project, or casting shade on an area the property owner would prefer to be sunny? Many would respond by pruning the tree, or by destroying it entirely. Often, a better solution is to move it.

Tree spades were invented in the 1800s, and early versions could move trees more than 30 feet tall. Modern-day tree spade operators say they have moved trees well over 100 feet tall, as well as trees with trunks more than 5 feet across.

Tree spade operators don’t really recommend moving trees that large, though, since survival of the transplanted tree depends on whether enough of its root system can be picked up and moved with it. A tree’s root system can be larger than the aboveground part of the tree, but even the biggest tree spades can only dig up a chunk of soil about 7 feet across and 4 feet deep.

Medium-sized trees, though, can be successfully moved, and it is not even very expensive to do so – especially when compared against the cost of destroying the tree, including the loss of the services that a relocated tree could have gone on providing for many years.

By creatively rearranging our yards, we can have room for everything we want, while respecting the needs of many species.

What is a tree spade?

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?

Why do leaves change color in the fall?

Fall has arrived in the northern hemisphere, and leaves will soon be changing colors. We’ve all heard why they do this: the green color is created by chlorophyll, a substance key to photosynthesis, and when the chlorophyll is lost at the end of the summer, leaves reveal their true colors.

Now, some scientists think this story is wrong. They believe that, instead, trees actively work to create their brilliant fall colors.

Why would trees do this? One challenge plants face in life is being attacked by insects. To combat this, plants produce a variety of chemicals that deter insects from eating their leaves or burrowing in their bark.

Just as chlorophyll creates a green color in leaves, some of these insect-deterring chemicals create bright yellows, oranges, and reds. The more chemical a tree stores up, the more vibrant the colors.

In the same way that a male bird puts on showy colors in spring to prove that he is a healthy mate, trees display dramatic autumn hues to tell insects, “I’ve invested in defenses against you; don’t bother trying to attack me.”

Colorful fall leaves may therefore be not only a defensive strategy, but a method of communication – providing further evidence that plants are intelligent.

Why do leaves change color in the fall?