Forgotten Landscapes: Native Americans and Cool Shade

I have just published my sixth book, Forgotten Landscapes: How Native Americans Created Pre-Columbian North America and What We Can Learn from It. I am starting a series of essays and videos to promote portions of this book. The video that goes along with this essay is here.

In this book, I describe one ecological problem that white European and American villages had that Cherokee and other Native villages did not was the problem of how to stay cool in the summer.

The video shows my neighborhood in Alsace, France. It is densely populated, and there are hundreds of condominiums right next to my building. Our condominiums are very energy-efficient, even the ones built a long time ago, such as mine. Our windows and walls are thick, which keeps our units quiet as well as insulated. And this is how that efficiency is achieved:

• Sources of energy. Instead of each condominium having its own individual heating units, as is often the case in America, these buildings get their heat from hot water going through radiators. The hot water comes from a network from a centralized power plant where furnaces burn methane to heat the water. The methane (as I described in the previous video) comes partly from food wastes.
• Required energy efficiency. Whenever one of the condominiums is sold, the building owner has to meet new standards of energy efficiency, e.g. through improvement of insulation; the building owners pass the costs to condo owners who pass the costs on to renters.
• Little but deliberate things. In the middle of our courtyard there are a lot of trees, which provide cool shade. There are two reasons for this. First, the leaves absorb sunlight energy and emit the heat straight up into the air, away from where we live. Second, transpiration (the evaporation of water) uses up some of the heat energy. These îlots fraîcheurs (cool islands) are a deliberate part of urban planning. Some American cities do this also. Las Vegas recently decided to plant trees to help deal with the over-110-degree summer heat. The problem is, where are they going to get the water for the trees? It can be done, but has to be planned.
• Air conditioning. Very few French apartments have air conditioners. We are just lucky to live 48 degrees north latitude where we can get away with not having air conditioners.

In my book I describe Cherokee villages that were filled with trees that kept them cool. Meanwhile, white cities would, at the time, cut down all the trees. The Cherokees were probably puzzled that white people would work so hard to cut down the trees and then get, literally, hot under the collar about the summer heat.

I doubt that the Native Americans knew more ecology than white Americans. They just thought cutting the trees down was a lot of hard work with nothing to show for it.

Getting Peed on...by a Tree

Leaves evaporate billions of tons of water vapor every day all around the world during the late spring and summer. The evaporation of water (transpiration) allows the leaves to get rid of the heat burden from sunlight and send the heat up higher into the air. Some of this water vapor ends up in the clouds. What the trees are “trying to do” is to keep its leaves cool. But this also means that when we or our houses are down in the shade, it is cool shade, cooler than the shade of a carport or building. I explain this in my book Green Planet: How Plants Keep the Earth Alive.

One would think that trees have to do a lot of work to pump all this water up from the ground, through the roots, through the trunk, and up to the leaves. But, actually, the trees allow the laws of physics to do the work for them. The water molecules cohere to one another. As water molecules transpire from the leaves, they pull the water molecules behind them. The little columns of water in the pipe cells of the trunk are stretched like tiny rubber bands. In some cases, botanists have been able to measure the tree trunk getting narrower when transpiration begins. During the daytime in the growing season, the leaves simply open their pores and let the laws of physics pull the water up. The water is under tension, which is the opposite of pressure.

But before the leaves emerge from the buds, there is no transpiration. The water cannot be pulled up to the top of the tree. Yet, we all know that the sap rises. In this case, the tree generates water pressure (by accumulating sugar and minerals) in the roots, and this pushes the water up to eventually make the buds burst open.

If the tree generates water pressure but the buds are not yet open, the water can occasionally leak out of damaged wood. The damage can be intentional, for instance when the rising sap of maple or birch trees is made to drip into little buckets. Or it can be accidental. If you are standing underneath a birch tree when the sap is rising but before the buds have opened, some of this water might drip down on you. You may think a squirrel has peed on you, but it was actually the tree. This little drop of water, coming out of what seems like a clear blue sky, hitting your head can have some of the same effect that the apple had on Newton: opening your mind to discovering something new about the world of nature—in this case, trees.

I have posted a video about this: Darwin gets peed on by a tree.

Water cannot be pushed (pressure) and pulled (tension) at the same time. The tree pushes the water into the expanding leaves and other structures. Once transpiration starts, the pressure stops and tension takes over.

All this invisible activity is going on right before your eyes!

A Poem As Useful As a Tree

I wish everyone could enjoy the beauty of trees as much as I do. Each one is a better poem than anything written by a human. Even ugly trees (and there are a few) are interesting. But if someone just thinks trees are useful, I’ll settle for that.

I had my botany students do an exercise in trigonometry, in which they measured a tree (each group had its own tree) and estimate the number of twigs on the tree. The results were astonishing. It is not unusual for a big oak tree to have 50,000 twigs.

Then I had them estimate their own carbon footprints, using websites that do the calculations. A typical number would be 50,000 kg of carbon dioxide per year from their activities.

Next I had them do some simple arithmetic. Just some multiplying and dividing. I had them assume (from actual measurements made in previous years) that each twig had 100 square cm of leaf area. This would mean a typical tree had 5,000,000 square cm of leaf area, which is 500 square m (they had trouble with this). If you assume (also based on measurements in previous years) that a typical tree absorbed 10 grams per square meter per day of carbon dioxide, this would be 5,000 grams or 5 kg of carbon dioxide per day. This may seem low to you, but remember these are post oaks in a dry forest. This means that it would take their tree 10,000 days to absorb 50,000 kg of carbon dioxide. Assume a growing season is 180 days. It would take the tree over 55 years to absorb that much carbon dioxide.

Which means there has to be 55 mature oak trees to compensate for one person’s carbon emissions.