NATURE NOTE - Number 74                                                                            August 14, 2023

Introduction:  Frequently new words or terms get introduced into our society and many folks don’t know exactly what the new term means. Some of these new words or phrases show up in news reports about nature and environmental science.  One of these relatively new terms is:

Our current topic:   Heat Islands

What are they?  This is simply a new way of describing something we have been well aware of for years:  It’s hotter in cities and built up areas than in areas with more open spaces and trees.  However, today’s technology gives us the means to map these hotter areas and plot them on a map, making the discussion more precise as to location and possible cause.  This is being done around the country and it shows that built-up areas with many paved streets, sidewalks, parking lots, large buildings, etc. absorb more solar heat during hot, sunny days, and then radiate this heat after the sun sets.  These areas are referred to as “heat islands”.  Areas with tree cover, shade, grass, etc. (parks, suburban neighborhoods, business parks) are shown to be cooler on the maps.  So there is a rather large movement afoot for cities, civic groups, etc. to plant, or encourage the planting of trees in heat islands in order to produce shade and reduce the amount of heat accumulating in these heat islands.  It’s something that can be done to help combat climate change.  This is certainly an idea that gives some folks a “feel good” sensation.  How could planting trees not be a good thing?  I’m certainly in favor of more trees, either planted or naturally regenerated, but let’s think about this idea of planting trees in the urban setting.  Most of these thoughts are not covered in the information we read about on heat islands.

First, we are going to have to plant a LOT of trees to help mitigate the heat islands.  And it’s going to take a long time for them to reach a canopy size that can start providing shade to homes or streets.  (Note that Google says we already plant 1 billion trees per year in the U. S.)

Are these trees going to be planted on public (mostly city owned) or on private land?  If it’s the city, there is a cost associated with not only planting the trees, but with maintaining them for years to come.  As these trees grow, what is the impact on the spreading crowns and spreading roots on overhead and underground utilities as well as sidewalks and streets?  Urban forestry is not often a very high priority on the budget of most cities and adding more trees will require more urban forestry effort and cost.  Interestingly, $1.5 Billion federal funds for urban forestry has been provided from the Inflation Reduction Act.  How this will be spent is not yet clear in most cases.

If it’s to be done on private land such as home lots, what is the incentive for the homeowner to spend the money to plant and maintain these trees? Then there is the space issue: is there room for the roots among the buried utilities?  The crown?  Does the owner want more leaves to rake and dispose of in the fall?  Will the owner need to water the young sapling during the summer?  Does the shade fall on his yard or his neighbor’s?  What happens if the owner decides to sell the home?  Will the new owner want that tree? Some cities have established a monetary incentive to encourage owners to plant shade trees in their yard.  Unfortunately many of the areas needing shade to off-set heat islands are in urban areas where the homeowner (often lower income, elderly, or single-parent) may not have the means to plant and care for the trees in their yard.  Yet the heat island mapping often shows these neighborhoods are the hottest and suffer the greatest from the heat – no air conditioners, more health issues, less opportunity to escape the heat, etc.   Also, what species do we plant? Does the city or the homeowner select the species?  Let’s be careful here, as some species have specific habitat requirements, some are fast growing, but not desirable, and some are invasive. 

Another issue that needs heavy thought is the city policy (and expense) of planting trees vs. that of allowing and often encouraging  development of already tree-covered land to be cleared and developed into more homes, apartments or businesses. This also adds more parking lots and streets to support the new development. It seems counterproductive to plant a hundred little saplings around the city at taxpayer’s expense (how many will survive?) while the city planning board allows a developer to clear 25 acres of mature forest to build an apartment complex.  Food for thought.


This heat island concept is a good way to have scouts – and adults – talk about and think about the difficult solutions to some of the problems we encounter with nature.  While we can blame global warming on human actions if we want, the fact is that these heat islands are, to some extent, a local problem.  The quick answer on the internet or newspaper is to plant more trees in urban areas.  While this sounds good, and to some may be easy to do, there are many questions that need answers.  These few trees are not going to have much impact on global warming anytime soon.  While they might mitigate to some extent the impact of heat in some parts of some cities, the basic problem is nothing that came about because of recent global warming. I knew seventy years ago when I visited my cousins in the city that it was hotter there than out in the country where I lived. 

More food for thought.

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Bob Garst