City Trees versus Country Trees

Posted by: Alison O'Connor, Larimer County Extension

Remember the book "Town Mouse Country Mouse" by Jan Brett? It was one of my favorites growing up. The plot was simple: a mouse from the country and a mouse from town swap lives and homes...and adventures follow!

Book cover from Amazon.com

An article from "Scientific Reports" from nature.com (published in November 2017; https://www.nature.com/articles/s41598-017-14831-w) reminded me of this story--but it focused on city (urban) trees and country (rural) trees and how they've differed in growth since the 1960s.

There's been a lot of research on how climate change (warming temperatures, increase carbon dioxide, longer growing seasons) have affected trees in the forest, but little research has looked at trees in urban areas. In general, warmer temperatures and increased carbon is good for trees. Carbon is used in the process of photosynthesis and warmer temperatures make photosynthesis more productive (to a point). City trees, compared to rural trees, often live among warmer temperatures (due to increased infrastructure and people) and increased carbon levels (due to pollution and industry).

Photo courtesy of Portland State University.

And yes, it comes down to climate change. But instead of getting political, let's just discuss the study and what they found.

The study looked at trees in 10 metropolises worldwide: Sappora, Japan; Price George, Canada; Berlin, Germany; Munich, Germany; Paris, France; Santiago de Chile, Chile; Cape Town, South Africa; Hanoi, Vietnam; Brisbane, Australia; and Houston, Texas. These cities differ in latitude, mean annual precipitation, mean annual temperature and climate zone (ranging from boreal to subtropical). The purpose was to determine how climate change and the urban heat island effect affect the vitality and growth of urban plants.

Metropolis locations for the research study.

The study looked at nearly 1400 mature trees of varying species: Sachalin fir, spruce, littleleaf linden, horsechestnut, London planetree, black locust, English oak, African mahogany, hoope pine and water oak. The data collected took two cores of the trees (from the north and east) and measurements like diameter at breast height (DBH), height and height to crown base. From these measurements, the researchers developed equations to test for overall growth trend and overall urban zone effect. And the researchers also wanted to get more detail about the combined effects of urban zone affiliation and period-specific growth trends, translating this across climate zones. Phew!

To sum it up (if you want to stop reading here): City trees grew better, overall, compared to their rural counterparts and climate change increases growth for city trees. But, this increased growth may lead to shorter overall lifespans and more rapid aging.

Since 1960, the study found that overall tree growth (in both urban and country locations) has increased. The study also found that urban trees will be larger and reach a greater size than a rural tree of the same age. But as the trees grow older (100+ years), the growth rate decreases for city trees, resulting in urban trees that are only 18% larger than rural trees after 100 years.

It also varies by climate zone. Colorado would likely be classified as a "temperate" climate and the research found that urban trees in this climate (in the cities of Paris, Berlin and Munich) grew significantly slower than rural trees. This was the only climate where urban trees were found to grow more slowly.

The City of Fort Collins

As urbanization continues and more people gravitate to living in urban areas, having trees in these areas becomes of utmost importance. I don't need to belabor the benefits of trees, but they do a lot for our local communities, climate and overall well-being. Interestingly, this study found similar results to what is being found in forest regions--accelerated growth (studies conducted in central Europe and Japan).

Another major thing the study didn't specifically address, but something Coloradans should have on the forefront of our minds is water and drought. While increasing temperatures can mean accelerated growth for trees, it cannot happen if there is drought or water shortages. As a result, drought will reduce growth and/or contribute to tree death. Urban trees can suffer from water stress due to higher temperatures and modified precipitation patterns...along with unfavorable soil conditions (compaction, impervious surfaces, etc.).

Boy, don't I feel like a downer! In short, studies like this one can open our eyes to the effects of climate and how trees respond in urban areas. It really is interesting...and my only advice to you is to plant your trees properly, select the correct species and care for and maintain them as best you can. Celebrate trees in our communities!

Town Mouse meets Country Mouse