The COVID-19 Domino: How Life Under the Pandemic Poses a Threat to Our Forests
By: Andrew Reinmann
Tags: Urban Planning, Covid 19, Cuny Graduate Center, Climate Change, Carbon Emissions

Central Park in New York City (credit: Getty Images)

For many of us, managing life under the COVID-19 pandemic has included leaving densely populated urban centers for suburban and rural areas in search of more living space and less-cramped public outings. But our efforts to seek more space and an end to the cacophony of endless Zoom meetings echoing throughout our small apartments may unwittingly increase the indirect effects of the virus on the planet well beyond the pandemic’s timeline.

The high costs of city living and a burgeoning work-from-anywhere culture will make many people’s departure from the city permanent. Existing housing stock will absorb some of the surge in demand, but the exodus from large cities like New York will undoubtedly necessitate new development that will bring about land clearing and spur greater forest fragmentation. This is one of the many ways in which the pandemic can have a lasting impact on Earth’s ecosystems. To understand how, we need to take a brief trip back into the city.

Cities are masses of concrete and pavement where trees and vegetation once covered the land. Trees move large amounts of water from the soil to the atmosphere through a cooling process known as transpiration, which is an earth surface analog to the way that sweating cools us off. Of course, transpiration is not an act of altruism by the tree, but rather a necessary process to provide trees with essential nutrients and the building blocks needed to synthesize sugars through photosynthesis. Replacing trees with materials that not only don’t transpire, but also convert a lot more of the sun’s incoming radiation to heat elevates air temperatures considerably. In fact, this urban heat island effect makes cities like New York, Boston, and Washington, D.C. 5–10 degrees Fahrenheit hotter than they were as heavily vegetated areas of land — an amount of warming on par with what is expected from climate change over the next hundred years. When urban expats set out for greener pastures and clear forestland for new homes, a little piece of the city can come along in the form of a small-scale urban heat islands and fragmented forests adjacent to development.

A study that my colleagues and I recently published in the journal Environmental Research Letters shows that forest fragmentation and urbanization have some surprising effects on the forests that we leave behind. We took to the woods in urban and rural landscapes of New England to measure growth rates of hundreds of trees along transects extending from a forest edge (the boundary between the forest and the adjacent development or pasture) to the forest interior. Trees growing near the forest edge have greater access to light and maybe nutrients too, and it seems that these improvements in growing conditions stimulate tree growth up to 65 feet into the forest! In fact, we find that forests growing along an edge remove twice as much carbon dioxide from the atmosphere than interior forests.

We learn in kindergarten that plants grow faster when they have more access to light, so why is this new discovery a surprise? Well, it turns out that our temperate broadleaf forests across the northeastern U.S. are a bit unique. Fragmentation in tropical and boreal forests actually increases tree mortality and reduces the amount of carbon these forests can store. This is partially because the tall trees in the tropics and the shallow-rooted trees in boreal forests (think spruce and fir) make them more likely to blow over in the face of the stronger winds encountered by trees along the forest edge. By contrast, the trees in broadleaf forests of the northeastern U.S. do not seem to suffer the same fate, probably because they are shorter than tropical trees and more deeply rooted than boreal trees.

So, humans escaping the city and subsequently promoting forest fragmentation in the country makes the forests sequester more carbon — good news, right? Well, not really, and here’s why: The forest cleared for a new home releases a lot of carbon dioxide back into the atmosphere, and without trees the capacity of the cleared land to remove carbon dioxide in the future is eliminated. While the forest surrounding the newly developed land might be growing faster, it is not fast enough to compensate for the forest that was lost. In fact, we estimate that enhancements of forest growth along edges across all of southern New England can only offset one-third of the lost carbon sequestration capacity the region had when it was completely forested.

Fragmentation can also have adverse impacts on biodiversity by reducing wildlife habitat, and we are finding that development can change how trees in the adjacent forest respond to environmental stress. Like many of us, the forests of the northeastern U.S. are not particularly fond of hot summers, but fragmentation and urbanization/development exacerbate tree growth declines in response to heat. Declines in tree growth during hot summers are two to three times greater in forests within 65 feet of the forest edge compared to trees in the interior. Our research also indicates that this sensitivity to heat is greater in forests surrounded by urban development than those surrounded by vegetation like pastures and meadows.

The upshot is, on one hand, the highly fragmented nature of our urban forests (those in our cities and dense suburbs) makes them more productive per acre than the large expanses of interior forests that we might find in the Catskills or Adirondacks. On the other hand, because they are more sensitive to heat they are likely to be more adversely impacted by climate change.

Perhaps now more than ever we are turning to trees and forests to provide valuable ecosystem services like cooling and carbon dioxide sequestration in efforts to mitigate climate change and make our cities more sustainable. The New York State Climate Leadership and Community Protection Act — which aims to reduce greenhouse gas emissions at least 85% by 2050 — is one of the most ambitious climate change mitigation programs in the world. This policy and many others like it rely, in part, on our forests ability to continue removing carbon dioxide from the atmosphere. Further, cooling by trees can help us to reduce emissions associated with electricity production to run our air conditioners. Forests can be resilient and enormously capable of meeting the demands we place on them. However, further fragmentation and development could hinder their resilience and capacity in this regard, particularly in the face of a changing climate. As many of us look to move to the suburbs and country we might want to consider what aspects of the city come with us. Can we have our little piece of the country without losing forests or making them more vulnerable to climate change? If we choose to make our new homes from existing houses or by rehabilitating old structures, perhaps we can have the country and resilient forests too.