Talking Trees — Part 2: What are the trees trying to say?

Image by © Stephen Chambers
Published by RA Editions
Photography: FS@SCS

In my new series, Talking Trees, I’m translating findings from my PhD dissertation, called the "Internet of Nature", into seedling-sized blogs. My research advances ecological engineering by exploring the potential of novel technologies to monitor urban ecology, particularly urban soils and forests. I believe technology can be a powerful tool—and in this series, I hope to illustrate how digitising our urban ecosystems into useful data can help us build healthier, happier, and more resilient places to live.

In Part 1 of Talking Trees (read here if you missed it), we asked: Do city trees talk to each other? In short, they probably don’t talk, and when they do, it’s a whisper. Even when there are tree-mycorrhizal associations, there typically aren’t enough of them to support the tree’s health.

In Part 2, I want to unpack why the life of a city tree is so difficult. There is a lot to unpack. For one, Bainard, Klironomos, and Gordon (2011) found mycorrhizal associations to be 37% lower in urban areas, as compared to rural areas [1]. Why? Mycorrhizae are elusive and prefer not to ‘deal’ with urban soil issues like compaction, pollution, nutrient content, pH levels, or a lack of oxygen. For example, if polluting aluminium levels establish in the soil, the pH of the soil may be too low for effective tree–mycorrhizal associations to establish.

Tree-mycorrhizal associations may be the least of our worries, though. Even to apply the term ‘soils’ to the earth I sampled during my fieldwork is to be awfully polite. The urban soils I saw can only be described as highly variable. Often due to soil compaction, the varying contents of organic matter, or the patchy distribution of coarse natural or human-made materials such as coarse gravel or construction waste, as often even the parent material is anthropogenic in origin. In general, urban forest soil health has been little investigated due to the complexity of urban environments (e.g., variations in soil cover, land-use history, pollution, and degradation) and the difficulties of soil monitoring (e.g., limited training in accurate methods, lack of investment, and tedious and time-consuming work). Nevertheless, urban-forest soil health is the primary determinant of urban-forest health, and vice versa.

Although healthy soil is fast becoming a limited commodity in cities, healthy soil, with ample microbes, should provide the foundation for healthy urban forests. However, urban trees face very different growing conditions than do forest trees; as a result, urban trees live for only a fraction of the time forest trees do. Many studies have catalogued the plight of dying urban trees, and inadequate soil quality is an important cause of this premature mortality.

Once the healthy-soil foundation has been established, water availability is the next consideration. Everyone knows that trees need water to live: the question is, how much? 

If a tree has too little water during a period of drought, the tree becomes stressed. It may wilt for a little while and then bounce back—or the tree could permanently wilt. When this happens, the tree has to use more energy to recover—or it may die back completely. Watering newly planted trees is especially important, as the root zone of such a tree is not as large as that of an established tree. 

Trees that get too much water and have ‘wet feet’ for long periods can also be severely damaged. The tree will not be able to access oxygen from the soil, and the roots may begin to rot.

Phillip Craul, a Harvard landscape ecologist once said: ‘street trees die for many reasons, but the best place to start looking for causes is in the soil.’‍

In cities, trees live fast and die young [2] for a number of reasons: not only smog, dog urination, insufficient soil volumes, compacted soil, and bad pruning practices, but also the fact that there is no network of fellow trees to share nutrients and information with via their roots, as mentioned above. Tree health is severely impacted by the absence of a network of close family nearby to communicate with and supply nutrients to. Some urban trees are orphans; we must foster these trees. If only we could find a way to hear what the trees had to say...

References

[1] Bainard, L. D., Klironomos, J. N., & Gordon, A. M. (2011). Arbuscular mycorrhizal fungi in tree-based intercropping systems: a review of their abundance and diversity. Pedobiologia, 54(2), 57-61.

[2] Smith, I. A., Dearborn, V. K., & Hutyra, L. R. (2019). Live fast, die young: Accelerated growth, mortality, and turnover in street trees. PloS one, 14(5), e0215846.

Acknowledgements

Thank you to Stephen Chambers for the beautiful illustrations. My research is made possible by the generous support of the Connecting Nature H2020 project, University College Dublin, Trinity College Dublin, Stichting Fulbright Commission the Netherlands, and MIT Senseable City Lab.