Hot in the City: urban heat and the future of trees

In this guest post our PhD student, Emily Meineke, discusses her new paper in PLoS One and the the city life of plants, pests, and people…..

Most people live in cities. We walk between tall buildings and eat food grown thousands of

Emily hard at work counting scales in climate chambers. Photo by Becky Kirkland/NC State University Communications

Emily hard at work counting scales in climate chambers. Photo by Becky Kirkland/NC State University Communications

miles away. Urbanization brings big changes, and these changes have consequences—good and bad—for the creatures live with us, from microbes to mammals. Two big questions are: How do we change the species around us, and how do they change us? Now that most people live in cities, urban species are at the center of these questions.

More so than in any other ecosystems, humans engineer the conditions in which other species live in cities. This engineering includes intentional consequences (streets and their traffic) but also unintended ones, such as the urban heat island effect.

Imagine what cities used to look like before industrialization: a cart, a horse, and a dirt road. Imagine that cart becomes a car. Then the dirt under that car becomes a sidewalk, and the wooden shop beside the car becomes a modern building. Every part of the built environment that was wood or some other soft material becomes plastic, cement, or metal. Most of the materials in cities now are impermeable, meaning water can’t get in, and heat gets trapped. Cities are seas of man-made hardscape that hold in heat during the day and slowly release it at night, so that when surrounding rural areas are cooling off for the evening, cities are still heating themselves up from everywhere. This excess heat, the urban heat island effect, is changing the urban fauna, including us.

Thermal map of the Raleigh urban heat island.

Thermal map of the Raleigh urban heat island.

Hotter cities disadvantage certain groups of our own species that are sensitive to high temperatures. They can also kill some plants, whether via drought or the direct effects of heat. However, the vast majority of animals that live around us need heat from outside their bodies to survive. Some urban warming could be a good thing for some of these species, including some insects, spiders, and roly polies, just to name a few.

In this big story, I study one small but (I think) consequential piece: how urban warming affects scale insects (photo), tree pests closely related to aphids that insert their straw-like mouthparts into trees and suck out their phloem. Sometimes they kill street trees, but more often they weaken them.

My new research, published in PLoS One, shows that urban warming causes scale insects

Oak lecanium scale on willow oak twigs. Photo by Becky Kirkland/NC State University Communications

Oak lecanium scale on willow oak twigs. Photo by Becky Kirkland/NC State University Communications

to become more abundant in cities. Warm a street tree and the scale insects on it become much, more dense. This happens even in the lab, though the scale insects from warm trees seem better able to take advantage of the heat. It may be they have evolved so as to like it hot, or at least hotter. Warming-driven scale insect outbreaks are likely to be bad news for urban plants. And because urban plants remove pollutants from the air and cool cities, they might be bad news for us too.

Knowing what urban heat does to pests can help us predict changes as global temperatures climb. Overall, between killing or weakening trees via drought and pest insects and causing human health issues, urban warming seems like bad news.

This is not to say I begrudge the scale insects. They do what they have long done; they suck (plant juices). They are fascinating and poorly understood, and I plan on spending the next years of my life figuring out their mysteries. The good news is, we know a lot about how to mitigate the effects of urban warming. Plant a tree. Paint your roof white. Or, better yet, plant a tree on your roof. But when you do, keep an eye on the scale insects and tell me what you see. They will be there, I guarantee you. They always are – the real question is how abundant they will be, whether or not they have prospered in the luxuriance of urban warmth.

There will be more to this story (I’m just starting my thesis), so stay tuned. Or if you aren’t the sit and wait type, go outside and start looking at what else is going on in the ecology of the city. Scales are just the tip of the urban life-berg. There are beneficial microbes living inside the scales. Sometimes the scale may be the unlucky victim of a parasitoid wasp. Go deeper and you might find that the offspring of that parasitoid wasp growing inside the scale could also be unlucky, themselves the host of a hyper-parasitoid wasp. No one said urban life was simple, it is just relevant and more so every day.

This work was supported by a grant from the USGS Southeast Regional Climate Science Center to RRD and SDF. RRD was also supported by NASA Biodiversity Grant (ROSES-NNX09AK22G) and an NSF Career grant (0953390). SDF was also supported by grants from USDA Southern Region IPM (2010-02678), North Carolina Nursery and Landscape Association, the Horticultural Research Institute, and the USDA IR-4 Project. EKM was also funded by the NCSU Department of Entomology, and an EPA STAR Fellowship.

The research described in this paper has been funded wholly or in part by the United State Environmental Protection Agency (EPA) under the Science to Achieve Results (STAR) Graduate Fellowship Program. EPA has not officially endorsed this publication and the views expressed herein may not reflect the views of the EPA.

The project described in this publication was supported by Grant/Cooperative Agreement Number G10AC00624 from the United States Geological Survey.  Its contents are solely the responsibility of the authors and do not necessarily represent the official views of the USGS

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