Dying trees in cities? Blame it on the concrete

March 4, 2019 by Mick Kulikowski, North Carolina State University
Scale insects accumulate on a maple tree in Raleigh. Scale insects wreak havoc on maples in the midrange of studied cities in the Southeast. Credit: Adam Dale

A North Carolina State University study examining urbanization, scale-insect abundance and latitudinal warming on tree health in the Southeast captured a few surprising results.

The study showed more scale insects on red maple in the midrange of eight cities within a 10-degree latitudinal difference, from Newark, Delaware, to Gainesville, Florida.

Cities in that midrange, including Raleigh and Asheville, showed poorer tree health, due mostly to these high volumes of tree-destroying gloomy scale insects (Melanaspis tenebricosa), which appear as tiny bumps on tree branches and leaves.

"Impervious surfaces—basically concrete and pavement—near trees was a better predictor of scale-insect abundance than temperature, and thus a better predictor of poor tree health in the study area," said Michael Just, an NC State postdoctoral entomology researcher and corresponding author of a paper describing the research.

The finding was surprising, Just said, as the study's original hypothesis predicted higher scale-insect abundance at lower latitudes—the study's southernmost areas.

"What we've learned over the years in like forests didn't translate in this study, which means we may need to consider if other natural-system theories can be used in urban areas," Just said. "That's important if we want to have reliable predictive ecological models."

The study appears in the journal Oikos.

Explore further: Urban warming slows tree growth, photosynthesis

More information: Michael G. Just et al, Urbanization drives unique latitudinal patterns of insect herbivory and tree condition, Oikos (2019). DOI: 10.1111/oik.05874

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5 / 5 (1) Mar 05, 2019
As an Urban Forester, I have seen many varied outcomes of the abiotic stress imposed on our urban trees by environmental conditions. Impervious surface such as concrete is a major contributor to a high incidence of disease but it's not the only factor. All too often we are selecting trees which are substandard - not true to species architecture, too deep in the root ball, etc. The site we plant into has not been prepared with growing a tree in mind. The biggest limiting factor to healthy growth is the lack of an adequate, accessible volume of living soil. The reason concrete is so problematic is the engineering spec that has to be applied to the soil substrate. The level of compaction that is used destroys macro & micro pores, rendering the soil inhospitable for the biological activity which constitutes a living soil organism. Many strategies are available to address these issues but they have to be implemented. Until then, urban environmental stress will increase disease & failure.
not rated yet Mar 06, 2019
Urban areas are also deficient in predatory insects that keep infestations from increasing to the point where the plant is damaged.

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