Hotspots of pest-induced US urban tree death: culprits, impacted tree species, and spatial hotspots

In their latest research Emma J. Hudgins, Frank H. Koch, Mark J. Ambrose, and Brian Leung, discuss the economic implications of pest-induced tree deaths in the US.

Urban trees are key to the wellbeing of city dwellers but are at high risk of mortality from insect pests, due to having high rates of exposure to invasive species as enabled by trade, travel, and other human activities. Urban forests may also provide habitat that is more conducive to the survival of those invaders. In many municipalities, parts of their urban forests (including their street trees) are dominated by a single tree species or genus, which means that a newly arrived insect for which those trees are a host can spread rather easily.

In the US, 82% of the population lives in urban settings and relies on the benefits derived by these trees, like lowering cooling costs and improving mental health outcomes. Our study identified where and when the greatest impacts of invasive insects are expected to occur, which can provide the basis for prioritizing management efforts such as wood product quarantines and biological control release sites. 

We used a series of four models to estimate the impacts of invasive insects on street trees in the United States over 30 years. These included a model that forecasted insect spread, a model of where susceptible trees were located, a model of how deadly each insect was on each tree species, and a model of the cost of removing and replacing trees of different sizes. We put these together to get forecasts of the precise locations, species, and cause of death of each future dead tree, as well as the associated cost to remove and replace it.

We estimate that 1.4 million United States (US) street trees will be killed by invasive insects in the next 30 years, costing over US$ 900 million in replacement costs. Roughly 90% of all impacts will be due to a wood-boring beetle called the emerald ash borer, which is expected to kill 99% of ash trees in more than 6000 communities. We predict that less than a quarter of US communities will bear 95% of the impacts of these insects on street trees.

We identify mortality hotspot cities including Milwaukee, WI; Chicago, IL; and New York, NY; which will each see impacts of up to US$ 13M. These locations have very high numbers of ash trees planted on their streets, and they are in the recent or near-future path of the emerald ash borer. Note that there are many villages, towns, and small cities that will lose comparable proportions of their street trees but that have lower absolute losses.

Our study points to several factors that affect the magnitude of impact among invasive forest insects in the United States. The most impactful insect species have been those native to Asia, likely due to high import volumes of Asian commodities. Wood-boring species like emerald ash borer have the most invasive style of feeding on trees, so they are the deadliest. A third factor increasing the risk of an invasive insect for street trees, is whether an insect feeds on common street trees like oaks, maples, or ash trees; if they do, these pests have a much larger pool of susceptible trees to impact.

We predict that a new establishment of an insect possessing these characteristics could cost US$ 4.9B over 30 years. The citrus long-horned beetle Is a species fitting this description. This species was discovered in a nursery in Tukwila, WA in 2001 and has since been eradicated. However, because not all import shipments are inspected, and despite current wood treatment protocols, the US remains vulnerable to invasion and subsequent spread of this species and other high-risk insects. 

For several reasons, our predictions are underestimates of the true future impact of invasive insects in cities. Our study focuses on impacts to street trees, which are a small fraction of all urban trees. Also, note that we were looking at economic impacts, and specifically the costs to municipalities of dealing with the street trees killed by an invasive insect. We did not look at the ecological impacts of losing urban trees, nor the ecological implications if an invader were to move from urban to natural forests. Thirdly, our models focus on species we know are currently established in the U.S.

More generally, these results can provide a cautionary tale against planting a single species or genus of tree throughout entire cities, as has been done with ash trees in North America. Increasing urban tree diversity provides resilience against pest infestations. While we recognize the importance of diversity for agriculture, it is also a critical consideration for urban forests.

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Read the full Open Access article Hotspots of pest-induced US urban tree death, 2020-2050 in Journal of Applied Ecology

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