Uncovering Urban Forests

Trees in urban areas lack the structural and species diversity of natural forests. This makes them ideal hosts for invasive pests, and the subsequent damage can be a major cost for cities and municipalities. Photo by Lance Cheung, USDA.

The scope of forest-pest risk analysis research is often limited to natural forests. Neglected are those tree communities called urban forests: trees within the boundaries of a city or populated area.

Urban trees see a significant proportion of the impacts from invasive pests. Their unnatural distribution and close proximity to transported goods and other means of pest dispersal make them highly susceptible to invasions.

USDA Forest Service research ecologist Frank Koch found a noticeable information gap while tackling this issue. “We don’t know the full composition of urban forests: what trees there are, where they are, and how many. We need to be able to represent their distribution if we want to track pests,” says Koch.

“To address any issue in urban forestry – be it pests, ecosystem services, or damage resilience — you need to know what trees you have in the first place,” adds Koch.

Koch and a team of scientists decided to fill in the lack of urban tree data using a three-step modeling approach. With the main goal of proving the model’s feasibility, they mapped distributions of maple, ash, and oak trees across the eastern and central U.S. Their results were published in Forest Ecology and Management.

“Some communities in the U.S. do in fact have whole tree inventories. Other communities have inventories of only their street trees. Most communities have none,” explains Koch.

Listen to a brief audio clip by author Frank Koch describing this publication. • Text Transcript

The research approach utilized communities that had both whole tree and street tree datasets to model the relationship between the two types of tree inventories. The scientists modeled relationships between tree inventories and other variables – including canopy cover, climate factors, and demographic trends.

These modeled relationships allowed Koch to estimate the total number of each tree type throughout the eastern and central U.S., even if the urban area had no tree inventory or an incomplete one.

“We found that maple and oak are sort of everywhere,” says Koch. “Ash is the funny one. They have really high distributions in the upper Midwest but are far less prominent in southern urban environments. This made them harder to model. Future studies should model them more regionally to better capture their variation.”

Palm trees aren’t actually trees — they’re flowering plants that sometimes take a tree form. Palms are grown in 14 states and are planted at much lower densities than other types of street trees. Photo by Mohamed Almari.

And though the model worked, there remain several components that can be improved. “One big constraint we faced was a lack of standardization: people record, sample, and report their tree inventories very differently. We also didn’t really deal with the West, but we need to. That’s where you can see the biggest differences between urban and natural forests: think Phoenix and Las Vegas as compared to their desert surroundings,” says Koch.

Both of these constraints are currently being addressed. The Forest Inventory and Analysis (FIA) program is working to scale up their surveys in urban areas. This will help improve standardization.

Deemed a success, the three-step model can now be put to a variety of uses.

“We are also using the model to map the distribution of street palm trees around the U.S. ahead of any invasion by a major pest such as the coconut rhinoceros beetle or the red palm weevil,” says Koch.

This work is preemptive, as neither insect has become established on the U.S. mainland. The insects wreak agricultural havoc around the globe, but their U.S. impact would be largely constrained to urban areas where palm trees line the streets. The costs would be in removing and replacing landscape trees – numbers which add up to significant amounts.

This effort represents the need to be prepared and take urban forests seriously.

“While urban areas are gateways for infestations, their problems rarely stay in urban areas,” says Koch. “It is important to be aware of how your forest interfaces and interacts with urban forests. We need to have a similar mindset about the pest interface as we have about the fire interface.”

Read the full text of the article here.

For more information, email Frank Koch at frank.h.koch@usda.gov.

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