The Forest Service is promoting its efforts to protect urban forests [see the Northeast Region’s “Roots in Research” in mid-December 2024]. The rationale is that urban forests provide substantial environmental and economic benefits that deserve more attention. These include air purification, temperature regulation and energy savings, water absorption, and improved public health. At the same time, urban forests face multiple and overlapping threats – including the one of greatest concern to us, introduction of tree-killing non-native insects and pathogens.
The article on which the Roots in Research “Science Brief” is based was actually published in 2022 in the Journal of Forestry. In it, David Nowak, Eric Greenfield, and Alexis Ellis evaluated historical and current threats to urban forests across the contiguous states and projected them 50 years into the future. Threats included urban expansion, climate change, insect infestation, and extreme weather events. Their goal was to help urban forest managers and policymakers prioritize resources and planning efforts.
I believe stakeholders should view these projects as underestimates because the sources Nowak et al. relied on for both future climatic conditions and non-native pest impacts are incomplete or outdated. I am not criticizing the choice of sources – they are the standard ones. But events have raised questions about their accuracy.
Nowak, Greenfield, and Ellis expected that urban tree cover will decline significantly by 2060. The principle cause is urban expansion — development of previously wooded areas. Development has traditionally been the leading cause of urban forest loss.
Newer threats have become obvious in recent decades – i.e., pest and disease attacks and extreme weather events.
The most troubling example of the sources’ weaknesses is the Alien Forest Pest Explorer (AFPE), on which the authors rely for their list of non-native insects and pathogens present in the United States. However, the compilers of this database decided not to include pests that are native to some parts of North America but are behaving as bioinvaders in other regions. The premier example is the goldspotted oak borer (GSOB), Agrilus auroguttatus. This insect kills three species of oaks native to southern California – coast live oak (Quercus agrifolia), California black oak (Q. kelloggii), and canyon live oak (Q. chrysolepis). Twelve years ago scientists estimated that GSOB had killed at least 100,000 trees in San Diego County; it has since been detected in widespread infestations in four other counties in southern California.
Not including GSOB (or Mediterranean oak borer; see below) skews the findings because of the importance of the oaks in California’s urban forests. Their genus is the second most-abundant native genus in the state’s urban forest, making up 6.5% of the trees. Because many of these trees are large, they contribute significantly to the ecosystem benefits provided by urban forests. Out of the 152,594 coast live oaks in 287 cities statewide, at least 30,000 of them meet GSOB’s preferred size limit (DBH greater than 18 – 20 inches [~45 cm]) (Love et al. 2022). The highest presence of oaks in urban forests in the South is in Santa Barbara – which has not yet been invaded by GSOB. However, built-up sections of Los Angeles – which are heavily invaded already — have between 250,000 and 300,000 coast live oak trees.
The Alien Forest Pest Explorer also does not include pests of palms. Palms are the first and second most the abundant species in urban areas of both the Southern California Coast and Southwest Desert regions (Love et al. 2022). Of course, palms contribute little to the ecosystem benefit associated with urban forests, but they are iconic symbols of the region. California’s palms are under attack by the South American palm weevil. https://cisr.ucr.edu/invasive-species/south-american-palm-weevil
More difficult to understand is the AFPC’s failure to include the Mediterranean oak borer, (MOB) (Xyleborus monographus). MOB has been introduced from Europe, so it fits the AFPE’s criteria for inclusion. MOB is killing valley (Quercus lobata) and blue oaks (Q. douglasii) in Lake, Napa, Sacramento, and Sonoma counties in California and Oregon oak (Q. garryana) in Troutdale, Salem, and other towns in Oregon.
As to the data sources relied on for projections of future climatic factors, several measurements of the changing climate already exceed projections in the models. They expect intensified threats from changes in air temperature, precipitation, aridity, wildfire risk, flooding, and sea level rise. By 2060, temperatures in urban areas are expected to increase by 1.2 – 3.5° C. Nowak and colleagues expected this warming to exacerbate threats from heat stress, flooding, increased salinity, drought, and wildfire. Less certain but possible are more intense storms and pest outbreaks. As I noted above, perhaps even these projections understate the threats.
For example, in discussing flooding the authors relied on measurements of the historic 100-year flood plain. I understand that experts now say this standard is inadequate, given existing records and projected further increases in precipitation (especially high-intensity storms). Urban areas in 98% of the 2,424 counties Nowak et al. analyzed contain flood-prone areas.
Nowak et al. do mention two additional elements exacerbating the flood risk: the spread of impervious surfaces and location of many cities next to bays or wide rivers. In these latter cases, risks might include salt intrusion linked to higher water levels, even in the absence of flooding. The National Oceanographic and Atmospheric Administration’s “intermediate high” scenario projects sea level will rise 61 cm by 2060.
Nowak, Greenfield, and Ellis said the greatest overall threat is in the eastern states, especially New England other than Vermont and Maine; the mid-Atlantic; South Carolina; and Ohio. They say this arises from the combination of high levels of urbanization and accumulation of several threats. The specific threats include projected precipitation changes, storms (hurricanes in the southeast; ice storms in the Appalachians); sea level rise; and the abundance of non-native pests. I think that reliance on data from the past results in understating the hurricane risk in the Northeast (especially the Hudson and Connecticut river basins) and in North Carolina.
Nowak, Greenfield, and Ellis reminded us that a healthy urban forest canopy can help mitigate some of the threats associated with climate change. This applies particularly to local air temperatures. Reducing urban heat islands not only addresses a direct threat; it can also moderate such other threats as pest infestations, wildfire, aridity, and storm damage, especially runoff. They advocate science-based tree management programs including preserving existing trees and planting species that can thrive in the expected new local and regional environment, e.g., withstand droughts, flooding, saltwater exposure, or extreme temperatures.
I think their recommendation on pest threats is lame: they suggested “monitoring and managing local pest threats.” Non-native pests demand additional actions at all levels of authority — local, state, and federal. (See the “Fading Forests” reports linked to at the end of this blog, and earlier blogs under the category “invasive species policy”.) I have already noted troubling exclusion of some pests already present in urban areas of the continental United States. I understand that it is impossible to predict which additional pests might be introduced in the next 50 years. But I would have appreciated a sentence stating the near certainty that more pests will be introduced and cause damage to urban forests in the next 50 years.
Given the recent fires in the Los Angeles region, I believe we need new analyses of the risk of wildfire in cities and the positive and negative interactions with the urban forest.
SOURCES
Threats to Urban Forests in the United States Roots in Research Issue 45 | December 2024 https://research.fs.usda.gov/nrs/ products/rooted-research/threats-urban-forests-united-states?utm_source=MarketingCloud&utm_medium=email accessed 24-12/31
Nowak, D.J., E.J. Greenfield, and A. Ellis. 2022. Assessing Urban Forest Threats across the conterminous United States. Journal of Forestry, 2022, Vol. 120, No. 6
Posted by Faith Campbell
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For a detailed discussion of the policies and practices that have allowed these pests to enter and spread – and that do not promote effective restoration strategies – review the Fading Forests report at https://treeimprovement.tennessee.edu/
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