Risks to Your Urban Forest
Climate change works both ways. Trees can help mitigate its impact, but climate change creates risks for urban forests. That risk varies community by community, even site by site.
- To identify species at risk, check out USDA’s Plant Hardiness Zone Map, and the Climate Change Response Framework.
- The FOREcast project website can help identify species whose ranges may be shifting dramatically, and identify potential replacements.
- For guidance on long-term planning and best practices consult Urban Forests: A Climate Change Adaptation Guide and the Template for Assessing Climate Change Impacts and Management Options.
- Baltimore’s Hazard Mitigation and Resilience Plan represents one of the most comprehensive regional attempts to anticipate and prepare for disasters and post-disaster recovery.
There are a number of tools to assess long-term and short-term fire risk on a variety of scales. Programs like FireWise Communities provide tools and examples of preventative planning for wildfires on scales from single homeowner to the municipality. In managing urban forests for wildfire risk, focus efforts on fire prevention because response following a wildfire will be focused on human and monetary impacts with the rehabilitation of the urban forest lagging far behind.
When planning for wildfire and urban forestry, be sure to look outside the city itself. Many resources on which your residents depend – in particular, water – can be degraded by fires that never touch the community itself.
Natural Life Cycle Changes in Your Urban Trees
Age matters, especially in the urban forest. Like many of our elders in society, mature trees contribute a disproportionate share of the benefits of urban forestry. But maintaining old trees can be time-consuming and expensive – especially as age makes them more susceptible to pests and disease.
- Consider potential liability risks and take care to compare costs and benefits of maintaining old trees against planting new trees.
- Over the long haul, strive for a tapestry of trees of different ages and species. Uneven age distribution is important for sustainability because it spreads out the timing of all management activities – planting, maintenance, removal, and replacement – so they won’t all come due at once.
- Uneven-aged forest stands also help pace the delivery of ecosystem services, or tree benefits, so there will be a steady supply at all times. A newly-planted tree needs decades of growth before it can provide the same level of benefits as its elders.
Even more important, trees of similar ages are – surprise – likely to die at about the same time. Organize your “big number” tree planting campaign wisely. Or your community may one day face a huge surge in expenses and a simultaneous deficit in ecosystem services.
Sea Level Rise
Of the 25 most populous counties in the US, 23 are considered coastal. NOAA predicts with “…very high confidence that global mean sea level will rise at least 0.2 meters and no more than 2.0 meters by 2100.” And while it’s impossible to predict precise impacts, some facts are certain. Even the low estimate has the potential to flood significant portions of these communities’ urban forests.
- To assess your community’s vulnerability to rising waters, check out NOAA’s Sea Level Rise and Coast Flooding Viewer and The Nature Conservancy’s Coastal Resilience Project.
- Low-lying coastal zones that already deal with the threat of sea level rise have begun to implement forest-based practices to mitigate flooding. Check out the Virginia Beach Urban Forest Management Plan to explore these strategies.
Pests, Disease, and Invasive Plants
Most urban forests are already stressed – and the dangers posed by pests, disease, and invasive species continue to increase, with cities paying the bills. As always, planning for perils costs far less than waiting until the dust settles, or the trees fall. Don’t be surprised, be prepared.
- The U.S. Forest Service hosts a portal for forest insect and disease reporting along with the ForWarn change detection system.
- Field methods designed for pest detection and tree health monitoring are also under development by the Healthy Trees, Healthy Cities initiative of The Nature Conservancy.
- At more local scales, it can be useful to engage community groups and engaged citizens to help identify new threats and monitor existing populations.
- Stock your toolkit from our resource library. Many communities have already developed ways to identify threats, prepare responses, and move into action at the right moment.
To remove and replace the dying, dead, and dangerous trees from just one pest, the emerald ash borer, the Forest Service estimates municipalities across the nation will likely spend more than $10.7 billion dollars.