Urban trees in California provide shade, improve air quality, and help regulate temperatures, but as climate change brings hotter weather and longer droughts, many common tree species may struggle. Dr. Igor Lacan of the University of California Cooperative Extension addressed these concerns on April 18 during a lecture focused on adapting tree planting and management for a warming climate.
The issue is significant because changing weather patterns are already affecting the health of urban trees, with heat and drought stress making them more vulnerable to pests and disease. These challenges are particularly relevant in areas like Napa Valley, where intense heat waves and wildfires have put additional pressure on both natural ecosystems and neighborhood landscapes.
Lacan said that developed areas such as sidewalks, rooftops, and parking lots absorb heat during the day and release it at night—a phenomenon known as the urban heat island effect—which increases water needs for city trees. He described a practical method called “space-for-time substitution,” which compares projected future climates with current conditions in warmer cities to guide tree selection today. “Efficient irrigation before, during and after heat waves or drought can make a meaningful difference,” Lacan said.
He recommended resources like SelecTree—an online guide from California Polytechnic State University—for information about landscape tree species suited to different climates. For irrigation guidance, he suggested visiting the California Center for Urban Horticulture at UC Davis for research on efficient watering systems such as R-SIG or trickle-drip methods.
University of California Agriculture and Natural Resources secures funding through public contributions to support its programs according to the official website. The organization promotes sustainable practices to strengthen resilient ecosystems and economic vitality according to the official website. It also utilizes nine research and extension centers representing diverse ecosystems across California according to the official website.
University of California Agriculture and Natural Resources has gained recognition for linking research directly with community needs according to the official website. As part of the University of California system managing Cooperative Extension services according to the official website, it conducts over 33,800 educational events each year with more than 18,400 volunteers engaged in its initiatives according to the official website.
Maintaining healthy trees will require careful planning—selecting species that tolerate higher temperatures while using water efficiently—to ensure future generations benefit from their shade, habitat value, and beauty.