Forests have long been credited with reducing flood risk. But when it comes to the largest and most destructive floods, many scientists have argued that trees make little difference. A new study from the University of British Columbia suggests that conclusion may be based on flawed measurements rather than flawed forests.
Published in the journal Ambio, the research challenges the widely used “before-and-after” method of comparing single flood peaks following logging, wildfire, or land-use change. According to the authors, floods are shaped by complex and variable conditions including soil moisture, snowpack, and storm intensity making simple peak-to-peak comparisons unreliable.
Instead, the team argues that flood risk should be evaluated probabilistically. In other words, rather than asking whether one specific flood was smaller or larger, researchers should examine how forests change the likelihood and frequency of floods over time. When viewed this way, forests appear to play a far greater role even in major flood events.
Lead author Samadhee Kaluarachchi and co-author Younes Alila contend that forests in headwater regions act as natural infrastructure. By storing rainfall, slowing runoff, and gradually releasing water, they influence how often downstream rivers reach dangerous levels. Forests may not eliminate catastrophic floods, but they can reduce how frequently they occur and how severe they become.
The findings carry important policy implications. Flood management strategies often prioritize dams, dikes, and engineered barriers near cities, where damage is most visible. But upstream land management including forest conservation may significantly shape flood risk long before rivers reach urban areas.
As climate change intensifies rainfall extremes, the researchers argue that forests should be treated not as a peripheral benefit but as a core component of flood risk reduction. They may not replace built infrastructure, but they could meaningfully lower the odds of disaster quietly, and upstream.