Researchers from the University of British Columbia argue that a widely used method to understand and predict flood risk has led scientists to miscalculate how forests can prevent major flooding.
The paper, published in Ambio , synthesizes decades of research to explain why the standard approach used to evaluate how forests impact flooding – comparing individual flood peaks before and after disturbance – fails to capture how floods actually develop.
The authors say earlier studies oversimplified flood behaviour and consequently underestimated the role of forests in mitigating flood risk.
“When we look at flood risk in probabilistic terms - how trees and forests change the likelihood of a flood - the picture changes,” said Samadhee Kaluarachchi , lead author and PhD student. “Forests are part of the solution, even for big floods.”
Rethinking forests and floods
For decades, many studies have concluded that forests mostly influence small floods in small basins, with limited impact on larger events. Those findings have influenced policy discussions, making governments hesitant to rely on forests for flood mitigation strategies.
But authors Kaluarachchi and Dr. Younes Alila, a professor at UBC’s faculty of forestry and environmental stewardship, argue these findings stem from flawed “before-and-after” comparisons that ignore the probabilistic nature of flooding.
“When studies focus only on peak flows from individual events, it overlooks how forests influence the broader distribution of food risk over time,” said Dr. Alila. “Our synthesis shows forests can alter the frequency and probability of floods, including major events. This doesn’t mean forests alone will stop catastrophic floods - but they can reduce flood risk at the source, making floods not only smaller but also rarer in cities and communities downstream.”
Forests as a natural flood defence
Previous research by Kaluarachchi and Dr. Alila has empirically shown that B.C.’s natural landscape – including lakes, wetlands and forests – offers built-in flood protection by storing and gradually releasing runoff, ultimately reducing peak flows and flood frequencies downstream. The new paper explains why this effect is scientifically sound and why these findings, and others like them, should be trusted over the conventional approach that dominates forest hydrology. It challenges the long-standing scientific assumption that forests do not influence large flooding events.
The authors emphasize that while infrastructure like dams and dikes remain essential, ignoring the role of forests and land cover in upstream areas risks poor land-use decisions downstream.
“It’s about broadening the toolbox,” says Kaluarachchi. “Engineering infrastructure is part of the solution, but it cannot address the root causes of flooding. When land management and forest removal in the headwaters increase flood risks downstream, forests and healthy ecosystems must be a core part of flood management.”
Implications for flood management
The researchers call for a shift in how flood impacts are evaluated – moving away from single flood event comparisons and toward assessing how forests affect the likelihood of floods over time.
Drawing on their own studies as well as research across the field, the authors aim to clarify the scientific basis for considering forests as a core component of long-term flood resilience. They also call for policy guidance to be updated to reflect this more rigorous, causal science.
They suggest that adopting a broader lens could improve how governments integrate land-use management into flood mitigation strategies.
Interview languages: English (Kaluarachchi, Alila), French (Alila)
Please contact charlotte.fisher@ubc.ca or lou.bosshart@ubc.ca to arrange interviews with Dr. Alila or Samadhee Kaluarachchi.
AMBIO
Literature review
Not applicable
Why forests can mitigate floods of all sizes: Evaluating the scientific basis for forest-based flood mitigation
1-Feb-2026