A patchwork forest landscape highlights how forest loss and fragmentation can change the way watersheds store and release water, according to new UBC Okanagan research.
Forest loss does more than reduce tree cover. A new global study involving UBC Okanagan researchers shows it can fundamentally change how watersheds hold and release water.
The research, published in Proceedings of the National Academy of Sciences , analyzed data from 657 watersheds across six continents.
It found that both forest loss and changes in forest landscape pattern cause watersheds to release a higher proportion of "young water"-rain and snowmelt that moves through a watershed within roughly two to three months of falling.
"Young water is a signal that water is moving quickly through a system," says Ming Qiu, lead author of the study and a doctoral student in UBC Okanagan's Earth and Environmental Sciences program.
"When the young-water fraction is high, it means less water is being stored in soils and groundwater for use during drier periods."
The study was co-authored by Qiu and Dr. Adam Wei, professor in UBCO's Irving K. Barber Faculty of Science . Together, they examined how forest cover and landscape configuration interact to influence watershed hydrology at a global scale.
The findings have direct implications for forest and watershed management, particularly in regions where timber harvesting is economically important. Rather than framing decisions as a binary choice between conservation and development, the research suggests there is room for more nuanced planning.
"Forest loss clearly reduces a watershed's ability to retain water," Dr. Wei says. "But our results also show that how forests are arranged on the landscape can either worsen or help mitigate that impact. Landscape planning can be part of the solution."
Previous research has largely focused on forest quantity, or how much forest is removed. This study adds a crucial new dimension: spatial arrangement. In watersheds with relatively low forest cover, typically below 40 to 50 per cent, how the remaining forest patches are arranged strongly influences how water moves.
In these sparsely forested landscapes, an increase in forest edges-where forest meets cleared or open land-was linked to lower young-water fractions. Forest edges experience more solar radiation, lower humidity and altered microclimates, which can increase evapotranspiration and reduce runoff.
By contrast, in watersheds with higher forest cover, forest pattern had little effect. When forests are dense and contiguous, edges are closer together and microclimate changes are muted, limiting their influence on water partitioning.
"This was one of the most surprising findings," Qiu says. "Forest pattern matters most when forest cover is already low. Above a certain threshold, its influence largely disappears."
As extreme weather increases pressure on water resources, understanding how land-use decisions affect long-term water availability is becoming increasingly urgent.
"Watersheds are nature's water-storage systems," Qiu says. "If we want water that lasts through dry seasons-for communities, ecosystems and industry-we need to think beyond how much forest we lose and start thinking carefully about how the remaining forest is laid out."
