DLNR explores wildfire impacts on forest watersheds and communities

While flames are no longer visible at the site of the Mililani Mauka fire on Oʻahu, the 1,600-acre burn area left behind is a reminder of the delicate landscape and its connection to the precious watershed below.

Native trees including koa, ‘ōhi‘a, and the native uluhe fern were burned and are interspersed with invasive ones, like albizia. 

During a survey flight this week, DLNR State Protection Forester Mike Walker with the Division of Forestry and Wildlife described the devastating impacts wildfires have on native forests, which serve as both suppliers and protectors of Hawai‘i watersheds. 

“A fire can cause the plant composition to change, allowing invasive weeds and plants to literally spread like wildfire. Fire also impacts the water intake into the ground. Depending on the burn severity, you can create hydrophobic soils. That means after rains the soil will shed the water rather than pull it in. So, we’re really concerned about the loss of the watershed’s ability to collect water for our aquifers,” Walker said. 

Underground aquifers store and provide Hawai‘i’s fresh water supplies. With drought conditions forecast into the future it is even more important to protect forests so they can continue to collect water, according to DLNR.

“Forests are the main way we can protect our water supplies, because native forests in particular are amazing at absorbing rain and cloud water, even when it’s not raining,”  said DOFAW’s Emma Yuen. “As clouds pass by, their moisture is captured by all the leaves, mosses, and ferns and it slowly drips into our groundwater supplies. By protecting native forests we’re dramatically increasing the ability to protect our island’s fresh water. It’s a huge loss when we lose these types of forests to wildfire, because that ability to absorb water is dramatically reduced as well.” 

“When a wildfire burns through a native forest, you damage the ability of the land to capture and absorb water,” Yuen said. “When you lose vegetation, a native-forest-to- bare-ground scenario, you see 15 times slower absorption of rainwater into the ground. This means when it rains there’ll be more flooding because the burned soil becomes water repellent, and the rain runs off instead of soaking in.” 

Some of the other negative impacts of wildfire on native forests and watersheds include: 

• Plants and animals unable to escape the fire, and the loss of large tracts of habitat for unique species, only found in Hawai‘i. Native plants and animals hold an important place in native Hawaiian culture. 
• Escalating costs and conflicts over water. In the Pearl Harbor aquifer, where the Mililani Mauka fire occurred, groundwater levels have dropped by half in the last half-century. This means it costs more to pump water. The loss of forests and the resulting loss of water supplies increases the cost of living in Hawai‘i. 

DOFAW has wildland firefighting responsibilities on one-million acres of state land under its jurisdiction. DLNR is seeking additional funding from the legislature next year for updated firefighting equipment, improved and/or new firebreaks and shaded fuel breaks, creation of water sources for fire suppression, replanting of native trees and plants, and seed storage. 

“Every fire really matters. Native forests are so critical for our culture, for our water supplies, and for preventing flooding and erosion,” Yuen said.