It’s widely known that nonnative trees are a growing presence in Hawaii’s forests. A new study, however, suggests that the invasion has progressed much further than previously believed.
The study by U.S. Forest Service scientists found that while natives still prevail among the largest trees in island forests, invasive species now dominate the understory, the next generation of trees.
The trajectory of the transformation of Hawaii forests, according to the study, suggests a future in which 75% or more of the forests across the island chain will be nonnative species.
“We have a bit of a problem,” said paper co-author Flint Hughes, a Forest Service ecologist based in Hilo. “There are a whole lot of nonnative individuals primed to replace the trees that make up the canopy.”
The study, “How invaded are Hawaiian forests? Nonnative understory tree dominance signals potential canopy replacement,” was published in the journal Landscape Ecology.
The Forest Service scientists who led the study examined the agency’s forest inventory and analysis data from 238 standardized forest plots systematically located throughout Hawaii.
Looking at species’ abundance and size, they found that while native trees dominated the canopy, nonnative species generally dominated the understory, even in high-elevation forests previously thought to be fairly insulated from the invasion of invasives.
“The results were sobering,” the paper says. “While 29% of the large trees across Hawai‘i were not native, that proportion more than doubles to 63% for saplings and 66% for seedlings, indicating the potential for accelerating change in the canopies of Hawai‘i’s native forests.”
The domination will increase as larger native trees die off, permanently changing the composition of native forests and undermining the cultural and ecological resources they provide.
“People think their water comes from the faucet,” Hughes said. “It comes from the forest.”
Native forests evolved over millions of years to act as sponges, capturing rain and cloud mist to help recharge island aquifers. The nonnatives, such as strawberry guava, that are replacing natives in the forest don’t come close to matching the incumbents in their ability to capture precipitation.
Native forests have come under increasing threat in recent years. The threats include the fungal disease known as rapid ohia death, which was discovered about a decade ago and has since caused a die-off of hundreds of thousands of trees.
Ohia, which is found almost everywhere in native ecosystems, has been described as the signature tree of the Hawaiian forest.
While deaths from the disease have largely occurred in the forests of Hawaii island, it has been detected on the other major islands.
Another big threat are ungulates, or hoofed animals such as pigs, whose activity in rooting, forging trails and tusking tree trunks in the forest aids the spread of invasives and leads to a decline in the natives.
Hughes said surveys of the Hawaii forest inventory have revealed a doubling of ungulate activity statewide over the last seven years and a quadrupling of activity on Oahu alone.
Other native forest threats include the spread of nonnative grasses, a changing climate and an increasing number of wildland fires.
“These are problems, and they represent more evidence of the need to better manage our forest for weed species and ungulates,” Hughes said.
Management tools include fences to keep hoofed animals out and allowing more hunting pressure. More bio-control measures are needed as well, Hughes said, to stunt the growth of any number of invasives.
The degree of forest invasion differed with elevation and ownership type, according to the paper. Nonnative trees tended to dominate lower-elevation forests, while higher-elevation forests were less invaded.
Forests on publicly owned lands, in conservation reserves or in fenced areas were less affected by nonnative species.
Understanding these patterns, the paper said, can help land managers prioritize actions for different forest types and provide insights into how invasion trajectories may play out in other forests, as invasions are altering forests all over the world.
Overall, the study’s findings suggest the native forests of Hawaii face a bleak future if efforts are not greatly expanded to manage the spread of invasive species.
“Our comprehensive analyses across forest layers signal a more dire future for native plants in Hawaiian forests than has been previously described,” the paper says.
