Eucalyptus species fail to support native understory

Much of native Hawaiian vegetation is understory vegetation, growing under canopies of 'ōhi'a and koa—but in restoring natives, will just any canopy species do?

Perhaps not, according to a study performed by Rebecca Ostertag at the University of Hawai'i at Hilo, and Christian Giardina and Susan Cordell, if of the USDA's Institute of Pacific Islands Forestry in Hilo. Their work was published in the journal Restoration Ecology under the title, “Understory colonization of eucalyptus plantations in Hawaii in relation to light and nutrient levels.”

They studied what was growing under a seven-year-old forest of Eucalptus saligna trees on former agricultural land. The forest had sections that were fertilized and others that were unfertilized, and herbicides had not been used for at least five years.

Saligna is also known as Sydney bluegum, and is a fast-growing, straight-grained tree popular for forestry applications.

The understory vegetation turned out to be heaviest in areas where more light was able to get to the surface. But generally, it contained mostly alien plants—not natives.

The most common tree under the fast-growing eucalypts was a weedy tree, Citharexylum caudatum, also known as fiddlewood or juniper berry. It is a tree often planted along streets, but which becomes a dangerous weed in the Hawaiian wild.

The researchers further studied a series of other plantations containing several different Eucalyptus species. Including E. saligna, E. grandis (rose gum), E. cloeziana (Gympie Messmate or cloeziana gum), and E. microcorys (Australian tallowwood).

“Again, very few native species were present in any of the stands, indicating that within certain landscapes and for native species with certain life history traits, exotic plantations may be ineffective nursery ecosystems for the regeneration of native species,” they wrote.

Some eucalyptus species are also known to contain growth inhibitors, which can limit the ability of other plants to colonize the ground under eucalypt plantings.

©2008 Jan TenBruggencate