Many people believe that getting rid of cows and goats is key to restoring Hawaii’s native ecosystems. But this is not nearly enough for the islands’ dry forests: canopy trees failed to regenerate even in an ungrazed preserve, according to new findings published in the April issue of Conservation Biology.
“Tropical dry forests are in general far more threatened than rainforests,” says Robert Cabin of the U.S. Forest Service in Hilo, Hawaii, who co-authored this paper. Hawaii has lost about 40% of its rainforests but more than 90% of its dry forests, which get only about 20 inches of rain per year.
One of Hawaii’s largest remaining areas of dry forest is in the north Kona region. However, the forest is broken into small fragments and most of these have been grazed for more than 150 years by cattle and feral goats.
To see how grazing affects the dry forests, Cabin and his colleagues compared regeneration of canopy trees in the Kaupulehu Dry Forest Preserve, which has not been grazed for 40 years, to an adjacent area that has been grazed continuously. The researchers found the preserve had almost no native canopy tree seedlings, showing that getting rid of grazing is not enough to conserve Hawaii’s dry forest fragments.
The researchers identified two other factors that might suppress canopy tree regeneration in the dry forest fragments: they are overrun with non-native rodents and covered by non-native fountain grass, a waist-high bunch grass that has invaded thousands of acres in west Hawaii. Cabin and his colleagues observed that the rodents ate most of the seeds while the fountain grass smothered any seedlings.
To control the rodents, the researchers used bait traps laced with rodenticide. To control the fountain grass, they weed-whacked the entire six-acre preserve in 1995 and then sprayed five times with a grass-specific herbicide. In 1996 the fountain grass was reduced by 90%. Since then, the researchers have sprayed and pulled out individual clumps of fountain grass, and today it covers only 4% of the preserve.
In 1997 Cabin and his colleagues found that canopy tree regeneration had increased tremendously. Before the fountain grass and rodents were controlled, virtually none of the preserve’s 16X16-foot plots had seedlings; afterwards nearly 40% of the plots did.
However, there was a downside to these restoration efforts. The researchers found 16 non-native plants new to the preserve, suggesting that controlling fountain grass and rodents facilitated invasion of non-native species.
These mixed results show that restoring Hawaii’s dry forests will require active management both to control non-native species and to reintroduce key native species, says Cabin. To date, the North Kona Dry Forest Working Group and local volunteers have planted more than 2000 native dry forest plants in the preserve.
Cabin’s co-authors are Stephen Weller and Ann Saki of the Department of Ecology and Evolutionary Biology at the University of California at Irvine; David Lorence, Tim Flynn and Lisa Hadway of the National Tropical Botanical Garden in Lawaii, Hawaii; and Darren Sandquist of the Department of Biological Sciences at Stanford University in Stanford, California.
For more Information
Cabin, R.J., S.G. Weller, D.H. Lorence, T.W. Flynn, A.K. Sakai, D. Sandquist, and L.J. Hadway. 2000. Effects of long-term ungulate exclusion and recent alien species control on the preservation and restoration of a Hawaiian tropical dry forest. Conservation Biology 14(2):439-453.