What impact will global warming have on Hawai’i’s land-based plants and animals? Among those scientists who have looked into the subject, the consensus seems to be that global warming may be a final, irreversible insult to many of the Islands’ native animals and plants that already are under extreme pressures from human development and introduction of alien, competing species.
In recent years, several scientific papers have been published describing possible impacts of global warming on Hawaiian ecosystems. In one article,1 published in 1992, Dieter Mueller-Dombois of the Department of Botany of the University of Hawai’i, described the possible effects that increasing levels of carbon dioxide would have on vegetation. Mueller-Dombois has spent much of his life studying the phenomenon known as ‘ohi’a dieback, in which large stands of ‘ohi’a trees (Merrosideros polymorpha) gradually lose vigor and die without apparent disease or other cause.
Mueller-Dombois’ study of the diebacks of ohi’a and other trees elsewhere (including Australia and New Zealand) led him to conclude that “there is a strong anthropogenic component that overrides the natural stresses” on the “biologically simplified forests” that experience this phenomenon. Climate change, Mueller-Dombois writes, may now be considered as driving forest dynamics in three ways: by CO2 fertilization, global warming, and increases in weather extremes.” Several studies support the notion that an increase in atmospheric carbon dioxide may have a fertilizing effect on some plants, while delaying maturity in others. Among the latter are tropical grasses (including sugar, sorghum, make and many savanna grasses), in which CO2 is already present at saturation levels. For such plants, increases in atmospheric CO2 may serve to delay the aging process, actually giving them a survival edge in periods of global warming.
For other species (including all trees known to have experienced dieback), increased levels of CO2, in the atmosphere will result in the plants absorbing more CO2 generating leafier canopies and increasing rates of plant metabolism, Mueller-Dombois writes. But this is not an unmitigated good. “An accelerated metabolism translates into faster growth rates and probably also shorter lifespans,” Mueller Dombois adds. “When trees have reached peak maturity, senescence is likely to be hastened by increased atmospheric CO2. An accelerated rate of maturation and senescence through atmospheric CO2 fertilization can be considered an additional stress factor in forest decline at the global level.”
Montane Cloud Forests
In 1998, Lloyd Loope, with the U.S. Geological Service’s Biological Resources Division unit at Haleakala National Park, and Tom Giambelluca of the University of Hawai’i’s Department of Geography, described the vulnerability of tropical montane cloud forests to climate change.2
“Unlike most ecosystems at lower elevations in Hawai’i, cloud forests have largely escaped impacts of humans and their introduced animals and plants until recently,” write Loope and Gianibelluca. “Cloud forests of windward Haleakala volcano, East Maui, are among the most intact remaining ecosystems in the Hawaiian Islands. The prime sites lie within Haleakala National Park, Hanawi (State) Natural Area Reserve, and Waikamoi Preserve (managed by The Nature Conservancy)…. Unfortunately, the spectre of global climate change and its potential effects on East Nui cloud forests presents a severe challenge to positive future conservation scenarios.
Trade-wind patterns, with corresponding rainfall on the eastern slopes of Haleakala, account for the types of vegetation found in the various “microclimates” that Loope and Giambelluca describe. They note that in 1994, more than 14 meters of rain fell at a weather station 1,650 meters above sea level, “ranking among the world’s greatest measured 1-year rainfall totals.” In addition, they write, Haleakala’s east slope may boast the steepest rainfall gradient in relation to horizontal distance ever recorded: a difference of 1.66 meters per kilometer measured between the elevations of 1,610 meters and 2,260 meters. (At 1,610 meters elevation, annual rainfall of almost 11 meters was recorded, while 4 kilometers away, at 2,260 meters elevation, the comparable figure was about 4.5 meters.)
Should trade wind patterns or elevation of the trade wind inversion be altered, rainfall distribution on the eastern slope of Haleakala would be disturbed. “The cloud forest zone, usually under cloud cover, is located below the trade wind inversion and above the lifting condensation level,” Loope and Giambelluca write. “Whether this zone will shrink, widen, disappear altogether, or be entirely unaffected remains to he determined.”
“Increased interannual variability of rainfall with increased incidence and/or intensity of drought could have profound effects upon cloud forest,” they continue. In the 1992 El Nino, precipitation at sites near the trade wind inversion on Mauna Loa, Hawai’i island, was 70 to 95 percent below normal, they note. “Substantial mortality among shrub species was absorbed. Severe drought is a likely mechanism causing or reinforcing a shift from dominance by woody species to dominance by herbaceous species.”
In general, Loope and Giambelluca predict that global warming will have the following effects on the montane cloud forests of Haleakala: the “species temperature tolerance zones” for native and introduced species would likely be shifted upward in elevation, about 360 to 450 meters. (In other words, with temperature decreasing with elevation, plants and animals that find warmer temperatures inhospitable will have to relocate upslope.)
Also, there will be some change in rainfall. “Complex changes in precipitation patterns maybe expected, with increases in the mean values for some sites and decreases for others; because of steep gradients in rainfall and other climatic parameters, slight shifts in ocean/atmospheric parameters could result in substantial change in precipitation for any location.”
The predicted increase that global warming would cause in the variability of year-to year rainfall rates “would almost certainly be deleterious” to island tropical montane cloud forests. And, “if there is a higher frequency and intensity of El Nino/Southern Oscillation conditions with global warming, increased drought frequency might be expected,” subjecting montane forests to water stress and fire.
With global warming predicted to bring more frequent and more intense hurricanes, the result for tropical montane cloud forests would be “drastic changes in forest canopy structure,” Giambelluca and Loope write.
Finally, they conclude, “the above-mentioned changes will generally favor the invasion of non-native species to higher elevations and favor their penetration of cloud forest in competition with native/endemic island species. The greater the magnitude of the changes, the more the invasive species will be favored… Changes in frequency and intensity of extreme events windstorms, drought, and fire may have much more drastic effects on invasion of non-native species into island montane cloud forests than changes in average climate. A particularly damaging sequence might involve a hurricane, followed by drought and fire. Anything promoting canopy opening will favor plant invasions in the forest understory. A more insidious mechanism may involve environmental stress to native species by extreme events, sometimes exacerbated by disease or predator attack, which may lead to displacement of native species by more broadly adapted (opportunistic) non-native species.
Insect Predation’?
Recently, an article published in Science magazine suggests another possible threat to native plants that could develop in a warmer climate. The article3 bolts at fossil records from the Paleocene and Eocene periods to see whether insect predation on plants changed as the climate grew warmer. According to the authors of the article, the fossils examined, found in an area of western Wyoming, bore out predictions of increasing insect damage with increased temperatures.
Efforts to mitigate global warming by increasing the volume of carbon held in the Earth’s soils and plants may also have adverse impacts if not undertaken with care. According to an article in Science published in 1996,4 fertilization with nitrogen can cause a shift from native species to invasive plants. “EA’s weedier species took over the nitrogen-fed plots, they absorbed less carbon for a given nitrogen input. And when scientists looked below ground, they found that the weedier plots had stored less carbon in soils.”
In addition to failing to hold carbon, when the plants died, soluble nitrogen, in the form of nitrates, began to leak into the soil. Should the nitrates end up in aquifers or surface runoff into streams and near-shore waters, the result could spur the growth of algal blooms or impair water quality.
1. “Potential Effects of the Increase in Carbon Dioxide and Climate Change on the Dynamics of Vegetation,” Water, Air, and Soil Pollution 64: 61-79 (1992).
2. “Vulnerability of Island Tropical Montane Cloud Forests to Climate Change, with Special Reference to East Maui, Hawai`i,” Climate Change 39: 503-517 (1998).
3. Peter Wilf and Conrad C. Labandeira, “Response of Plant-Insect Associations to Paleocene-Eocene Warming,” Science, 284, June 25, 1999.
4. David A. Wedin and David Tilman, “Influence of Nitrogen Loading and Species Composition on the Carbon Balance of Grasslands,” Science, 274, Dec. 6, 1996.
— Patricia Tummons
Volume 10, Number 2 August 1999