Hawaiian Lava Life
The colonization of newly available habitats is one of the more dramatic processes in Nature. Those of us who have had the opportunity to follow the impressive return of vegetation to the area devastated by the 1980 eruption of Mount St. Helens in Washington State have seen how quickly plants regain their hold on the land, although it will be a very long time indeed before the forests regain the stature they enjoyed before that blast. Perhaps even more striking, and relevant to the series of articles on Hawaiian plants to appear on this site, is the colonization of newly deposited lava. Coming out of the Earth at about 2,000°F Photo 1, the new soil-to-be is sterile-free of any living thing-and hostile in the extreme. But upon cooling, this initially inhospitable material assumes its new role that could be advertised as “Habitat available-ready for immediate occupancy!” Lava is rich in minerals, there is usually plentiful rainfall, and the temperature is ideal for growth. [Remember, we're talking about subtropical Hawai’i here; the situation would have been quite different on Surtsey, a volcanic island south of Iceland that first broke the surface in 1963.]
There are some problems with the Hawaiian scene, however-nothing is ever perfect! One of the problems associated with living on lava is the heat-not volcanic heat-but heat from the sun. Think about walking barefoot on a tarred surface on a summer’s day and you get the idea. Seeds and spores (we have to consider the ferns and other spore-bearing plants here too) do not survive long in direct sunlight on a black surface. But lava has a tendency to shrink as it cools, and the resulting cracks and crevices provide just the protection needed for a seed or spore to germinate. Some plants are better at this than others; they are the ones we call pioneers-not much different from human pioneers who took advantage of whatever their new environments offered.
There's also the matter of wind, which on islands can be quite brisk and steady; the northeast Trade Winds blow almost constantly. Seeds and spores that fall on an open surface would soon get blown away, while those that fall into a crack or crevice can settle in and get down to the business of growing. Photo 2 shows the results of colonization of a small crack on the floor of Kilauea Iki crater (literally, little Kilauea) in Hawai‘i Volcanoes National Park on Hawai`i Island. From right to left one sees the fern Polypodium pellucidum var. vulcanicum, the silversword relative Dubautia scabra subsp. scabra, more ferns, a small patch of the whitish lava lichen, another Dubautia plant looking a bit sad and in the smaller crack, on the left, a young ‘ohi‘a plant (Metrosideros polymorpha). The growth of ferns in the larger crevice Photo 3 probably represents at least 30 year's growth. The ‘ohi‘a Photo 4, growing on the floor of Kilauea Iki, could be that old as well.
Cinder fields also provide an excellent site for colonization where seeds and spores can immediately find places to nestle within the loose substrate. One of the best places to see colonization of a cinder field is in the immediate vicinity of the Devastation Trail in the Park. In late Autumn 1959 an eruption in Kilauea Iki deposited a massive amount of cinder Photo 5 that wiped out the `ohi`a forests on the eastern rim of the crater, along with all of the undergrowth. A boardwalk, visible on the right hand side of the photograph, leads to the Kilauea Iki Overlook and provides an easy access to this area. On visits in the mid to late-1960s one saw very few plants-only dead trees-whereas today the result of some 40 year's growth can be seen, with the hillside dotted with plants. In addition to alien grasses, readily recognizable plants include a species of endemic Hawaiian blueberry Vaccinium reticulatum, (`ohelo), characterized by bright red berries Photo 6; and Dubautia scabra subsp. scabra, the scruffy, white-flowered shrub lying just beyond the dead tree in Photo 5.
The effect of molten lava or very hot cinders on a forest is obvious. What might not be so obvious are the casts left by burned-out trees. These tree-molds, as they are also known, offer a convenient opportunity for a plant to take root in a sheltering hole. Photo 7 shows a species of Nephrolepis growing in a tree hole beside the Devastation Trail boardwalk. Some readers will recognize this genus as the Boston ferns of commerce.
Visitors are often surprised to see lava fields that have a grayish coloration rather than the stark black appearance normally associated with lava. Closer inspection reveals the presence of a very pale greenish-gray lichen Stereocaulon vulcani, here seen with the fern Polypodium pellucidum var. vulcanicum Photo 8. The lichen performs two critical functions in helping to prepare its surroundings for eventual habitation, by both ferns and flowering plants.
The first service that the “lava lichen” provides is to act as a sunlight reflector, which results in a lower surface temperature, one less harmful to spores and seeds. Analyses in the field have demonstrated that the lava lichen has the capacity to fix atmospheric nitrogen, and do so at a level as high as 0.45 kilogram (ca. a half pound) nitrogen per hectare (1 hectare = 2.47 acres) per year. To growers who are knowledgeable about nitrogen content in commercial fertilizers, the amount fixed by the lichen is very small, but it must be appreciated that lava on its own has an extremely low nitrogen content. In this case, every little bit helps. Thus, this lichen not only provides shade for potential colonizers, but an enriched habitat for them as well.
Bruce A. Bohm October 10, 2006