Study of the Ecology of Pioneer Lichens, Mosses, and Algae on Recent Hawaiian Lava Flows

Abstract

The ecology of pioneer lichens, mosses, and blue-green algae on some recent Hawaiian lava flows was investigated quantitatively. Up to an elevation of at least 3,000 feet, the major variables of the physical environment are rainfall, rock texture, and sea breezes. The lichen Stereocaslon vulcani, the most abundant and widespread pioneer organism, shows a marked preference for regions of higher rainfall, but all species of Parmelia and Cladonia, together with an unidentified crustose lichen, were found only in areas of lower rainfall. The mosses and blue-green algae prefer relatively humid regions, but Campylopus densifolius is able to grow in some areas that are too dry to permit growth of Rhacomitrium lanuginosum, Rough aa lava provides a more favorable substrate for Stereocaulon vulcani than does the smoother pahoehoe, but this effect becomes less pronounced with increasing rainfall. Thus, aa creates a more moist environment than does pahoehoe, probably because its highly irregular, pitted surface is better able to trap and retain rainwater. A possible contributing factor is the greater susceptibility of aa to chemical weathering. On some lavas, lichens and mosses preferentially colonize seaward-facing rock surfaces. This is ascribed to water vapor conveyed inland by sea breezes. Nutrients in wind-borne ocean salts may play a secondary role. The net effect of rainfall , rock texture, and, in some cases, sea breezes determines the abundance and gross vegetative morphology of Stereocaulon vulcani, its ability to gain a foothold , and the level of maturity which it can attain. The successfulness of S. vulcani in colonizing lava can be ascribed to its ability to invade vesicles and narrow recesses in the rock, its ability (or that of its associated microflora, or both) to accelerate the chemical weathering of the rock, and its rapid rates of dispersal, establishment, and growth. Under optimal conditions, S. vulcani spreads rapidly over a fresh rock surface, and dominates the pioneer community, probably by preempting space which might otherwise be occupied by slower-growing species. In one particularly damp area, mosses and blue-green algae increase at the expense of S. vulcani, In one exceptionally dry area, Stereocaulon is initially the most abundant lichen on the aa flows, but it never attains maturity, and its numerical importance is gradually superseded by that of Parmelia and Cladonia, which are better adapted to dry conditions. No evidence of "mat" formation was found. Vascular plants spring up in crevices, while lichens, mosses, and algae occupy the intervening rock surfaces.