Remarks on the Succession of on Hawaiian Lava Flows

H ARVEY ALFRED MILLER l

THE HAWAllAN ISLANDS are well known for growing list of pioneer bryophytes. The Decem­ their volcanic origin and for continuing vol­ ber, 1958, collections contained Camp ylopus canic activity on the largest island, Hawaii. On exasperatus and hygrometrica, along May 13, 1955, Hawaii experienced a volcanic with species previously found on the flows. eruption and lava flow of considerable magni­ In general, exposed lava flows support few tude. Professor Maxwell Dory undertook a series species of bryophytes and these are restricted to of surveys to outline the succession on protected crevices or small shaded niches. The this new flow.2 several species of Campylopus establish them­ The first was observed and col­ selves rather readily in lava under climatic condi­ lected at about a 950-ft. elevation on December tions of the type favorable for the growth of 20, 1955, when a tiny juvenile colony of Metrosideros. It is of interest that Campylopus Campylopus introflexus (perhaps C. boswelli, fumarioli becomes established in the steam fis­ though this species was not collected later ) was sures at Kilauea. In Hawaii, species of Campylo­ discovered in the folds of lava blisters. A sub­ pus appear to play a significant pi oneer role in stantial portion of the collection was sent to me rock succession and are often found on rocks in half of a foil chewing-gum wrapper-which devoid of other . Rhacopilum cuspid­ indicates the extreme youth of the colony. The igerum is the most ubiquitous in Hawaii indi vidual plants were 3- 5 mm. high, or about and with its obviously great range of tolerance the length of 1Y2 leaves. On February 22, 1956, would be expected to be a pioneer species on Campylopus densifolius, in about the same stage new lava as well as on older rocks. It produces an of development as the first moss collected, was abundance of spores throughout the year, which taken at about 900 fr., in the Kamaili Hom e­ is another factor favoring rather rapid establish­ steads at Puna. In late March more colonies of ment in unoccupied habitats in frost -free re­ C. densifolius were discovered on the Iilewa gions . flow, and C. exasperatus was discovered grow ing Rhacomitrium lanuginosum var. pruinosum on old lava adjacent to the new flow. Specimens has a much greater altitudinal range than Rhac­ of apparently living Macromitrium owahiense opilum and is found in more exposed situations. were collected from the bark of a Metrosideros The upper half of each leaf is hyaline and the (Ohia lehua ) tree which had been killed by striking silver-grey tufts of Rbacomitrisem. can the encroachment of hot lava. In August, Cam­ be easily seen and recognized. Above 8,000 ft. pylopus introflexus was found on the flow about on the barren lava flows of Mauna l oa this Y2 mi. east of McKenzie Park. In subsequent species of Rhacomitrium and Grimmia -tricbo­ collections this species seemed to be established phylla are the only plants to be found . I have in several places on the new flow. Two species collected both of these on the unexposed previously unrecorded for the Iilewa flow were sides of lava clinkers at 11,000 ft. The Rha­ discovered at about 950 ft. in April, 1957, and comitrium has been collected at the summit of Rhacopilum cuspidigerum and Rhacomitrium Mauna loa by several people. In spite of its lanttginosum var. pruinosum were added to the great tolerance of seemingly arid conditions, Rhacomitrium plays an important role in bog 1 Department of Botany, Miami Un iversity, Oxford, formation in the Alakai Swamp on Kauai. It is Ohio. Publ ication of this pap er has been aided by NSF Grant G7115 . Manu script received March 4, suggested that the acid nature of the bog may 1959. represent a physiologically, though not phys­ 2 This study was aided by NSF Grant G 1922. ically, arid habitat with the cloud moisture

246 Bryophytes on Lava Flows-MILLER 247 characteristic of mountain tops. Comparative become established in suitable microhabitats physiological studies of several populations and never form an extremely conspicuous part would be most interesting in this regard . of the vegetation. In fact, it would seem to one The species of Campylopus, Rbacopilum, who has observed Hawaiian lava flows that Rhacomitrium, and Grimmia previously men­ plants are established wherever and whenever tioned are true pioneers which establish them­ suitable habitats exist for the disseminules which selves on bare lava rock. However, other mosses reach these habitats. Thus, no exact plant suc­ appear on relatively new lava flows in pockets cession of one plant replacing another can be where dust collects to form a minute patch of established. With the establishment of vascular soil. The plants which colonize this type of plants, new microhabitats become available and microhabitat are mosses of world-wide distribu­ such moss species as Macromitrium brevisetum, tion which might aptly be called "weeds." M. emersulum, Fissidens spp., Bryum spp., Pohlia Funaria bygrometrica is the only such species spp., Ectropothecium sandwichense , and Ves­ found in Dory's collections, although Bryum icularia graminicolor become established on rock in mesophytic regions . Often the leafy liverwort argenteum and Ceratodon purpu1'eus have been Brachiolejeunea sandvicensis becomes estab­ found on essentially barren lava flows in sim­ lished at the same time as the mosses listed ilar circumstances. above. Another liverwort, Riccio recbingeri, was The classical concept of lichen-mass-herb­ recently discovered in soil pockets in the Kau shrub-tree succession does not apply in Hawaii desert and may be discovered in similar hab­ on new lava flows. Mosses are true rock pioneers itats on newer flows. As soil is necessary for and lichens are rather limited in their establish­ the growth of this plant, it probably should not ment. In general, a few species of bryophytes be considered a true pioneer species.