HAWAI`I VOLCANOES Invasion and Recovery of Vegetation after a Volcanic Eruption in Hawaii NPS Scientific Monograph No. 5

Plant Cover Development (continued)

Habitat 5

Figure 11 shows that the greatest cover was produced by the phanerogamic life forms. The sPM column indicates the recovery of the surviving sclerophyllous Metrosideros trees. The trees over 2 m in height covered about 50% of the habitat surface in year 7. Their cover did not increase to year 9. The sPN group contained several surviving native shrubs, such as Dubautia scabra, D. ciliolata, Vaccinium reticulatum, Wikstroemia sandwicensis, and Styphelia tameiameiae. Together with the small Metrosideros trees (between 25 and 200 cm height) they covered about 13% of the habitat in year 9. The dPN cross-hatched column shows the cover of the invading Rubus rosaefolius and R. penetrans shrubs, which increased from about 20% in the year 4 to 30% in year 7. Thereafter, their cover did not increase much. Several native survivors in the dPN category increased in combined cover up to 3% (W. sandwicensis, Dodonaea viscosa, and V. calycinum). The dPN scand form refers to tree-climbing individuals of R. penetrans.

Fig. 11. Life-form spectra chronology—habit 5. (Symbols explained in Appendix VII).

The herbaceous life forms were much less important in habitat 5. the geophytes (G rhiz) covered a little under 10% of the surface. This group included the surviving Astelia menziesiana, Tritonia crocosmiflora¹, Hedychium coronarium¹, and the fern Nephrolepis hirsutula¹. It also included the native ferns N. exaltata, Polypodium pellucidum, and Pteridium decompositum. The lichens Stereocaulon volcani and Cladonia skottsbergii (LCh) together attained a little over 5% cover in year 9. The chamaephytes were fourth in rank of cover importance, with two groups attaining almost 5% cover each in year 9. These were the sclerophyllous, low shrubs (sCh frut) which included Coprosma ernodeoides and most of the low-growing (up to 25 cm high) specimens of the species mentioned as the sclerophyllous nanophanerophyte ( = sPN) group in habitat 5. The soft-leaved, woody chamaephytes (dCh frut) included Osteomeles anthyllidifolia and low growing individuals of Rubus rosaefolius¹, R. Penetrans¹, Wikstroemia sandwicensis, and Dodonaea viscosa.

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¹Exotics.

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The recovery of the surviving Metrosideros trees in a 1.5 m deep ash deposit in habitat 5 is documented by a sequence of four photographs (Figs. 12.1, 12.2, 12.3, and 12.4). Figure 12.1 shows the trees in year 1 (1960). Nearly all foliage was slashed off, but bark remnants and short branch-stubs were left along the trunks. Figure 12.2 shows the same stand-segment in year 2 (1961). A profusion of new leaves had developed all around the trunks from top to base from adventitious buds formed from the surviving tissue (i.e., bark on branchlets and main stems). Figure 12.3 was photographed in year 4 (1963). The cylindrical crowns still covered less than 30%. Several smaller trees did not recover. Figure 12.4 shows the same stand segment in year 7 (1966). The crowns had expanded radially to about 50% cover, but hardly any significant new plant invasion had occurred on the surface between the trees.

Fig. 12.1. Segment of habitat 5 in area of 1.5-m-deep pumice deposit photographed in year 1 (1960) after the ash fallout.

Fig. 12.2. The same habitat segment photographed in year 2 (1961).

Fig. 12.3. The same habitat segment (as shown in Figs. 12.1 and 12.2) photographed in year 4 (1963).

Fig. 12.4. The same habitat segment photographed in year 7 (1966).

Figure 13.1 shows a cross-section of a Metrosideros tree, which stood near the area photographed in Fig. 12. The cross-section was made in year 7 (1966). It exhibits a spontaneous radial increase in diameter from the time of the volcanic explosion. The latter is indicated by the blast injury. Figure 13.2 is a photograph of a similarly sized, uninjured tree that grew in the adjacent forest. The comparison demonstrates that the sudden increase in radial increment of the injured tree was correlated with the volcanic disturbance. The reason for the sudden spurt in diameter growth is probably related to the sudden increase in leaf-area from top to base and the full exposure of the new crowns to light.

Fig. 13.1. Cross-section of Metrosideros stem from surviving stand photographed in Fig. 12, habitat 5.

Fig. 13.2. Cross-section of Metrosideros stem uninjured in forest adjacent to habitat 5.

Figure 14.1 and 14.2 illustrate a section in habitat 5, where the pumice blanket was only 20-30 cm deep. Here, even a few leaves remained on the trees after the fallout in year 1 (Fig. 14.1). Also the crowns retained a greater number of fine branches. The result was a more uneven, but also thick, refoliation. Figure 14.2 shows the situation in year 3 (1962). Here the trees did not assume the right cylindrical crown shape shown in Fig. 12. Also many defoliated shrubs were seen under the trees in year 1 in Fig. 14.1. These recovered among the trees (sPN group in Fig. 11). But also many exotic Rubus rosaefolius and R. penetrans shrubs invaded in this general area (dPN group in Fig. 11). Also some grasses recovered and invaded the area as seen in the foreground in Fig. 14.2. The barren area is a trail that was buried under ash.

Fig. 14.1. Segment of habitat 5 in area of shallow (20-30 cm deep) pumice deposit photographed in year 1 (1960).

Fig. 14.2. The same habitat segment photographed in year 3 (1962).

Figure 15 shows a section in habitat 5, where the Metrosideros trees developed abundant aerial roots that grew like thick, reddish "brooms." Such aerial roots appeared on many of the more vigorous trees standing in 50 cm and deeper pumice deposits. Their function is probably in balancing or replacing the respiration of the buried root system. But this has not yet been studied. Several low-growing roots made contact and grew into the soil, but ground-contact was not very common.

Fig. 15. Aerial roots on Metrosideros trees that survived ash burial of 50-100 cm depth. Photograph taken in year 7 (1966).