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

Plant Cover Development

The cover of the newly invaded plant species in habitats 1 and 2 was still under 1% in year 9 following the eruption. In contrast, the plant cover of some new invaders and of the surviving species became significant in habitats 3, 4, 5, and 6 during the period of observation. the development of the new plant cover in these habitats is shown by life-form spectra chronologies and photographs.

Life-form spectra are lists or diagrams that show the life-form composition of an area (Ellenberg 1956). In this case, the diagrams show the life-form composition of each habitat along the abscissa, while the height of each bar shows the percent cover for each life form from year 1 to 9 (Fig. 8). The percent cover was obtained from the Braun-Blanquet cover-abundance estimates given for each species in each plot. The mean cover value was first calculated for each species. Thereafter, these values were added for the species belonging to each life-form group.

Classification of each species into more detailed life-form groups appeared useful for several reasons. It was a means of identifying the developing synusiae, which are defined as groups of species of similar life form growing together in the same habitat (Ellenberg 1956). These species, because of similar morphological characteristics, are also probably the closest competitors for the same general niche (Mueller-Dombois and Sims 1966). Moreover, a detailed life-form classification focuses attention on the adaptive mechanisms of plants that are prevalent on different, new volcanic habitats. The life-form classification used for this purpose is based on the well-known life-form classification of Raunkiaer (1918).

Each diagram (Figs. 8, 10, 11, and 16) shows on the abscissa the five basic Raunkiaer life forms for vascular plants (phanerophytes, chamaephytes, hemicryptophytes, geophytes, and therophytes) plus two life-form groups for thallophytes. The latter include algae, mosses, and lichens. Subdivisions of each of these seven major life forms are based on the more detailed plant life-form key of Ellenberg and Mueller-Dombois (1967).

Macrophanerophytes (symbol PM) were here defined as trees over 2 m tall; Nanophanerophytes (symbol PN), as woody plants from 25 to 200 cm tall. Chamaephytes (symbol Ch) were defined as low shrubs or woody plant seedlings that were up to 25 cm tall at the time of observation. Chamae-hemicryptophytes [symbol Ch (H)] were defined as herbaceous perennials whose shoot system remained alive above the surface also when part of it died or dried up. Geophytes (symbol G) were defined as herbaceous perennials that could reproduce vegetatively from bulbs or rhizomes after shoot die-back. Therophytes (symbol T) were defined as herbaceous plants without the capacity of vegetative shoot reproduction. Thallo-chamaephytes included the cushion-forming bryophytes (symbol BrCh) and fruticose lichens (symbol LCh). The term "thallo-hemicryptophytes" was applied to flat-appressed mosses (symbol BrH) and the macroscopic algae (symbol Phyc H).

The subdivisions within these seven major life-form groups refer to more detailed morphological characteristics of the plants that suggest their structural adaptation to cope with the new volcanic environments. These symbols are defined in Appendix VII.