Influence of substrate on the distribution of the Hawaiian Silversword (Argyroxiphium sandwicense DC.) in Haleakala (Maui, HI)

The spatial patterns of the Hawaiian silversword (Argyroxiphium sandwicense DC.) were studied in Haleakala (Maui, HI). The silversword is a “giant” rosette plant nearly brought to extinction by human impact and goat browsing during the 1920s, but stern protection has resulted in the resurgence of plant populations. Silversword regeneration is occurring vigorously in soils with surficial layers of volcaniclastic fragments. Ten sites with sizable silversword populations and the associated substrates were examined in Haleakala's crater between 2175 and 2755 m. At each site, the population structure of 100 plants was determined along wandering-quarter transects, which limit sampling bias. Substrates where rosettes were rooted and the size of the biggest stone fragments upslope and downslope from the plant's base were determined. Volcaniclastic substrates were examined with photographs along 12-m-long transverse transects. Fine-debris samples (gravel and pebbles) were analyzed by mechanical sifting of surficial fragments gathered from 15×15 cm miniplots. Volcaniclastic fragments in silversword areas have different sources. (i) Most were ejected during eruptions yielding pyroclastic materials (ash to blocks) that built cinder cones on Haleakala's crater. (ii) On steep slopes, weathering of (olivine alkali) basalt outcrops and subsequent mass wasting of rock fragments contributed a mantle of clasts to slopes below. (iii) On some cinder cones, welded spatter was produced during late eruptive stages; this coalesced into ruff-like agglutinate layers around crater rims, called Pohaku-o-Hanalei (“wreath of stones”). Agglutinate deposits weather gradually, supplying clasts that roll downhill and accumulate on cone flanks. (iv) On weathered aa lava flows, stone fragments and rocky crags abound along eroding lava-flow ridges, associated with vegetation; but the intervening troughs, covered with fine ash and cinder, remain bare. Silversword population structures indicate healthy regeneration. All populations display a typical pattern of decreasing silversword numbers with increasing size (=age). Nearly 65% of the rosettes are rooted near cobbles (5–10 cm long) or pebbles (2.5–5 cm), and ∼23% grow at the base of blocks (>10 cm) or outcrops; 9% of the plants are on gravel (<2.5 cm), but only 1% have germinated on sandy soil without granules or stones; 2% of the silverswords grow on organic litter of dead rosettes. Plants on six sites showed noticeable differences in clast size between their upslope and downslope sides; particles are significantly larger upslope from rosettes. This suggests that successful seedlings germinate below clasts, which deflect finer downslope-moving sediment, thus protecting young plants from burial by descending debris. Statistical tests indicate a few blocks above plants may have been stopped by adult rosettes. Such a relationship underscores the dynamic nature of steep cinder and rubble (talus) slopes, regularly affected by dry debris slides, frost creep, and other mass-wasting processes. Slope instability is also evinced by (i) common bending and deformation of silversword roots—particularly in seedlings—which trail upslope; (ii) pronounced downhill tilting and asymmetry of tall rosettes on steep gradients; (iii) presence of accumulated fine-debris steps (9.9–13.8 cm thick) upslope from plants and blocks; (iv) ubiquitous fine-earth flags exposed below embedded blocks, which intercept descending debris and deflect it laterally; (v) widespread miniature sorted stripes.