Steady as a rock: Biogeomorphic influence of nurse rocks and slope processes on kupaoa (Dubautia menziesii) shrubs in Haleakala Crater (Maui, Hawai'i)

摘要

This study examines biogeomorphic interactions between nurse rocks, slope processes, and 300 kupaoa (Dubautia menziesii) shrubs in Haleakala Crater (Maui, Hawai'i). Research objectives were to: assess the association of lalpaoa with substrates upslope and downslope of plants, and proximity to the closest rock uphill; contrast shrub/substrate relationships with site frequency of sediment types; measure surface soil shear-strength and compressibility on 50 paired locations near boulders; and investigate the aggregation characteristics and spatial patterns of kupaoa in relation to rock and substrate variation. Data analyzed came from three 100-plant surveys at 3 sites: a plant census at 2720-2975 m altitude, and wandering-quarter transects (WQTs) across two areas (2610-2710 m); ground sediment cover was estimated along four phototransects on these sites. Data for the three 100-plant surveys included substrate type outcrops, blocks, cobbles, pebbles, exposed soil, organic litter upslope from each plant, and distance to the largest rock upslope. The two surveys examined along WQTs included substrate type found downslope from lalpaoa, plant height, plant diameters across and along the slope, and distance between successively censused plants. Most plants grew downslope of nurse rocks; >74% were adjacent to blocks or outcrops, and >17% near cobbles. Plants showed avoidance for finer substrates; only similar to 53% and 2.7% grew onear bare soils and pebbles, respectively. About 92% of kupaoa were cm down slope of rocks; >89% grew cm away, and similar to 83% in direct contact with a rock. Some seedlings also grew on pukiawe (Leptecophylla tameiameiae) nurse plants. Several stable rock microsites protected plants from disturbance by slope processes causing debris shift. Site sediments were significantly finer than substrates near plants; shrubs grew preferentially adjacent to boulders >20 cm wide, which were more common near plants than across sites. Soils downslope of 50 boulders mean 413 cm showed higher shear strength and compressibility than soils along rock sides. These boulder dams stop descending debris, steadily deflecting them toward rock sides, whereas protected soils downslope of blocks are infrequently disturbed and develop into downslope elongated, clast-free fine-earth flags. Kupaoa were uniformly dispersed across slopes, showing a single-phase mosaic arrangement, without apparent aggregation. Slopes where kupaoa grows had a relatively uniform distribution of surface sediments; observed ualpaca patterns may have resulted from the array of microsites generated by surface rocks and sediments, as shrubs simply responded to such spatial structure. (C) 2017 Elsevier B.V. All rights reserved.