Spatial aggregation patterns and population structure of the Haleakalā silversword (Argyroxiphium sandwicense DC. subsp. macrocephalum), Maui, Hawai’i

Abstract

Spatial aggregation, density, and population structure were studied in 10 populations (patches) of Haleakalā silversword rosettes (Argyroxiphium sandwicense DC. subsp. macrocephalum) between 2175 and 2755 m at Haleakalā Crater (Maui, Hawai’i). Rosette diameter, height, and inter-rosette distances were determined for 100 silverswords at each location along wandering-quarter transects (WQTs). Plant density was also assessed from 10 100-m² quadrats. Spatial patterns showed a typical two-phase mosaic of small silversword first-level patches separated from neighboring clusters by larger intervening barren gaps. Patches showed pronounced clumping; average distance between plants (~205 cm) was ~71–324 cm. First-level patches exhibited internal hierarchical structure, with nested second-level patches about the center (core) and around the edge (periphery). Core inter-plant distances were shorter (~132 cm) than at the periphery (~268 cm); cores contained greater percentages of small plants. Aggregation patterns resulted in sparsely vegetated sites. WQT density was ~13–284 rosettes/100 m² (average: 83.6); quadrats showed slightly greater densities (average: 119.5 plants/100 m²). Some sites showed size-class distributions with high percentages (42–59%) of plants ≤7.5-cm diameter, but half the locations had ≤23% within this cohort. Significant pattern characteristics, and substrates and biotic factors influencing them, are discussed; overall, silverswords grow wherever sufficient stones are exposed at the ground surface.

Keywords:

• plant aggregation
• population structure
• rosette plants
• spatial patterns
• stones
• volcanic substrates
• Haleakalā silversword

Acknowledgments

I appreciate the help of Elizabeth Gordon, Ronald J. Nagata, and Patti Welton at HNP, for providing valuable information about local vegetation and for research permits. I specially thank Lloyd L. Loope (USGS, Haleakalā Field Station) for sharing publications and unpublished data of ongoing ecological projects. I am grateful to Alejandro Pérez-Bergquist and Martha Kowalak-Pérez for enthusiastic help during fieldwork and enjoyable company while trekking across the crater. The paper benefited from thorough reviews by Paul Krushelnycky (University of Hawai’i, Mānoa) and an anonymous reviewer. I dedicate this work to the memory of my father, Francisco Pérez Conca (1918–2003); he contributed generously to my research in many useful, loving ways, and his constant and unstinting support helped me greatly over the years.