Regeneration of the Mauna Kea silversword Argyroxiphium
sandwicense (Asteraceae) in Hawaii
Lawrence R. Walker *, Elizabeth Ann Powell
Department of Biological Sciences, University of Nevada, Las Vegas, Box 454004, 4505 Maryland Parkway, Las Vegas, NV, USA
Received 3 June 1997; received in revised form 8 September 1998; accepted 10 September 1998
Abstract
The Mauna Kea silversword Argyroxiphium sandwicense is an endangered plant endemic to the high elevation cinder deserts of
Mauna Kea on the Island of Hawaii. Restoration eorts have increased the total naturally occurring population of ca. 50 adult
plants by ca. 500 individuals, primarily by the outplanting of silverswords initially grown under controlled conditions. We evaluated
the direct sowing of seeds into ®eld sites as an alternative to outplanting. Gravel-covered surfaces enhanced germination and
growth of seedlings and increased soil moisture. The presence of shrubs and trees was associated with increased silversword ger-
mination but decreased silversword growth and reproduction. The best environment for silverswords varied throughout the life
cycle of the plant, but critical stages are pollination, seed production, germination, and seedling survival. Sowing and outplanting
each have unique advantages. High elevation, moist, gravel surfaces that can be protected from ungulate browsing should be the
focus of future sowing and outplanting eorts to maximize restoration success. # 1999 Elsevier Science Ltd. All rights reserved.
Keywords: Competition; Germination; Hawaii; Microhabitats; Restoration; Silversword
1. Introduction
The Mauna Kea silversword Argyroxiphium sandwi-
cense DC is a federally-listed endangered plant endemic
to the high elevation cinder deserts of Mauna Kea on
the island of Hawaii. It is of particular interest to con-
servation biologists, evolutionists, and ecologists because
it is rare, monocarpic, and primarily self-incompatible.
Silverswords are members of the subtribe Madiinae of
the family Asteraceae, which represents a spectacular
example of adaptive radiation (Carr, 1985). The Mauna
Kea silversword is a giant rosette plant with one or
more showy ¯ower stalks that reach 2±3 m in height and
have >100 capitula per stalk (Fig. 1). The demography
and pollination biology of the silversword are relatively
well-studied (Powell, 1992; Walker and Powell, 1995).
Because of the concerns about how to manage rare
plant populations (Elias, 1987; Falk and Holsinger, 1991;
Bowles and Whelan, 1994; Given, 1994), the silversword
provides an excellent example for experimentally testing
principles of conservation biology and the management
and restoration of rare plant populations.
The Mauna Kea silversword may have covered large
areas of Mauna Kea in a broad band above treeline
(2600±3800 m a.s.l.) before sheep Ovis aries, mou¯on
sheep O. musimon, and goats Caprus hirtus were intro-
duced to Hawaii. By 1892, the silversword was noted as
being ``nearly extinct'' on Mauna Kea (Alexander,
1892), primarily due to browsing damage by ungulates.
Today there are ca. 50 naturally occurring adult silver-
swords remaining and ca. 500 established adults that are
the result of an outplanting program begun by the
Hawaii State Division of Forestry in 1973. Despite
ongoing sheep removal on Mauna Kea (S. Bergfeld,
pers. comm.), the silversword is still being aected by
ungulate browsing. The only reproductive populations
among the outplanted individuals are those that have
been planted in three ungulate exclosures (Fig. 2;
Powell, 1992). Outplanting is costly and only moder-
ately successful (30±75% survival for 1 year in 1996; E.
Powell, unpubl. data). Furthermore, the favorable sha-
dehouse environment under which outplants have been
grown may select for characteristics neither common in,
nor advantageous to, natural populations (Powell,
1992). Therefore, we examined the feasibility of the
direct sowing of seeds. However, the Mauna Kea sil-
versword is primarily self-incompatible (Carr et al.,
0006-3207/99/$Ðsee front matter # 1999 Elsevier Science Ltd. All rights reserved.
PII: S0006-3207(98)00132-3
Biological Conservation 89 (1999) 61±70
* Corresponding author. Tel.: +1-702/895-3196; fax: +1-702/895-
3956; e-mail: walker@nevada.edu