Only four populations of the fern, Hawaiian pepperwort Marsilea villosa,are known, three on O'ahu and one on Moloka’I. The most extensive population is at Koko Head (O'ahu), occurring in an ephemerally open wet area of about 1l0 × 50 m. 1983 to 1986 were unusually dry (El Niño) years and this coupled with damage by off-road vehicles, reduced the population. After flooding in 1988, the Hawaiian Botanical Society and The Nature Conservancy initiated a conservation program to protect the site and enhance the fern population.
The unusually dry years combined with vehicle damage resulted in litter layer loss and exposed soil was colonized mainly by alien weeds. To determine composition and extent of weed invasion, the Marsilea sward was divided into 10 × 10 m quadrats and cover of each species estimated.
To test the effectiveness of weeding, six pairs of 1 × 1 m quadrats were established and species cover was determined (using a 50 point-intersect frame). The quadrat of each pair with least Marsilea was selected for weeding. All plants except Marsilea were removed by hand. Quadrats were surveyed during the growing seasons of 1988-89, 1989-90 and 1990-91.
From 1988 to 1990 the total number of plant species in quadrats rose from 29 to 34, declining to 22 after flooding in 1991. Considering total cover values, four trends were evident among commoner species:
i) only Marsilea,dominant during each survey, consistently increased. After the 1991 flood, a statistically significant increase in cover occurred;
ii) the grasses Echinochloa colona and Setaria verticillata (the two most frequent aliens in 1988), exhibited statistically significant decreases by 1990 and remained low. Five non-native annuals showed a similar but less marked trend;
iii) four perennials (Panicum maximum, Digitaria insularis, Sida fallax, Asystasia gangetica) and the annual grass Chloris barbata, had statistically significant increases from 1988 to 1990 followed by a significant decrease in 1991 after the flood;
iv) only Cynodon dactylon showed no significant change.
In 1988 two weeks after weeding, Marsilea cover in weeded plots was significantly greater; this difference persisted until the end of the growing season. In the second and third growing seasons there was no significant difference in Marsilea cover between weeded and control plots; it showed marked increases in both, growing vigorously and excluding most competitors. Whilst weeding initially stimulated Marsilea growth, the long-term effect was negligible. Recovery occurred under conditions of normal rainfall and protection from off-road vehicle damage.
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