PARENT SESSION
Oral Session #66: Invasions: Plants. Mechanisms, patterns, controls. Presiding: R. Mack.
Thursday, August 9, 2001. 1:00 PM to 4:45 PM. Hall of Ideas E.

Germination and seedling survival of an invasive weed (Verbascum thapsus) along an elevational gradient in Hawai`i.

ANSARI, SHAHIN¹, DAEHLER, CURTIS¹, ¹

ABSTRACT- Demographic studies of invaders over space can help explain their patterns of abundance and spread. Mullein (Verbascum thapsus) was introduced ca. 1910 to the island of Hawai`i, where it invades upper montane and subalpine habitats. We investigated whether differences in adult mullein densities across a 1000m elevational gradient could be explained by differences in seedling germination and survival. At each of seven sites spanning 1690m to 2700m elevation, we quantified mullein population densities, then removed all mullein from 30 m² plots and monitored natural mullein germination and seedling survival. Considering plants > 5 cm in diameter, population densities ranged from 0.6 to 2.8 plants per m² and generally increased with elevation. Natural germination in cleared plots ranged from 0.3 to 6.2 seedlings per m² and was significantly correlated with established population density at each site (r = 0.98, P <0.01). Survival over 4 months (September through December) was not correlated with established population density but generally declined with increasing elevation. Mean annual rainfall decreases from 2500 mm at the lowest site to <1000 mm at the highest site and probably limits survival at the highest site. At the lower sites, greater rainfall supports a denser weed community, which could lead to greater mortality of mullein over time and account for generally lower population densities at the lower elevations, in spite of higher early seedling survival at these sites.

KEY WORDS: Invasion , Demography, Germination, Seedling survival