Life history variation, population dynamics, and impact of the introduced weed, mullein (Verbascum thapsus) on the island of Hawai'i

The goal of this research was to explore whether adaptive changes have occurred in Verbascum thapsus in Hawai'i, and to quantify other ecological factors associated with the invasion dynamics of this Eurasian weed and its potential impacts. Life history traits of over 3000 seedlings were followed at seven elevations, spanning 1030 m, along the slopes of Mauna Kea. Seedling density, rosette survival, rosette size, number of leaves, mature plant height and the length of the life cycle generally increased with elevation. Differences in abundance of neighboring vegetation were correlated with differences in size-dependent survival of V. thapsus rosettes and probably affected the observed variation in life history traits along the elevational gradient. A common garden experiment revealed that none of the life history parameters of growth, survival, size, and time to bolting and flowering showed ordered differentiation with elevation. This suggested that phenotypic plasticity, rather than genetic differentiation accounts for the observed life history variation in V. thapsus on Mauna Kea. Observed differences in time to germination and seedling size across elevations when grown under common greenhouse conditions suggested that these early life history traits might be under the influence of maternal effects. The common garden experiment also revealed that fasciation---an abnormal morphological growth form, in V. thapsus, does not follow a monogenic pattern of inheritance. Inoculation experiments showed that bacteria do not induce fasciation in V. thapsus , and physical damage reduced fasciation rates in V. thapsus . Fasciation in V. thapsus is likely a result of a complex interaction of genetics and some environmental factor(s) other than damage. Experimental removal of V. thapsus and/or its litter in the subalpine plant community on Mauna Kea did not consistently result in net changes in the abundance of all herbaceous species, but removal of V. thapsus reduced the relative competitive pressure on grasses and led to their increase, which in turn reduced the abundance of herbs in the removal treatments. V. thapsus probably appears to be important in structuring the plant community on Mauna Kea by controlling the competitive interactions between herbs and grasses.