Growth trajectories and size-dependent reproduction in the highly invasive grass <Emphasis Type="Italic">Microstegium vimineum</Emphasis>

To better understand the ecological and evolutionary processes that underlie the ability of invasive plants to colonize and spread in a region, basic data on quantitative genetic variation in growth and reproduction of plants from contrasting microsites are desirable. The annual grass Microstegium vimineum is an invasive colonizer of the herbaceous layer in forests of the eastern United States. This greenhouse study utilized seeds collected from maternal plants in a shady forest or sunny edge microsite in central New Jersey, USA, to examine variation in growth trajectories and size-dependent reproduction. Questions addressed were (1) do growth trajectories vary significantly among families and between microsites? and (2) does reproductive mass scale with vegetative size? The trajectory for shoot dry mass over time was significantly related to microsite, due apparently to greater growth over the final 8 weeks of plants from the interior compared to those from the edge. It is suggested that a growth increase late in the season enables plants to maximize reproduction when light conditions improve following canopy leaf senescence. Number of tillers exhibited significant variation among families within microsites, but growth trajectories did not. Positive phenotypic and genetic correlations were detected between reproductive and vegetative mass per tiller; thus selection may favor large tillers to increase seed output, a critical life history trait for an invasive weed.