Effects of disturbance and environment on yellow toadflax (Linaria vulgaris) invasiveness

The response of yellow toadflax (Linaria vulgaris) populations to disturbance was evaluated in four distinct environments – Clearcut, Meadow, Riparian and Wildfire in the Gallatin National Forest, Montana.  Disturbances (0.75 m²) of burning, digging, vegetation clipping and herbicide application (Tordon K, 1 kg ha^-1) were created in the spring of 2004 in plots on the edge and just outside of existing L. vulgaris patches.  Disturbed plots were monitored for three growing seasons (2004 – 2006) and L. vulgaris stem counts were recorded each spring.  From 2004 to 2005, with the exception of herbicide application, all treatments at all sites resulted in positive L. vulgaris growth rates.  By spring 2006, differences in growth rate were apparent.  Regardless of treatment, Riparian (λ = 8.1) and Clearcut (λ = 7.0) populations had the highest mean growth rates, while the Wildfire (λ = 2.7) and Meadow populations (λ = 2.5) had the lowest.  Between 2005 and 2006, at the Meadow site, control (no treatment), clipping and digging produced L. vulgaris growth rates significantly greater than burning and herbicide treatment (P < 0.001).  At all other sites, herbicide treatment, which reduced L. vulgaris growth rates in 2005, resulted in higher growth rates than the other treatments (P <0.001) in 2006.  Calculations of L. vulgaris invasiveness were made based on 2005 to 2006 changes in cell occupancy and stem density.  Invasiveness values indicated that in the Meadow, clipping resulted in the most invasive populations.  For all other sites, populations treated with herbicide were most invasive.