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Abundance of common mullein along the quarry trails

Laura Crawford, Gaby Duong, fall 2013

Introduction

Common mullein (Verbascum thapsus) is a weed that grows in inhospitable areas.  This plant species is a good invader because it colonizes environments where other plants cannot survive.  Common mullein is native to Europe but is now found throughout most of the United States.  Common mullein is common in the quarry bottom of the DePauw Nature Park.  There are two trails in the quarry bottom, Q1 and Q2.  Common mullein seems be more abundant along the edges of these two trails than in areas away from the trails.  The goal of our research was to test this observation.  

Our hypothesis is that common mullein will be more abundant along trail edges than in areas away from trails.  We think this pattern is due to more human activity and more disturbance along trails.  In this study, we quantify the distribution and abundance of common mullein in the quarry bottom.  

methods

We mapped the locations of rosettes (first year plants) and fruiting (second year) common mullein plants on the two quarry trails (Q1, Q2) and on four "shadow" trails.  The "shadow" trails were located in the quarry bottom in areas with no trails.  The substrate conditions (exposed rock, limestone gravel) were similar between the quarry trails and the shadow trails. 

Results

There were more patches of common mullein plants along the Q1 trail, but the densest patches of plants were on the Q2 trail.  We observed 9 patches of fruiting plants on the quarry trails.  Most of the fruiting plants were found on the western end of the Q1 trail, close to the pond.  We only found 2 plants along the shadow trails.  Both plants were rosettes, in their first year of growth.  No fruiting plants were found on the shadow trails.  

discussion

We found support for our hypothesis that common mullein would be more abundant along trails compared to open areas with no trails.  Trails appear to have a significant effect on common mullein populations.  Common mullein was abundant along the Q1 and Q2 trails, especially north and east of the quarry pond.  We expected common mullein to be abundant along trails because of high human activity in terms of traffic and trail maintenance.  Common mullein seeds are probably carried along trails in clothing and dog fur.  Increased disturbance levels, due to human trampling, mowing, and gravel deposition, may also be contributing to the abundance of common mullein along the trails.  Common mullein thrives under high disturbance regimes, partly because it is good at colonizing areas that other plant species cannot survive in.  Many species cannot survive in disturbed areas, so common mullein probably benefits from decreased competition.

Common mullein was rare along the shadow trails in the quarry bottom.  There are many potential reasons for the lack of mullein in these areas.  There may not be any seeds, there may be other plant species outcompeting common mullein, or there may be differences in water or nutrient availability. 

There were nine patches of fruiting plants along the Q1 and Q2 trails.  Rosettes were much more abundant than fruiting plants.  Perhaps fewer plants reach reproductive age because of insufficient resources, mowing, and competition.  Common mullein plants store resources during their rosette stage.  If resources are insufficient, the plants die before they can reproduce.  We hypothesize that many plants do not survive the rosette stage because of resource pressures.