Dust and proximity to unpaved roads correlated with decreased reproduction of an endangered Utah endemic shrub

Background/Question/Methods

Energy development on the Colorado Plateau has led to increased fragmentation of open space by roads with negative consequences for native plant species. Roads reduce available habitat, spread exotic species, and create barriers to dispersal. In addition, unpaved roads also increase dust loads on leaves and floral structures, which may significantly reduce the growth and reproduction of nearby plants. We studied the effects of an unpaved road on the successful reproduction of the endangered Utah endemic shrub Hesperidanthus suffrutescens(shrubby reed-mustard). We measured the size and reproductive output of 156 plants and dust deposition at increasing distances from the road. We hand outcrossed 240 flowers on 80 plants to determine whether reduced reproduction, if any, is due to pre or post-pollination mechanisms. Additionally, we experimentally dusted 3 leaves on 30 plants (n=90) and measured stomatal conductance pre-dust, post-dust, and after washing. We also dusted 3 flowers on 10 plants (n=30) prior to hand pollination and measured fruit set. Differences were analyzed using generalized linear mixed models to determine significant correlations between reproduction and dust deposition.

Results/Conclusions

Dust deposition was greatest near the road and decreased as distance from the road increased. When controlling for plant size and distance, fruit set was negatively correlated with increasing levels of dust deposition (ƒ_1,15 = 5.26, p = 0.0366). The number of seeds per plant, the mean plant seed weight, and the proportion of hand-pollinated flowers that set fruit were also negatively correlated with dust, although not significantly. Although correlated with dust, the observed pattern of reduced reproduction could be due to other factors. Roads have been shown to interrupt complex plant-pollinator interactions, resulting in reduced reproduction. Stomatal conductance was significantly reduced (ƒ_{1, 58} = 87.56, p < 0.001) by the application of road dust. Eighty percent (24/30) of hand pollinated flowers set fruit after dusting, suggesting that dust did not prevent pollination. However, the process of applying pollen by hand could have removed any dust on the stigma. Overall, these results suggest that dust may impact fruit set through reduced physiological processes. The results also highlight the need for further research into the effects that roads and dust have on nearby plants while suggesting negative consequences for the conservation of an endangered shrub in Utah’s Uinta Basin.