Does urbanization increase nitrogen availability for Dalmatian toadflax?

Background/Question/Methods

Introduced from the Mediterranean to North America in the early 1900’s as an ornamental species, Dalmatian toadflax (Linaria dalmatica) has spread dramatically throughout the Eastern Washington and Northern Idaho region. Considered a class B invasive weed in Spokane County, toadflax growth is especially vigorous in urban areas. Nitrogen is important to to toadflax growth, and urbanization is expected to increase nitrate content in urban areas (for example, due to industrial runoff and increased fertilization). The study that we performed aimed to evaluate urbanization effects on nitrogen availability for Dalmatian toadflax. We studied fourteen field sites from May to August in 2012. The sites varied in urbanization, which we quantified by estimating the percentage of land cover that was maintained vegetation or impermeable surfaces within a 100-meter radius around each site. We also measured the the average height of toadflax stems and the amount of nitrate (NO₃^-) from soil samples collected at each site.

Results/Conclusions

We found a positive correlation between the average heights of toadflax and the percent of maintained area surrounding each of the sites (r² = 0.36, p < 0.05). The amount of nitrate (parts per million) at each field site was not significantly correlated with the percent of maintained surrounding the site (r² = 0.22 and p > 0.05). There was also no significant correlation observed between the nitrate content at each field site and average toadflax height (r² = 0.13 and p > 0.05). Our results are consistent with previous work that suggests that urbinzation has a positive effect on Dalmatian toadflax growth. However, we did not find support for the hypothesis that urbanization increases the nitrogen available to toadflax. One limit to our study is that our 2012 nitrogen measurement were collected relatively late in the growing season. Thus it is possible that toadflax growth had depleted available nitrogen to similar levels at the different sites. We plan to measure nitrogen at our sites early in the 2013 growing season to better evaluate whether urbanization increases nitrogen availability.