Experimental test of soil nutrient influences on a demographic model of the exotic invasive Alliaria petiolata

Background/Question/Methods

Organismal life histories display a wide variety of themes and variations in survival and reproductive patterns.  This diversity is associated with variation in population persistence, growth and the life history stages most closely regulating population size.  While we know that species differ in regulation mechanisms, do populations of a single species vary throughout their range?  Answering this question may be especially important to regulating the growth and spread of exotic invasive species. We have been investigating how regulation varies among populations of Alliaria petiolata, a biennial exotic invasive species currently spreading rapidly throughout forest understories throughout the eastern U.S.  By demographically tracking over 30 permanent populations of Alliaria petiolata for the past five years, we have found that variation in population growth rate can be most strongly attributed to changes in soil NO₃^- and NH₄⁺ content. These resources are strongly associated with overwinter plant survival.  We tested the robustness of these associations by experimentally manipulating the availability of soil NO₃^- and NH₄⁺ using slow-release fertilizers and sawdust in a set of permanent populations. 

Results/Conclusions

We tracked individuals in these manipulated populations for 2 years and found mixed support for the observations made in uncontrolled populations.  While increased soil N availability improved overwinter plant survival over controls as predicted by our model, decreasing soil N availability had no effects on survival when compared to controls.  LTRA comparisons revealed that while increasing N improved per capita population growth rate through its effects on overwinter survival, decreasing N depressed population growth rate through effects on post-winter survival and fecundity.  Thus, our observational studies revealed environmental qualities that will be associated with rapid per capita population growth rate in this invasive species.  They also show that the demographic reasons for rapid population growth can change with environmental conditions. However, the experimental results suggest that reducing soil N while keeping all other resources constant may not have desirable population control effects.  It is clear that further experimentation involving the manipulation of multiple environmental factors in conjunction with demographic data collection is necessary to develop resource-based control strategies for A. petiolata.