Growth of an exotic legume on a restored prairie depends on light, soil fertility, and root parasitism

Background/Question/Methods

Exotic legumes can pose a special challenge to managers of nutrient-poor grasslands, including many restored prairies.  Low-nutrient sites support sparse vegetation with ample light for nitrogen fixing invaders and these exotic legumes may shade out less competitive species. Fertilizing to deter legumes may encourage invasion by non-legume exotics.  Low-nutrient/high-light environments also tend to be suitable habitat for hemiparasitic plants.  Hemiparasites photosynthesize and form attachments to neighbors’ roots to extract minerals, water, and organic compounds. Previously we demonstrated that the native hemiparasite Pedicularis canadensis significantly depressed aboveground productivity, especially of grasses, on a nutrient-poor restored prairie and this effect was not altered by light and fertilizer manipulation.  We continued this experiment to determine how fertilizer, light, and parasitism affect growth of Lespedeza cuneata as it invades.  Starting in 2006, 1-m² quadrats were given one of eight treatments that were combinations of three factors:   fertilizer (none/ addition of 70 g N-P-K), shade (full sun/50% shade in years 1-4), or hemiparasite (present/removed). Lespedeza cuneata shoots were harvested from the center of the plots after years 1, 3, 4, and 5. Using ANCOVA we analyzed the effects of treatments on the difference in log-transformed dry mass harvested after the fifth and first seasons.  

Results/Conclusions

Total cover of vegetation in plots prior to the onset of treatments was used as a covariate, and this variable was negatively associated with L. cuneata growth.   ANCOVA revealed a three-way interaction:  Shade and fertilizer treatments significantly affected the mean difference in mass of L. cuneata only in the absence of the hemiparasite.  In these removal plots, shade reduced the difference in mass significantly (i.e., reduced growth of L. cuneata) when plots were fertilized, probably due to greater competition from non-legume forbs.  Growth of L. cuneata was not deterred by shade regardless of fertilizer if the hemiparasite was present, probably because this hemiparasite strongly reduces growth of grasses as well as non-legume forbs.  Hemiparasites have been found and used to increase species diversity in grasslands.  Our research suggests that parasites may influence the success of an invasive species indirectly by altering competitive interactions between existing species and therefore have unexpected effects on communities.