The role of resource mutualisms and intraspecific variation during succession

Background/Question/Methods

Mutualists may have profound effects on community assembly during succession by altering biotic and abiotic conditions.  For example, mutualistic symbioses between legumes and nitrogen-fixing rhizobia may result in a competitive advantage for early successional legumes which may slow down succession or alternatively can increase nitrogen availability which may accelerate succession. In this way, the legume-rhizobium resource mutualism could be a key driver of succession. In addition, there is substantial intraspecific genetic variation in how legumes interact with rhizobia, including variation in rhizobia population size, nitrogen availability to plants, and legume fitness. Therefore, not only may the presence of particular legume influence community dynamics, but the colonizing genotype could also influence these processes when populations vary in traits related to ecologically important interactions with rhizobia. To explore how resource mutualisms and intraspecific variation in a legume-rhizobium mutualism can influence community dynamics during succession, I created experimental plots manipulating rhizobia availability, nitrogen availability, and Chamaecrista fasciculata population identity in an early successional old-field.

Results/Conclusions

I found that species richness is reduced when plots are inoculated with rhizobia, but these effects were only evident in fertilized plots (F_1,30=14.4, p<0.001).  Moreover, in the absence of rhizobia, fertilization increases richness (F_1,31=5.6, p=0.02), but in the presence of rhizobia fertilization decreases richness (F_1,31=4.3, p<0.05). Therefore, both abiotic and biotic soil factors of a site that a legume is colonizing may impact the subsequent community interactions. Intraspecific variation among C. fasciculata populations also led to marginal changes in richness between plots (F_3,62=2.5, p=0.06), indicating that populations which I have previously shown to vary in ecologically important traits may alter community patterns depending on which population establishes. Overall, the role of mutualisms in differing environments and variation in the legume-rhizobium relationship among different potentially colonizing populations may be important components of succession, especially in the early stages of establishment. These results help provide a rigorous field test of the ecological importance of both mutualism and population variability to successional dynamics.