The extent to which symbiotic nitrogen-fixing plants (hereafter “N fixers”) are able to persist and modify the amount of nitrogen (N) in circulation determines an ecosystem’s ability to respond to perturbations and changes in atmospheric CO2 concentration. N fixers and symbiotic N fixation (SNF) are sometimes common early in succession, but as forests age, SNF within an ecosystem is typically negligible. Is the transition from SNF to no SNF within an ecosystem gradual and reversible or does it follow more complex dynamics? To explore possible answers to this question, we constructed a theoretical model of plant biomass, available soil N, and organic soil N where competing N fixers and nonfixers can be limited by either N or light. We compared obligate and facultative fixation strategies to investigate the conditions under which we expect multiple stable states (MSS, a.k.a. alternative stable states) between N fixers and nonfixers along a successional gradient where N supply is allowed to accumulate over time. Using timescale separation techniques, we determined which species-level traits lead to different successional dynamics when N fixers have a higher N demand than nonfixers.
Results/Conclusions
Possibilities for MSS arise when N fixers and nonfixers experience asymmetric light and N competition. When MSS are possible, the realized successional dynamics are determined by the initial abundance of co-occurring N fixers and nonfixers (founder effects). Species-level traits such as N demand, uptake of available soil N, N use efficiency, and maximum biomass under non-N limited conditions are particularly important in creating conditions for MSS. With MSS, the N fixer’s stable state is maintained when the N fixer is N limited because it grows more rapidly than its competitor as N supply increases. The N fixer promotes its own growth through SNF, however, this positive feedback turns negative when the N fixer stops being N limited. As a result, the N fixer can be invaded by the nonfixer. Simultaneously, there is a dramatic pulse of available N losses, leading to a reduction in N supply and equilibrium conditions where N fixers are unable to reestablish. Compared to obligate SNF, facultative SNF delays successional N fixer exclusion and extends the N supply range over which MSS are possible. This work has implications for resilience to disturbance under different scenarios of resource limitation.