Manifold drivers of regional variation in successional timing: Modeling the impacts of latitudinal variation in growth rate and herbivory on herbaceous-woody competition

Background/Question/Methods

During old-field succession in the Eastern Deciduous Forest, the switch from herbaceous to woody species dominance marks a major change in ecosystem functioning. The timing of this shift is known to vary greatly, from <10 to >50 years. Latitude explains approximately a third of this variation, but the remaining variation cannot be explained by edaphic or climatic factors alone. We expect that a combination abiotic and biotic factors, including herbivory, play a role in altering competition and therefore affect the timing of the shift, but the relative importance of these factors is unknown.

In order to identify the parameters which are responsible for the observed competitive dynamics, we developed a two individual model that uses an increasing exponential decay function to simulate the growth of competing herbaceous and woody species. Using a range of realistic growth rates derived from existing data, three relationships of growth rate with latitude were developed: both species’ growth rates decrease northward (climate model), both increase northward (nutrient model), or herbs increase while trees decrease northward (mixed model). These models were then run under varying herbivory regimes. We sought to identify under what conditions we could recapitulate the full range of timing variation documented empirically. 

Results/Conclusions

We found that multiple parameter settings can reproduce the observed variation in timing. In the absence of herbivory, only the mixed model yields realistic results. The climate model produces the correct qualitative relationship, but does not achieve a wide enough range of values for the time to woody dominance. In the presence of constant herbivory with greater herbaceous tolerance, the climate model produces a more accurate range of values than the mixed model. Realistic results can also be achieved with the climate model when herbaceous tolerance is greater than woody and herbivory pressure increases northward and with the mixed model when herbaceous tolerance is greater than woody and herbivory pressure decreases northward. At all levels of herbivory, the nutrient model produced the wrong qualitative relationship.

These results indicate climate or nutrients alone are not responsible for the regional variation in successional timing. Future research will be necessary to derive better data that jointly considers resource competition, climate, and herbivory in order to understand the dynamics of old-field succession.