Disturbance and dispersal mechanism influence seedling establishment rates in the Eastern United States

Background/Question/Methods

The aim of this project was to investigate the influence of disturbance type, species’ dispersal syndrome, species’ shade tolerance level, and weather in predicting seedling establishment rates both within and beyond the individual species’ range extent. We were explicitly interested in tree harvesting as a disturbance, as forests in the eastern USA are primarily dense, secondary regrowth. We used the United States Forest Service’s Forest Inventory and Analysis (FIA) data for eastern United States tree species and ran generalized linear models for seedling presence/absence and seedling number as response variables. Disturbance categories included no disturbance, natural disturbance, harvest, and both. Weather variables investigated were spring minimum temperature and summer maximum vapor pressure deficit.

Results/Conclusions

The probability of seedling establishment increased in response to harvest within FIA plots, but not in response to natural disturbances. This effect was seen across species, regardless of their shade tolerances. Additionally, bird-dispersed species had a higher probability of colonizing new plots outside their range, compared to mammal-dispersed or wind-dispersed tree species. There was no consistent significant response to either weather variable. Our results suggest that stand competition, not climate, is the primary limiting factor for seedling establishment in the dense, secondary forests of the eastern United States. Harvesting creates sizeable gaps that promote seedling establishment and species turnover. Bird-dispersed species establish more readily in these harvested gaps than mammal- or wind-dispersed species, especially in plots located outside of their range.