Competition is reduced by tornado disturbance but does not predict tree survival in an eastern deciduous forest

Background/Question/Methods

Climate change is causing an increase in the frequency and intensity of natural disturbances, especially within eastern deciduous forests. These novel disturbance regimes can modify niche distribution, competitive exclusion pressures, and spatial occupation. Competition shapes species distribution and is likely affected by the changing environmental conditions. Ecologists have been studying how these processes interact and their influence on community resilience. To explore how competitive potential contributes to resistance to a large-scale disturbance, we examined the level of competition and survival of trees in a mature temperate forest stand following a windthrow disturbance. Tree community structure data was obtained at the Temple Forest Observatory (TFO), which was hit by an EF-2 tornado in 2021. Trees were censused in a 2.56-hectare section of the forest before and after the storm. To quantify competition experienced by individual trees, we calculated Hegyi’s competition index, which is a distant-dependent analysis used to indicate plant competitive interactions. We tested the hypothesis that the tornado would reduce competition index values of trees, and that lower pre-storm values would predict tree survival to the storm.

Results/Conclusions

Many of the dominant tree populations experienced thinning due to the windthrow disturbance. Preceding the tornado, there were 911 trees within the 2.56-hectare section of the forest, and only 18% of these trees survived. Competition values pre-tornado indicate a highly competitive ecosystem with distinct mechanisms and life-history characteristics contributing to the coexistence of strong and weak competitors. The stand-replacing disturbance, however, increased resources that were previously limiting (e.g., space and light). The substantial losses of tree individuals due to the storm reduced competition within the forest. Mean competition index shifted from 4.74 before the storm to 1.38 after, indicating an over three-fold decline in inter-specific competitive potential due to the thinning of tree populations. While competition index did not predict survivorship in our forest site, adaptations to other ecological pressures, however, have potential to influence species resistance in similar ecosystems. With climate-change driven disturbances favoring species with selected characteristic, maladapted species are more likely to become locally extinct, permanently altering species interactions and ecosystem dynamics. Determining factors that contribute to the susceptibility of dominant species to disturbance will help advance eco-evolutionary understanding of the impacts of climate change on biodiversity and ecosystem stability.