Root traits are poor predictors of the response of the understory plant community to widespread spruce mortality

Background/Question/Methods

Epidemic outbreaks of native beetle populations in western coniferous forests have become increasingly prevalent. The mortality of spruce, a canopy dominant tree, provided an opportunity for understory plant communities to increase in cover and richness 10 years after the spruce beetle outbreak. Above ground functional traits (plant height, leaf turgor loss point, specific leaf area) and priority effects were important predictors of understory species responses to canopy mortality. However, that is potentially only half the story; we know little about the role of root traits in predicting the response of species to disturbance, despite the importance of roots for resource acquisition and fungal collaboration. We asked the following research questions: Do root traits predict understory species level responses to spruce beetle disturbance? Do newly colonizing post-disturbance species express significantly different trait values from pre-disturbance community members? How do community weighted mean trait values vary along a gradient of spruce mortality? We conducted our study in the Snowy Range of southeastern Wyoming and collected above and below ground functional traits for 59 species. We used generalized linear models to elucidate the importance of root traits for predicting the response of the understory plant community.

Results/Conclusions

Specific root length, root tissue density, root diameter, and root nitrogen were poor predictors of the response of understory species to spruce beetle disturbance at the plot scale, explaining less than 10% of variance. In contrast, priority effects explained over 40% of the variance. Species that were only present post-disturbance did not have trait values (leaf dry matter content, specific leaf area, height at maturity, leaf turgor loss point, percent leaf nitrogen, specific root length, root tissue density, root diameter, percent root nitrogen) that significantly differed from trait values of species that were present both before and after the disturbance. Additionally, community weighted trait values for the traits measured did not vary along a gradient of spruce mortality. This research suggests that priority effects which presumably influence competition for space and light are more important for post-disturbance community assembly than root functional traits at this scale and for at least a decade post-disturbance.