From the Mississippi to the Rockies: Do species’ suites track shifts in disturbance and climate regimes?

Background/Question/Methods

Forecasting community responses to disturbance has always been a central tenet of ecology. Numerous studies have shown species-specific responses to climate change and disturbance; however, species in a community may respond as a unit due to analogously evolved strategies for particular conditions and interspecific dependency. We examined this possibility with two case studies: a flooding disturbance gradient of the Mississippi River, and a 700 m elevation gradient of Colorado Front Range forests. The objective of this research was to determine if shifts in the disturbance regime (Mississippi River Islands) or climate (Colorado Front Range) result in predictable shifts in plant communities. For each case study, we assessed elevational shifts based on vegetation data from two sampling times (Islands: 43 plots sampled in 2007 & 2008; Mountains: 89 plots sampled in 1973/74 & 2013/14) using similarity indices and tested for significance with permutation analyses. We hypothesized that a shift in disturbance or climate regime up the elevation gradient would be mirrored by an upward shift in plant communities adapted to that regime, and that shifts would depend on site-specific conditions (thalweg vs. backwater for Mississippi Islands, mesic vs. xeric for Colorado Front Range forests).

Results/Conclusions

Although dominant trends were either a lack of change or species-specific responses (i.e., Gleasonian or individual phase shifts), we found some evidence of community phase shifts (i.e., Clementsian) in both data sets, partially supporting our hypothesis. Although dissimilarities were generally lowest between plots at the same elevation for the Colorado Front Range data, the vegetation in mesic sites from 1973/74 at 2440 to 2515 m elevation was more similar to the vegetation from 2013/14 at 2515-2590 m elevation, and this similarity was higher than by chance suggesting an upslope community phase shift. For the Mississippi River Islands data, highest dissimilarities were observed between plots at mid-elevations where species richness was highest. The highest dissimilarity occurred at lower elevation in backwater plots compared to thalweg plots, indicative of disturbance severity differences. The only evidence for community phase shifts was found in thalweg vegetation at 8 and 9 m elevation, both downward in elevation. Our analysis proved to be applicable to and useful for both long-term comparisons of plots at irregular elevation intervals that are subjected to long-term gradual change (Colorado Front Range), and short-term dynamics of regularly delineated plots with severe and abrupt disturbances (Mississippi Islands).