Environmental determinants of bryophyte community change over time

Environmental factors mediating community change over time have been well documented for vascular plants, but not for their non-vascular relatives, bryophytes. Elevation is one variable that has been explored regarding bryophyte community change wherein as elevation increases, bryophyte communities increase in diversity and abundance, whereas diversity and abundance decline for vascular plant communities. This begs the question, how might other environmental variables such as slope and aspect influence bryophyte communities, and how might these variables drive community changes over time? Our study focused on bryophyte communities in high elevation, spruce-fir zones in the Great Smoky Mountains National Park (GSMNP), located in the Southeastern United States to answer this question. Plots established in 2007 were revisited and reinventoried in 2020. Comparisons were drawn to determine whether changes in bryophyte coverage corresponded to canopy cover change and environmental factors including elevation, slope, and aspect. Diversity and abundance at the two timepoints were analyzed. Alpha diversity and turnover across orders of q were compared for different elevation zones. Bryophyte alpha diversity at q = 0 significantly declined over time in association with elevation, but at q = 2, changes in alpha diversity varied according to changes in gap fraction, a measure of canopy cover, and slope. Bryophyte coverage remained stable except in the highest elevation zone dominated by fir trees where there was a decline in coverage predicted by aspect. There were high rates of turnover across all elevation zones regardless of Hill number. In contrast with other studies which examine how environmental variables mediate community changes at q = 0, the species richness of a community, our study shows that the environmental drivers of bryophyte community change vary depending on how abundance is weighted.