Quantifying species turnover and compositional shifts of the Longleaf Pine ecosystem in North Carolina after 18 years

Background/Question/Methods

For centuries, botanists, plant ecologists, and taxonomists have been fascinated with the high degree of biodiversity, endemism, and rarity associated with the Longleaf Pine ecosystem of the southeastern United States. While many studies have documented the importance of fire and edaphic properties in structuring species richness, diversity, and composition, few have tracked shifts of these parameters over the long term. In addition, there has been little research quantifying the amount of species turnover within this system; however turnover is considered to be an important mechanism in the maintenance of biodiveristy in other frequently disturbed grasslands.  To assess changes in composition and quantify species turnover over time, vegetation plots in the North Carolina Coastal Plain were re-sampled in 2009 after 18 years. Abundance data was recorded at 7 spatial scales within 0.1 ha plots.  We used NMDS ordination and clustering techniques to examine compositional shifts over time and generalized linear models to assess temporal turnover as a function of environment and disturbance history.

Results/Conclusions

Over time, species composition has shifted substantially on some sites and minimally on others.  On the most nutrient-rich and frequently burned sites, composition has remained fairly consistent, while sterile or fire suppressed sites have experienced dramatic shifts in species abundance.  At the plot scale, a significant amount of species are turning over, although the magnitude of turnover varies depending on the local environment and fire frequency. The most nutrient rich and diverse sites are experiencing the most turnover as exemplified by the combined loss and addition of 42 species from a single plot. On average, twice as many species are being gained as those being lost, with an average plot turnover of 19 species. The degree of compositional change and turnover seems to result from an interaction between soil fertility and fire frequency.  Productive, species-rich sites are burned more frequently by land managers than their species-poor counterparts, and experience repeat opportunities for species colonization and extinction.  However, frequent fire on these sites prevents secondary succession from processing.  Thus, fire frequency may explain in part why species turnover is higher and compositional change is lower on productive sites. The Longleaf Pine ecosystem is a dynamic system characterized by frequent disturbance, high diversity, and many rare species.  Understanding how community dynamics shift in time is vital to inform management agencies on the effectiveness of current fire regimes in maintaining biodiversity in an ever-changing landscape.