Long-term vegetation change in contrasting North Carolina ecosystems

Background/Question/Methods

Long-term vegetation change is driven by multiple processes with some well understood and others unknown.  In an effort to provide a framework for assessing vegetation change we examined composition and diversity over time in three contrasting North Carolina ecosystem types. In the interest of assessing options for long-term monitoring we explored how spatial scale of observation influences detection of vegetation change. We also examined the utility of comparing change across recognized vegetation types.  Piedmont upland forests are known to be changing as a consequence of long-term succession coupled with recent increases in deer and exotic plant species. Coastal Plain riparian forests are changing from both natural succession and altered hydrologic regimes. Longleaf pine woodlands are changing in response to altered fire regimes. In each case we re-sampled 1000m² permanent, nested vegetation plots. For Piedmont forests we resampled 77 plots after 22 years and again after 10 more.  On the Coastal Plain we resampled 29 riparian forests after 14 years and 59 longleaf woodlands after 17 years.

Results/Conclusions

Piedmont hardwood forests are changing in a consistent direction across all vegetation types, in part driven by deer browse and arrival of exotics.  In successional pine stands the change is significantly different from that predicted by a space-for-time substitution using original survey data.  Species richness did not change significantly at 1000m², but declined at 25m² and 1 m². The rate of change in diversity and composition has been substantially greater during the most recent 10 years as compared to the previous 22. Coastal Plain riparian forests have experienced loss of high flood levels coupled with extended periods of inundation at low elevations. As a consequence the, higher elevation sites show increases in flood intolerant species and low-elevation sites show declines. Species richness has generally increased. Species richness in longleaf pine plots increased over time across all spatial scales on parcels that have experienced increased fire frequency in the last 20 years, whereas fire-suppressed parcels have lost a substantial proportion of species at smaller spatial scales. Compositional turnover was relatively low in vegetation types associated with dry, sandy sites and high on silty woodlands and savannas. Overall, our results show that vegetation change is pervasive and much greater than expected, multi-scale sampling allows enhanced perception and quantification of change, and comparison of change across vegetation types in a region increases interpretability.