The Smithsonian Institution Global Earth Observatory (SIGEO) is a SI-led initiative to develop a network of temperate large forest dynamics plots to complement the highly regarded network of tropical forest plots of its Center for Tropical Forest Science (CTFS).   In 2008, a new 25 hectare (ha) SIGEO forest dynamics plot was established at the Smithsonian National Zoological Park’s Conservation and Research Center (NZP-CRC) in mature secondary mixed deciduous forest near Front Royal, VA, USA.  This plot encompassed two 1 ha monitoring plots that were established in 1993 and 1997 by the NZP’s Center for Conservation Education and Sustainability (CCES) under the Monitoring and Assessment of Biodiversity program.  This study’s objective was to investigate recent forest compositional changes by comparing the current and historical census data from these plots.  During these censuses, all woody stems ≥ 1cm dbh (SIGEO) and ≥ 4cm dbh (CCES) were identified to species, tagged with individual numbered metal tags, measured at breast height, and mapped on a 20-meter quadrat gridded coordinate system.  We compared only stems ≥ 4cm dbh in our analyses.

Results/Conclusions

Results showed a nearly 20% decline in species richness on the two 1 ha plots.  The canopy continued to be dominated by Carya and Quercus species, but Nyssa sylvatica remained the single most abundant species over the census period.  Only Carya glabra increased in abundance from 1993 to 2008.  The most dramatic change occurred in the mid-story tree layer, where flowering dogwood (Cornus florida) underwent a major die-off.  Of the six major mid-story species, C. florida comprised 57% of all stems in this class in 1993 but declined to 23% by 2008.  The average increase in dbh for the ten most common species over the 15-year period was 5.5cm (range 9.8 – 2.9cm), with Q. rubra showing the highest growth rate and C. glabra having the lowest.  The dearth of sapling recruitment for most species may be attributable to persistently high browsing pressure from white-tailed deer (Odocoileus virginianus), while the loss of flowering dogwood as the major mid-story tree species is due to mortality from the dogwood anthracnose pathogen.  Detection of these changes demonstrate the value of long-term, large forest dynamics plots for assessing the effects of multiple stressors such as disease, herbivory and climate change on forest composition, structure and function.