Deaths of individual trees, singularly or in small clumps, provide opportunities for canopy accession in closed-canopy forests like the Eastern Deciduous Forest (EDF). Yet paleoecological evidence indicates simultaneous, regional-scale changes in composition, even in regions where gap-dynamics is thought to be the prevailing disturbance regime. These compositional changes are associated with abrupt climate change. Here we present tree-ring data from 76 populations consisting of 12 species in the Central Hardwood Forest region (CHF). Our objectives are to evaluate: 1) the occurrence of simultaneous canopy accession events, and 2) recruitment of Quercus prior to European settlement. Investigation of regional-scale stand dynamics was conducted on seven new collections from old-growth forests in southeastern Kentucky and central Tennessee and an additional 69 collections from the International Tree-ring Data Bank. We assessed all populations using Principal Components Analysis. The resulting second and third eigenvectors each identified a geographic region that experienced distinct step-changes in raw ring widths resembling canopy accession events. Populations from each region were then analyzed for canopy disturbance and accession events using standard techniques. Recruitment analysis is limited to Quercus samples from Kentucky as they are the only samples that we can determine pith dates.
Results/Conclusions
Principal components analysis revealed a step-change in average ring width during the 1770s in the southeastern CHF and in the mid-1800s in the northeastern CHF. While disturbance occurs somewhat regularly throughout both regions, disturbance detection analysis suggests a spike in canopy accession events in the 1770s in the southeastern CHF. These events were evident in Kentucky Quercus alba and Q. montana, and central Tennessee Tsuga candensis. The spike comes at the end of a 30-year drought, the worst in the last 300 years. Trees that entered the canopy during the 1770s event experienced a decade-long pluvial. Throughout the last 400 years, evidence suggests episodic Quercus recruitment in Kentucky, including an establishment episode across eastern Kentucky. Spatiotemporal patterns within our high-resolution data set corroborate paleoecological records indicating climate as an important factor of stand dynamics in the EDF. While evidence of gap-phase dynamics is present in this analysis, the striking patterns observed in establishment, radial growth and stand dynamics of these records (despite the various investigators, objectives and sampling schemes under which the raw data were collected) suggest stand structure and composition in the CHF region are also strongly influenced by episodic, regional-scale events.
