A pervasive species composition shift has been reported from oak (Quercus)-dominated stands throughout the temperate zone. The regeneration layer of oak stands across all but the most xeric sites often contains a high abundance of shade-tolerant and mesophytic species and lacks advanced oak reproduction. Based on quantitative data reported from stands across the temperate zone, many researchers have concluded that the replacement of oak by more shade-tolerant and mesophytic species is inevitable. The projected successional replacement of oak by mesophytes (primarily Acer and Fagus species), is based largely on composition of the regeneration layer. Questions remain on the ability of these species to positively respond to changing growing environments caused by canopy disturbances. To this end, we reconstructed canopy disturbance history for 51 oak-dominated stands in the eastern US. We also quantified discrete canopy disturbances and plant responses to a range of events from singletree canopy gaps to intermediate-severity disturbances. Our results improve the mechanistic understanding of the relationships between canopy disturbances and successional pathways in oak-dominated systems.
Results/Conclusions
In the oak stands we studied, the disturbance regimes were characterized by frequent asynchronous, localized canopy disturbances punctuated by intermediate-severity events. The return interval of intermediate-severity disturbances ranged from 20–40 years and on average, a localized canopy disturbance occurred every 2–4 years. These overstory disturbances influence species composition and successional patterns. We found that canopy disturbances (ranging from singletree gaps to patches of 0.5 ha) typically allowed for the recruitment of saplings and small trees to larger size classes. In the stands sampled, the majority of stems in these size classes were shade-tolerant species. Disturbances are strong drivers of successional patterns in forest ecosystems and most oak stands in the eastern US have a canopy disturbance regime that favors shade-tolerant stems that can withstand suppression and maintain the ability to recruit when released from competition. To maintain an oak component in these systems, managers will need to modify the composition of the regeneration layer to increase the abundance of competitive oak stems. As the abundance of oak stems in the regeneration layer increases, so does the probability that an oak stem will be in position to recruit following a stochastic or planned canopy disturbance event.