Scale and strength oak-mesophyte interactions in a transitional oak-hickory forest.

Forests in eastern North America are undergoing rapid compositional changes as they experience novel climate, disturbance, and pest conditions. One striking pattern is the replacement of canopy oaks (Quercus spp.) by mesic, fire-sensitive, shade-tolerant species like red maple (Acer rubrum). To gain insight into the successional patterns driving stand-level canopy oak replacement we ask two questions: (i) What is the spatial association of oak and mesophyte recruitment compared to oak and mesophyte overstory individuals, and (ii) How do oaks and mesophytes differentially respond to canopy openings. We analyzed census data from a 23 ha forest plot surveyed in 2003, 2008 and 2014. We show that oak recruits are negatively associated with overstory red maples and black cherries (Prunus serotina), while mesophytic recruits were positively associated with overstory oaks. Second, we found that proximity to a dead overstory tree increased growth and survival for black cherries, increased growth for red maples, but had no effect on oaks. Black cherries and red maples are therefore better suited than oaks to take advantage of canopy openings and the moderate light available under adult oaks. These same fine scale competitive processes are contributing to canopy oak replacement across eastern North America.