Many forests in eastern North America are undergoing rapid successional change as they regrow from abandoned agricultural land and experience different disturbance regimes from those pre-European colonization. One striking pattern in some of these forests is the replacement of oak trees, which had been canopy dominants, by mesic, shade-tolerant, fire-sensitive species like red maple. The processes responsible for the change have been proposed, but rarely explicitly measured. Here we measure the strength and scale of two of these processes: (i) how the canopy density of oaks and their competitors affects the recruitment of those species, and (ii) how those species differentially respond to canopy openings. We analyze data from a 23ha forest research plot which has been censused three times over the past decade to measure these processes.
For process (i) we find that oak recruits are negatively associated with canopy red maples and black cherries, while recruits of those species are positively associate with canopy oaks. The scale of this association is measured with an inhomogeneous bivariate pair correlation function and compared to the expectation under a simple null dispersal model for the the recruits. For process (2) we find that proximity to a dead canopy tree increased survival and growth for black cherries, increased survival and marginally increased growth for red maples, but had no effect on oaks. So black cherries and red maples are better suited than oaks to take advantage of canopy openings. Taken together these results give explicit measures of two processes responsible for the replacement of oaks in forests where they were previously canopy dominants.