By what mechanisms do early-successional tree species such as pines persist in late-successional forests in regions where recurrence intervals for stand-replacing disturbance are much longer than maximum tree life spans? How are recruitment patterns affected by habitat variation?
Old-growth, mixed species stands of Pinus strobus, Pinus resinosa, Tsuga canadensis and Acer saccharum at 36 locations in three subregions of northeast Wisconsin and west upper Michigan, USA.
Four to nine stands were located in each of six habitats and sampled in 200-m2 plots. Increment cores were collected from 1456 trees, and charcoal collected and dated to characterize stand age structure and disturbance history. Stands on upland mesic sites, with strong successional trends toward late-successional species, were compared to those on ‘marginal’ habitats (rock outcrops, sandy inclusions and lake, river and wetland edges), where competition is comparatively low. We developed a new methodology that utilizes uncertainty in establishment dates to more rigorously distinguish even-aged stands (the null hypothesis) from those with multiple age classes.
Analysis of tree germination dates suggests that some stands with an apparently broad range of pine ages primarily reflect uncertainty in age estimates rather than actual periods of extended pine recruitment. Nevertheless, multi-aged populations of pine (rejection of the even-aged null hypothesis) were found in 86% of the stands and in all habitats. Marginal habitats had more age classes than mesic sites (mean 2.8 vs 1.7; P = 0.007) and greater age diversity. Disturbances that initiated pine cohorts were more often severe and more often caused by fire on mesic sites than on marginal habitats. Pine establishment was somewhat episodic in all three subregions, but age-class peaks across subregions had limited synchrony and were not strongly correlated with severe drought.
These findings are consistent with growing evidence that age structures of early-successional tree species in primary forests are often more complex than commonly believed, and that population structure and persistence are strongly influenced by habitat variation. Pine age structures provide indirect support for ‘refuge-based’ landscape persistence, with dispersal limitation overcome by persistent seed sources in nearby marginal habitats.