The intermediate disturbance hypothesis (IDH) postulates that maximum levels of diversity are maintained by disturbances occurring at the frequencies and severities that prevent both competitive exclusion by and extinction of late-seral species.  The IDH was originally articulated as a mechanism explaining high diversity of tropical forest and coral reef ecosystems; however, evidence suggests that such processes may operate in other ecosystems including northern hardwood forests.  Here we examine the role of the IDH using a large-scale, replicated, manipulative field experiment in a second-growth northern hardwood forest in north central Wisconsin.  Experimental treatments consisted of a range of gap sizes observed in old-growth forests (small = 50 m², medium = 200 m², large = 380 m²) and untreated controls.  We report on trends observed over three years prior to, and during the first three years following, treatment implementation (six years total).

Results/Conclusions

Plant species richness increased within large and medium gaps due to the immigration of primarily wind and bird-dispersed forb and shrub species with late-summer leafing phenology.  Increases in richness within gaps corresponded to increases in productivity and environmental heterogeneity.  Rates of immigration were greater within medium and large gaps than in small gaps and control plots.  Rates of extinction were lower than rates of immigration in gaps and did not vary among gap treatments, the closed canopy matrix and control plots.  Rates of immigration in medium and large gaps were greatest during the second growing season following gap creation. Our results suggest that moderate-severity disturbances resulting in medium-large canopy gaps in northern hardwood forests prevent competitive exclusion of wind-dispersed and seed-banking forb and shrub species while sapling recruitment and canopy closure may reduce the abundances of these species over time.  These disturbances increase both alpha diversity locally within gaps and beta diversity within the forest landscape.