Harvest-created canopy gap size influences niche partitioning of the ground-layer plant community in a northern hardwood forest

Background/Question/Methods

The Gap Partitioning Hypothesis (GPH) states that gaps in the forest canopy create variation in understory microclimate and resources that facilitate tree species coexistence.  Yet, empirical support for the GPH is mixed for tree species composition. We used a ground-layer plant community of a temperate deciduous forest in northern Wisconsin, U.S.A, as a model system to test whether the GPH extends to the relatively species-rich ground layer.  Specifically, we used an experimental approach to isolate how gap size related to ground-layer plant composition and whether the resulting composition provided evidence for niche partitioning.  Our well-replicated experimental design included a gap opening gradient (five gap sizes, 6–46 m diameter, and undisturbed reference areas), a within-gap location gradient (gap edge to center), and a temporal gradient (0, 2, 6, and 13 years after gap creation). 

Results/Conclusions

Ground-layer plant composition differed across gap sizes, within gap location, and over time.  Small gaps (6 to 10m dia.) did not differ in composition but intermediate to large gaps (20–46m dia.) differed from small gaps and among each other, suggesting a minimum threshold to detect gap partitioning in this model system.  Large gaps also displayed within-gap compositional gradients from gap edge to center locations (from 2.5 m to 17.5 m).  Compositional differences in gap size were evident two years after gap creation and grew more pronounced over the 13 year period.   Species’ functional traits and microenvironmental conditions were related to variation in ground-layer composition.  Species with smaller seeds, lower shade tolerance, later bloom times, shorter stature, and longer leaves were associated with higher light, more central gap locations, larger gap sizes, and greater time since gap creation.  The correlation between gap size and ground-layer plant composition suggests that the GPH concept is applicable for this forest layer and that the GPH framework can be used to achieve management goals in community composition and functional diversity at larger regional scales.