Diversifying the composition and structure of managed, late-successional forests with harvest gaps: What is the optimal gap size?

Background/Question/Methods

Managing forests for resilience is crucial in the face of uncertain future environmental conditions.  Because harvest gap size alters the species diversity and the vertical and horizontal structural heterogeneity, there may be an optimum gap size for conferring resilience to environmental uncertainty.  We examined the impacts of different harvest gap sizes on structure and composition in northern hardwood forests of the upper Great Lakes region, USA using a robustly-designed experiment consisting of six harvest gap sizes replicated12 times with over a decade of repeated measurements.  We asked the following questions: How does gap size influence tree seedling recruitment over time? How does gap size influence tree sapling composition and diversity over time?  How does gap size influence sapling layer height development and future canopy composition?

Results/Conclusions

We found that germinant (trees <0.15 m tall) densities were greatest in year 2 (71.6 stems m^-2) but were present in all years (0, 2, 6, and 12 years post-harvest), suggesting a continuous influx of tree seedlings on site.  Sapling (>0.6 m tall) densities were greatest (3.9 stems m^-2) in year 12 and were composed primarily of three species already present on site.  The 6 and 10 m diameter (dia.) harvest gaps had, on average, the minimum number of saplings (~2 stems m^-2 ) needed to fill small canopy openings, but, by year 12, small gaps were closed by crown extension of mature edge trees.  Medium (20 m dia.) to large harvest gaps (30 and 46 m dia.) had fewer saplings than small gaps. The understory layer of medium and large gaps was dominated by shrubs that dampened growth of some saplings. Consequently, gap-filling by saplings would not occur in the near future, and, from a management perspective would be considered a regeneration failure within larger gaps.  Our study suggests that the perfect gap size does not exist and using harvest gaps to create resilient forests with structural and compositional diversity alone is challenging on productive habitats.