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

Abstract

Managing forests for resilience is crucial in the face of uncertain future environmental conditions. Because harvest gap size alters the species diversity and vertical and horizontal structural heterogeneity, there may be an optimum range of gap sizes 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 replicated 12 times with over a decade of repeated measurements. We found that germinant (trees<0.15m tall) densities were greatest in year 2 (71.6stemsm−2) but were present in all measurement years (0, 2, 6, and 12years post-harvest), suggesting a continuous influx of tree seedlings on site. Sapling (>0.6m tall) densities were greatest (3.9stemsm−2) at the end of the study period (year 12) and were composed primarily of three species (Acer saccharum, Fraxinus americana, Ostrya virginiana) already present on site. The 6 and 10m diameter (dia.) harvest gaps had, on average, the minimum number of saplings (∼2stemsm−2) needed to fill dominant positions in small canopy openings, but, by year 12, small gaps were closed by crown extension of mature edge trees. Medium (20m dia.) to large harvest gaps (30 and 46m dia.) had fewer saplings than small gaps. The understory layer of medium and large gaps were dominated by shrubs (Rubus sp.) that dampened growth of species such as F. americana. Consequently, gap-filling from the sapling layer 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 optimum gap size does not exist and using harvest gaps to create resilient forests with structural and compositional diversity alone is challenging on productive habitats. In particular, medium to large gaps will likely require microsite specific placement of desired advance regeneration or additional treatments (e.g., vegetation control) to cultivate a dense sapling layer in an acceptable timeframe after harvest.