Harvested opening size affects cohort development and failures in a second-growth northern hardwood forest
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Forest Ecology and Management
Group selection is one potential tool in contemporary, uneven-aged silviculture for mitigating increased dominance by shade-tolerant species while continuing to meet stocking goals and harvest quotas. However, recent experimental group-selection openings in northern hardwood forests on some sites in the Upper Great Lakes region have failed to meet management goals of increased tree diversity and timely natural regeneration. Few observational studies have documented the long-term stand development of nascent cohorts following group-selection prescriptions. This study revisits a long-term group-selection experiment which previously found that opening size had little effect on regeneration diversity and found that large areas devoid of saplings and dominated by understory vegetation were common more than a decade following harvest. Our goals were to reassess the state of stand development and diversity within harvested openings, as well as look for early postharvest indicators of regeneration success or failure after 23 years. At 23 years post-harvest, we found that opening size had little effect on regeneration diversity but rather influenced patterns of cohort development. Smaller openings (6–20 m diameter) showed signs of self-thinning, while larger openings (30–46 m diameter) showed divergent developmental patterns including both self-thinning and ongoing sapling recruitment. Most areas of regeneration failures persisted through 23 years, and the largest openings (46 m diameter) were most likely to display regeneration failure, with 42% of subplots devoid of saplings but containing dense understory vegetation. The density of understory vegetation at 2 years post-harvest had no effect on year-23 sapling density. Instead, only seedling/sapling density in year 2 correlated with regeneration success after 23 years. These results suggest that in northern hardwood forests in the upper Great Lakes region, opening creation alone may not produce the desired effects of increased canopy diversity and sustained timber yields. Canopy openings >30 m diameter appear more likely to result in partial, multi-decade regeneration failures compared to smaller openings; however, our results highlight the importance of advance regeneration for preventing these failures. Thus, simultaneously achieving management goals of increased tree diversity and adequate regenerative stocking with group selection likely requires additional inputs (e.g. herbivore protection, planting, scarification, herbicides, repeated thinning, etc.) beyond the current status quo.
Knapp, Samuel P.; Kern, Christel C.; Webster, Christopher R. 2021. Harvested opening size affects cohort development and failures in a second-growth northern hardwood forest. Forest Ecology and Management. 482: 118804. 10 p. https://doi.org/10.1016/j.foreco.2020.118804.