The influence of cutting cycle and stocking level on the structure and composition of managed old-growth northern hardwoods

Abstract

Forest management approaches are increasingly being focused on restoring or retaining structurally complex old forest conditions. Evaluations of the long-term impacts of different management regimes on the maintenance and restoration of these conditions are critical for informing and refining current management approaches. To this end, we examined the long-term effects of differing intensities of single-tree selection harvest on stand structural attributes within northern hardwood forests in the Upper Peninsula of Michigan, USA. Stands in this study were classified as old growth at the onset of the experiment and were repeatedly treated and maintained at three residual stocking levels (11.5, 16.1, and 20.7 m 2 ha −1 in trees greater than 24 cm DBH) with three different cutting cycle lengths (5, 10, and 15 years) over a 57-year period. Conditions within an adjacent, unmanaged old-growth system were used as a benchmark for comparing stand structural attributes, including abundance of downed woody debris (DWD), regeneration, and large diameter trees. Stand stocking level was the primary factor affecting the long-term trends in stand structure and composition observed among stands. In particular, stands maintained at a 11.5 m 2 ha −1 stocking level had significantly lower amounts of DWD, snags, and large overstory trees compared to the 20.7 m 2 ha −1 stocking level. Similarly, the highest stocking level treatment had the greatest abundance of Betula alleghaniensis relative to the other two stocking level treatments. In addition, long-term trends in diameter distribution forms indicated that the two highest stocking level treatments largely displayed increasing q and negative exponential curve forms over time, whereas the 11.5 m 2 ha −1 stocking level was primarily characterized by concave distribution forms. Ordination analyses of stand structural attributes indicated that stands managed with the highest residual stocking levels contained stand structural characteristics approximating those found within the unmanaged, old-growth benchmark stand, whereas the lowest stocking level was much less complex. These results suggest that single-tree selection systems that maintain high stand residual basal areas, a portion of which is made up of large diameter, below-grade, defective trees, could serve as a potential means to integrate or retain old-growth forest elements in managed stands.