Sugar maple (Acer saccharum) age structure reveals limited establishment and development of age cohorts in response to selection management in northern hardwood forests

Single-tree selection (STS) silviculture has dominated management of northern hardwood forests (NHFs) in the Great Lakes region since introduction the 1960's. Periodic partial cuttings are assumed to promote and recruit well-stocked natural regeneration, resulting in balanced uneven-aged stand structure with sustainable harvest volumes. However, low sapling density of tree species desirable for management, including typically dominant and economically valuable sugar maple (Acer saccharum), suggests STS may not be effectively recruiting sugar maple in some regions. Few studies have analyzed sugar maple size-age structure to determine whether recruitment failure has been chronic since partial cutting began or is a more recent phenomenon. Here, we analyzed sugar maple age structure, regional patterns, and possible drivers for 51 selection-managed NHF stands located in northern Michigan, USA. Our data were 1499 aged basal discs collected from stumps of sugar maple trees >5 cm diameter at breast height (DBH) harvested winter 2017-2018. Our results offer little evidence to suggest that partial cuttings have promoted well-stocked ingrowth of sugar maple saplings. For most stands, age and diameter were non-linearly related, with a wide range of larger-diameter trees having similar ages. Among stands, sugar maple sawtimber (>24 cm DBH) averaged 106 years old and poletimber (11.4-24 cm DBH) 91 years, along with older than expected saplings (5-11.4 cm DBH) averaging 67 years. Most stands were dominated by a similar to 100-year-old upper canopy cohort, with a majority having <= 2 younger cohorts typically of low density and >50 years old. Areas with greater snow water equivalent and lower January minimum temperatures, where deer populations have been historically low, generally have younger sugar maple saplings. Low density and generally old age of smaller size classes led to much lower stocking of age classes <70 years old than expected under STS. Together, this suggests long-term STS management for sugar maple may be unsustainable over large portions of the study region if current trends in forest and deer management continue. Our age structure analysis supports the growing consensus that alternative management may be needed for NHF in parts of the Great Lakes region.

Automated summary

Single-tree selection (STS) silviculture may not effectively recruit sugar maple in northern hardwood forests (NHFs) in the Great Lakes region, leading to recruitment failure. Age structure analysis of sugar maple supports the need for alternative management methods in parts of NHFs in the study region.