Can natural disturbance-based harvest increase tree species diversity in a northern hardwood forest?

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Claudia I. Bartlick, Robert E. Froese and Christopher R. Webster, College of Forest Resources and Environmental Science, Michigan Technological University, Houghton, MI, Yvette L. Dickinson, Scion, Rotorua, New Zealand

Background/Question/Methods
Long-term implementation of uniform silvicultural systems in northern hardwoods is one example of how management can reduce species diversity. Widespread application of single-tree-selection in the Great Lakes region has favored sugar maple (Acer saccharum Marsh) as shade and leaf-litter tolerant species and caused regeneration failure in other species like yellow birch (Betula alleghaniensis Britton) and eastern hemlock (Tsuga canadensis Carrière). Although sugar maple is a valuable timber species, diversity loss leads to less resilient forest ecosystems, especially with future challenges like climate change and increasing spread of tree diseases. Implementation of natural disturbance-based management could be beneficial for biodiversity by emulating natural stand conditions that prevailed before the species decline. Can artificial soil disturbance and the removal of canopy trees help to create suitable seedbeds for declining tree species in a managed northern hardwood forest?
To address these objectives an experiment was established in 2017 with a gradient of seedbed and light environment including different forms of soil disturbance (control, artificial tip-up mounds, scarification) nested within different light environments (single-tree-selection, shelterwood-cutting with each 30% or 60% residual canopy, clear-cutting). In 2019 and 2020 seedling counts were taken and in addition environment factors like cover of coarse woody debris, bare mineral soil, and ground-layer vegetation were determined.
Results/Conclusions
Although evaluation of the data is still ongoing, preliminary results indicate that seedlings of the targeted species benefit from the artificially created tip-up mounds. Germination and early establishment seem linked to the availability of bare mineral soil as well as reduced ground-layer vegetation and accumulated leaf-litter. Soil scarification alone as a treatment does not appear to have that much of an impact on germination and establishment, probably due to leaf-litter that accumulated again after the treatment was applied. Light-availability does not seem to play as much of a role as expected, but the effect may be masked by factors like cover of ground-layer vegetation in particularly light-rich plots. As expected, sugar maple continued to dominate in the untreated controls. If further analysis confirms the early findings, interesting management implications may emerge. Winter harvest is often applied in northern hardwoods, which inter alia causes little soil disturbance. If our assumptions are confirmed, it could be worthwhile to supplement the current harvest regime with techniques that allow more soil exposure while still protecting soils from severe disturbances and compaction. Forest sites that are particularly affected by regeneration failure could benefit from the implementation of artificial seedbeds.