**5. Conclusions**

This paper reiterates the point that significant compaction measured as an increase in bulk density is related to soil water potential, which can be measured in the field with a handheld penetrometer. The value applies to a specific weight class of forestry machines. Heavier machines will compact soil to a higher bulk density and do so across a wider range of soil wetness. Hence, the applicable value of soil water potential will be lower because heavier machines can significantly compact soil at a lower soil water content.

Harvesting and soil trafficking resulted in a slight but statistically significant increase in bulk density after 1 year on these sites. Older mature forests are viewed as providing a deeper forest floor and mature canopy that protects natural processes that have tended to loosen soil over an extended period of time, which cannot be maintained after harvesting. The bulk density of all trafficked soil increased by the same small amount, which is attributed to soil rebound. These are natural soil mechanical and biophysical responses to a soil perturbation; they have always happened. Only the size and quality of our database allowed us to document them.

This project confirms that small, significant increases in bulk density can have site- and soil-specific ecological consequences. The ecological consequence on these sites was that compaction prolonged anaerobic soil conditions in soils with partially impaired drainage. The dynamics of the process indicate that the shift will be temporary. Reforestation will shift the water balance of these sites back to a less anaerobic environment, but in the interim, more aggressive forms of site preparation will be necessary to promptly reforest the sites.

Monitoring of air and soil temperatures found that subzero soil temperature is not indicative of a freeze–thaw process that could decompact soil. As a decompaction process, the freeze–thaw process is complex with exacting requirements of soil texture and mineralogy, soil permeability, temperature gradients in the profile, and external supply of water. These requirements were not met, and the bulk density of trafficked soil remained unchanged. A snow depth of 40 cm in these ecotypes effectively moderated soil temperatures with and without an intact forest floor.

Recent changes simplifying the measurement of bulk density using the core method and videos showing the ease with which sample rings can be hammered into the ground are misleading. The collection of high-quality core samples for bulk density and other physical tests has never been easy. When bulk density is used as an indicator of soil health, monitoring the sustained impact of soil compaction on ecosystem services, and soil carbon storage, the precision of the values of bulk density obtained by the core method becomes paramount. The protocol for the collection of bulk density using the core method must be updated with an emphasis on the use of better practices and specifications for the core method and a rigorous commitment to a quality control and assurance program in the field.

**Author Contributions:** Conceptualization, D.H.M.; methodology, D.H.M.; formal analysis, A.S.; investigation, all; resources, D.H.M.; data curation, A.S.; writing, D.H.M.; visualization, D.H.M.; supervision, all; project administration, all; funding acquisition, all. All authors have read and agreed to the published version of the manuscript.

**Funding:** The data collection and original publications were performed while both authors were employed by the Alberta Environmental Centre, Alberta Environment at Vegreville, Alberta, which became part of the Alberta Research Council toward the end of the data collection period. Research sites and forest harvesting equipment for the field trials were provided by Canadian Forest Products Ltd., Weldwood of Canada Ltd. (Hinton Division), Weyerhaeuser Canada Ltd. (Grande Prairie), Sundance Forest Products Ltd., Sunpine Forest Products Ltd., Millar Western Industries Ltd., and Alberta Newsprint Company. These companies also provided direct funding for the project via the Alberta Forest Development Trust, Government of Alberta. Funding for the first 3 years was also provided by Foothills Model Forest (now fRI Research), Hinton, Alberta. The analyses and preparation of a draft report of these data were initially made possible by a gran<sup>t</sup> from the Forest Resources Improvement Association of Alberta (FRIAA), Open Funds Initiative, Government of Alberta, to ForestSoil Science Ltd. Final paper and interpretations are personal contributions.

**Institutional Review Board Statement:** Not applicable.

**Data Availability Statement:** Data may be available from the senior author. **Acknowledgments:** The authors thank Susan Paquin and Rod Kusiek for their technical and laboratory support and as crew leaders for the collection of soil samples, and the field crew; Michelle Hiltz for advice on statistical analyses; and reviewers.

**Conflicts of Interest:** The funders had no role in the design of the study; in the collection, analyses, and interpretation of data; in the writing of the manuscript; or in the decision to publish the results. DM has also done research and development on soil restoration implements and practices, which has resulted in Canada Patent No 2586933, and U.S. Patent No 8176993 B2, a 'Ripper Plough for Soil Tillage'.
