Fifteen-year tree growth on standard long-term soil productivity trials and various adjacent amelioration treatments at Interior Cedar-Hemlock sites in southeastern British Columbia and northern Idaho

Abstract Understanding the relationship between soil disturbance and tree growth is key to sustainable forest management. We report fifteen-year tree growth results of Douglas-fir (Pseudotsuga menziesii var. glauca [Beissn.] Franco) and western white pine (Pinus monticola Dougl. ex D.Don) growth following clearcut harvesting and installation of three replicates of the international Long-Term Soil Productivity (LTSP) trial protocols. Additional amelioration plots were also established on each replicate site to study rehabilitation of the most severe disturbance, treatment of Armillaria root disease, and enhancement of soil organic matter. Stark differences were found among the various treatments and species. Vegetation control resulted in significantly better growth for both tree species across the core LTSP treatments and amelioration treatments. On the core LTSP treatments, organic matter removal had a significant negative effect on Douglas-fir growth. Amelioration treatments resulted in similar survival and similar or better volume growth than the core LTSP treatments, with the exception of poor volume growth after compaction rehabilitation with no organic matter returned into the soil. Compaction significantly influenced Douglas-fir survival and was often higher on moderate compaction plots. On some sites there was decreased survival and growth associated with heavy compaction that will likely influence longer-term outcomes with a more variable and changing climate. Overall, results to date on the core LTSP treatments, amelioration treatments, and adjacent miniplots reported elsewhere suggest that forest floor removal causes negative growth and, combined with severe compaction, caused both negative growth and poor survival. In the Interior Northwest it is still prudent to minimize soil disturbance and leave organic matter onsite unless there is site specific evidence to the contrary. Additional work is required to reap the full, long-term benefit of the installations. Recommendations are provided highlighting the critical work that should be completed in the near future before biological evidence is lost to decay, including suggestions for further data analysis and extension, the need to continue LTSP core, amelioration and miniplot measurements to demonstrate and calibrate sustainable forest resource management. Our data suggest considering changes to current soil disturbance policy and forest operations based on the current results regarding forest floor displacement.