Indicators of site sensitivity to the removal of forest harvest residues at the sub-continental scale: Mapping, comparisons, and challenges

Abstract Many jurisdictions have put forward guidelines to identify sites at risks of soil degradation with the extraction of logging residues. Most guidelines are based on expert opinion and use the precautionary principle because of the lack of strong understanding of what makes a site sensitive to intensive biomass removal. Two main approaches are used: 1-identifying thresholds for specific site properties 2- developing nutrient budget indicators to rate the potential for nutrient deficit. Thanks to the development of digital soil mapping, it is becoming easier to develop maps for such indicators. A crucial question is the reliability of the different indicators. One way of evaluating their reliability is to test the coherence between the geographic locations of sites rated as sensitive by different indicators. In this study, we developed maps of key soil properties and of biogeochemical fluxes for the managed forest land of Canada at a resolution of 250 m. We used three site properties (slope, pH and sand content), as well as three nutrient budget indicators (N Budget, Base cation (BC) Budget and N Stability ratio) that were mapped and compared. The results indicated very little overlap between the areas identified as sensitive by the three site property indicators and very little concordance between sites rated as sensitive by site property indicators and those identified as sensitive by nutrient budget indicators while nutrient budget indicators showed coherence among themselves. Because nutrient budget indicators were found to be more dependent on the amount of nutrient extracted in the harvested biomass, than on the rates of nutrient inputs from the soil or the atmosphere, they tended to rate productive sites as sensitive, to the opposite of site property indicators. These results suggest that different indicators are assessing different processes and different aspects of site sensitivity. Because of the lack of coherence amongst the indicators, it is advisable 1) to use indicators based on the results of long-term monitoring plots, 2) to maintain such long-term observations and 3) to leave on the ground a substantial proportion of harvest residues, especially on sites evaluated as sensitive.