Soil Nitrogen Leaching in Logged Beetle‐Killed Forests and Implications for Riparian Fuel Reduction

Abstract

Recent extensive forest mortality and subsequent salvage logging may threaten the water supply from headwater catchments in western North America. Land managers confronting current insect outbreaks and projected increases in forest disturbance require information about the potential water quality consequences of these changes. This study is a hillslopescale evaluation of soil N and C leaching in lodgepole pine forests that have experienced 80% overstory tree mortality. I measured extractable inorganic and ion resin exchangeable-N forms, in situ net mineralization and nitrification, and leaching in upland and riparian forests with the following treatment combinations: (i) uncut upland with uncut riparian, (ii) harvested upland with uncut riparian buffer, and (iii) harvested upland with harvested riparian. This design permitted comparison of N cycling and leaching in 30-m-wide riparian buffers and a riparian fuel break designed to remove canopy fuels and reduce wildfire concerns in areas with extensive bark beetle infestation. Harvesting increased NO 3 -N, total dissolved N, and dissolved organic C leachate concentrations in upland landscapes but had little effect on net N transformations. Leachate N and C concentrations were 1.5 times higher in riparian buffers downslope of harvested uplands than those in riparian zones downslope of uncut uplands. Riparian forest harvest increased N leaching relative to uncut buffers, although postharvest concentrations remained well below regional water quality standards. Thus, while this study provides evidence that N leaching from dead buffers is low, it also suggests that riparian fuel reduction may complement wildfire mitigation objectives without compromising watershed protection.

Current Insights into the Effectiveness of Riparian Management, Attainment of Multiple Benefits, and Potential Technical Enhancements

“…Hence, gains for forested buffers were relatively small when considering the investments for tree planting and fencing. In terms of managing forested buffers, Rhoades (2019) found that riparian harvesting significantly increased dissolved organic C (DOC), total dissolved N, and NO 3 leaching when this harvesting was performed in beetle‐infested stands (where >80% of trees were already dead). However, these postharvest effects were mediated by rapid regeneration of herbaceous vegetation and aspen shoots.…”

“…However, the implications of this management prescription on biodiversity require further research. Harvesting management is also important where buffer zone vegetation comprises trees (Dal Ferro et al, 2019; Zak et al, 2019; Rhoades, 2019) or other biomass (Brown et al, 2019). These issues highlight the important role that clear, robust agricultural and environmental policy can play a part in influencing management and determining the effectiveness of riparian buffer zones.…”

“…It was earlier also noted by Ferrarini et al (2017) that the limited operating space for machinery was an issue for bioenergy buffers (as opposed to larger‐scale plantations) and that spatial fragmentation across and between farms added to biomass handling and processing costs. This aspect of mechanized harvesting was also considered by Rhoades (2019) in managing mature forest dead wood in a 30‐m‐wide stream buffer zone where either specialist machinery (beam cutters) or labor‐intensive manual cutting and winching out were used to protect the stream‐side zone. The interactions of harvesting with biodiversity have been less studied here and remain a gap in setting management guidance.…”