Fungal Inoculations and Mechanical Wounding of Trees Have Limited Efficacy for Enhancing Structural Diversity Two Decades after Treatment

Standing dead trees (snags) support multiple functions within forest ecosystems by providing vertical structure, contributing to nutrient flows and carbon cycling, and serving as habitat elements for a diversity of organisms. In many forest landscapes, managers often enhance structural diversity via the creation of snags, particularly in areas where snag loss is high and snag recruitment is low. Despite snag creation being used across a range of forest types on both private and public lands, a dearth of long-term studies has led to uncertainty about which techniques work best to create snags and support deadwood-dependent organisms over long (>15 y) timescales. In this study, I assessed the long-term consequences of varied snag creation treatments, encompassing chainsaw topping, fungal inoculation, topping + inoculation, and mechanical wounding at the base, which were applied to live Douglas-fir (Pseudotsuga menziesii) trees located in two study areas near Coos Bay, in southwestern Oregon, USA. Trees were revisited 18-20 y after treatment and the great majority of relocated trees (97.6%, n = 809) remained standing in both study areas. Markers of decay – including whether a tree was broken, cracked along the bole, had peeling bark, or harbored shelf fungi – were most pronounced on trees that had experienced one of two chainsaw topping treatments. In contrast, limited decay was observed on trees subjected to fungal inoculation and mechanical wounding, likely due to the slow pace of decay processes in inured trees. The same pattern held for both bark cover and cavity cover, the latter being an index of prior use by woodpeckers, a keystone group within forest ecosystems. Finally, treatments that were a combination of inoculation and chainsaw topping led to little additional increase in decay relative to chainsaw topping alone. This study indicates that managers should choose snag creation methods that align with the timeframe that they require decaying trees to be available to deadwood-dependent organisms. It also shows that combining different snag creation methods that lead to differences in the timing of tree mortality and decay is likely provide a longer window of use, which has the additional benefit of lower implementation costs stand. Despite snag creation being widespread management tool, additional research is needed to ascertain how snag treatments vary across space and time, and to develop a contemporary understanding of the trade-offs between the ecological benefits and financial costs that stem from implementing snag creation treatments.