Fungal inoculations and mechanical wounding of trees have limited efficacy for snag creation two decades after treatment

Abstract

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 use snag creation to enhance structural diversity, particularly in areas where snag loss is high and natural 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 applied to live Douglas-fir (Pseudotsuga menziesii) trees in two study areas near Coos Bay, in southwestern Oregon, USA. Treatments included chainsaw topping, fungal inoculation, topping + inoculation, and mechanical wounding at the base. Trees were revisited 18–20 y after treatment and nearly all focal trees (97.6%, n = 809) remained standing when relocated. 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, differentiated by the number of retained branch whorls. In contrast, limited decay was observed on trees subjected to fungal inoculation and mechanical wounding treatments, likely due to the slow pace of decay processes in injured trees. The same pattern held for both bark cover and cavity cover, the latter an index of created snag use by woodpeckers, a keystone group within forest ecosystems. Finally, adding fungal inoculation to chainsaw topping of trees led to little additional decay relative to chainsaw topping alone. These findings indicate that managers should choose snag creation methods that align with the timeframe they require decaying trees to be available to deadwood-dependent organisms. They also show that combining different snag creation approaches that vary in timing of tree mortality and decay is likely to provide a longer window of use by wildlife and require lower implementation costs. Although snag creation is a widespread management tool, additional research is needed to ascertain the effectiveness of snag treatments across space and time, and to quantify the tradeoffs between the ecological benefits and financial costs that result from intentional snag creation.