Contemporary wildfires further degrade resistance and resilience of fire-excluded forests

Abstract

In fire-dependent forest landscapes, frequent low- to moderate-severity fire maintained vegetation patterns that limited the severity of droughts, wildfires, and insect and pathogen activity. More than a century of fire exclusion, in combination with intensive timber management, has altered these spatial patterns and eroded resistance. Today’s much denser forests and highly connected fuel loads are increasingly vulnerable to drought and fire, especially under a rapidly warming climate. Current policies favor continued fire exclusion and suppression. Consequently, most area burns at the peak of the fire season when hot, dry, and often windy conditions foster uncontrollable high-intensity wildfires in today’s fire-excluded forests. As a result, high-severity fire effects are replacing the once abundant influence of low- to moderate-severity fire. For three subwatersheds in central Oregon, we assessed the vulnerability of contemporary conditions to drought and fire by quantifying their departure from the conditions that continue to demonstrate greater resistance and resilience today. We also compared contemporary conditions on recently (1984–2016) burned and unburned sites to evaluate the potential of contemporary peak-season wildfires to foster a diversity of forest and nonforest conditions in fire-excluded landscapes. In the early 20th-century, extensive high-severity fire effects were relatively common (4–20% of area) in mesic forests, but essentially absent (0–1% of area) in intermediate and dry forests. In recent wildfires, however, high-severity fire effects were evenly distributed (6–7% of area) across forest zones. After these fires, nonforest cover was more abundant and in larger patches than in the early to mid-20th century. Until at least the mid-20th century, open-canopy overstories with medium and/or large (>40 cm dbh) trees strongly dominated forest cover (>75% of area) in mesic to dry forests. By 2016, overstory cover dominated by medium and/or large trees was less than half as common, and trees > 63.5 cm dbh were essentially absent on recently burned sites. Open-canopy forest dominated by medium and large ponderosa pine trees has declined substantially since the mid-20th century, especially on recently burned sites. These changes demonstrate the magnitude and extent of departure of fire-excluded forests and landscapes from the conditions that continue to foster resistance and resilience to drought and fire today. In fire-excluded landscapes, peak-season wildfires compound the legacy of past management and further reduce options for conserving or restoring cultural, social, and ecological functions of multi-aged forests.