Forest-landscape structure mediates effects of a spruce bark beetle (Dendroctonus rufipennis) outbreak on subsequent likelihood of burning in Alaskan boreal forest

Abstract

Characterizing how variation in forest landscape structure shapes patterns of natural disturbances and mediates interactions between multiple disturbances is critical for anticipating ecological consequences of climate change in high-latitude forest ecosystems. During the 1990s, a massive spruce bark beetle (Dendroctonus rufipennis) outbreak took place in boreal spruce forest on the Kenai Peninsula, Alaska allowing us to ask (1) How did the extent and duration of bark beetle outbreak differ between a homogenous landscape dominated by white spruce (Picea glauca), and a landscape in which white spruce and black spruce (Picea mariana) were intermixed? (2) How has the occurrence and duration of bark beetle outbreak influenced the likelihood of subsequent burning in these two landscapes? Forest landscape structure had a substantial effect on disturbance patterns and interactions between disturbances in this study. The spruce bark beetle outbreak was smaller in extent and duration where white spruce, the beetle’s primary host tree, was intermixed with more beetle-resistant black spruce. However, likelihood of subsequent burning increased where outbreak did occur. Surface fuel loads increased substantially in this landscape following the outbreak, potentially increasing the flammability of white spruce where they once served as fire breaks. In contrast, the outbreak was larger and lasted longer in the landscape with homogeneous stands of white spruce, but was not related to likelihood of subsequent burning, which is consistent with the fire history. Our results suggest that bark beetle outbreaks may have different effects on subsequent patterns of burning than in other systems, such as the Rocky Mountains. These results could inform more effective and targeted management strategies to ameliorate fire risk in beetle-killed stands of Alaska and may help us anticipate the dynamics and consequences of future boreal bark beetle outbreaks as climate warms at high latitudes.