Disturbance history and its influence on drought tolerance of interior Douglas-fir (Pseudotsuga menziesii var. glauca (Beisn.) Franco) in the Cariboo-Chilcotin region of British Columbia, Canada

Disturbance—the death of trees due to external forces such as wildfire or windstorms—drives forest stand dynamics and shapes ecosystems. Natural disturbances arising from the interaction of climate, topography, and established tree species have often occurred with some regularity, resulting in apparently perpetual renewal of particular forest structures. Minor changes in climate can have profound impacts on these disturbance regimes, breaking historically observed cycles and introducing novel stand conditions. Long-term historical baselines are critical to understanding such changes. Observational records are often inadequate, especially in western North America, where 400 year-old stands are common but reliable data are generally unavailable prior to the 20th century. I use tree ring analysis to investigate the history of Douglas-fir beetle and western spruce budworm infestations, and the influence of partial disturbances on the drought tolerance of surviving trees, developing baseline understanding of disturbance interactions in interior British Columbia. No evidence is found of any outbreaks of western spruce budworm or Douglas-fir beetle that exceed the magnitude of outbreaks in the early 21st century, suggesting that recent outbreaks represent historically high levels of insect activity. Both natural and anthropogenic partial disturbances are demonstrated to positively affect the drought tolerance of surviving trees in old-growth remnants and younger managed stands, respectively. Access by roots to areas with uninterrupted precipitation throughfall appears to be the driving force behind observed drought resistance, and will likely become more important under climate scenarios where droughts are prolonged and intensified by warmer temperatures. A major growth release ca. 1800 brought previously suppressed trees from a number of age classes into preeminence within their respective stands across the study area, forming the basis of the structure we now consider to be representative of old forests in the region. I propose that mountain pine beetle is the leading cause of this regionally synchronous growth release, based on the number of sites affected and the survival of sapling-sized Douglas-fir. Silvicultural prescriptions designed to provide open growing space to residual trees may help reverse overstocking resulting from wildfire exclusion and enhance resilience of stands within the timber harvesting area to increased temperatures.

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