Secondary disturbances of low and moderate severity drive the dynamics of eastern Canadian boreal old-growth forests

Map of the study area. Black dots represent the sample sites, and blue lines and polygons represent rivers and standing waterbodies, respectively. The inset maps indicate the position of the study area map in Canada (left) and in the province of Quebec (right)

Both low- and moderate-severity secondary disturbances are drivers of eastern Canadian boreal old-growth forests dynamics. Moderate-severity disturbances reflect mainly spruce budworm outbreaks. Low-severity disturbances are produced by both spruce budworm outbreaks and random events such as windthrow. Each level of disturbance severity has a specific impact on stand dynamics, and both shape the diversity of boreal old-growth forests.

Context A regular succession of low-severity disturbances is seen as determining the dynamics of the old-growth stage (gap dynamics); however, recent studies suggest that moderate-severity secondary disturbances also play an important role in the dynamics of eastern Canadian boreal forests.
Aims This study aims to determine if eastern Canadian boreal old-growth forests are driven by a combination of low- and moderate-severity secondary disturbances.
Methods We reconstructed the 200-year disturbance history of 20 boreal old-growth stands using dendrochronological analysis. We discriminated low- from moderate-severity disturbances based on their respective influence on mean stand growth.
Results The secondary disturbance regime of eastern Canadian boreal old-growth forests varies highly over time, reflected by disturbance peaks in the chronological record. Most peaks occurred during spruce budworm outbreaks related to both low- and moderate-severity disturbances. Between each peak, low-severity disturbances dominate. Each level of disturbance severity has specific consequences for stand dynamics.
Conclusion Both low and moderate secondary disturbances are drivers of forest dynamics in eastern Canadian boreal old-growth stands and shape the structural diversity of these stands. The complexity of these dynamics should be recognized in management planning to ensure the efficiency of old-growth forest conservation policies.

Low severity, Moderate severity, Natural disturbance, Succession, Spruce budworm, Windthrow

Martin, M., Morin, H. & Fenton, N.J. Annals of Forest Science (2019) 76: 108.

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Data availability
The datasets generated and/or analyzed during the current study are available from the corresponding author on reasonable request.

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