Linking radial growth patterns and moderate-severity disturbance dynamics in boreal old-growth forests driven by recurrent insect outbreaks: A tale of opportunities, successes, and failures.

Maxence Martin,Hubert Morin,Cornélia Krause

doi:10.1002/ece3.7080

Abstract

In boreal landscapes, emphasis is currently placed on close‐to‐nature management strategies, which aim to maintain the biodiversity and ecosystem services related to old‐growth forests. The success of these strategies, however, depends on an accurate understanding of the dynamics within these forests. While moderate‐severity disturbances have recently been recognized as important drivers of boreal forests, little is known about their effects on stand structure and growth. This study therefore aimed to reconstruct the disturbance and postdisturbance dynamics in boreal old‐growth forests that are driven by recurrent moderate‐severity disturbances. We studied eight primary old‐growth forests in Québec, Canada, that have recorded recurrent and moderately severe spruce budworm (Choristoneura fumiferana [Clem.]) outbreaks over the 20th century. We applied an innovative dendrochronological approach based on the combined study of growth patterns and releases to reconstruct stand disturbance and postdisturbance dynamics. We identified nine growth patterns; they represented trees differing in age, size, and canopy layer. These patterns highlighted the ability of suppressed trees to rapidly fill gaps created by moderate‐severity disturbances through a single and significant increase in radial growth and height. Trees that are unable to attain the canopy following the disturbance tend to remain in the lower canopy layers, even if subsequent disturbances create new gaps. This combination of a low stand height typical of boreal forests, periodic disturbances, and rapid canopy closure often resulted in stands constituted mainly of dominant and codominant trees, similar to even‐aged forests. Overall, this study underscored the resistance of boreal old‐growth forests owing to their capacity to withstand repeated moderate‐severity disturbances. Moreover, the combined study of growth patterns and growth release demonstrated the efficacy of such an approach for improving the understanding of the fine‐scale dynamics of natural forests. The results of this research will thus help develop silvicultural practices that approximate the moderate‐severity disturbance dynamics observed in primary and old‐growth boreal forests.

Highlights

Anthropogenic activities over the last centuries have increased pressure on forest ecosystems, causing a significant loss of natural forest areas (Aksenov et al 1999; Achard et al 2009; Watson et al 2018)
This study focused on the boreal forests of eastern Canada, and within a territory subjected to moderate-severity secondary disturbances caused by spruce budworm outbreaks over the 20th century (Morin, 1994; Morin & Laprise, 1990; Navarro, Morin, Bergeron, & Montoro Girona, 2018)
This study aims to reconstruct the disturbance and post-disturbance dynamics in boreal old-growth forests driven by moderate-severity disturbances

Summary

Introduction

Anthropogenic activities over the last centuries have increased pressure on forest ecosystems, causing a significant loss of natural forest areas (Aksenov et al 1999; Achard et al 2009; Watson et al 2018). The desire to reduce differences between managed and natural forests has led to a heightened focus on old-growth forests—forests in the final stage of forest succession, driven by secondary disturbances (Oliver and Larson 1996; Wirth et al 2009). These forests are often the most threatened by human activities, their area greatly reduced through deforestation and intensive forest management (Potapov et al 2017; Grondin et al 2018; Martin et al 2020a). These structural features provide essential habitats for many species (Winter and Möller 2008; Boudreault et al 2018; Tremblay et al 2018)

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