Interactions between large high-severity fires and salvage logging on a short return interval reduce the regrowth of fire-prone serotinous forests

Abstract

New fire disturbance regimes under accelerating global environmental change can have unprecedented consequences for ecosystem resilience, lessening ecosystem natural regeneration. In the Mediterranean Basin, fire-dependent obligate seeder forests that are prone to increasingly frequent stand-replacing fires and then salvaged logged repeatedly can be vulnerable to additional disturbances for decades. In this study, we investigated, for the first time, the cumulative and interactive effects of two large high-severity fires at a short (<15-year) return interval and the subsequent burned timber harvesting with biomass removal on the post-disturbance recovery of such forests. We further assessed the type and amount of the material legacies (deadwood) that persisted through the different post-disturbance successional trajectories, as well as the influence of these legacies on forest regeneration. The early recovery of the studied forests after two consecutive large fires and post-fire logging was, in the first place, driven by fire repetition, which led to reduced seedling recruitment and enhanced regrowth of resprouter shrubs. Despite no interactive effects between fire and logging were detected after a single large fire event, two repeated fires followed by salvage harvesting had a greater negative impact than two fires alone (synergistic effects) on seedling establishment; while a lower positive impact (subadditive effects) on the recovery of resprouter shrubs. There was also an interaction modification effect in which fire repetition worsened the per-unit impact of salvage logging on forest regeneration. Nonetheless, the residual legacies, i.e., fine and coarse woody debris (unburned needles, downed branches, pieces of deadwood, and burned pine cones) that remained after the manual harvesting of the burned trees, aided seedling re-establishment and hindered the regrowth of the shrubby understorey. These findings indicate that high-intensity salvage logging after two large high-severity fires at a short return interval is inadvisable in fire-prone serotinous pine forests, unless it explicitly retains the key material legacies that help tree natural regeneration and enhance ecosystem resilience to the next disturbance.

Ecological consequences of compound disturbances in forest ecosystems: a systematic review

“…Similarly, Knapp and Ritchie (2016) attributed reduced shrub cover and richness to mechanical impacts of salvage logging, but noted that, of the multiple functional groups and life-history categories assessed, only shrubs were significantly impacted. Taboada et al (2018) reported that, after a single fire, salvage logging did not impact forest recovery, but with two fires, reduced seedling establishment was amplified by salvage logging. Soil nutrient levels and ecologically important exchangeable cations were reduced as well (Bowd et al 2019).…”

“…For instance, reduced postfire regeneration was attributed to seed source reduction by a preceding fire (Taboada et al 2018), bark beetle outbreak (Harvey et al 2013), and clearcut logging operation (Ton and Krawchuk 2016). For instance, reduced postfire regeneration was attributed to seed source reduction by a preceding fire (Taboada et al 2018), bark beetle outbreak (Harvey et al 2013), and clearcut logging operation (Ton and Krawchuk 2016).…”

“…Disturbance frequency also influenced whether fireand-salvage logging exhibited interactions. Taboada et al (2018) reported that, after a single fire, salvage logging did not impact forest recovery, but with two fires, reduced seedling establishment was amplified by salvage logging.…”

“…Commonly reported disturbance legacies that facilitated compound interactions included seed source reduction, deadwood accumulation and reduction, and increased light and growing space availability. For instance, reduced postfire regeneration was attributed to seed source reduction by a preceding fire (Taboada et al 2018), bark beetle outbreak (Harvey et al 2013), and clearcut logging operation (Ton and Krawchuk 2016). Salvage-mediated deadwood extraction resulted in reduced black carbon formation (Buma et al 2014) and delayed carbon and soil organic matter recovery (Kishchuk et al 2015).…”

“…Inter-disturbance recovery time also influenced the occurrence of compound interactions (Pidgen and Mallik 2013, Carlson et al 2017, Taboada et al 2018. In particular, Bowd et al (2018) and Taboada et al (2018) described how on-site seeders had insufficient time to mature and reproduce between successive fires. Although compound interactions generally involved five or fewer years between successive disturbance events, and this five-year benchmark may help characterize compound disturbances, it should not be considered a defining principle.…”