Can fire exclusion zones enhance postfire tree regeneration? A simulation study in subalpine conifer forests.

Climate change and novel fire regimes increasingly challenge stewardship of forests adapted to infrequent, stand-replacing fire. Novel fire regimes may disrupt mechanisms that sustained postfire regeneration historically, and whether fire management can promote forest resilience to future fires is uncertain. We used the individual-based forest simulation model iLand to explore how fire exclusion zones that mimic historical burn mosaics may affect postfire tree regeneration in conifer forests of Grand Teton National Park (Wyoming, USA). We asked: (1) How do the amount and configuration of potential fire exclusion zones influence postfire tree regeneration throughout the 21st century under alternative climate scenarios? (2) How do "operational" fire exclusion zones affect postfire tree regeneration within burned patches and across the landscape by the end of the 21st century? We first conducted a factorial simulation experiment with varying amounts (10%, 30%, 50% of the landscape) and configurations (dispersed vs. clumped) of fire exclusion zones. Informed by this experiment and logistical firefighting considerations, we developed an operational scenario in which we designated mature forests surrounded by defensible fuel breaks as fire exclusion zones. Simulations were conducted under four future climate scenarios (warm-wet, hot-wet, warm-dry, hot-dry), and postfire tree regeneration densities with fire exclusion zones were compared to reference scenarios without fire exclusion zones. Regeneration of fire-avoiding conifers (subalpine fir, Abies lasiocarpa and Engelmann spruce, Picea engelmannii) was consistently greater with fire exclusion zones, especially with ≥30% of the landscape in dispersed configuration. Fire exclusion zones had minimal effects on regeneration of fire embracers (lodgepole pine, Pinus contorta var. latifolia) and fire resisters (Douglas-fir, Pseudotsuga menziesii var. glauca). In the operational scenario, postfire regeneration of fire-avoiding species was greater compared to the reference scenario, especially in hot climate scenarios. Although regeneration of fire avoiders declined in operational and reference scenarios throughout the 21st century, regeneration densities were up to 10 times greater in the operational relative to the reference scenario. Our results suggest that mimicking historical burn mosaics by establishing fire exclusion zones could sustain seed sources and afford more time for subalpine conifer forests to adapt to a warmer world with more fire.