LB 23-205 - Changing rates of regeneration portend reorganization of subalpine forests of Greater Yellowstone during the 21st century

Successful postfire tree regeneration is key for sustaining forests as fire frequency, size, and severity increase. The reorganization phase (short window of time after disturbance) is especially pivotal after stand-replacing fire because trees that establish in the first decades shape forest structure and composition for centuries. Thus, early postfire regeneration can be a sensitive indicator of forest resilience and recovery pathways. We simulated subalpine forests of Greater Yellowstone (Wyoming, USA) to ask whether postfire tree regeneration rates declined under future climate scenarios in two contrasting regions: (1) a mid-elevation landscape dominated by serotinous lodgepole pines (Pinus contorta var. latifolia) that re-seed in situ following fire, and (2) a high-elevation landscape dominated by fire-sensitive conifers [Engelmann spruce (Picea engelmannii), subalpine fir (Abies lasiocarpa), and non-serotinous lodgepole pine] that depend on ex situ seed input from mature live trees. Using the spatially explicit forest landscape model iLand, fire and forest dynamics were simulated to 2100 under four climate scenarios that varied in aridity (wet-dry) and temperature (warm-hot), with 20 iterations for each climate scenario and landscape. Within areas of stand-replacing fire, we quantified early postfire tree seedling (5 yrs postfire) and sapling (20 yrs postfire) density by species. Under wet scenarios (CanESM2) that were warm or hot (RCP 4.5 or 8.5), fire remained within its historical range of variability (HRV), and postfire tree regeneration was sustained in both landscapes. Postfire regeneration of serotinous lodgepole pine remained dense in the mid-elevation landscape, and tree regeneration and species composition were sustained in the high-elevation landscape. In contrast, under dry scenarios (HadGEM2-ES), fire activity exceeded the HRV. Postfire tree regeneration declined through 2100 in both landscapes, but the magnitude and timing differed. In the mid-elevation landscape, there were modest declines in regeneration with the warm-dry future and gradual but substantial declines in the hot-dry future (e.g., from >20,000 to < 200 lodgepole pine seedlings/ha). In the high-elevation landscape, tree seedlings established with a warm-dry future but did not recruit well to saplings. With a hot-dry future, however, regeneration of Engelmann spruce and subalpine fir declined precipitously at mid-century, and postfire sapling densities dropped below 50/ha by 2100. Declines in postfire tree regeneration will slow rates of forest recovery, and tracking postfire tree regeneration following future fires can identify pathways of forest reorganization as climate and fire regimes continue to change.