OOS 23-4 - Sparse or failed postfire tree regeneration alters plant communities and reduces carbon stocks in Greater Yellowstone

Climate change is driving more frequent and extensive wildland fire in forests of western North America. Recent fires in both mature, high-elevation forests dominated by fire-sensitive tree species and young subalpine forests have contributed to sparse or failed postfire tree regeneration. However, subsequent effects of poor forest recovery on understory plant communities and ecosystem processes are not well-understood. We asked (1) How does sparse or failed tree regeneration following high-severity fire influence plant community composition? (2) Do areas of sparse or failed tree regeneration appear to be persistent, or is there evidence of forest recovery? (3) What are the effects of sparse or failed tree regeneration on aboveground carbon (C) stocks? Plant communities following recent fires were sampled across 31 plot pairs in 2000 and 2021, with each pair including one plot burned at historical fire return intervals (FRIs; 100-300 yr) and one burned at anomalously short FRIs ( <30 yr) in the same most recent fire across the Greater Yellowstone Ecosystem, USA. In 2022, we measured tree regeneration, plant community composition, and aboveground C stocks in areas of sparse or failed tree regeneration (n=55) 34 years after the 1988 Yellowstone Fires. Across all plots in both years, native perennial species common to both forest and meadow habitats dominated plant communities. Relative to plant communities following historical FRIs, however, those following short-interval fire were increasingly composed of shade-intolerant species, and species typical of lower elevations and adapted to drier climate conditions. Ordination techniques also showed short-interval fires homogenized postfire plant communities, as those following two recent short-interval fires in different regions of Greater Yellowstone were more similar than those following the previous long-interval fires despite comparable time since fire. Additionally, many areas of poor postfire tree regeneration in fire-sensitive forests persisted after 34 years, with stem (tree, sapling, and seedling) densities ≤500 stems/ha in 45% of plots. However, other areas may yet recover to forest, with 38% of plots possessing >1000 stems/ha. Aboveground tree C stocks averaged (±SE) ~3.20 (±0.57) Mg/ha, more than an order of magnitude lower than that in 24-45-year-old recovering forests. With extensive fire- and climate-driven forest loss expected through the end of this century, areas of current sparse or failed postfire forest recovery may foreshadow the future of these burned landscapes. Forest change will drive marked changes to community structure and ecosystem function, with implications for other fire-prone regions across western North America.