OOS 23-1 - Reburning before recovery: Effects of short-interval fire on subalpine forest nitrogen stocks and fluxes

Fire is transforming western conifer forests as climate warms. In subalpine forests adapted to infrequent (100-300-yr fire-return interval, FRI) stand-replacing fires, novel short-interval ( <30-yr FRI) fires are burning young postfire forests before they fully recover. Postfire tree regeneration is reduced and soils are hotter and drier following short-interval fire, but consequences for biogeochemical cycling are poorly understood. Recovery of nitrogen (N) lost to combustion takes decades, suggesting that N cycling may be altered with short-interval fire. We asked how postfire N stocks, N fixation and N availability varied in lodgepole pine (Pinus contorta var. latifolia) forests burned at long and short FRI in Grand Teton National Park (Wyoming, USA). In 2021 and 2022, we sampled 0.25-ha plots that burned as long-interval stand-replacing fire in 2000 (n=3) or 2016 (n=3) and nearby plots of short-interval fire (16 yr) that burned as stand-replacing fire in both years (n=6). Nitrogen stocks were calculated for live vegetation, forest floor litter, downed coarse wood and mineral soil (0-15 cm). Field-measured asymbiotic N fixation was assessed in midsummer 2021 in mineral soil and fire moss (Ceratodon purpureus), a potential N fixer, and resin bags were deployed for one year to index inorganic N availability. Forests recovering from long-interval fire in 2000 followed typical stand development trajectories, accruing substantial aboveground N stocks by 22 years postfire. Lodgepole pine N, litter N and understory N stocks averaged 169, 54 and 17 kg-N ha-1, respectively. Forests recovering from long-interval fire in 2016 were also on typical trajectories. At 5 yr postfire, lodgepole pine seedlings were dense, and lodgepole pine N, litter N and understory N stocks averaged 8, 7 and 31 kg-N ha-1, respectively. In contrast, forests burned as short-interval fire were recovering much more slowly. At 5 yr postfire, lodgepole pine N and litter N stocks were only 0.2 and 2.5 kg-N ha-1, respectively, but understory N stocks were 36 kg-N ha-1. Foliar N stocks in aspen (Populus tremuloides) that resprouted after the short-interval fire were 0.8 kg-N ha-1, 4x greater than lodgepole pines and comparable to 22-yr old aspen established from seed after the 2000 fire. Nitrogen inputs via asymbiotic fixation were modest across all plots ( <0.5 kg-N ha-1 yr-1). Short-interval fire reduced N stocks and altered partitioning of N in the ecosystem. Effects of fire on ecosystem processes are likely to compound as novel fire regimes disrupt disturbance-recovery processes in subalpine forests.