Reburning Before Recovery: Effects of Short-Interval Fire on Subalpine Forest Nitrogen Stocks and Fluxes

In forests adapted to infrequent (> 100-year) stand-replacing fires, novel short-interval (< 30-year) fires burn young forests before they recover from previous burns. Postfire tree regeneration is reduced, plant communities shift, soils are hotter and drier, but effects on biogeochemical cycling are unresolved. We asked how postfire nitrogen (N) stocks, N availability and N fixation varied in lodgepole pine (Pinus contorta var. latifolia) forests burned at long and short intervals in Grand Teton National Park (Wyoming, USA). In 2021 and 2022, we sampled 0.25-ha plots that burned as long-interval (> 130-year) stand-replacing fire in 2000 (n = 3) or 2016 (n = 3) and nearby plots of short-interval (16-year) fire that burned as stand-replacing fire in both years (n = 6 ‘reburns’). Five years postfire, aboveground N stocks were 31% lower in short- versus long-interval fire (77 vs. 109 kg N ha−1, respectively) and 76% lower than 21-year-old stands that did not reburn (323 kg N ha−1). However, soil total N averaged 1,072 kg N ha−1 and dominated ecosystem N stocks, which averaged 1,235 kg N ha−1 and did not vary among burn categories. Annual resin-sorbed nitrate was highest in reburns and positively correlated with understory species richness and biomass. Lupinus argenteus was sparse, and asymbiotic N fixation rates were modest in all plots (< 0.1 kg N ha−1 y−1). Although ecosystem N stocks were unaffected, high-severity short-interval fire reduced and repartitioned aboveground N stocks and increased N availability. These shifts in N pools and fluxes suggest reburns can markedly alter N cycling in subalpine forests.