An alternate vegetation type proves resilient and persists for decades following forest conversion in the North American boreal biome

Climate change and natural disturbances are catalysing forest transitions to different vegetation types, but whether these new communities are resilient alternate states that will persist for decades to centuries is not known. Here, we test how changing climate, disturbance and biotic interactions shape the long-term fate of a deciduous broadleaf forest type that replaces black spruce after severe wildfires in interior Alaska, USA. We simulated postfire deciduous forest that replaced black spruce after severe fires in 2004 for tens to hundreds of years under different climate scenarios (contemporary, mid 21st century, late 21st century), fire return intervals (11-250 years), distances to seed source (50-1,000 m) and browsing intensities (background, moderate, chronic). We identified combinations of conditions where deciduous forest remained the dominant vegetation type and combinations where it returned to black spruce forest, transitioned to mixed forest (where deciduous species and black spruce co-dominate) or converted to nonforest. Deciduous forest persisted in 86% of simulations and was most resilient if fire return intervals were short (<= 50 years). When transitions to another vegetation type occurred, mixed forest was most common, particularly when fire return intervals were long (>50 years) and the nearest seed source was 500 m or farther. Moderate and chronic browsing also reduced deciduous sapling growth and survival, helping black spruce compete if fire return intervals were long and seed source was distant. Dry soils occasionally caused conversion to nonforest following short-interval fire when simulations were forced with a late 21st-century climate scenario that projects warming and increased vapor pressure deficit. Return to black spruce forest almost never occurred. Synthesis. Conversion from black spruce to deciduous forest is already underway at regional scales in interior Alaska, and similar transitions have been widely observed throughout the North American boreal biome. We show that this boreal deciduous forest type is likely a resilient alternate state that will persist through the 21st century, which is important, because future vegetation outcomes will shape biophysical feedbacks to regional climate and influence subsequent disturbance regimes.

Automated summary

The study shows that the conversion from black spruce to deciduous forest is already occurring on a regional scale in interior Alaska. The boreal deciduous forest type is likely to persist as a resilient alternate state through the 21st century, which will have important implications for regional climate and disturbance regimes.