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

Abstract

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.

Bottom-up drivers of future fire regimes in western boreal North America

“…Simulated vegetation response to shifting climate differed based on ecoregion (figure 6) and physiographic condition (figures S5 and S6), with drier sites showing overall decreases in biomass due to drought stress (figure S7), and wetter sites showing overall increases in biomass with climate change, as in Foster et al (2019). These changes in vegetation are also similar to what has been predicted in other vegetation modeling studies in boreal North America (Mekonnen et al 2019, Hansen et al 2021. A similar pattern was predicted with changing fire intensity across different physiographic conditions (figure S8): drier sites, which also tended to have a higher deciduous fraction and greater decreases in biomass, had lower fire intensity with climate change than wetter sites.…”

“…Such 'fire self-regulation' (Parks et al 2015) may limit fire ignition and progression in young post-fire stands or in stands that have converted to deciduous due to climate change. Thus, with a predicted shift towards more deciduous forests due to fire activity and climate warming (Beck et al 2011, Foster et al 2019, Mekonnen et al 2019, Hansen et al 2021, the ultimate fate of the fire regime is dependent on the interplay between increasing fuel drying and changing fuel loading, type, and flammability.…”

Ecohydrological modelling in a deciduous boreal forest: Model evaluation for application in non‐stationary climates

“…Early snowmelt in northern latitudes is negatively correlated with vegetation greenness, suggesting that productivity may be limited by summer soil moisture, even at relatively high latitudes in interior Alaska (Barichivich et al, 2014). These deciduous forest types are projected to become increasingly prevalent as wildfire frequency increases (Hansen et al, 2020; Johnstone et al, 2010; Mann et al, 2012) and therefore a quantitative understanding of energy and water balances in these forest types is critical to understanding consequences of climate change at high latitudes.…”

Comparative drought sensitivity of co‐occurring white spruce and paper birch in interior Alaska

“…Coupled changes in the climate and wildfire regime of interior Alaska are hypothesized to promote conversion of forests from coniferous to deciduous dominance (Beck et al., 2011; Hansen et al., 2021). An implicit assumption in this ‘biome shift’ hypothesis is that deciduous tree species (paper birch and trembling aspen) are less sensitive to moisture limitation and therefore able to outcompete the coniferous species (black and white spruce) when both tree functional types establish after fire.…”

“…More severe wildfires that burn deeply and consume most of the soil organic matter in spruce forests have been shown to favour post‐fire establishment of deciduous tree species, such as paper birch and trembling aspen, especially in drier parts of the landscape, and even when the deciduous species were absent from the pre‐fire forest (Johnstone et al., 2010). Drought‐induced spruce growth declines, increasing spruce mortality and changes in post‐fire successional trajectories have led to predictions of a boreal biome shift in which deciduous forests will overtake coniferous forests as the dominant forest type (Beck et al., 2011; Hansen et al., 2021). The biome shift hypothesis implicitly assumes that deciduous tree species are better adapted to warm and dry conditions than spruce.…”