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Abstract Fire frequency is increasing with climate warming in the boreal regions of interior Alaska, with short fire return intervals (< 50 years) becoming more common. Recent studies suggest these “reburns” will reduce the insulating surface organic layer (SOL) and seedbanks, inhibiting black spruce regeneration and increasing deciduous cover. These changes are projected to amplify soil warming, increasing mineral soil organic carbon (SOC) decomposition rates, and impair re-establishment of understorey vegetation and the SOL. We examined how reburns changed soil temperature, heterotrophic soil respiration (RH), and understorey gross primary production (GPP), and related these to shifts in vegetation composition and SOL depths. Two distinct burn complexes previously covered by spruce were measured; both included areas burned 1x, 2x, and 3x over 60 years and mature (≈ 90 year old) spruce forests underlain by permafrost. A 2.7 °C increase in annual near-surface soil temperatures from 1x to 3x burns was correlated with a decrease in SOL depths and a 1.9 Mg C ha−1increase in annual RH efflux. However, near-surface soil warming accounted for ≤ 23% of higher RH efflux; increases in deciduous overstorey vegetation and root biomass with reburning better correlated with RH than soil temperature. Reburning also warmed deeper soils and reduced the biomass and GPP of understory plants, lessening their potential to offset elevated RH and contribute to SOL development. This suggests that reburning led to losses of mineral SOC previously stored in permafrost due to warming soils and changes in vegetation composition, illustrating how burn frequency creates pathways for accelerated regional C loss.
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The Permafrost and Organic LayEr module for Forest Models (POLE-FM) 1.0
Abstract. Climate change and increased fire are eroding theresilience of boreal forests. This is problematic because boreal vegetationand the cold soils underneath store approximately 30 % of all terrestrialcarbon. Society urgently needs projections of where, when, and why borealforests are likely to change. Permafrost (i.e., subsurface material thatremains frozen for at least 2 consecutive years) and the thicksoil-surface organic layers (SOLs) that insulate permafrost are importantcontrols of boreal forest dynamics and carbon cycling. However, both arerarely included in process-based vegetation models used to simulate futureecosystem trajectories. To address this challenge, we developed acomputationally efficient permafrost and SOL module named the Permafrost andOrganic LayEr module for Forest Models (POLE-FM) that operates at finespatial (1 ha) and temporal (daily) resolutions. The module mechanisticallysimulates daily changes in depth to permafrost, annual SOL accumulation, andtheir complex effects on boreal forest structure and functions. We coupledthe module to an established forest landscape model, iLand, and benchmarkedthe model in interior Alaska at spatial scales of stands (1 ha) tolandscapes (61 000 ha) and over temporal scales of days to centuries. Thecoupled model generated intra- and inter-annual patterns of snowaccumulation and active layer depth (portion of soil column that thawsthroughout the year) generally consistent with independent observations in17 instrumented forest stands. The model also represented the distributionof near-surface permafrost presence in a topographically complex landscape.We simulated 39.3 % of forested area in the landscape as underlain bypermafrost, compared to the estimated 33.4 % from the benchmarkingproduct. We further determined that the model could accurately simulate mossbiomass, SOL accumulation, fire activity, tree species composition, andstand structure at the landscape scale. Modular and flexible representationsof key biophysical processes that underpin 21st-century ecologicalchange are an essential next step in vegetation simulation to reduceuncertainty in future projections and to support innovative environmentaldecision-making. We show that coupling a new permafrost and SOL module to anexisting forest landscape model increases the model's utility for projectingforest futures at high latitudes. Process-based models that representrelevant dynamics will catalyze opportunities to address previouslyintractable questions about boreal forest resilience, biogeochemicalcycling, and feedbacks to regional and global climate.
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- PAR ID:
- 10466858
- Publisher / Repository:
- Geoscientific Model Development
- Date Published:
- Journal Name:
- Geoscientific Model Development
- Volume:
- 16
- Issue:
- 7
- ISSN:
- 1991-9603
- Page Range / eLocation ID:
- 2011 to 2036
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.5194/gmd-16-2011-2023
