| 英文摘要: | Wildfires play a key role in the boreal forest carbon cycle1, 2, and models suggest that accelerated burning will increase boreal C emissions in the coming century3. However, these predictions may be compromised because brief observational records provide limited constraints to model initial conditions4. We confronted this limitation by using palaeoenvironmental data to drive simulations of long-term C dynamics in the Alaskan boreal forest. Results show that fire was the dominant control on C cycling over the past millennium, with changes in fire frequency accounting for 84% of C stock variability. A recent rise in fire frequency inferred from the palaeorecord5 led to simulated C losses of 1.4 kg C m−2 (12% of ecosystem C stocks) from 1950 to 2006. In stark contrast, a small net C sink of 0.3 kg C m−2 occurred if the past fire regime was assumed to be similar to the modern regime, as is common in models of C dynamics. Although boreal fire regimes are heterogeneous, recent trends6 and future projections7 point to increasing fire activity in response to climate warming throughout the biome. Thus, predictions8 that terrestrial C sinks of northern high latitudes will mitigate rising atmospheric CO2 may be over-optimistic. The Arctic has experienced rapid climate change in recent decades and is projected to warm 4–5 °C—more than twice the global average—during the twenty-first century under moderate anthropogenic emissions scenarios9. High-latitude ecosystems impose critical feedbacks to global climate change by modulating the rise in atmospheric concentration of greenhouse gases. In particular, the vast boreal forest biome is estimated to serve as a net C sink of ~0.5 Pg C yr−1 (ref. 10), contributing substantially to a global terrestrial sink of 1–1.5 Pg C yr−1 in recent decades9. Longer growing seasons and rising atmospheric CO2 concentration (pCO2) could enhance boreal forest productivity in the twenty-first century, and Earth system models (ESMs) indicate that these effects will strengthen the boreal C sink8. However, observed recent trends have been heterogeneous11, and the sustainability of continued C uptake by the biome depends on many interacting factors that remain poorly understood, including changing disturbance regimes, thawing permafrost, and nutrient limitation. To constrain models of the global C cycle, it is critical to understand how these processes operate within boreal ecosystems and to scale their behaviour to the entire biome. Wildfire plays a dominant role in the C dynamics of boreal forests2, 3. In recent decades, climate warming has been linked to increased boreal forest burning, including record-breaking fire years and unprecedented regional fire regimes5, 6, 7, and future climate change is expected to increase fire activity throughout the biome7. The potential for these changes to feed back to the climate system has not been formally evaluated using ESMs, because the inclusion of fire in such models is a relatively new development8, 12. However, ecosystem models suggest that C emissions resulting from even moderate increases in burning could offset enhanced productivity caused by CO2 fertilization and climate change13, potentially converting the boreal biome from a sink to a source of C within the next century3, 14. Efforts to model fire effects on boreal C cycling may be compromised by the brevity of observational fire records, which span only the past several decades in most boreal regions. The ‘spin-up’ procedure commonly used to initialize ecosystem models often requires hundreds to thousands of model years to reach an approximately steady state, and, for lack of empirical data, prehistoric fire regimes are typically assumed to be stationary and similar to modern for the purpose of the spin-up. Model results depend strongly on this assumption3, 4, 15, and recent palaeoecological studies have challenged it by revealing striking variability in past boreal forest fire activity16, 17. In particular, a fire history reconstruction from the Yukon Flats ecoregion of Alaska indicates transition to a new fire regime within the past several decades, providing unambiguous evidence that the modern fire regime is unrepresentative of prehistoric variability5. The Yukon Flats region has experienced among the most extensive burning of any North American boreal forest in recent years18, and may therefore be indicative of widespread future change if predictions of increased burning are realized. Here we use palaeoecological data from this region as a basis for ecosystem modelling experiments to elucidate the implications of past fire-regime change to present and future C balance. We modelled C dynamics of the past millennium (850–2006) for ~2,000 km2 of boreal forest in the Yukon Flats (Supplementary Fig. 1) using the dynamic organic soil version of the Terrestrial Ecosystem Model (DOS-TEM), a process-based model designed to simulate the cycling of carbon and nitrogen through the soil and vegetation of terrestrial ecosystems (Methods and Supplementary Fig. 2). We forced the model with fire frequency and severity proxies derived from sediment charcoal records5, palaeoclimate simulations generated by the Max Planck Institute for Meteorology Earth System Model (MPI-ESM; ref. 19), and ice-core pCO2 records20. Simulated total ecosystem carbon storage (CECO) was highly variable over centennial timescales (Figs 1 and 2a), ranging from 9.6 kg C m−2 in 1230 to a maximum of 12.5 kg C m−2 in 1870. Model experiments in which each forcing was allowed to vary or held stationary reveal that the majority (83.5%) of CECO variability was due to shifts in fire frequency, and most of the remainder (14.6% of total) was accounted for by fire severity. The direct effects of climate and pCO2 were minor (1.6% and <0.1% of CECO variance, respectively). Thus, long-term C dynamics of the past millennium were almost entirely dictated by patterns of fire-regime variability.
| http://www.nature.com/nclimate/journal/v6/n1/full/nclimate2832.html
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