Ninth International Conference on Permafrost ... - IARC Research

Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s tFire disturbance moduleThe fire disturbance module is run at an annual timestep.When a fire happens, it is used to calculate the fire severitybased on the drainage (poorly drained and moderatelydrained), fire season (early season or late season), and firesize (small, large to ultra-large). The severity is used tocalculate the burned organic layer thickness, which is thenpassed to the dynamic soil layer module.The above- and below-ground living vegetation is alsokilled by wildfire. It is assumed that only 1% of abovegroundvegetation remains alive, while the fraction of livingbelow-ground vegetation depends on the burned organiclayer thickness and the fine root distribution.Dynamic soil layer moduleThe dynamic soil layer module is used to manipulatesoil layer structures to maintain stability and efficiencyof the soil temperature and moisture calculations, whenthe thickness of the organic layer is changed by wildfiredisturbance and ecological processes. There are at most 2moss layers, 3 shallow organic layers, and 3 deep organiclayers. The minimum thickness of a soil layer is set to 2 cm.When an organic layer is too thin, it may be either removedor combined to an adjacent soil layer of the same type.When an organic layer is too thick, it will be divided intotwo layers.DatasetsThe input datasets include nonspatial datasets (atmosphericCO 2concentration and fire size), grid-level datasets (0.5°by 0.5°, climate, fire return interval, fire season and soiltexture), and pixel-level datasets (1 km by 1 km vegetationtype, drainage, and fire history).Three different cohort-level datasets have been created,based on grid- and pixel-level information. To initializeDSL-TEM, each cohort is run to an equilibrium state usingthe 1901–1930 mean climate, without disturbances. Theequilibrium cohort is a unique combination of drainage,vegetation, and climate. During the next phase of simulation,the spinup stage, each cohort is run using 1901–1930atmospheric data in a cyclic fashion that considers firedisturbance over the period 1001–1900. Thus the spinupcohort is a unique combination of equilibrium cohort and firehistory during period 1001–1900. To save computing time,the fire history has been reclassified into several categoriesbased on the first recorded fire occurrence after 1900. Thefinal phase of the simulation results in a transient cohort thatis a unique combination of a spinup cohort and both climateand fire history from 1901–2006.Overall, for black spruce in the Yukon River Basin, thereare 1,167 equilibrium cohorts, 6,858 spinup cohorts, and40,738 transient cohorts. The total black spruce area in theYukon River Basin is 213,513 km 2 .Model ExperimentThe DSL-TEM was first run to equilibrium state in year1000 and then through the spinup phase over the period from1001–1900. A factorial experiment of 8 simulations was thenperformed over the period from 1901–2006, consideringthe effects of CO 2fertilization (constant vs. transient CO 2),climate (constant vs. transient climate), and fire disturbance(with and without fire disturbance).Results and Discussion1. The mean annual air temperature increased 0.39 to1.14°C over the Yukon River Basin (YRB) between 1950–1975 and 1976–2000. Winter precipitation (DJF) increased4–20 mm at the eastern and western ends of the YRB, anddecreased by 7–14 mm in the Tanana River, Eastern CentralYukon, and Koyukuk River sub-basins between the same timeperiods. The unfrozen column, which is defined as the meanthickness of unfrozen soil layer over a year, increased at boththe eastern and westerns ends of the YRB, but decreased inthe central YRB. This suggests that winter snowfall playsa more important role than air temperature in affectingpermafrost dynamics between the two time periods.2. For the whole YRB, climate plays a dominant rolein determining the thermal state of soil. The effects of fireon soil thermal state are relatively small, in part due to thesmall fraction of burn area, and in part due to the decrease ofwinter snowfall in areas with high fire return interval.3. Fire plays a dominant role in determining the netcarbon flux between the land surface and the atmosphere,especially after 1985. The difference between simulationswith and without fire can be 140 gC m -2 . However, theindirect effect of fire through increasing soil temperature onsoil decomposition is relatively small, usually less than 10gC m -2 .AcknowledgmentsWe would like to thank other coauthors, including EricKasischke, Kristen Manies, Larry Hinzman, Anna Liljedahl,Jim Raderson, Heping Liu, Vladimir Romanovsky, SergeyMarchenko, and Kim Yongwon.ReferencesKasischke, E.S. & Turetsky, M.R. 2006. Recent changesin the fire regime across the North American borealregion – Spatial and temporal patterns of burningacross Canada and Alaska. Geophysical ResearchLetters 33: doi:10.1029/2006GL025677.Yi, S., McGuire, A.D., Harden, J., Kasischke, E., Manies,K., Hinzman, L., Liljedahl, A., Randerson, J., Liu, H.,Romanovsky, V. & Marchenko, S. 2007. A dynamicsoil layer model for assessing the effects of wildfireon high latitude terrestrial ecosystem dynamics. The2007 Fall Meeting of American Geophysical Union,December 10–14, 2007, San Francisco, CA.358