Implementing a dynamic representation of fire and harvest including subgrid-scale heterogeneity in the tile-based land surface model CLASSIC v1.45

Abstract

Abstract. Canada’s forests play a critical role in the global carbon (C) cycle and are responding to unprecedented climate change as well as ongoing natural and anthropogenic disturbances. However, the representation of disturbance in boreal regions is limited in pre-existing land surface models (LSMs). Moreover, many LSMs do not explicitly represent subgrid-scale heterogeneity resulting from disturbance. To address these limitations, we implement harvest and wildfire forcings in the Canadian Land Surface Scheme Including Biogeochemical Cycles (CLASSIC) land surface model alongside dynamic tiling that represents subgrid-scale heterogeneity due to disturbance. The disturbances are captured using 30-m spatial resolution satellite data (Landsat) on an annual basis for 33 years. Using the pan-Canadian domain (i.e. all of Canada south of 76° N) as our study area for demonstration, we determine the model setup that optimally balances detailed process representation and computational efficiency. We then demonstrate the impacts of subgrid-scale heterogeneity relative to standard average individual-based representations of disturbance and explore the resultant model biases. Our results indicate that the modeling approach implemented can balance model complexity and computational cost to represent the impacts of subgrid-scale heterogeneity resulting from disturbance. Subgrid-scale heterogeneity is shown to have impacts 1.5 to 4 times the impact of disturbance alone on gross primary productivity, autotrophic respiration, and surface energy balance processes in our simulations. These impacts are a result of subgrid-scale heterogeneity slowing vegetation re-growth and affecting surface energy balance in recently disturbed, sparsely vegetated, and often snow-covered fractions of the land surface. Representing subgrid-scale heterogeneity is key to more accurately representing timber harvest, which preferentially impacts larger trees on higher quality and more accessible sites. Our results show how different discretization schemes can impact model biases resulting from the representation of disturbance. These insights, along with our implementation of dynamic tiling may apply to other tile-based LSMs. Ultimately our results enhance our understanding of, and ability to, represent disturbance within Canada to facilitate a comprehensive process-based assessment of Canada’s terrestrial C cycle.

Canada's wildfire future: climate change below a 2°C global target avoids large increases in burned area by the end of the century

“…These additional processes could include consideration of fire severity (e.g. smoldering versus crown fire), depth of fire burn, vegetation fuel quality, disturbance-mediated subgrid-scale heterogeneity, shifts in vegetation cover, nutrient cycling, peatland hydrology, peatland soils, overwintering fire, landscape fragmentation, and fire suppression policy 4,12,20,48,[67][68][69][70][71][72][73][74][75][76] . Nevertheless, by representing the influence of fuel moisture content, atmospheric humidity, and ignition on wildfire our FWI-based module realistically represents wildfire in Canada.…”

“…It couples the Canadian Land Surface Scheme (CLASS; physics) [78][79][80][81] and the Canadian Terrestrial Ecosystem Model (CTEM; biogeochemistry) 64,82 . The site level performance of CLASSIC v1.0 was evaluated by Melton et al 83 , the global performance was evaluated by Seiler et al 84 , and model updates and improvements since v1.0 are detailed in Asaadi et al 85 , MacKay et al 86 , Meyer et al 87 , and Curasi et al 22,75 . We carry out simulations of the pan-Canadian domain at 0.22°spatial resolution using 14 biogeochemical plant functional types (PFTs) suitable for Canada 22 and reflective of the boreal vegetation of other circumboreal regions.…”

“…Carbon emissions to the atmosphere are calculated from each PFT burned area using pre-defined PFT-specific fire emission fractions for each live vegetation component (i.e. both structural and non-structural leaves, stems, and roots) as well as the litter pool (ʊ; Table S3) 64,75 . The PFT-specific fire emission fractions for soil account for the distribution of soil carbon within the soil profile, as well as its combustibility under average conditions [90][91][92] .…”

“…The PFT-specific fire emission fractions for soil account for the distribution of soil carbon within the soil profile, as well as its combustibility under average conditions [90][91][92] . The quantity of live vegetation carbon transferred to the litter pool as a result of fire-related mortality is calculated from each PFT burned area using pre-defined PFT-specific mortality fractions (ϴ; Table S3) 64,75 .…”

“…CLASSIC requires seven climate forcing variables: incoming shortwave radiation, incoming longwave radiation, air temperature, precipitation rate, air pressure, specific humidity, and wind speed (See Table S1 for details of the simulations utilizing specific forcings). For model evaluation, we use a bias-corrected forcing based upon reanalysis datasets (GSWP3-W5E5-ERA5), which is described in detail by Meyer et al 87 and Curasi et al 22,75 . It combines the 1901 -1978 portion of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) GSWP3-W5E5 and the 1979-2018 portion of the ERA5 time series bias corrected to match the means of the 310 overlapping period in the GSWP3-W5E5 [93][94][95][96] .…”