FATES projections of forest structure and composition (1830-2098) at a mixed conifer site in the southern Sierra Nevada

This data repository contains model source code, parameter sets, driver data, and model output for simulations run with the Functionally Assembled Terrestrial Ecosystem Simulator (FATES) in a mixed conifer forest ~35 miles northeast of Fresno (37.0717, 119.2265) in California’s southern Sierra Nevada. The code and data provided here were used to 1) hindcast historical fire regimes, forest structure and function, including responses to logging and fire suppression and 2), project future changes in fire regimes and vegetation structure, including conifer and oak basal area and shrub cover, under future climate (2015-2098) and forest management scenarios. Model output for five validated parameterizations are provided. The code and data were designed to answer the following research questions: 1) how will projected future climate (2015-2098) affect forest structure, composition, and persistence of a representative mixed conifer forest in the Sierra Nevada; 2) how does management in the form of fuel reduction treatments and thinning influence these outcomes? The manuscript associated with this research is titled: "Will thinning and fuel reduction treatments save dry conifer forests from climate change?"

Methods
FATES was run with 5 different validated parameter files (M1-M5) available in “validated_parameterizations_070224.zip”. All simulations were forced with hourly meteorology from the midpoint of the elevation range of yellow pine and mixed conifer (YPMC) forests in the southern Sierra Nevada (1730 m; Safford & Stevens, 2017) at a point approximately 35 miles northeast of Fresno (37.0717, -119.2265). Pre-1870 simulations were initialized from bare ground and forced with pre-industrial CO2 (280 ppm) and dynamically downscaled (Rahimi et al., 2022) ERA5 reanalysis meteorology (Hersbach et al., 2020) for 700 simulation years. Pre-industrial meteorology was represented by recycling 1951-1980 meteorology because this time period had the same mean temperature in the southern Sierra Nevada as the pre-industrial period. The meteorological driver data are available in “met_driver_wrf_1950_2020.tar.gz”.

To recreate present-day forest conditions we used our pre-industrial simulations to represent the year 1870 and then logged the forest in accordance with early logging operations (Knapp et al., 2013; Stephens, 2000) by removing 95% of all large (> 75 cm dbh) conifers and varying fractions of medium-sized (45-75 cm dbh) pines, cedars, and firs (73%, 13%, and 33% respectively). We then forced the model with 20th century transient climate and CO2 (1870-2015; (Hersbach et al., 2020; Meinshausen et al., 2017; Rahimi et al., 2022). Transient climate (1951 to 2015) is in the “met_driver_wrf_1950_2020.tar.gz” file. We represented fire suppression during this period by only allowing fires to spread if fire intensity exceeded the threshold required to escape suppression efforts (1700 kW m-1) where a “hand line cannot be relied on to hold a fire” (Andrews et al., 2011). We calibrated the ignition rate so that annual burned area in the early 21st century aligned with observations (Williams et al., 2023).

To simulate future climate we forced FATES with dynamically downscaled (Rahimi et al., 2022) FGOALS-g3 climate model projections (Li et al., 2020) under the SSP3-7.0 emissions scenario (2015-2098). Future meteorological data are provided in “met_driver_wrf_ssp370.tar.gz”. We used three forest treatment scenarios: 1) “no treatment” 2) a “single treatment” in 2015 and 3) “continuous treatment” where the forest was treated every 14 years. Based on recommended intensive thinning protocols (North et al., 2007, 2022), our treatments removed small trees (< 50 cm dbh) until relative stand density index (rSDI) was below 25%. If rSDI was still above 25% after removing small trees we thinned larger trees (50-75 cm dbh). Trees larger than 75 cm dbh were not removed and pines larger than 50 cm were not removed. Based on prior analyses of the effects of prescribed fire on fuel loads (Knapp et al., 2017; Stephens and Moghaddas, 2005; Vaillant et al., 2009) we removed 64% of surface fuels in each treatment. All future scenarios assumed that wildfire suppression would continue in the future. All model output is available as a timeseries (1870-2098) in “processed_fates_output_timeseries_1830-2098.tar.gz”

References

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Knapp, E.E., Lydersen, J.M., North, M.P., Collins, B.M., 2017. Efficacy of variable density thinning and prescribed fire for restoring forest heterogeneity to mixed-conifer forest in the central Sierra Nevada, CA. Forest Ecology and Management 406, 228–241. https://doi.org/10.1016/j.foreco.2017.08.028

Knapp, E.E., Skinner, C.N., North, M.P., Estes, B.L., 2013. Long-term overstory and understory change following logging and fire exclusion in a Sierra Nevada mixed-conifer forest. Forest Ecology and Management 310, 903–914. https://doi.org/10.1016/j.foreco.2013.09.041

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North, M.P., Tompkins, R.E., Bernal, A.A., Collins, B.M., Stephens, S.L., York, R.A., 2022. Operational resilience in western US frequent-fire forests. Forest Ecology and Management 507, 120004. https://doi.org/10.1016/j.foreco.2021.120004

Rahimi, S., Krantz, W., Lin, Y.-H., Bass, B., Goldenson, N., Hall, A., Lebo, Z.J., Norris, J., 2022. Evaluation of a Reanalysis-Driven Configuration of WRF4 Over the Western United States From 1980 to 2020. Journal of Geophysical Research: Atmospheres 127, e2021JD035699. https://doi.org/10.1029/2021JD035699

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Williams, J.N., Safford, H.D., Enstice, N., Steel, Z.L., Paulson, A.K., 2023. High-severity burned area and proportion exceed historic conditions in Sierra Nevada, California, and adjacent ranges. Ecosphere 14, e4397. https://doi.org/10.1002/ecs2.4397