Land use and carbon modeling in the Sierra Nevada Mountains

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The goal of this study was to develop an integrated, regional-scale terrestrial carbon model, which can project changes in ecosystem carbon dynamics resulting from both changing biophysical conditions (e.g. CO2 fertilization, changes in climate) and land-change processes (e.g. urbanization, agricultural intensification, wildfire, harvest).

Our objective was to develop a modeling framework which could reliably reproduce estimates of carbon stocks and fluxes from a process-based biogeochemical model while increasing transparency and achieving significant computational and parameterization efficiencies.

For more information visit the Land Use and Climate Change Team website.

Using an integrated framework, we conducted simulations for a calibration scenario, and two future projections (climate only, climate and land change) based on the A1B scenario from the IPCC’s Fourth Assessment Report. Additionally, we conducted a simple sensitivity analysis of key model parameters, in an effort to demonstrate the usefulness of the integrated structure of the model. For each of the future scenarios we analyzed the impact on forest carbon storage and flux in the Sierra Nevada Mountains Ecoregion.

Project Description

Sierra Nevada study area - An integrated approach to modeling changes in land use, land cover, and disturbance and their impact

Study site map of the Sierra Nevada Ecoregion(Public domain.)

There are large uncertainties in how land and climate systems will evolve and interact to shape future ecosystem carbon dynamics. To address this we developed the Land Use and Carbon Scenario Simulator (LUCAS) to track changes in land use, land cover, land management, and disturbance, and their impact on ecosystem carbon storage and flux within a scenario-based framework.

We have combined a state-and-transition simulation model (STSM) of land change with a stock and flow model of carbon dynamics. Land-change projections downscaled from the Intergovernmental Panel on Climate Change’s (IPCC) Special Report on Emission Scenarios (SRES) were used to drive changes within the STSM, while the Integrated Biosphere Simulator (IBIS) ecosystem model was used to derive input parameters for the carbon stock and flow model.

The model was applied to the Sierra Nevada Mountains ecoregion in California, USA, a region prone to large wildfires and a forestry sector projected to intensify over the next century.

Methods

We refer to our integrated land-use and carbon model as the Land Use and Carbon Scenario Simulator (LUCAS). The motivation behind development of LUCAS was to have an integrated modeling platform capable of efficiently and robustly evaluating the effects of land-use and management actions on regional carbon dynamics. Within LUCAS, a STSM was used to project land use, land cover, and ecosystem disturbance based on future global change scenarios.

Integrated within the STSM, a stock and flow (SF) model was developed to calculate carbon storage and fluxes. The SF projects “automatic” fluxes, such as those associated with biomass growth, litterfall, and decomposition, as well as “event-based” fluxes, which can occur when a simulation cell within the STSM experiences an abrupt change, such as wildfire, harvest, or conversion into a new land use. To parameterize the SF portion of LUCAS, we ran a series of simulations using the process-based IBIS ecosystem model to generate carbon flux coefficients.

Findings and Results

Calibration scenario

The purpose of the CALIB scenario was to develop a basic set of carbon stock and flow parameters for the Sierra Nevada Mountains ecoregion, which could be used within the LUCAS framework to reproduce output from a process-based biogeochemical model. Here we compare carbon stock and flux estimates from LUCAS to those of IBIS for the CALIB scenario.

In general, the LUCAS age-structured accounting approach is able to closely replicate estimates produced by IBIS, particularly for the living biomass and soil pools:

Carbon stock projections for the Sierra Nevadas

Comparison of carbon stock estimates between the IBIS (black lines) and LUCAS (red lines) models over the 300 year calibration simulation.(Public domain.)

  • LUCAS projects an increase of total forest carbon stock density in the Sierra Nevada from 1.126 kg C/m2 in the beginning of the simulation to 10.63 kg C/m2 at the end of the 300 year simulation, compared to IBIS which projected forest carbon at 10.73 kg C/m2 at the end of the simulation.
  • Soil carbon was projected to increase from 11.09 kg C/m2 to 17.59 kg C/m2 in both models.
  • For NPP, LUCAS is able to replicate closely the NPP projection from IBIS, however, this is expected since IBIS NPP is used as an input to drive the SF model.

Climate and land-change scenarios

A future climate change scenario (IBIS-CLIM) was simulated in IBIS to generate projected growth (i.e. NPP) on an annual time-step for the Sierra Nevada Ecoregion. For the IBIS-A1B scenario, monthly climate data from the Canadian Centre for Climate Modelling and Analysis Coupled Global Climate Model (CGCM3) [41,42,43] downscaled by the Canadian Forest Service and updated in 2009 (ftp. nofc. cfs. nrcan. gc. ca) [44], were used within IBIS to generate a time-series of projected biomass growth (i.e. NPP) for the Sierra Nevada Mountains ecoregion for the period 2000-2100.

Under the LUCAS-CLIM scenario:

  • Total ecosystem carbon increased from 535.3 Tg C to 604.2 Tg C in 2100, an increase of 13%.
  • Of the total ecosystem carbon in 2100, soils accounted for approximately 45% of the stored carbon, biomass accounted for 42%, deadwood accounted for ~ 7%, and litter ~ 6%.
  • In 68% of the years, Sierra Nevada Forests were a net sink of carbon.

 

Biome productivity under two SRES scenarios for the Sierra Nevadas

Comparison of net biome productivity under two LUCAS generated scenarios. The left panel shows projected annual NBP for the climate-only (LUCAS-CLIM) scenario, and the right panel shows projected annual NBP for the climate and land change (LUCAS-LUD) scenario.(Public domain.)

Within the STSM we used a combination of transition probabilities and transition targets (i.e. explicitly defined targets for the area to be transitioned) to drive the land-use change associated with the LUCAS-LUD scenario. Transition probabilities were used to characterize natural disturbance (wildfire) and vegetation change pathways, and were based on recent historical data. Land use and land-use change transitions were based on area targets from downscaled future projections.

Under the LUCAS-LUD scenario, the major land changes were associated with wildfire and harvest, and to a lesser degree, vegetation change and urbanization:

  • Projections of forest wildfire were consistent over the projection period and averaged 78 km2/yr−1. Forest harvest averaged 122 km2/yr−1, while vegetation change was projected at 12 km2/yr−1.
  • Urbanization was rare, averaging less than 1 km2/yr−1.
  • In total, forest area in the Sierra Nevada Mountains Ecoregion was projected to decline by 1.1% (350 km2) between 2000 and 2100.
Carbon emissions associated with land use, land use change, and disturbance

For the A1B-LUD scenario, cumulative emissions associated with land use, land use change, and disturbance (left) and projected land use, land cover, and disturbance area (right).(Public domain.)