Scaling net ecosystem production and net biome production over a heterogeneous region in the western United States

Abstract

Abstract. Bottom-up scaling of net ecosystem production (NEP) and net biome production (NBP) was used to generate a carbon budget for a large heterogeneous region (the state of Oregon, 2.5×10 5 km 2 ) in the western United States. Landsat resolution (30 m) remote sensing provided the basis for mapping land cover and disturbance history, thus allowing us to account for all major fire and logging events over the last 30 years. For NEP, a 23-year record of distributed meteorology (1 km resolution) at the daily time step was used to drive a process-based carbon cycle model (Biome-BGC). For NBP, fire emissions were computed from remote sensing based estimates of area burned and our mapped biomass estimates. Our estimates for the contribution of logging and crop harvest removals to NBP were from the model simulations and were checked against public records of forest and crop harvesting. The predominately forested ecoregions within our study region had the highest NEP sinks, with ecoregion averages up to 197 gC m −2 yr −1 . Agricultural ecoregions were also NEP sinks, reflecting the imbalance of NPP and decomposition of crop residues. For the period 1996-2000, mean NEP for the study area was 17.0 TgC yr −1 , with strong interannual variation (SD of 10.6). The sum of forest harvest removals, crop removals, and direct fire emissions amounted to 63% of NEP, leaving a mean NBP of 6.1 TgC yr −1 . Carbon sequestration was predominantly on public forestland, where the harvest rate has fallen dramatically in the recent years. Comparison of simulation results with estimates of carbon stocks, and changes in carbon stocks, based on forest inventory data showed generally good agreement. The carbon sequestered as NBP, plus accumulation of forest products in slow turnover pools, offset 51% of the annual emissions of fossil fuel CO 2 for the state. Statelevel NBP dropped below zero in 2002 because of the combination of a dry climate year and a large (200 000 ha) fire.Correspondence to: D. P. Turner (david.turner@oregonstate.edu) These results highlight the strong influence of land management and interannual variation in climate on the terrestrial carbon flux in the temperate zone.

Evaluation and improvement of the Community Land Model (CLM4) in Oregon forests

“…The regional total NPP for forests is 57.6 Tg C yr −1 , a value very close to the observed total of 58.2 × 6.5 Tg C yr −1 calculated from inventory data . The regional modeled total NEP for 2001-2006 is also close to the reported value from inventory data (12.8 versus 15.2 ± 1.6 Tg C yr −1 ; , and Biome-BGC modeled results (17.0 ± 10 Tg C yr −1 ; Turner et al, 2007), but lower than uncertainty of an inverse modeling estimate (35.4 Tg ± 11 Tg C yr −1 ; Göeckede et al, 2010).…”

“…Stem biomass density is affected by removal rates through fire and harvest. The regional input harvest rates in current decades match very closely with current estimates (Turner et al, 2007), but the spatial variation varies with overestimates in the West Cascades and Klamath Mountains, which could be contributing to underestimation of stem biomass overtime. Correcting the historical rates in the input file would require thorough review of historical records and is beyond the scope of this study.…”

“…We use the PFT mean for each of these parameters in the calibrated physiology file for default model configuration evaluation, hereafter indicated as "CLM4" in figures and text. Prior modeling studies in this region indicate that a dynamic mortality rate that varies with age and/or disturbance type is necessary to predict the correct seasonal and annual carbon fluxes (Turner et al, 2007), especially in the drier forest types where mortality decreases as a percentage of live biomass as stands age. Mortality rates in Oregon forests range from 0.5 to 2 % (Turner et al, 2007;Hudiburg et al, 2009) in the absence of stand replacing disturbance.…”

“…Prior modeling studies in this region indicate that a dynamic mortality rate that varies with age and/or disturbance type is necessary to predict the correct seasonal and annual carbon fluxes (Turner et al, 2007), especially in the drier forest types where mortality decreases as a percentage of live biomass as stands age. Mortality rates in Oregon forests range from 0.5 to 2 % (Turner et al, 2007;Hudiburg et al, 2009) in the absence of stand replacing disturbance. Since dynamic mortality algorithms have not been incorporated into CLM4, we chose to use 1.0 % as a static mean value for the default configuration simulations.…”

Multiple constraint analysis of regional land-surface carbon flux

“…Thus a variety of observational datasets may be used for model calibration and validation (Wang et al., 2009), and the modelling framework allows for numerical experiments that facilitate examining the mechanisms and uncertainties associated with the simulated stocks and fluxes over daily to seasonal and interannual timescales. Notable in terms of validation is that in areas covered by forest inventory programs, bottom‐up estimates of carbon stocks and changes in stocks can be compared to aggregated inventory data (Turner et al., 2007). Opportunities for evaluation of simulated wall‐to‐wall carbon fluxes, including croplands and other non‐forest ecosystems, over a regional domain are more limited because monitoring programs in non‐forest cover types are less developed.…”