[1] Forest disturbances greatly alter the carbon cycle at various spatial and temporal scales. It is critical to understand disturbance regimes and their impacts to better quantify regional and global carbon dynamics. This review of the status and major challenges in representing the impacts of disturbances in modeling the carbon dynamics across North America revealed some major advances and challenges. First, significant advances have been made in representation, scaling, and characterization of disturbances that should be included in regional modeling efforts. Second, there is a need to develop effective and comprehensive process-based procedures and algorithms to quantify the immediate and long-term impacts of disturbances on ecosystem succession, soils, microclimate, and cycles of carbon, water, and nutrients. Third, our capability to simulate the occurrences and severity of disturbances is very limited. Fourth, scaling issues have rarely been addressed in continental scale model applications. It is not fully understood which finer scale processes and properties need to be scaled to coarser spatial and temporal scales. Fifth, there are inadequate databases on disturbances at the continental scale to support the quantification of their effects on the carbon balance in North America. Finally, procedures are needed to quantify the uncertainty of model inputs, model parameters, and model structures, and thus to estimate their impacts on overall model uncertainty. Working together, the scientific community interested in disturbance and its impacts can identify the most uncertain issues surrounding the role of disturbance in the North American carbon budget and develop working hypotheses to reduce the uncertainty.

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Abstract.A new process-based model TRIPLEX-GHG was developed based on the Integrated Biosphere Simulator (IBIS), coupled with a new methane (CH 4 ) biogeochemistry module (incorporating CH 4 production, oxidation, and transportation processes) and a water table module to investigate CH 4 emission processes and dynamics that occur in natural wetlands. Sensitivity analysis indicates that the most sensitive parameters to evaluate CH 4 emission processes from wetlands are r (defined as the CH 4 to CO 2 release ratio) and Q 10 in the CH 4 production process. These two parameters were subsequently calibrated to data obtained from 19 sites collected from approximately 35 studies across different wetlands globally. Being heterogeneously spatially distributed, r ranged from 0.1 to 0.7 with a mean value of 0.23, and the Q 10 for CH 4 production ranged from 1.6 to 4.5 with a mean value of 2.48. The model performed well when simulating magnitude and capturing temporal patterns in CH 4 emissions from natural wetlands. Results suggest that the model is able to be applied to different wetlands under varying conditions and is also applicable for global-scale simulations.

Syngas (CO/H 2 ) produced from coal, natural gas, or biomass has attracted much attention as alternative to petroleumderived fuels and chemicals. Syngas can be selectively converted to oxygenates, such as alcohols, aldehydes, and carboxylic acids, or hydrocarbons by Fischer-Tropsch synthesis (FTS). [1] Industrially, rhodium- [2] and cobalt-based [3] catalysts are often used for production of C 2 oxygenates and hydrocarbons. Despite numerous studies, the exact mechanism remains in debate, and represents a major challenge in catalysis. [4] Formyl, formed by CO hydrogenation, has been implicated as of the key reactive intermediates in syngas conversion, [4e, 5] and it was proposed that hydrogenation of HCO followed by C = O bond scission leads to the formation of a CH x monomer. Then chain growth proceeds by CO insertion into CH x , by carbene coupling, or by condensation of C 1 oxygenates with elimination of water, with formation of C n (n ! 2) oxygenates or hydrocarbons. However, the short lifetime of HCO prevents its characterization, which typically requires elevated pressures, and identification of its role in syngas conversion. [6] Recently, direct evidence for HCO as the key intermediate for CO methanation was obtained by in situ spectroscopic experiments on supported Ru catalysts. [7] Herein we report on the use of DFT calculations (for computational details, see Methods) to explore the role of HCO in syngas conversion and its dependence on the catalyst. Insertion of HCO was revealed to be an efficient alternative for chain growth on Rh(111) and Co(0001) surfaces in syngas conversion for the first time. Since HCO was proposed to be a prerequisite for the formation of a CH x monomer (the key intermediate involved in chain growth), there should be a sufficiently high concentration of HCO for the formation of C 2 oxygenates and hydrocarbons. The results were compared to reaction pathways of CO insertion and carbene coupling. This work offers a mechanistic understanding of syngas chemistry, by achieving fundamental insight that could be used to design and develop improved catalysts for these and other important reactions of technological interest.We first investigated competitive CO versus HCO insertion into CH x (x = 1-3) on Rh(111), as shown in Figure 1 a. The calculated activation energy barriers for CO insertion into CH, CH 2 , and CH 3 of 1.34, 1.25, and 1.55 eV, respectively, are significantly higher than the corresponding barriers for HCO insertion (0.89, 0.75, and 1.02 eV). Compared to the most commonly studied CO insertion pathway, the kinetic preference for the HCO insertion pathway is immediately apparent. Moreover, HCO insertion into CH x is slightly endothermic or exothermic, with reaction energies of 0.27, À0.10, and À0.04 eV, whereas CO insertion is endothermic by 1.11, 0.69, and 0.35 eV, respectively. Therefore, the HCO insertion pathway is preferred on thermochemical grounds. Regardless of the pathway, insertion into CH 2 (CH 3 ) is the most kinetically favorable (unfavorable) step among al...

While wetlands are the largest natural source of methane (CH4) to the atmosphere, they represent a large source of uncertainty in the global CH4 budget due to the complex biogeochemical controls on CH4 dynamics. Here we present, to our knowledge, the first multi‐site synthesis of how predictors of CH4 fluxes (FCH4) in freshwater wetlands vary across wetland types at diel, multiday (synoptic), and seasonal time scales. We used several statistical approaches (correlation analysis, generalized additive modeling, mutual information, and random forests) in a wavelet‐based multi‐resolution framework to assess the importance of environmental predictors, nonlinearities and lags on FCH4 across 23 eddy covariance sites. Seasonally, soil and air temperature were dominant predictors of FCH4 at sites with smaller seasonal variation in water table depth (WTD). In contrast, WTD was the dominant predictor for wetlands with smaller variations in temperature (e.g., seasonal tropical/subtropical wetlands). Changes in seasonal FCH4 lagged fluctuations in WTD by ~17 ± 11 days, and lagged air and soil temperature by median values of 8 ± 16 and 5 ± 15 days, respectively. Temperature and WTD were also dominant predictors at the multiday scale. Atmospheric pressure (PA) was another important multiday scale predictor for peat‐dominated sites, with drops in PA coinciding with synchronous releases of CH4. At the diel scale, synchronous relationships with latent heat flux and vapor pressure deficit suggest that physical processes controlling evaporation and boundary layer mixing exert similar controls on CH4 volatilization, and suggest the influence of pressurized ventilation in aerenchymatous vegetation. In addition, 1‐ to 4‐h lagged relationships with ecosystem photosynthesis indicate recent carbon substrates, such as root exudates, may also control FCH4. By addressing issues of scale, asynchrony, and nonlinearity, this work improves understanding of the predictors and timing of wetland FCH4 that can inform future studies and models, and help constrain wetland CH4 emissions.

Six commonly used nonlinear growth functions were fitted to individual tree height-diameter data of nine major tree species in Ontario's boreal forests. A total of 22,571 trees was collected from new permanent sample plots across the northeast and northwest of Ontario.The available data for each species were split into two sets: the majority (90%) was used to estimate model parameters, and the remaining data (10%) were reserved to validate the models. The performance of the models was compared and evaluated by model, R2, mean difference, and mean absolute difference. The results showed that these six sigmoidal models were able to capture the height–diameter relationships and fit the data equally well, but produced different asymptote estimates. Sigmoidal models such as Chapman–Richards, Weibull, and Schnute functions provided the most satisfactory height predictions. The effect of model performance on tree volume estimation was also investigated. Tree volumes of different species were computed by Honer's volume equations using a range of diameters and the predicted tree total height from the six models. For trees with diameter less than 55 cm, the six height-diameter models produced very similar results for all species, while more differentiation among the models was observed for large-sized trees (e.g., diameters > 80 cm). North. J. Appl. For. 18:87–94.

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