Kevin P. Hosman

On the multi-temporal correlation between photosynthesis and soil CO2 efflux: reconciling lags and observations.

Although there is increasing evidence of the temporal correlation between photosynthesis and soil CO(2) efflux, no study has so far tested its generality across the growing season at multiple study sites and across several time scales. Here, we used continuous (hourly) data and applied time series analysis (wavelet coherence analysis) to identify temporal correlations and time lags between photosynthesis and soil CO(2) efflux for three forests from different climates and a grassland. Results showed the existence of multi-temporal correlations at time periods that varied between 1 and 16 d during the growing seasons at all study sites. Temporal correlations were strongest at the 1 d time period, with longer time lags for forests relative to the grassland. The multi-temporal correlations were not continuous throughout the growing season, and were weakened when the effect of variations in soil temperature and CO(2) diffusivity on soil CO(2) efflux was taken into account. Multi-temporal correlations between photosynthesis and soil CO(2) efflux exist, and suggest that multiple biophysical drivers (i.e. photosynthesis, soil CO(2) diffusion, temperature) are likely to coexist for the regulation of allocation and transport speed of carbon during a growing season. Future studies should consider the multi-temporal influence of these biophysical drivers to investigate their effect on the transport of carbon through the soil-plant-atmosphere continuum.

New Particle Formation and Growth in an Isoprene-Dominated Ozark Forest: From Sub-5 nm to CCN-Active Sizes

Particle Investigations at a Northern Ozarks Tower: NOx, Oxidant, Isoprene Research (PINOT NOIR) were conducted in a Missouri forest dominated by isoprene emissions from May to October 2012. This study presents results of new particle formation (NPF) and the growth of new particles to cloud condensation nuclei (CCN)-active sizes (∼100 nm) observed during this field campaign. The measured sub-5 nm particles were up to ∼20,000 cm−3 during a typical NPF event. Nucleation rates J1 were relatively high (11.0 ± 10.6 cm−3 s−1), and one order of magnitude higher than formation rates of 5 nm particles (J5). Sub-5 nm particle formation events were observed during 64% of measurement days, with a high preference in biogenic volatile organic compounds (BVOCs)- and SO2-poor northwesterly (90%) air masses than in BVOCs-rich southerly air masses (13%). About 80% of sub-5 nm particle events led to the further growth. While high temperatures and high aerosol loadings in the southerly air masses were not favorable for nucleation, high BVOCs in the southerly air masses facilitated the growth of new particles to CCN-active sizes. In overall, 0.4–9.4% of the sub-5 nm particles grew to CCN-active sizes within each single NPF event. During a regional NPF event period that took place consecutively over several days, concentrations of CCN size particles increased by a factor of 4.7 in average. This enhanced production of CCN particles from new particles was commonly observed during all 13 regional NPF events during the campaign period. Copyright 2014 American Association for Aerosol Research

Testing a land model in ecosystem functional space via a comparison of observed and modeled ecosystem flux responses to precipitation regimes and associated stresses in a Central U.S. forest

1 Testing complex land surface models has often proceeded by asking the question: does 2 the model prediction agree with the observation? Such an approach has yet to produce a 3 solution to the ‘spaghetti problem of terrestrial models’. Here we test the Community 4 Land Model (CLM) by asking the question: does the model behave like an ecosystem? 5 We pursue its answer by testing CLM in the ecosystem functional space (EFS) at the 6 Missouri Ozark AmeriFlux (MOFLUX) forest site in the central USA, focusing on 7 carbon and water flux responses to precipitation regimes and associated stresses. In the 8 observed EFS, precipitation regimes and associated water and heat stresses controlled 9 seasonal and interannual variations of carbon uptake and water use in this deciduous 10 forest ecosystem. Such controls were exerted more strongly by precipitation variability 11 than by the total precipitation amount per se. A few simply constructed climate variability 12 indices captured these controls, suggesting a high degree of potential predictability. 13 While the interannual fluctuation in carbon uptake was large, a net carbon sink was 14 maintained even during an extreme drought year, suggesting a high degree of resilience 15 of this forest ecosystem to environmental stresses. Although CLM predicted seasonal and 16 interanual variations in evapotranspiration reasonably well, its predictions of carbon 17 uptake were too small across the observed range of climate variability. Also, the model 18 systematically underestimated the sensitivities of carbon uptake and evapotranspiration to 19 climate variability and overestimated the coupling between carbon and water fluxes. 20 Consequently, the modeled and observed trajectories of ecosystem fluxes did not overlap 21 in the EFS and the model did not behave like the ecosystem it attempted to simulate. We 22 suggest that future model improvements should focus on better representation and 23 parameterization of process responses to environmental stresses and on more complete 24 Subsurface Biogeochemical Research and Terrestrial Ecosystem Science Joint Meeting April 26-27, 2016 http://doesbr.org/PImeetings/2016/ and http://tes.science.energy.gov/PImeetings/2016/