and Fall, Ray, "Leaf isoprene emission rate as a function of atmospheric CO2 concentration" (2009 Leaf isoprene emission rate as a function of atmospheric CO 2 concentration AbstractThere is considerable interest in modeling isoprene emissions from terrestrial vegetation, because these emissions exert a principal control over the oxidative capacity of the troposphere. We used a unique field experiment that employs a continuous gradient in CO 2 concentration from 240 to 520 ppmv to demonstrate that isoprene emissions in Eucalyptus globulus were enhanced at the lowest CO 2 concentration, which was similar to the estimated CO 2 concentrations during the last Glacial Maximum, compared with 380 ppmv, the current CO 2 concentration. Leaves of Liquidambar styraciflua did not show an increase in isoprene emission at the lowest CO 2 concentration. However, isoprene emission rates from both species were lower for trees grown at 520 ppmv CO 2 compared with trees grown at 380 ppmv CO 2 . When grown in environmentally controlled chambers, trees of Populus deltoides and Populus tremuloides exhibited a 30-40% reduction in isoprene emission rate when grown at 800 ppmv CO 2 , compared with 400 ppmv CO 2 . P. tremuloides exhibited a 33% reduction when grown at 1200 ppmv CO 2 , compared with 600 ppmv CO 2 . We used current models of leaf isoprene emission to demonstrate that significant errors occur if the CO 2 inhibition of isoprene is not taken into account. In order to alleviate these errors, we present a new model of isoprene emission that describes its response to changes in atmospheric CO 2 concentration. The model logic is based on assumed competition between cytosolic and chloroplastic processes for pyruvate, one of the principal substrates of isoprene biosynthesis.

Coupled surface-atmosphere models are being used with increased frequency to make predictions of tropospheric chemistry on a 'future' earth characterized by a warmer climate and elevated atmospheric CO 2 concentration. One of the key inputs to these models is the emission of isoprene from forest ecosystems. Most models in current use rely on a scheme by which global change is coupled to changes in terrestrial net primary productivity (NPP) which, in turn, is coupled to changes in the magnitude of isoprene emissions. In this study, we conducted measurements of isoprene emissions at three prominent global change experiments in the United States. Our results showed that growth in an atmosphere of elevated CO 2 inhibited the emission of isoprene at levels that completely compensate for possible increases in emission due to increases in aboveground NPP. Exposure to a prolonged drought caused leaves to increase their isoprene emissions despite reductions in photosynthesis, and presumably NPP. Thus, the current generation of models intended to predict the response of isoprene emission to future global change probably contain large errors. A framework is offered as a foundation for constructing new isoprene emission models based on the responses of leaf biochemistry to future climate change and elevated atmospheric CO 2 concentrations.

Amid a worldwide increase in tree mortality, mountain pine beetles (Dendroctonus ponderosae Hopkins) have led to the death of billions of trees from Mexico to Alaska since 2000. This is predicted to have important carbon, water and energy balance feedbacks on the Earth system. Counter to current projections, we show that on a decadal scale, tree mortality causes no increase in ecosystem respiration from scales of several square metres up to an 84 km2 valley. Rather, we found comparable declines in both gross primary productivity and respiration suggesting little change in net flux, with a transitory recovery of respiration 6–7 years after mortality associated with increased incorporation of leaf litter C into soil organic matter, followed by further decline in years 8–10. The mechanism of the impact of tree mortality caused by these biotic disturbances is consistent with reduced input rather than increased output of carbon.

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A recent unprecedented epidemic of beetle-induced tree mortality has occurred in the lodgepole pine forests of Western North America. Here, we present the results of studies in two subalpine forests in the Rocky Mountains, one that experienced natural pine beetle disturbance and one that experienced simulated disturbance imposed through bole girdling. We assessed changes to soil microclimate and biogeochemical pools in plots representing different post-disturbance chronosequences. High plot tree mortality, whether due to girdling or beetle infestation, caused similar alterations in soil nutrient pools. During the first 4 years after disturbance, sharp declines were observed in the soil dissolved organic carbon (DOC) concentration (45-51 %), microbial biomass carbon concentration (33-39 %), dissolved organic nitrogen (DON) concentration (31-42%), and inorganic phosphorus (PO4(3-)) concentration (53-55%). Five to six years after disturbance, concentrations of DOC, DON, and PO4(3-) recovered to 71-140 % of those measured in undisturbed plots. Recovery was coincident with observed increases in litter depth and the sublitter, soil O-horizon. During the 4 years following disturbance, soil ammonium, but not nitrate, increased to 2-3 times the levels measured in undisturbed plots. Microbial biomass N increased in plots where increased ammonium was available. Our results show that previously observed declines in soil respiration following beetle-induced disturbance are accompanied by losses in key soil nutrients. Recovery of the soil nutrient pool occurs only after several years following disturbance, and is correlated with progressive mineralization of dead tree litter.

Plant isoprene emissions have been linked to several reaction pathways involved in atmospheric photochemistry. Evidence exists from a limited set of past observations that isoprene emission rate (I ) decreases as a function of increasing atmospheric CO concentration, and that increased temperature suppresses the CO effect. We studied interactions between intercellular CO concentration (C ) and temperature as they affect I in field-grown hybrid poplar trees in one of the warmest climates on earth - the Sonoran Desert of the southwestern United States. We observed an unexpected midsummer downregulation of I despite the persistence of relatively high temperatures. High temperature suppression of the I :C relation occurred at all times during the growing season, but sensitivity of I to increased C was greatest during the midsummer period when I was lowest. We interpret the seasonal downregulation of I and increased sensitivity of I to C as being caused by weather changes associated with the onset of a regional monsoon system. Our observations on the temperature suppression of the I :C relation are best explained by the existence of a small pool of chloroplastic inorganic phosphate, balanced by several large, connected metabolic fluxes, which together, determine the C and temperature dependencies of phosphoenolpyruvate import into the chloroplast.

Hybrid-poplar tree plantations provide a source for biofuel and biomass, but they also increase forest isoprene emissions. The consequences of increased isoprene emissions include higher rates of tropospheric ozone production, increases in the lifetime of methane, and increases in atmospheric aerosol production, all of which affect the global energy budget and/or lead to the degradation of air quality. Using RNA interference (RNAi) to suppress isoprene emission, we show that this trait, which is thought to be required for the tolerance of abiotic stress, is not required for high rates of photosynthesis and woody biomass production in the agroforest plantation environment, even in areas with high levels of climatic stress. Biomass production over 4 y in plantations in Arizona and Oregon was similar among genetic lines that emitted or did not emit significant amounts of isoprene. Lines that had substantially reduced isoprene emission rates also showed decreases in flavonol pigments, which reduce oxidative damage during extremes of abiotic stress, a pattern that would be expected to amplify metabolic dysfunction in the absence of isoprene production in stress-prone climate regimes. However, compensatory increases in the expression of other proteomic components, especially those associated with the production of protective compounds, such as carotenoids and terpenoids, and the fact that most biomass is produced prior to the hottest and driest part of the growing season explain the observed pattern of high biomass production with low isoprene emission. Our results show that it is possible to reduce the deleterious influences of isoprene on the atmosphere, while sustaining woody biomass production in temperate agroforest plantations.

Summary1 Measuring CO 2 concentrations and fluxes is key to the evaluation of terrestrial ecosystem carbon dynamics. Both the high cost and low portability of currently available sensors and field instruments are constraints to achieving adequate spatial and temporal coverage in characterizing ecosystem CO 2 fluxes and point concentrations. 2 We used commercially available, low-cost and low-power non-dispersive infrared (NDIR) CO 2 sensors to develop: (i) a soil CO 2 efflux system (K-33 ELG sensor, 0-1% or 10 000 ppm(v)) and (ii) a point CO 2 concentration system (K-33 BLG sensor, 0 to 30% or 300 000 ppm(v)). 3 We first calibrated the sensors against benchmark instruments (LI-COR LI-6400 efflux system and Vaisala GMP343 probe). The K-33 ELG sensor tracked the LI-6400 well during a steady reduction from~4000 ppm to background CO 2 levels (RMSE = 176 ppm). The K-33 BLG point sensor were less favourable (RMSE = 424 ppm) because of its broad range of detection, but were suitable for proof-of-concept testing at elevated CO 2 levels (> 1500 ppm). 4 In field tests of soil CO 2 efflux on locations with and without leaf litter, the K-33 efflux system yielded mean surface efflux rate values significantly lower (by~27%) than those obtained with a LI-COR LI-6400 for the bare (P = 0Á006) and litter (P = 0Á002). We explain the systematic difference in terms of flux chamber geometry and potential leakage from the prototypical system. 5 In a test on leaf cutter ant nest vents, the K-33 BLG point system yielded comparable spatial and temporal patterns and slightly higher (~10-15%) CO 2 concentrations in comparison with a Vaisala GMP343 probe. 6 The results provide proof-of-concept for the use of two low-cost, portable CO 2 sensing systems to enable terrestrial ecologists to substantially improve the characterization of CO 2 fluxes and concentrations in heterogeneous environments.