Comparing the modeled deposition of PM2.5 with the Eddy Covariance flux and SEM analysis of an urban forest in Naples

Trees can remove particles from the air through the physical deposition on the leaf surface. This process depends on pollution concentration and weather conditions as wind speed and precipitation, in addition to leaf characteristics. Wind speed increases at the same time the deposition velocity and the resuspension of PM deposited, instead, the rain washes off into the soil the particles accumulated on the leaf. The PM flux removed by trees has been modeled in the i-Tree Eco model considering the effect of wind speed on deposition velocity and resuspension and fixing a threshold of leaf washing (0.2 mm x LAI). However, the results of the model have not been validated with measured data and especially the washing threshold and resuspension classes based on wind speed still remain uncertain. In this study, we compared the modeled deposition of PM2.5 with the Eddy Covariance flux measured in an urban forest in Naples. The results of the model have been further validated by comparing the expected PM2.5 accumulations on the leaf (net flux integral) with the average PM load experimentally determined in the same site where the model input data (i.e., PM concentration, wind speed and rain) have been collected. The model and Eddy Covariance presented a good agreement in assessing the deposition flux on leaves but we show that also precipitation events higher than the threshold are not able to wash all particles accumulated on leaves as confirmed by the higher accumulation of PM2.5 measured with the SEM analysis. Furthermore, a wind speed above 20 m s-1 strongly affects the deposition because of the high resuspension back to the atmosphere. Finally, we highlight the importance of including a species-specific parametrization in the model to take into account the influence of leaf characteristics on the deposition velocity, resuspension, and leaf washing

Plant respiration : Controlled by photosynthesis or biomass?

Abstract Two simplifying hypotheses have been proposed for whole-plant respiration. One links respiration to photosynthesis; the other to biomass. Using a first-principles carbon balance model with a prescribed live woody biomass turnover, applied at a forest research site where multidecadal measurements are available for comparison, we show that if turnover is fast the accumulation of respiring biomass is low and respiration depends primarily on photosynthesis; while if turnover is slow the accumulation of respiring biomass is high and respiration depends primarily on biomass. But the first scenario is inconsistent with evidence for substantial carryover of fixed carbon between years, while the second implies far too great an increase in respiration during stand development ? leading to depleted carbohydrate reserves and an unrealistically high mortality risk. These two mutually incompatible hypotheses are thus both incorrect. Respiration is not linearly related either to photosynthesis or to biomass, but it is more strongly controlled by recent photosynthates (and reserve availability) than by total biomass.Peer reviewe

Temperature Dependence of Soil Respiration Modulated by Thresholds in Soil Water Availability Across European Shrubland Ecosystems

Soil respiration (SR) is a major component of the global carbon cycle and plays a fundamental role in ecosystem feedback to climate change. Empirical modelling is an essential tool for predicting ecosystem responses to environmental change, and also provides important data for calibrating and corroborating process-based models. In this study, we evaluated the performance of three empirical temperature–SR response functions (exponential, Lloyd–Taylor and Gaussian) at seven shrublands located within three climatic regions (Atlantic, Mediterranean and Continental) across Europe. We investigated the performance of SR models by including the interaction between soil moisture and soil temperature. We found that the best fit for the temperature functions depended on the site-specific climatic conditions. Including soil moisture, we identified thresholds in the three different response functions that improved the model fit in all cases. The direct soil moisture effect on SR, however, was weak at the annual time scale. We conclude that the exponential soil temperature function may only be a good predictor for SR in a narrow temperature range, and that extrapolating predictions for future climate based on this function should be treated with caution as modelled outputs may underestimate SR. The addition of soil moisture thresholds improved the model fit at all sites, but had a far greater ecological significance in the wet Atlantic shrubland where a fundamental change in the soil CO2 efflux would likely have an impact on the whole carbon budget

Temperature dependence of soil respiration modulated by thresholds in soil water availability across European shrubland ecosystems

Soil respiration (SR) is a major component of the global carbon cycle and plays a fundamental role in ecosystem feedback to climate change. Empirical modelling is an essential tool for predicting ecosystem responses to environmental change, and also provides important data for calibrating and corroborating process-based models. In this study, we evaluated the performance of three empirical temperature–SR response functions (exponential, Lloyd–Taylor and Gaussian) at seven shrublands located within three climatic regions (Atlantic, Mediterranean and Continental) across Europe. We investigated the performance of SR models by including the interaction between soil moisture and soil temperature. We found that the best fit for the temperature functions depended on the site-specific climatic conditions. Including soil moisture, we identified thresholds in the three different response functions that improved the model fit in all cases. The direct soil moisture effect on SR, however, was weak at the annual time scale. We conclude that the exponential soil temperature function may only be a good predictor for SR in a narrow temperature range, and that extrapolating predictions for future climate based on this function should be treated with caution as modelled outputs may underestimate SR. The addition of soil moisture thresholds improved the model fit at all sites, but had a far greater ecological significance in the wet Atlantic shrubland where a fundamental change in the soil CO2 efflux would likely have an impact on the whole carbon budget

Shrubland primary production and soil respiration diverge along European climate gradient

Above- and belowground carbon (C) stores of terrestrial ecosystems are vulnerable to environmental change. Ecosystem C balances in response to environmental changes have been quantified at individual sites, but the magnitudes and directions of these responses along environmental gradients remain uncertain. Here we show the responses of ecosystem C to 8–12 years of experimental drought and night-time warming across an aridity gradient spanning seven European shrublands using indices of C assimilation (aboveground net primary production: aNPP) and soil C efflux (soil respiration: Rs). The changes of aNPP and Rs in response to drought indicated that wet systems had an overall risk of increased loss of C but drier systems did not. Warming had no consistent effect on aNPP across the climate gradient, but suppressed Rs more at the drier sites. Our findings suggest that above- and belowground C fluxes can decouple, and provide no evidence of acclimation to environmental change at a decadal timescale. aNPP and Rs especially differed in their sensitivity to drought and warming, with belowground processes being more sensitive to environmental change

Isoprenoids emission in Stipa tenacissima L.: Photosynthetic control and the effect of UV light

Fluxes of CO2 and isoprenoids were measured for the first time in Stipa tenacissima L (alfa grass), a perennial tussock grass dominant in the driest areas of Europe. In addition, we studied how those fluxes were influenced by environmental conditions, leaf ontogeny and UV radiation and compared emission rates in two contrasting seasons: summer when plants are mostly inactive and autumn, the growing season in this region. Leaf ontogeny significantly affected both photosynthesis and isoprenoids emission. Isoprene emission was positively correlated with photosynthesis, although a low isoprene emission was detected in brown leaves with a net carbon loss. Moreover, leaves with a significant lower photosynthesis emitted only monoterpenes, while at higher photosynthetic rates also isoprene was produced. Ambient UV radiation uncoupled photosynthesis and isoprene emission. It is speculated that alfa grass represent an exception from the general rules governing plant isoprenoid emitters.The research was also supported by NPU I programme (LO1415) and by Ministry of Education, CZ (LD13031).Peer Reviewe

Linking photosynthetic performances with the changes in cover degree of three Mediterranean shrubs under climate manipulation

Understanding how different combinations of plant functional traits contribute to species fitness is a question of considerable ecological interest, that can give insights into the mechanisms controlling community assembly, and into the processes by which climate change can modify plant community structure and composition. We investigated the changes in cover degree of three shrubs (Cistus monspeliensis, Dorycnium pentaphyllum and Helichrysum microphyllum) growing in a Mediterranean garrigue subjected for 11 years to a reduced rainfall regime, following a conceptual framework based on the two-phase resource dynamic model: considering the seasonal drought typical of the Mediterranean climate, the two-phases were identified based on high (pulse phase) and low (interpulse phase) soil water availability. We developed a parameter proportional to the whole plant photosynthesis (plant photosynthetic index, PPI), scaling up the leaf photosynthesis to canopy level, taking into account the different canopy densities and the fluctuations in leaf biomass due to summer leaf shedding. PPI was used to derive plant performance estimators for both pulse (maximum value reached by PPI, PPImax) and interpulse phase (duration of the exhaustion phase, Durep, when drought constrains PPI below the plant carbon compensation point determining carbon starvation). For each species the ratio between PPImax and Durep (named PPIred) was used as an index of plant performance. The reduced rainfall regime mainly decreased the performances of the dominant species C. monspeliensis, both limiting PPImax and extending Durep. Under both natural and the manipulated rainfall regime, PPIred was proportional to plant success, measured as the cover degree variation rate of the species. This result suggests that a mechanistic approach using functional traits to quantify the different performance of co-occurring species can be used to investigate 1) the drivers of the medium-term changes in species abundance and 2) the processes responsible for change in plant community composition under climate change.4n