Temperature dependence of emission of volatile organic compounds (VOC) from litters collected in two Mediterranean ecosystems determined before the flaming phase of biomass burning.

Fire represents a major threat to Mediterranean terrestrial ecosystems because of the high temperatures reached during summer. While massive loads of organic, inorganic compounds and particulate matter are known to be emitted into the atmosphere from forest wildfires, less is known about the emission from vegetation surrounding fires where air temperatures higher than 100 °C can be reached. Little information exists on the emission from dead vegetation accumulated as litter over forest soils, from which fires often starts. In this study, the response of litter to heatwaves generated by nearby fires was investigated under controlled conditions. Litter samples collected in a Mediterranean maquis and a Holm oak stand during summer were placed in an enclosure flushed with a continuous flow of air, the temperature of the enclosure was progressively risen to 125 °C, until some smog developed but no flaming occurred. The gas from the enclosure was analysed for the content of CO2, H2O, and volatile organic compounds (VOC) to assess the dependence of emission from the air temperature. VOC emission was continuously determined by Proton-Transfer-Reaction mass spectrometry with time of flight (PTR-TOF-MS). Data obtained were complemented with those obtained by collecting VOC on traps that were later analysed by Gas chromatography-mass spectrometry (GC-MS). Results provided useful information to understand the emission mechanism of VOC and other gases from dead vegetation present in the litter of two Mediterranean ecosystems, both dominated by evergreen vegetation species. The study demonstrated that low molecular weight VOC and aromatic hydrocarbons (arenes) produced mostly by thermal oxidation of the wood biopolymers are emitted in addition to isoprenoids typically associated to storage organs and photosynthetic pathway. Moreover, our results support parameterization of litter VOC emission processes in air quality models.

Upside down and the game of C allocation.

Non-structural carbohydrates (NSCs) represent the primary carbon (C) reserves and play a crucial role for plant functioning and resilience. Indeed, these compounds are involved in the regulation between C supply and demand, and in the maintenance of hydraulic efficiency. NSCs are stored in parenchyma of woody organs, which is recognized as a proxy for reserve storage capacity of tree. Notwithstanding the importance of NSCs for tree physiology, their long-term regulation and trade-offs against growth were not deeply investigated. This work evaluated the long-term dynamics of mature tree reserves in stem and root, proxied by parenchyma features, and focusing on the trade off and interplay between the resources allocation in radial growth and reserves in stem and coarse root. In a Mediterranean beech forest, NSCs content, stem and root wood anatomy analysis, and eddy covariance data, were combined. The parenchyma fraction (RAP) of beech root and stem was different, due to differences in axial parenchyma (AP) and narrow ray parenchyma (nRP) fractions. However, these parenchyma components and radial growth showed synchronous inter-annual dynamics between the two organs. In beech stem, positive correlations were found among soluble sugars content and nRP, and among starch content and the AP. Positive correlations were found among Net Ecosystem Exchange (NEE) and AP of both organs. In contrast, NEE was negatively correlated to radial growth of root and stem. Our results suggest a different contribution of stem and roots to reserves storage, and a putative partitioning in the functional roles of parenchyma components. Moreover, a long-term trade-off of C allocation between growth and reserve pool was evidenced. Indeed, in case of C source reduction, trees preferentially allocate C towards reserves pool. Conversely, in high productive years, growth represents the major C sink.

Above- and belowground interplay: Canopy CO2 uptake, carbon and nitrogen allocation and isotope fractionation along the plant-ectomycorrhiza continuum.

In forests, mycorrhizal fungi regulate carbon (C) and nitrogen (N) dynamics. We evaluated the interplay among ectomycorrhizas (ECM), ecosystem C fluxes, tree productivity, C and N exchange and isotopic fractionation along the soil-ECM-plant continuum in a Mediterranean beech forest. From bud break to leaf shedding, we monitored: net ecosystem exchange (NEE, a measure of the net exchange of C between an ecosystem and the atmosphere), leaf area index, stem growth, N concentration, δ13 C and δ15 N in rhizosphere soil, ectomycorrhizal fine root tips (ERT), ECM-free fine root portions (NCR) and leaves. Seasonal changes in ERT relative biomass were strictly related to NEE and mimicked those detected in the radial growth. The analysis of δ13 C in ERT, leaves and NCR highlighted the impact of canopy photosynthesis on ERT development and an asynchronous seasonal C allocation strategy between ERT and NCR at the root tips level. Concerning N, δ15 N of leaves was negatively related to that of ERT and dependent on seasonal 15 N differences between ERT and NCR. Our results unravel a synchronous C allocation towards ERT and tree stem driven by the increasing NEE in spring-early summer. Moreover, they highlighted a phenology-dependent 15 N fractionation during N transfer from ECM to their hosts. This evidence, obtained in mature beech trees under natural conditions, may improve the knowledge of Mediterranean forests functionality.

Unravelling resilience mechanisms in forests: role of non-structural carbohydrates in responding to extreme weather events.

Extreme weather events are increasing in frequency and intensity due to global climate change. We hypothesized that tree carbon reserves are crucial for resilience of beech, buffering the source-sink imbalance due to late frosts and summer droughts, and that different components of non-structural carbohydrates (NSCs) play specific roles in coping with stressful situations. To assess the compound effects on mature trees of two extreme weather events, first a late frost in spring 2016 and then a drought in summer 2017, we monitored the phenology, radial growth and the dynamics of starch and soluble sugars in a Mediterranean beech forest. A growth reduction of 85% was observed after the spring late frost, yet not after the drought event. We observed a strong impact of late frost on starch, which also affected its dynamic at the beginning of the subsequent vegetative season. In 2017, the increase of soluble sugars, associated with starch hydrolysis, played a crucial role in coping with the severe summer drought. Non-structural carbohydrates helped to counteract the negative effects of both events, supporting plant survival and buffering source-sink imbalances under stressful conditions. Our findings indicate a strong trade-off between growth and NSC storage in trees. Overall, our results highlight the key role of NSCs on beech trees, response to extreme weather events, confirming the resilience of this species to highly stressful events. These insights are useful for assessing how forests may respond to the potential impacts of climate change on ecosystem processes in the Mediterranean area.

Frost and drought: Effects of extreme weather events on stem carbon dynamics in a Mediterranean beech forest.

The effects of short-term extreme events on tree functioning and physiology are still rather elusive. European beech is one of the most sensitive species to late frost and water shortage. We investigated the intra-annual C dynamics in stems under such conditions. Wood formation and stem CO2 efflux were monitored in a Mediterranean beech forest for 3 years (2015-2017), including a late frost (2016) and a summer drought (2017). The late frost reduced radial growth and, consequently, the amount of carbon fixed in the stem biomass by 80%. Stem carbon dioxide efflux in 2016 was reduced by 25%, which can be attributed to the reduction of effluxes due to growth respiration. Counter to our expectations, we found no effects of the 2017 summer drought on radial growth and stem carbon efflux. The studied extreme weather events had various effects on tree growth. Even though late spring frost had a strong impact on beech radial growth in the current year, trees fully recovered in the following growing season, indicating high resilience of beech to this stressful event.

Winter's bite: beech trees survive complete defoliation due to spring late-frost damage by mobilizing old C reserves.

Late frost can destroy the photosynthetic apparatus of trees. We hypothesized that this can alter the normal cyclic dynamics of C-reserves in the wood. We measured soluble sugar concentrations and radiocarbon signatures (Δ14 C) of soluble nonstructural carbon (NSC) in woody tissues sampled from a Mediterranean beech forest that was completely defoliated by an exceptional late frost in 2016. We used the bomb radiocarbon approach to estimate the time elapsed since fixation of mobilized soluble sugars. During the leafless period after the frost event, soluble sugar concentrations declined sharply while Δ14 C of NSC increased. This can be explained by the lack of fresh assimilate supply and a mobilization of C from reserve pools. Soluble NSC became increasingly older during the leafless period, with a maximum average age of 5 yr from samples collected 27 d before canopy recovery. Following leaf re-growth, soluble sugar concentrations increased and Δ14 C of soluble NSC decreased, indicating the allocation of new assimilates to the stem soluble sugars pool. These data highlight that beech trees rapidly mobilize reserve C to survive strong source-sink imbalances, for example due to late frost, and show that NSC is a key trait for tree resilience under global change.

Do atmospheric CO2 concentration increase, climate and forest management affect iWUE of common beech? Evidences from carbon isotope analyses in tree rings.

Beech is one of the most important forest tree species in Europe, hence possible adverse factors affecting its physiology and productivity can have strong ecological and economic impacts. In this context, four beech forests along a latitudinal gradient from southern Apennines to middle European lowlands were selected for chronological determinations of carbon isotope composition (δ13C) in tree-ring cellulose. The main objectives of this study were to assess (i) the effect of climate on the carbon signature of tree-ring cellulose (δ13C); (ii) the physiological response to recent CO2 concentration increment and to climatic variation; and (iii) the relationship between intrinsic water-use efficiency (iWUE, here the average long-term ratio of net photosynthesis to stomatal conductance) and growth of trees in different sites since 1950. Our results demonstrated that site climatic conditions peculiarly affect δ13C. In northern sites, a climatic control of summer precipitation and temperature on stomatal conductance was demonstrated by their opposite correlations with δ13C, negative and positive, respectively. Furthermore, an 'earliness effect' was suggested by a significant relationship between spring temperature and δ13C in the coldest sites and by a positive one between winter temperature and δ13C in the warmest ones. In all the study sites, during the maturity phase, a positive correlation between the increment of CO2 and iWUE was observed, due to an active response of trees to CO2 increment. This increment of CO2 was the main driver of the long term increasing trend of iWUE, resulting by an active response of trees to CO2 fertilization. Moreover, precipitation mostly influences positively and negatively the inter-annual variations of iWUE of the southernmost and northernmost sites, respectively. Overall, we observed a mean increment of 40% of iWUE. Moreover, the sensitivity of iWUE to the increase of CO2 was different between the northernmost and southernmost sites. Increasing iWUE was correlated to growth in the two sites during the release phase and we hypothesize a positive effect of silvicultural treatments.

Which climate change path are we following? Bad news from Scots pine.

Current expectations on future climate derive from coordinated experiments, which compile many climate models for sampling the entire uncertainty related to emission scenarios, initial conditions, and modelling process. Quantifying this uncertainty is important for taking decisions that are robust under a wide range of possible future conditions. Nevertheless, if uncertainty is too large, it can prevent from planning specific and effective measures. For this reason, reducing the spectrum of the possible scenarios to a small number of one or a few models that actually represent the climate pathway influencing natural ecosystems would substantially increase our planning capacity. Here we adopt a multidisciplinary approach based on the comparison of observed and expected spatial patterns of response to climate change in order to identify which specific models, among those included in the CMIP5, catch the real climate variation driving the response of natural ecosystems. We used dendrochronological analyses for determining the geographic pattern of recent growth trends for three European species of trees. At the same time, we modelled the climatic niche for the same species and forecasted the suitability variation expected across Europe under each different GCM. Finally, we estimated how well each GCM explains the real response of ecosystems, by comparing the expected variation with the observed growth trends. Doing this, we identified four climatic models that are coherent with the observed trends. These models are close to the highest range limit of the climatic variations expected by the ensemble of the CMIP5 models, suggesting that current predictions of climate change impacts on ecosystems could be underestimated.

Blood donor deferral pattern in Iran.

BACKGROUND: Donor selection is still being emphasized, even in the era of sensitive laboratory screening. Therefore, a number of volunteers are deferred from blood donation. This study evaluated the deferral rate in Iran, as well as the frequency of each reason in order to monitor the impact of donor selection process on donor loss. METHODS: This study was carried out on data of all volunteers who were deferred from blood donation in Isfahan Blood Transfusion Services between the years 2007 - 2008. The deferral reasons were then classified into "donor safety" and "recipient safety". The total deferral rates within sociodemographic variables and donor status were compared. Then, the rates of various specific deferral reasons were calculated. RESULTS: Out of 197,757 blood donor volunteers, 50,727 (25.6 %) were deferred from donation; 88.6 % were temporarily deferred whereas 11.4 % were permanently deferred. The deferral rate for females and males was 54.6 % and 24.3 %, respectively. Forty-three percent of the first-time donors, 9 % of the lapsed donors, and 11.9 % of the regular donors were deferred. Sixty-one percent of all deferrals were due to donors' safety while 31 % were deferred due to recipients' safety. CONCLUSION: Blood donors' deferral rate in Iran was significantly higher than other countries. This may be due to special attention to donor selection procedures. However, revision of the national criteria of donor selection and rigorous adherence to these criteria should be considered. Also, it is emphasized to improve the blood donor retention strategies to boost the regular blood donors' pool.