The unknown third - Hydrogen isotopes in tree-ring cellulose across Europe

This is the first Europe-wide comprehensive assessment of the climatological and physiological information recorded by hydrogen isotope ratios in tree-ring cellulose (δ²H_c) based on a unique collection of annually resolved 100-year tree-ring records of two genera (Pinus and Quercus) from 17 sites (36°N to 68°N). We observed that the high-frequency climate signals in the δ²H_c chronologies were weaker than those recorded in carbon (δ¹³C_c) and oxygen isotope signals (δ¹⁸O_c) but similar to the tree-ring width ones (TRW). The δ²H_c climate signal strength varied across the continent and was stronger and more consistent for Pinus than for Quercus. For both genera, years with extremely dry summer conditions caused a significant ²H-enrichment in tree-ring cellulose. The δ²H_c inter-annual variability was strongly site-specific, as a result of the imprinting of climate and hydrology, but also physiological mechanisms and tree growth. To differentiate between environmental and physiological signals in δ²H_c, we investigated its relationships with δ¹⁸O_c and TRW. We found significant negative relationships between δ²H_c and TRW (7 sites), and positive ones between δ²H_c and δ¹⁸O_c (10 sites). The strength of these relationships was nonlinearly related to temperature and precipitation. Mechanistic δ²H_c models performed well for both genera at continental scale simulating average values, but they failed on capturing year-to-year δ²H_c variations. Our results suggest that the information recorded by δ²H_c is significantly different from that of δ¹⁸O_c, and has a stronger physiological component independent from climate, possibly related to the use of carbohydrate reserves for growth. Advancements in the understanding of ²H-fractionations and their relationships with climate, physiology, and species-specific traits are needed to improve the modelling and interpretation accuracy of δ²H_c. Such advancements could lead to new insights into trees' carbon allocation mechanisms, and responses to abiotic and biotic stress conditions.

Variations in the intrinsic water-use efficiency of north Patagonian forests under a present climate change scenario: tree age, site conditions and long-term environmental effects

The carbon isotope composition (δ13C) in tree rings were used to derive the intrinsic water-use efficiency (iWUE) of Araucaria araucana trees of northern Patagonia along a strong precipitation gradient. It is well known that climatic and ontogenetic factors affect growth performance of this species but little is known about their influence in the physiological responses, as iWUE. Thus, the main objective of this study was to assess the physiological reactions of young and adult trees from two open xeric and two moderately dense mesic A. araucana forests to the increases in atmospheric CO2 (Ca) and air temperature during the 20th century, and to relate these responses with radial tree growth. The results indicated that the iWUE and the intercellular CO2 concentration (Ci) increased 33% and 32% in average during the last century, respectively, but carbon isotope discrimination (∆13C) was more variable between sites and age classes. Trees from xeric sites presented greater iWUE and lower ∆13C and Ci values than those from mesic sites. In general, iWUE was strongly related with Ca and was significantly affected by mean summer maximum temperature. ∆13C from mesic sites seemed to be mainly affected by summer maximum temperature, while trees from xeric conditions did not show any influence. Tree age also presented a significant effect on iWUE. Adult trees showed higher iWUE values than young trees, indicating an incidence of the tree age and/or height, mainly in closed mesic forests. Moreover, some trees presented positive relationships between iWUE and radial tree growth, while others presented negative or no relationships, indicating that other factors may negatively influence tree growth. Broadly, the results demonstrate the incidence of climatic, environmental and ontogenetic variability in the tree responses; however, more studies are needed to better understand which forests will be more affected by actual and future climate changes.

Tree height strongly affects estimates of water-use efficiency responses to climate and CO2 using isotopes

Various studies report substantial increases in intrinsic water-use efficiency (W i ), estimated using carbon isotopes in tree rings, suggesting trees are gaining increasingly more carbon per unit water lost due to increases in atmospheric CO2. Usually, reconstructions do not, however, correct for the effect of intrinsic developmental changes in W i as trees grow larger. Here we show, by comparing W i across varying tree sizes at one CO2 level, that ignoring such developmental effects can severely affect inferences of trees' W i . W i doubled or even tripled over a trees' lifespan in three broadleaf species due to changes in tree height and light availability alone, and there are also weak trends for Pine trees. Developmental trends in broadleaf species are as large as the trends previously assigned to CO2 and climate. Credible future tree ring isotope studies require explicit accounting for species-specific developmental effects before CO2 and climate effects are inferred.Intrinsic water-use efficiency (W i ) reconstructions using tree rings often disregard developmental changes in W i as trees age. Here, the authors compare W i across varying tree sizes at a fixed CO2 level and show that ignoring developmental changes impacts conclusions on trees' W i responses to CO2 or climate.

A novel approach for the homogenization of cellulose to use micro-amounts for stable isotope analyses

Climate reconstructions using stable isotopes from tree-rings are steadily increasing. The investigations concentrate mostly on cellulose due to its high stability. In recent years the available amount of cellulose has steadily decreased, mainly because micro-structures of plant material have had to be analyzed. Today, the amounts of cellulose being studied are frequently in the milligram and often in the microgram range. Consequently, homogeneity problems with regard to the stable isotopes of carbon and oxygen from cellulose have occurred and these have called for new methods in the preparation of cellulose for reliable isotope analyses. Three different methods were tested for preparing isotopically homogenous cellulose, namely mechanical grinding, freezing by liquid nitrogen with subsequent milling and ultrasonic breaking of cellulose fibres. The best precision of isotope data was achieved by freeze-milling and ultrasonic breaking. However, equipment for freeze-milling is expensive and the procedure is labour-intensive. Mechanical grinding resulted in a rather high loss of material and it is also labour-intensive. The use of ultrasound for breaking cellulose fibres proved to be the best method in terms of rapidity of sample throughput, avoidance of sample loss, precision of isotope results, ease of handling, and cost.

[The agonist-antagonist nalbuphine prolongs gastro-cecal transit time and induces short-term pain following neuroleptanesthesia using fentanyl. A comparative study using a placebo]

Little is known about the effects of mixed opioid analgesics on gastrointestinal propulsion. In 20 patients, nalbuphine (0.1 mg/kg) was given after routine neuroleptanesthesia consisting of 70 micrograms/kg droperidol, 7 micrograms/kg fentanyl, and N2O/O2 (3:1) ventilation, to study its effect on gastrointestinal motility in the postoperative period. For comparison, another group of patients (n = 20) undergoing similar interventions received placebo (0.9% NaCl) at the end of the procedure. Gastrointestinal transit time was determined by measuring the exhaled H2 concentration following gastric lactulose administration. As lactulose is degraded only in the cecum, resulting in the release of hydrogen, the arrival of the polysaccharide at the terminal ileum could thus be determined. Compared to placebo, gastrointestinal transit was significantly longer in patients after nalbuphine (mean transit time 270 min vs 380 min). Pain estimation by visual analogue scale (VAS 0-10) suggested an antagonistic effect at the 10th and 20th min postoperatively, as pain scores in the nalbuphine group were higher when compared to placebo (3.5 vs 1.8 and 2.5 vs 1.4). There was a similar pain score in both groups (1.3 vs 1.4) 30 min after drug administration. However, there was significantly better pain relief after nalbuphine (0.7 vs 1.4 and 0.7 vs 1.1) in the late postoperative period (120th and 240th min). When given after potent opioids, it must be borne in mind that the antagonistic effect of nalbuphine is initially apparent. The agonistic potency of the compound will come into effect around the 30th min post-injection. Delayed gastrointestinal transit after nalbuphine is explained by agonist-like effects on peripheral opioid receptors in the gut.