Minimización del riesgo de extinción mediante el rescate genético

According to the genetic rescue hypothesis, immigrants can improve population persistence through their genetic contribution alone. We investigate the potential for such rescue using small, inbred laboratory populations of the bean beetle (Callosobruchus maculatus). We ask how many migrants per generation (MPG) are needed to minimize the genetic component of extinction risk. During Phase 1, population size was made to fluctuate between 6 and 60 (for 10 generations). During this phase, we manipulated the number of MPG, replacing 0, 1, 3, or 5 females every generation with immigrant females. During Phase 2, we simply set an upper limit on population size (.10). Compared with the 0–MPG treatment, the other treatments were equivalently effective at improving reproductive success and reducing extinction risk. A single MPG was sufficient for genetic rescue, apparently because effective migration rate was inflated dramatically during generations when population size was small. An analysis of quasi–extinction suggests that replicate populations in the 1–MPG treatment benefited from initial purging of inbreeding depression. Populations in this treatment performed so well apparently because they received the dual benefit of purging followed by genetic infusion. Our results suggest the need for further evaluation of alternative schemes for genetic rescue.Según la hipótesis del rescate genético, los inmigrantes pueden mejorar la persistencia de una población mediante su contribución genética. Hemos investigado el potencial de un rescate de este tipo, utilizando pequeñas poblaciones endogámicas de laboratorio del gorgojo del haba Callosobruchus maculatus. Nos preguntamos cuántos migrantes por generación (MPG) son necesarios para minimizar el componente genético del riesgo de extinción. Durante la Fase 1, se hizo fluctuar el tamaño de la población entre 6 y 60 (durante 10 generaciones). En dicha fase manipulamos el número de MPGs, reemplazando 0, 1, 3, o 5 hembras nativas por hembras inmigrantes en cada generación. Durante la Fase 2, nos limitamos a poner un límite superior al tamaño de la población (.10). Comparados con el tratamiento de 0–MPG, los otros tratamientos resultaron ser igualmente efectivos en la mejora del éxito reproductivo y la reducción del riesgo de extinción. Un único MPG era suficiente para el rescate genético, aparentemente debido a que la tasa de migración efectiva quedaba espectacularmente sobredimensionada durante generaciones, cuando el tamaño de la población era pequeño. Un análisis de cuasi–extinción sugiere que las poblaciones replicadas durante el tratamiento 1–MPG se beneficiaron de un saneamiento inicial por la disminución de la endogamia. Aparentemente, las poblaciones de este tratamiento se comportaron tan bien debido a que recibieron el doble beneficio del saneamiento seguido de la inyección genética. Nuestros resultados sugieren la necesidad de posteriores evaluaciones del rescate genético mediante esquemas alternativos

Increasing atmospheric CO2 concentrations outweighs effects of stand density in determining growth and water use efficiency in Pinus ponderosa of the semi-arid grasslands of Nebraska (U.S.A.)

This study investigated the impacts of environmental (e.g., climate and CO2 level) and ecological (e.g., stand density) factors on the long-term growth and physiology of ponderosa pine (Pinus ponderosa) in a semi-arid north American grassland. We hypothesized that ponderosa pine long-term growth patterns were positively influenced by an increase in atmospheric CO2 concentrations and a decrease in stand density. To test this hypothesis, comparison of long-term trends in tree-ring width and carbon and oxygen stable isotopic composition of trees growing in dense and sparse forest stands were carried out at two sites located in the Nebraska National Forest. Results indicated that tree-ring growth increased over time, more at the sparse than at the dense stands. In addition, the carbon and oxygen isotopic ratios showed long-term increases in intrinsic water use efficiency (WUEi), with little difference between dense and sparse stands. We found a clear trend over time in ponderosa pine tree growth and WUEi, mechanistically linked to long-term changes in global CO2 concentration. The study also highlighted that global factors tend to outweigh local effects of stand density in determining long-term trends in ponderosa pine growth. Finally, we discuss the implications of these results for woody encroachment into grasslands of Nebraska and we underlined how the use of long-term time series is crucial for understanding those ecosystems and to guarantee their conservation

The influence of traffic and wood combustion on the stable isotopic composition of carbon monoxide

Carbon monoxide in the atmosphere is originating from various combustion and oxidation processes. Recently, the proportion of CO resulting from the combustion of wood for domestic heating may have increased due to political measures promoting this renewable energy source. Here, we used the stable isotope composition of CO (δ<sup>13</sup>C and δ<sup>18</sup>O) for the characterization of different CO sources in Switzerland, along with other indicators for traffic and wood combustion (NO<sub>x</sub>-concentration, aerosol light absorption at different wavelengths). We assessed diurnal variations of the isotopic composition of CO at 3 sites during winter: a village site dominated by domestic heating, a site close to a motorway and a rural site. The isotope ratios of wood combustion emissions were studied at a test facility, indicating significantly lower δ<sup>18</sup>O of CO from wood combustion compared to traffic emissions. At the village and the motorway site, we observed very pronounced diurnal δ<sup>18</sup>O-variations of CO with an amplitude of up to 8‰. Solving the isotope mass balance equation for three distinct sources (wood combustion, traffic, clean background air) resulted in diurnal patterns consistent with other indicators for wood burning and traffic. The average night-time contribution of wood-burning to total CO was 70% at the village site, 49% at the motorway site and 29% at the rural site based on the isotope mass balance. The results, however, depend strongly on the pure source isotope values, which are not very well known. We therefore additionally applied a combined CO/NO<sub>x</sub>-isotope model for verification. Here, we separated the CO emissions into different sources based on distinct CO/NO<sub>x</sub> emissions ratios for wood combustion and traffic, and inserted this information in the isotope mass balance equation. Accordingly, a highly significant agreement between measured and calculated δ<sup>18</sup>O-values of CO was found (<i>r</i>=0.67, <i>p</i><0.001). While different proxies for wood combustion all have their uncertainties, our results indicate that the oxygen isotope ratio of CO (but not the carbon isotope ratio) is an independent sensitive tool for source attribution studies

Technical note: On uncertainties in plant water isotopic composition following extraction by cryogenic vacuum distillation

Recent studies have challenged the interpretation of plant water isotopes obtained through cryogenic vacuum distillation (CVD) based on observations of a large 2H fractionation. These studies have hypothesized the existence of an H-atom exchange between water and organic tissue during CVD extraction with the magnitude of H exchange related to relative water content of the sample; however, clear evidence is lacking. Here, we systematically tested the uncertainties in the isotopic composition of CVD-extracted water by conducting a series of incubation and rehydration experiments using isotopically depleted water, water at natural isotope abundance, woody materials with exchangeable H, and organic materials without exchangeable H (cellulose triacetate and caffeine). We show that the offsets between hydrogen and oxygen isotope ratios and expected reference values (Δ2H and Δ18O) have inversely proportional relationships with the absolute amount of water being extracted, i.e. the lower the water amount, the higher the Δ2H and Δ18O. However, neither Δ2H nor Δ18O values, were related to sample relative water content. The Δ2H pattern was more pronounced for materials with exchangeable H atoms than with non-exchangeable H atoms. This is caused by the combined effect of H exchange during the incubation of materials in water and isotopic enrichments during evaporation and sublimation that depend on absolute water amount. The H exchange during CVD extraction itself was negligible. Despite these technical issues, we observed that the water amount-dependent patterns were much less pronounced for samples at natural isotope abundance and particularly low when sufficiently high amounts of water were extracted (&gt;600 µL). Our study provides new insights into the mechanisms causing isotope fractionation during CVD extraction of water. The methodological uncertainties can be controlled if large samples of natural isotope abundance are used in ecohydrological studies.</p

Technical Note: On Uncertainties in Plant Water Isotopic Composition Following Extraction by Cryogenic Vacuum Distillation

Recent studies have challenged the interpretation of plant water isotopes obtained through cryogenic vacuum distillation (CVD) based on observations of a large 2H fractionation. These studies have hypothesized the existence of an H-atom exchange between water and organic tissue during CVD extraction with the magnitude of H exchange related to relative water content of the sample; however, clear evidence is lacking. Here, we systematically tested the uncertainties in the isotopic composition of CVD-extracted water by conducting a series of incubation and rehydration experiments using isotopically depleted water, water at natural isotope abundance, woody materials with exchangeable H, and organic materials without exchangeable H (cellulose triacetate and caffeine). We show that the offsets between hydrogen and oxygen isotope ratios and expected reference values (Δ2H and Δ18O) have inversely proportional relationships with the absolute amount of water being extracted, i.e. the lower the water amount, the higher the Δ2H and Δ18O. However, neither Δ2H nor Δ18O values, were related to sample relative water content. The Δ2H pattern was more pronounced for materials with exchangeable H atoms than with non-exchangeable H atoms. This is caused by the combined effect of H exchange during the incubation of materials in water and isotopic enrichments during evaporation and sublimation that depend on absolute water amount. The H exchange during CVD extraction itself was negligible. Despite these technical issues, we observed that the water amount-dependent patterns were much less pronounced for samples at natural isotope abundance and particularly low when sufficiently high amounts of water were extracted (\u3e600 µL). Our study provides new insights into the mechanisms causing isotope fractionation during CVD extraction of water. The methodological uncertainties can be controlled if large samples of natural isotope abundance are used in ecohydrological studies

Impact of different nitrogen emission sources on tree physiology as assessed by a triple stable isotope approach

The importance that nitrogen (N) deposition has in driving the carbon (C) sequestration of forests has recently been investigated using both experimental and modeling approaches. Whether increased N deposition has positive or negative effects on such ecosystems depends on the status of the N and the duration of the deposition. By combining d13C, d18O, d15N and dendrochronological approaches, we analyzed the impact of two different sources of NOx emissions on two tree species, namely: a broadleaved species (Quercus cerris) that was located close to an oil refinery in Southern Italy, and a coniferous species (Picea abies) located close to a freeway in Switzerland. Variations in the ci/ca ratio and the distinction between stomatal and photosynthetic responses to NOx emissions in trees were assessed using a conceptual model, which combines d13C and d18O. d15N in leaves, needles and tree rings was found to be a bioindicator of N input from anthropogenic emissions, especially at the oil refinery site.We observed that N fertilization had a stimulatory effect on tree growth near the oil refinery, while the opposite effect was found for trees at the freeway site. Changes in the ci/ca ratio were mostly related to variations in d13C at the freeway site and, thus, were driven by photosynthesis. At the oil refinery site they were mainly related to stomatal conductance, as assessed using d18O. This study demonstrates that a single method approach does not always provide a complete picture of which physiological traits are more affected by N emissions. The triple isotope approach combined with dendrochronological analyses proved to be a very promising tool for monitoring the ecophysiological responses of trees to long-term N deposition

The Distance Scale of Planetary Nebulae

By collecting distances from the literature, a set of 73 planetary nebulae with mean distances of high accuracy is derived. This sample is used for recalibration of the mass-radius relationship, used by many statistical distance methods. An attempt to correct for a statistical peculiarity, where errors in the distances influences the mass--radius relationship by increasing its slope, has been made for the first time. Distances to PNe in the Galactic Bulge, derived by this new method as well as other statistical methods from the last decade, are then used for the evaluation of these methods as distance indicators. In order of achieving a Bulge sample that is free from outliers we derive new criteria for Bulge membership. These criteria are much more stringent than those used hitherto, in the sense that they also discriminate against background objects. By splitting our Bulge sample in two, one with optically thick (small) PNe and one with optically thin (large) PNe, we find that our calibration is of higher accuracy than most other calibrations. Differences between the two subsamples, we believe, are due to the incompleteness of the Bulge sample, as well as the dominance of optical diameters in the ``thin'' sample and radio diameters in the ``thick'' sample. Our final conclusion is that statistical methods give distances that are at least as accurate as the ones obtained from many individual methods. Also, the `long' distance scale of Galactic PNe is confirmed.Comment: 15 pages, 9 figures, accepted for publication in A&

Estimating intraseasonal intrinsic water-use efficiency from high-resolution tree-ring delta C-13 data in boreal Scots pine forests

Intrinsic water-use efficiency (iWUE), a key index for carbon and water balance, has been widely estimated from tree-ring delta C-13 at annual resolution, but rarely at high-resolution intraseasonal scale. We estimated high-resolution iWUE from laser-ablation delta C-13 analysis of tree-rings (iWUE(iso)) and compared it with iWUE derived from gas exchange (iWUE(gas)) and eddy covariance (iWUE(EC)) data for two Pinus sylvestris forests from 2002 to 2019. By carefully timing iWUE(iso) via modeled tree-ring growth, iWUE(iso) aligned well with iWUE(gas) and iWUE(EC) at intraseasonal scale. However, year-to-year patterns of iWUE(gas), iWUE(iso), and iWUE(EC) were different, possibly due to distinct environmental drivers on iWUE across leaf, tree, and ecosystem scales. We quantified the modification of iWUE(iso) by postphotosynthetic delta C-13 enrichment from leaf sucrose to tree rings and by nonexplicit inclusion of mesophyll and photorespiration terms in photosynthetic discrimination model, which resulted in overestimation of iWUE(iso) by up to 11% and 14%, respectively. We thus extended the application of tree-ring delta C-13 for iWUE estimates to high-resolution intraseasonal scale. The comparison of iWUE(gas), iWUE(iso), and iWUE(EC) provides important insights into physiological acclimation of trees across leaf, tree, and ecosystem scales under climate change and improves the upscaling of ecological models.Peer reviewe

Estimating intra-seasonal photosynthetic discrimination and water use efficiency using delta C-13 of leaf sucrose in Scots pine

Comparison of assimilate delta C-13 values estimated from different leaf carbon pools, Picarro measurements, and modelled data demonstrates the potential for misinterpretation arising from bulk organic matter delta C-13 analysis.Sucrose has a unique role in recording environmental and physiological signals during photosynthesis in its carbon isotope composition (delta C-13) and transport of the signal to tree rings. Yet, instead of sucrose, total organic matter (TOM) or water-soluble carbohydrates (WSC) are typically analysed in studies that follow delta C-13 signals within trees. To study how the choice of organic material may bias the interpretation of delta C-13 records, we used mature field-grown Scots pine (Pinus sylvestris) to compare for the first time delta C-13 of different leaf carbon pools with delta C-13 of assimilates estimated by a chamber-Picarro system (delta C-13(A_Picarro)), and a photosynthetic discrimination model (delta C-13(A_model)). Compared with sucrose, the other tested carbon pools, such as TOM and WSC, poorly recorded the seasonal trends or absolute values of delta C-13(A_Picarro) and delta C-13(A_model). Consequently, in comparison with the other carbon pools, sucrose delta C-13 was superior for reconstructing changes in intrinsic water use efficiency (iWUE), agreeing in both absolute values and intra-seasonal variations with iWUE estimated from gas exchange. Thus, deriving iWUE and environmental signals from delta C-13 of bulk organic matter can lead to misinterpretation. Our findings underscore the advantage of using sucrose delta C-13 to understand plant physiological responses in depth.Peer reviewe