Neil J. Loader

Comparison of stable carbon isotope ratios in the whole wood, cellulose and lignin of oak tree-rings

Abstract The stable carbon isotope ratios (δ13C) of whole wood, cellulose and acid-insoluble lignin from annual latewood increments of Quercus robur L., from modern and sub-fossil wood, were measured and their potential use as palaeoenvironmental indicators examined. The resulting time series demonstrate a very high degree of coherence, with δ13C of cellulose isotopically enriched by approximately 3‰ compared to δ13C of lignin. The δ13C values of all three components are influenced by the climate of July and August. Modern whole wood retains the strongest climate signal, perhaps because its composition is closest to that of leaf sugars. In sub-fossil wood there is no evidence that differential decay leads to fractionation of carbon within either cellulose or lignin, but differential decay can alter the cellulose to lignin ratio.

A 500-year record of summer near-ground solar radiation from tree-ring stable carbon isotopes

Tree-ring stable carbon isotope ratios (d13C) in environments of low moisture stress are likely to be controlled primarily by photosynthetic rate. Therefore, sunshine, rather than temperature, represents the more direct controlling factor. Temperature reconstructions based on tree-ring d13C results thus rest on the assumption that temperature and sunshine are strongly coupled. This assumption is tested using a d13C series from pine trees in NW Norway, where there are long (>100 yr) records of both summer temperature and cloud cover. It is demonstrated that when summer temperature and d13C diverge, summer temperature and cloud cover also diverge, and that cloud cover/sunshine may provide a stronger and more consistent parameter with which to calibrate tree-ring d13C series in this area. When a 500-year reconstruction of summer cloudiness is compared with a published reconstruction of summer temperatures in northern Sweden based on tree-ring maximum densities, the two time-series are largely parallel, with high levels of annual—decadal coherence. We identify, however, three distinct periods of lower frequency divergence: two (AD 1600—1650 and AD 1900—1927) when we propose summers were cool but sunny and one during the first half of the sixteenth century when summers were warm but cloudy. These episodes where temperature and sunshine decouple may represent large-scale changes in circulation as recorded in the Arctic Oscillation (AO) index. Strongly negative values of the summer AO index, as occurred during the early twentieth century, are associated with persistent high pressure over northern Norway and Fennoscandia, bringing cool summers with clear skies. Long reconstructions of cloudiness (near-ground radiation), based on tree-ring d13C series from suitable sites, would be extremely valuable for testing General Circulation Models (GCMs), because the generation of cloud is a strong control on temperature evolution, but remains a major source of uncertainty.

Spatial variability and temporal trends in water‐use efficiency of European forests

The increasing carbon dioxide (CO2 ) concentration in the atmosphere in combination with climatic changes throughout the last century are likely to have had a profound effect on the physiology of trees: altering the carbon and water fluxes passing through the stomatal pores. However, the magnitude and spatial patterns of such changes in natural forests remain highly uncertain. Here, stable carbon isotope ratios from a network of 35 tree-ring sites located across Europe are investigated to determine the intrinsic water-use efficiency (iWUE), the ratio of photosynthesis to stomatal conductance from 1901 to 2000. The results were compared with simulations of a dynamic vegetation model (LPX-Bern 1.0) that integrates numerous ecosystem and land-atmosphere exchange processes in a theoretical framework. The spatial pattern of tree-ring derived iWUE of the investigated coniferous and deciduous species and the model results agreed significantly with a clear south-to-north gradient, as well as a general increase in iWUE over the 20th century. The magnitude of the iWUE increase was not spatially uniform, with the strongest increase observed and modelled for temperate forests in Central Europe, a region where summer soil-water availability decreased over the last century. We were able to demonstrate that the combined effects of increasing CO2 and climate change leading to soil drying have resulted in an accelerated increase in iWUE. These findings will help to reduce uncertainties in the land surface schemes of global climate models, where vegetation-climate feedbacks are currently still poorly constrained by observational data.

Recent summer precipitation trends in the Greater Horn of Africa and the emerging role of Indian Ocean sea surface temperature

We utilize a variety of climate datasets to examine impacts of two mechanisms on precipitation in the Greater Horn of Africa (GHA) during northern-hemisphere summer. First, surface-pressure gradients draw moist air toward the GHA from the tropical Atlantic Ocean and Congo Basin. Variability of the strength of these gradients strongly influences GHA precipitation totals and accounts for important phenomena such as the 1960s–1980s rainfall decline and devastating 1984 drought. Following the 1980s, precipitation variability became increasingly influenced by the southern tropical Indian Ocean (STIO) region. Within this region, increases in sea-surface temperature, evaporation, and precipitation are linked with increased exports of dry mid-tropospheric air from the STIO region toward the GHA. Convergence of dry air above the GHA reduces local convection and precipitation. It also produces a clockwise circulation response near the ground that reduces moisture transports from the Congo Basin. Because precipitation originating in the Congo Basin has a unique isotopic signature, records of moisture transports from the Congo Basin may be preserved in the isotopic composition of annual tree rings in the Ethiopian Highlands. A negative trend in tree-ring oxygen-18 during the past half century suggests a decline in the proportion of precipitation originating from the Congo Basin. This trend may not be part of a natural cycle that will soon rebound because climate models characterize Indian Ocean warming as a principal signature of greenhouse-gas induced climate change. We therefore expect surface warming in the STIO region to continue to negatively impact GHA precipitation during northern-hemisphere summer.

A 1200-year multiproxy record of tree growth and summer temperature at the northern pine forest limit of Europe

Combining nine tree growth proxies from four sites, from the west coast of Norway to the Kola Peninsula of NW Russia, provides a well replicated (> 100 annual measurements per year) mean index of tree growth over the last 1200 years that represents the growth of much of the northern pine timberline forests of northern Fennoscandia. The simple mean of the nine series, z-scored over their common period, correlates strongly with mean June to August temperature averaged over this region (r = 0.81), allowing reconstructions of summer temperature based on regression and variance scaling. The reconstructions correlate significantly with gridded summer temperatures across the whole of Fennoscandia, extending north across Svalbard and south into Denmark. Uncertainty in the reconstructions is estimated by combining the uncertainty in mean tree growth with the uncertainty in the regression models. Over the last seven centuries the uncertainty is < 4.5% higher than in the 20th century, and reaches a maximum of 12% above recent levels during the 10th century. The results suggest that the 20th century was the warmest of the last 1200 years, but that it was not significantly different from the 11th century. The coldest century was the 17th. The impact of volcanic eruptions is clear, and a delayed recovery from pairs or multiple eruptions suggests the presence of some positive feedback mechanism. There is no clear and consistent link between northern Fennoscandian summer temperatures and solar forcing.

Cloud response to summer temperatures in Fennoscandia over the last thousand years

Cloud cover is one of the most important factors controlling the radiation balance of the Earth. The response of cloud cover to increasing global temperatures represents the largest uncertainty in ...

Characterizing carbon isotopic variability in Sphagnum

To understand more fully the nature of isotopic fractionation in mosses and to explore the potential of stable isotope analyses of selected peat constituents for palaeoenvironmental research, we present results from a study of inter- and intra-plant δ13C variability in Sphagnum spp. Subdivisions of stem, pendant and horizontal branch elements of modern Sphagnum capillifolium plants revealed consistent and statistically significant differences in their isotopic composition. Sequential (downstem) analysis of a further cohort of four modern Sphagnum capillifolium plants also reveals evidence of common forcing on the isotopic composition of sequentially formed stem and branch increments. This relationship was tested further by analysis of a series of branch and stem samples manually recovered from Sphagnum fuscum preserved within a late Holocene (AD 2003—1970) peat monolith from a European mire. The high degree of isotopic coherence observed between plants supports the analysis of Sphagnum in palaeoecological investigations. However inter- and intra-plant variability between both branch and stem sections emphasize the need for representative sampling, replication and sample homogeneity when conducting palaeoecological studies.

Blue intensity in Pinus sylvestris tree-rings: developing a new palaeoclimate proxy

Minimum blue intensity measurements of resin-extracted Pinus sylvestris (L.) samples, conducted using a flat-bed scanner and commercially available software, are shown to provide a robust and reliable surrogate for maximum latewood density. Blue intensity data from 15 trees, from three stands, are reported relative to a standard blue-scale in a manner similar to grey-scale calibration in x-ray densitometry. The resulting time series are highly correlated with x-ray densitometry data generated from the same samples and preserve the same high level of signal strength. Sensitivity to summer climate variables is similar to that identified in the relative density record, demonstrating that minimum blue intensity can also be used for the study of climate change. While not a replacement for the powerful range of x-ray densitometry techniques, blue intensity provides an inexpensive and accessible alternative for accessing palaeoclimatic information.

Age trends in tree ring growth and isotopic archives: A case study of Pinus sylvestris L. from northwestern Norway

Measurements of tree ring width and relative density have contributed significantly to many of the large-scale reconstructions of past climatic change, but to extract the climate signal it is first ...

Extracting Climatic Information from Stable Isotopes in Tree Rings

Publisher Summary Trees provide within the physical characteristics of their rings (width, relative density, reflectance), a record of past environmental changes, which, when expressed strongly, may be used successfully to extract palaeoclimatic information. Such approaches are now well established and have been applied globally. Trees can live for many hundreds or even thousands of years; it is, therefore, possible using these physical parameters to reconstruct climatic change throughout the life of the tree. Along with these established physical proxies, the stable carbon, hydrogen, and oxygen isotopic analyses of tree ring series provide a powerful suite of additional climate proxies. In comparison with the measurement of the physical proxies, the analysis of stable isotope ratios in tree rings is demanding, in terms of both personnel and resources. Consequently, the stable isotope analysis of tree rings can only be justified if the resulting data can provide additional, reliable climate information that cannot be obtained through alternative methods such as ring width or relative density. This chapter provides an introduction to the application of modern stable isotope techniques for the reconstruction and study of past climate from tree rings. Current challenges and limitations are discussed with specific emphasis placed on the development of robust isotope-based palaeoclimate time series and their potential for both isotopic and multiproxy analysis, using examples from a well-replicated site located close to the Boreal tree line.