Gagen M.H

Gagen M.H
Swansea University | SWAN · Department of Geography

PhD

About

67
Publications
17,880
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Introduction
I am a broadly trained physical geographer with a research specialism in high-resolution palaeoclimatology. I am interested in developing reconstructions of past climates, primarily from the information stored in tree rings. I am developing dendroclimatology based palaeocloud reconstructions in the northern Boreal forest and in Sabah, Malaysian Borneo. I am interested in the use of stable isotope dendroclimatology in ‘non-traditional’ environments.
Additional affiliations
January 2006 - present
Swansea University
Position
  • Professor (Associate)

Publications

Publications (67)
Article
Tree-ring intra-annual stable isotopes (δ13C and δ18O) are powerful tools for revealing plant ecophysiological responses to climatic extremes. We analyzed interannual and fine-scale intra-annual variability of tree-ring δ13C and δ18O in Pinus massoniana from southeastern China to explore environmental drivers and potential trade-offs between the ma...
Chapter
In this chapter we introduce the climate signal in stable isotope tree-ring records, with the emphasis on temperate forests. The development of the subdiscipline is recapped followed by an exploration of isotope dendroclimatic records by geography and, broadly, by isotopic species. Whilst there are still questions to be answered around signal stren...
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Land-surface temperature changes lead to thermal contrasts between the land and the sea and have significant water cycle impacts particularly within global monsoon regions. Whilst such influence may dominate in the East Asian summer monsoon region, the long-term warm-season temperature dynamics in monsoonal China have not been effectively explored....
Data
Land-surface temperature changes lead to thermal contrasts between the land and the sea and have significant water cycle impacts particularly within global monsoon regions. Whilst such influence may dominate in the East Asian summer monsoon region, the long-term warm-season temperature dynamics in monsoonal China have not been effectively explored....
Data
The spatio-temporal variations of precipitation in the Tibetan Plateau (TP) have vital impacts on fresh water resources, and therefore the sustainable development, over a large part of Asia. Extending instrumental records of precipitation in the regional trans-boundary river basins is critical for resource managers and policy-makers to manage finit...
Article
Rising atmospheric CO2 (ca) is expected to promote tree growth and lower water loss via changes in leaf‐gas exchange. However, uncertainties remain if gas‐exchange regulation strategies are homeostatic or dynamical in response to increasing ca, as well as evolving climate and pollution inputs. Using a suite of tree‐ring‐based δ13C‐derived physiolog...
Article
Full-text available
The spatio-temporal variations of precipitation in the Tibetan Plateau (TP) have vital impacts on fresh water resources, and therefore the sustainable development, over a large part of Asia. Extending instrumental records of precipitation in the regional trans-boundary river basins is critical for resource managers and policy-makers to manage finit...
Article
Spatial learning is an ecologically important trait well studied in vertebrates and a few invertebrates yet poorly understood in crustaceans. We investigated the ability of European shore crabs, Carcinus maenas, to learn a complex maze over four consecutive weeks using food as a motivator. Crabs showed steady improvement during this conditioning pe...
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The long-term relationship between temperature and hydroclimate has remained uncertain due to the short length of instrumental measurements and inconsistent results from climate model simulations. This lack of understanding is particularly critical with regard to projected drought and flood risks. Here we assess warm-season co-variability patterns...
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Altitudinally separated bristlecone pine populations in the White Mountains (California, USA) exhibit differential climate-growth responses as temperature and tree-water relations change with altitude. These populations provide a natural experiment to explore the ecophysiological adaptations of this unique tree species to the twentieth century clim...
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Plain Language Summary Temperatures have increased globally over recent decades, strongly linked to increases in greenhouse gases. However, over Northern Fennoscandia summer temperatures have increased little over this period, although this region should be strongly affected by global warming. We suggest that changes in summer cloud cover, driven b...
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Declines are a distinctive category of tree disease, complex to define and quantify and challenging to mitigate due to their multiple causes and heterogeneous tree response patterns. In many parts of Europe oak decline syndromes are severely impacting tree health and having a measurable economic impact on forestry. In the UK the impact of periodic...
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Eocene paleoclimate reconstructions are rarely accompanied by parallel estimates of CO2 from the same locality, complicating assessment of the equilibrium climate response to elevated CO2. We reconstruct temperature, precipitation, and CO2 from latest middle Eocene (ca. 38 Ma) terrestrial sediments in the posteruptive sediment fill of the Giraffe k...
Article
Stable carbon isotope ratios from early-wood (EW) and late-wood (LW) are used to test competing models of carbon storage and allocation, providing a cost-effective alternative to measuring and dating non-structural carbohydrates in mature temperate broad-leaf forest trees growing under natural conditions. Annual samples of EW and LW from seven matu...
Article
Certain large, sustained anomalies in European temperatures in the past millennium are probably the result of internal variation. Such internal variations can modulate regional temperatures away from the expected response to greenhouse gas forcing. Here we assess the causes of European summer temperature variability over the past millennium using t...
Article
We present a drought reconstruction for southeastern China based on a tree-ring width chronology of Cryptomeria fortunei developed from two sampling sites in central Fujian. A reconstruction of July–February drought variability, spanning AD 1855–2011, was developed by calibrating total tree-ring width data with the self-calibrating Palmer drought s...
Article
We present a drought reconstruction for southeastern China based on a tree-ring width chronology of Cryptomeria fortunei developed from two sampling sites in central Fujian. A reconstruction of July–February drought variability, spanning AD 1855–2011, was developed by calibrating total tree-ring width data with the self-calibrating Palmer drought s...
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The spatial context is critical when assessing present-day climate anomalies, attributing them to potential forcings and making statements regarding their frequency and severity in a long-term perspective. Recent international initiatives have expanded the number of high-quality proxy-records and developed new statistical reconstruction methods. Th...
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One of the greatest challenges to developing time series from non-annual ring forming tropical trees arises before a sampling campaign can begin. Tropical trees can be exceptionally hard, often containing chemicals and minerals which make the wood near non-biodegradable. Such trees have considerable palaeoclimatic potential due to their longevity a...
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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 fore...
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A 520-year stable carbon isotope chronology from tree ring cellulose in high altitude larch trees (Larix decidua Mill.), from the eastern European Alps, correlates more strongly with summer temperature than with summer sunshine hours. However, when instrumental records of temperature and sunshine diverge after AD1980, the tree ring time series does...
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Tree-ring stable carbon isotope ratios (δ13C) often display a decline over the industrial period (post-AD1850) that is only partly explained by changes in the isotopic ratio of carbon dioxide (CO2) and may represent a response to increased atmospheric concentrations of CO2 (ca). If this is not addressed, reconstructions using long tree-ring stable...
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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...
Article
Accurate modelling of long-term changes in plant stomatal functioning is vital to global climate change studies because changes in evapotranspiration influence temperature via physiological forcing of the climate. Various stomatal models are included in land surface schemes, but their robustness over longer timescales is difficult to validate. We c...
Article
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 model estimates of future climate because the cloud response to temperature is not well-constrained. Here we present the first regional reconstruction of...
Article
There are some indications of large multi-decadal excursions in temperature in European climate over the recent past that do not match the estimated external forcing. Characterisation of these episodes of large internal variability, in terms of their duration, amplitude and spatial structure is important for informing the effort to develop secure c...
Article
Stable carbon isotope time-series (δ13C) from tree-rings are capable of providing valuable palaeoclimatic information, but analysis of individual tree-rings is time consuming and expensive. Pooling material from several tree-rings prior to isotopic analysis reduces costs, but does not allow the magnitude of uncertainty in the mean δ13C chronology t...
Article
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We developed a millennial length tree ring δ13C chronology from Pinus sylvestris at a site known as Laanila, in northern Finnish Lapland. In order to measure this series rapidly and with adequate replication, we adapted a multiple year pooling system. Using a combination of offset cut 5-year blocks, plus a novel method of joining tree cohorts toget...
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Stable isotope analysis of cellulose is an increasingly important aspect of ecological and palaeoenvironmental research. Since these techniques are very costly, any methodological development which can provide simultaneous measurement of stable carbon and oxygen isotope ratios in cellulose deserves further exploration. A large number (3074) of tree...
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The southeastern border of the European Alps is not well resourced with high-resolution climate proxies and experiences a distinct climatic regime from the northern and western Alpine zones. Here, we present new high-resolution climatic proxies (AD 1907–2006) from ring widths and stable carbon (δ13C), non-exchangeable hydrogen (δ2H) and oxygen (δ18...
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Stable carbon isotope (δ(13)C) series were developed from analysis of sequential radial wood increments from AD 1850 to AD 2009 for four mature primary rainforest trees from the Danum and Imbak areas of Sabah, Malaysia. The aseasonal equatorial climate meant that conventional dendrochronology was not possible as the tree species investigated do not...
Article
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Cloud cover currently represents the single greatest source of uncertainty in General Circulation Models. Stable carbon isotope ratios (δ13C) from tree-rings, in areas of low moisture stress, are likely to be primarily controlled by photosynthetically active radiation (PAR), and therefore should provide a proxy record for cloud cover or sunshine; i...
Article
We present the first near millennium-length, annually resolved stable isotope record from bristlecone pines (Pinus longaeva, D.K Bailey). The carbon isotope ratios from the cellulose of seven trees from the White Mountains of California, corrected for anthropogenic changes in atmospheric chemistry, are used to reconstruct growing season (June throu...
Article
Full-text available
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 model estimates of future climate because the cloud response to temperature is not well-constrained. Here we present the first regional reconstruction of...
Chapter
Full-text available
When trees grow, they assimilate carbon from atmospheric carbon dioxide, and hydrogen and oxygen from soil water. The stable isotope ratios of these three elements carry signals that can be interpreted in terms of past climate because isotope ratios are climatically controlled by the tree’s water and gas exchange budgets. The traditional tree-ring...
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Twentieth-century summer (July–August) temperatures in northern Finland are reconstructed using ring widths, maximum density and stable carbon isotope ratios (δ13C) of Scots pine tree rings, and using combinations of these proxies. Verification is based on the coefficient of determination (r2), reduction of error (RE) and coefficient of efficiency...
Article
Investigating the many internal feedbacks within the climate system is a vital component of the effort to quantify the full effects of future anthropogenic climate change. The stomatal apertures of plants tend to close and decrease in number under elevated CO2 concentrations, increasing water-use efficiency (WUE) and reducing canopy evapotranspirat...
Article
Investigating the many internal feedbacks within the climate system is a vital component of the effort to quantify the full effects of future anthropogenic climate change. The stomatal apertures of plants tend to close and decrease in number under elevated CO2 concentrations, increasing water-use efficiency (WUE) and reducing canopy evapotranspirat...
Article
Full-text available
Palaeoclimate information from the annual rings of trees dominates research on climates of the last thousand years because the proxy is unrivalled through its combination of geographical coverage, absolute dating and depth of study. The stable isotope treering proxies (carbon hydrogen and oxygen isotopes measured in wood cellulose extracted from an...
Article
Tree-ring stable isotope chronologies are now routinely based upon multiple samples with a high degree of replication. Recent technological advances have enabled climate to be reconstructed with a high degree of statistical veracity. Most of these pioneering studies have focused upon reconstructing the climate of the last 1000 years to enable recen...
Article
Full-text available
The absolutely dated bristlecone pine (Pinus longaeva) tree-ring chronology spans almost 9000 years, offering great potential for inferring past environmental change. Existing ring width chronologies have been widely used to produce some of the most influential millennial length temperature reconstructions for the Northern Hemisphere. A recently pu...
Article
Paleoenvironmental science provides important context for assessing ongoing and projected changes in Earth's climate and has informed research on central issues such as greenhouse gas variability, abrupt climate change, and ice sheet dynamics. Studies of the past have provided insight into natural climate variability at interannual to glacial/inter...
Article
This special issue of the Journal of Quaternary Science contains a set of 12 papers based upon a thematic session aimed at reconstructing climate of the last 1000 years from multi-proxy archives, held at the International Union for Quaternary Research (INQUA) XVII Congress in Cairns, Australia. New techniques are presented to enable the extraction...
Article
In a 2007 paper (Gagen et al., 2007) we presented a stable carbon isotope tree ring archive covering the period AD 1640-2002. The stable carbon isotope series was constructed using trunk cellulose from the annual rings of Pinus sylvestris trees sampled from close to the northern limit for the species, in Finnish Lapland. This chronology was used to...
Article
The Boreal forests contains 33% of the earth's forest cover and are located at the latitude where most of the estimated global warming is predicted to occur. Warming as a consequence of rising carbon dioxide will affect evapotranspiration within the biome, with significant consequences given that water vapour is an important greenhouse gas. However...
Article
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Non-climatic trends in tree ring δ13C series are investigated using highly replicated Pinus sylvestris δ13C series from Finnish Lapland. The declining trend in stable carbon isotopes over the industrial period, reflecting changes in the isotopic ratio of atmospheric CO2 is removed mathematically, without recourse to statistical de-trending. Regiona...
Article
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Across much of Europe, Eurasia and N. America there exist networks of long tree-ring chronologies which, under favourable circumstances, may be used to provide a record of palaeoclimate information. A proportion of these tree-ring archives, primarily those collected for archaeological dating purposes, represent a significant and largely untapped pa...
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To investigate the potential of Norway spruce (Picea abies L. Karst) as a palaeoclimate archive in the southeastern European Alps, tree ring chronologies were developed from trees growing at two sites in Slovenia which differed in their ecological and climatological characteristics. Ring width, maximum latewood density, annual height increment and...
Article
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...
Article
The trees of the northern forest have responded in an active way to the anthropogenic increase in atmospheric carbon dioxide levels. They have reduced their stomatal conductance to the extent that the ratio of internal to external carbon dioxide concentrations has remained near constant. The dominant effect is that trees have increased their water...
Article
Full-text available
Stable carbon isotope ratios from the latewood cellulose of 12 trees from two sites in northern Finland are used to construct an isotope chronology covering AD 1640 to 2002. By measuring isotopic ratios of every sample independently it is possible to identify and remove the juvenile portion of each δ 13C series, correct the individual series for an...
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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 mo...
Article
The detection and attribution of anthropogenic climate changes requires fundamental knowledge about the low- frequency variability of tropical climate including large-scale modes such as the El Niño-Southern Oscillation system (ENSO). In the tropics, a lack of instrumental climate records is compounded by a lack of high- resolution terrestrial clim...
Article
Full-text available
At present the most powerful tree-ring based climate reconstructions use high numbers of growth proxy series (ring width and density) to produce spatially smoothed estimates, such as average Northern Hemisphere summer temperatures. These single parameter reconstructions might be supplemented with regional climate reconstructions capable of capturin...
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Pine latewood width, density, and stable carbon isotope ratios were measured at two sites, separated in altitude by 400 m, close to the forest limit on a south-facing slope in the western French Alps. The signal to noise ratio in the δ13C series from each site is higher than that of either of the growth proxies. When the sites are combined, the hig...